covert judgements are sufficient to trigger subsequent task-switching costs

ORIGINAL ARTICLE

Covert judgements are sufficient to trigger subsequenttask-switching costs

Rachel Swainson • Douglas Martin

Received: 31 May 2012 / Accepted: 20 July 2012 / Published online: 12 August 2012

� Springer-Verlag 2012

Abstract This research examines whether we have a

tendency to repeat mental processes leading to decisions or

judgements that are not accompanied by overt behaviours.

We adapted the task-switching paradigm so that on selec-

ted trials task processing would be terminated prior to

response execution. Switch costs were present subsequent

to trials where task processing was terminated either at the

stage of response selection or at the earlier stage of making

a covert judgement (a mental decision) about the target

stimulus. These costs were residual, as they occurred

despite long preparation intervals, and they did not result

from cue-switching or feature-repetition effects. We con-

clude that the same type of control mechanism may be

recruited to select between potential alternative tasks

whenever a stimulus needs to be processed in a task-

specific way, regardless of whether or not an overt response

is required.

Introduction

Studies of sequential performance have provided consid-

erable evidence that our behaviour is strongly influenced

by our past actions (e.g., Allport, Styles, & Hsieh, 1994;

Bertelson, 1965; Pashler & Baylis, 1991; Rogers & Mon-

sell, 1995). Within the realm of task switching, where

participants must act according to one or other task rule

from trial to trial, it is clear that performance is generally

better when a repetition of the same task rather than a

switch between tasks is required (Monsell, 2003). Thus, it

seems that using a task leads to a situation where it is easier

subsequently to repeat than to switch tasks. In this series of

studies, we ask what constitutes ‘‘use’’ of a task. Must an

overt response be executed in order for subsequent action

to be affected, or could the same effect be produced by a

covert stage of task processing? To put it another way, is it

possible that our thoughts impact upon our subsequent

behaviour in the same way as our actions do?

Empirical examinations of our tendency to repeat

actions often employ task-switching paradigms, where the

bias towards repetition is evident as a ‘‘switch cost’’—i.e.,

an increase in response latency or errors on trials where the

task switches from that on the preceding trial compared

with trials where the previous task is repeated. For exam-

ple, when the stimuli of interest are words presented in

different colours and the tasks require either naming the

colour in which the word is displayed or reading the word

aloud, performance is typically poorer on ‘‘switch’’ trials

(e.g., a colour-naming trial following a word-reading trial),

than ‘‘repeat’’ trials (e.g., a colour-naming trial following a

colour-naming trial; Allport et al., 1994). An important

aspect of switch costs is that they are rarely eliminated by

preparation (Monsell, 2003). Thus, even when participants

are warned in advance which task will be required (e.g.,

through a visual cue such as ‘‘colour’’ or ‘‘word’’), per-

formance on switch trials is typically still poorer than on

repeat trials. The cost that remains is termed the ‘‘residual

switch cost’’ and it is remarkably resistant to the effects of

preparation, remaining with preparation intervals of 5 s

(Sohn, Ursu, Anderson, Stenger, & Carter, 2000), although

it can seemingly be abolished under some circumstances,

e.g., with particularly high motivation (de Jong, 2000;

Verbruggen, Liefooghe, Vandierendonck, & Demanet,

2007). Intriguingly, however, performing just one trial of

the new task can eliminate the entire cost completely

R. Swainson (&) � D. Martin

School of Psychology, William Guild Building,

University of Aberdeen, Aberdeen AB24 2UB, UK

e-mail: [email protected]

123

Psychological Research (2013) 77:434–448

DOI 10.1007/s00426-012-0448-6

(Rogers & Monsell, 1995). So clearly something happens

during the performance of a task which changes the state of

bias between alternative tasks. It is as if a task-set is

somehow instantiated through use in a way that it cannot be

through preparation.

Much work has been done to establish the nature of the

switch cost, both the part that can be reduced voluntarily

and the residual part (Allport et al., 1994; Allport & Wylie,

2000; de Jong, 2000; Meiran, 2000; Rogers & Monsell,

1995; Rubinstein, Meyer, & Evans, 2001). For example,

there has been a great deal of debate around whether the

cost results from a time-consuming process of reconfigu-

ration (Rogers & Monsell, 1995) or the time taken to

overcome positive and negative priming of stimulus–

response mappings applying to the previous task (Allport &

Wylie, 1999, 2000). Recently, however, a number of

authors have begun to ask a related but different question:

what determines whether there will be a switch cost when

subsequently switching from the current task to an alter-

native task? And more specifically, which stages of task

processing are necessary to bring about subsequent switch

costs? i.e., which parts of a task does one have to complete

on a trial for there to be a switch cost when subsequently

switching to an alternative task? It is this question which

we are attempting to answer here.

In most task-switching studies, the entire task would be

completed on every trial—i.e., task rules prepared, target

stimulus processed and appropriate response selected and

executed. Any of these stages could potentially be

responsible for producing the costs evident on a subsequent

trial. The first study to look at the possibility that switch

costs might arise from the completion of a stage prior to the

overt execution of a task-based response (e.g., pressing a

button to indicate that a stimulus is ‘‘yellow’’) was that of

Schuch and Koch (2003). That study introduced a design

whereby, on particular trials within a sequence of trials, no

response was required to ‘‘target’’ stimuli; i.e., the

sequence included ‘‘no-go’’ trials. On these ‘‘no-go’’ trials,

a cue was presented at the beginning of the trial, for either

100 ms or 1,000 ms, but at the time of target stimulus

presentation, either a ‘‘no-go’’ tone, indicating that no

response would be required, was presented concurrently

with the visual target, or an alternative stimulus was pre-

sented which did not map onto a response in either task. In

a third version of the study, the tone indicated that a

‘‘double-press’’ response was required regardless of the

task or target. In all of these conditions, there was no

significant subsequent switch cost (or ‘‘backward inhibi-

tion’’—a measure of the extent to which a task is inhibited

when it is switched away from, and which can only be

measured when three alternative tasks are used within a

block of trials) present on the trial which followed the ‘‘no-

go’’ (or ‘‘double-press’’) trial. These results led Schuch and

Koch to conclude that response selection (based on task-

specific processing of the target stimulus) was the stage of

task processing which was required to cause ‘‘selection’’ of

the current task (which had been ‘‘activated’’ by the cue).

The process of task selection was thought to involve

inhibition of alternative tasks which may have remained

active to some degree, such as the task which had been

selected on the preceding trial. But because the ‘‘no-go’’

(and ‘‘double-press’’) trials would have required no pro-

cessing of the target at all using the cued task, the absence

of subsequent switch costs in that study was not necessarily

due to the absence of task-based response selection per

se, but could have been due to the absence of any task-

processing stage occurring prior to response execution. The

question remains, then, whether earlier task-specific pro-

cesses may be sufficient to trigger subsequent switching

costs.

There is already some evidence on whether task-specific

processes prior to overt response execution may trigger

subsequent costs. The initial preparation stage alone (e.g.,

following a cue indicating that the current task is ‘‘colour’’

but prior to presentation of a specific target upon which the

task can be used) does not appear to be sufficient to trigger

subsequent costs, at least not costs which are residual. For

example, no switch costs tend to be found following cued

‘‘no-go’’ trials with long preparation intervals, where par-

ticipants are able to prepare fully the appropriate task

(Astle, Jackson, & Swainson, 2006; Schuch & Koch,

2003). Lenartowicz, Yeung, and Cohen (2011) have

recently provided evidence that processing a task cue alone

can lead to subsequent costs, but only non-residual ones.

There is also some evidence that switch costs can be

generated by an intermediate stage of task processing—i.e.,

categorisation of the target stimulus or selection of a

response (in the absence of execution)—but again, no

evidence yet, as far as we are aware, that these costs can be

residual. Verbruggen et al. (2006) used a ‘‘selective stop-

ping’’ paradigm in which a ‘‘stop’’ signal, presented after a

target stimulus on a proportion of trials, signalled that a

specific response (e.g., left hand) should not be executed;

correct stopping relied upon the task having been used to

select a specific response, and therefore the costs which

were present on the subsequent trial could have been due to

any task-processing stage up to and including response

selection. As only a short preparation interval (300 ms

CTI) was used, we do not know whether residual costs

would have been generated by this method. Philipp, Jo-

licoeur, Falkenstein, and Koch (2007) used a design similar

to that used in the current study (Experiment 1), in which

target stimuli were followed by a delayed ‘‘go’’ or ‘‘no-go’’

signal; while switch costs were reduced following ‘‘no-go’’

trials, they remained significant. Here again, residual

switch costs were not tested for as only a fixed, short CTI

Psychological Research (2013) 77:434–448 435

123

was used (100 ms). Residual costs are highly resistant to

voluntary efforts to switch tasks, suggesting that they

reflect the state of activity in non-declarative cognitive

systems; Rubinstein, Meyer, and Evans (2001) suggest that

the swapping of rules in procedural working memory

constitutes the ‘‘rule activation’’ stage of task-switching

and accounts for the residual switch cost. It may be that

only processes relating to an overt response (i.e., response

selection or execution) can alter the state of such systems,

and therefore that an earlier stage of processing such as a

mental judgement would be incapable of doing so; if that is

the case, mental judgements about target stimuli should not

produce subsequent residual switch costs.

The aim of the current study was to examine whether

intermediate stages of task processing are sufficient for the

generation of subsequent switch costs, and whether they

can produce residual costs. Knowing that response execu-

tion is sufficient to produce costs but that it might not be

necessary, we worked backwards to ask initially whether

selecting but not executing a response would trigger sub-

sequent costs (Experiment 1). Upon finding that it did, we

then asked whether an earlier stage might be sufficient. We

found significant costs following a mental judgement not

associated with any overt response (Experiment 2) and we

then examined whether these costs could be residual

(Experiment 3) and whether they could be the product of a

confound with cue-switching (Experiment 4) or due to the

presence of feature-repetitions (Experiment 5).

In all of our experiments, we presented participants with

blocks of trials in which most trials required an overt

response (‘‘go’’ trials), with a smaller proportion requiring

no overt response (‘‘no-go’’ trials). In order to determine

whether switch costs could be triggered by the task-pro-

cessing steps which took place on ‘‘no-go’’ trials, we

examined performance on ‘‘go’’ trials which followed ‘‘no-

go’’ trials. Performance was compared between trials

where the preceding (‘‘no-go’’) trial had cued the same task

as that required on the current trial (‘‘repetition trials’’) or

where it had cued a different task (‘‘switch trials’’), the

difference between these representing the ‘‘switch cost’’

(switch minus repetition). Any significant switch cost had

to be due to the effects of partially completing the cued

task on the preceding ‘‘no-go’’ trial.

In Experiments 1–4, we constrained our analysis to

three-trial sequences where the first and last trial of the

sequences would always be a ‘‘go’’ trial and would always

share the same task. The idea was that this should give

relatively large switch costs because switching back to a

recently inhibited task is particularly difficult (the ‘‘back-

ward inhibition’’ phenomenon; Mayr & Keele, 2000) and

repeating a task for three successive trials is likely to be

particularly easy. In Experiment 5, we relaxed this limi-

tation and used the more usual procedure of defining trials

as ‘‘switch’’ or ‘‘repetition’’ only in terms of the current and

preceding trial, so that our results could be applied more

generally to the rest of the task-switching literature.

By manipulating the format of our ‘‘no-go’’ trials we

were able to examine the extent (stage of processing) to

which a task had to be performed in order to effectively

instantiate it and therefore trigger subsequent costs. The

idea was that if the demands of the preceding trial did not

require participants to select or instantiate the task-set

fully, no switch costs would be seen on the current trial. So

clearly, if the task was not used at all on the preceding trial,

then we would expect to see no switch costs. We expected

that this would be the case for what we called ‘‘no-target’’

trials in Experiment 1—i.e., trials on which a neutral

stimulus was presented (akin to the ‘‘no-go’’ trials of

Schuch & Koch, 2003, and Astle et al., 2006). However,

given the findings of Verbruggen et al. (2006) and Philipp

et al. (2007), we expected that costs might be present fol-

lowing other types of ‘‘no-go’’ trial—i.e., ‘‘no-go’’ trials

which retained intermediate elements of task processing.

These trials were designed to allow use of the tasks only up

to the point of selecting a response (Experiment 1) or the

earlier stage of making a mental judgement about a stim-

ulus (Experiments 2–5). We then examined whether any

costs triggered by partial task completion were equivalent

to those triggered by full execution of the task by com-

paring costs following ‘‘no-go’’ to those following ‘‘go’’

trials.

In the first experiment, the key question was whether a

task had to be completed up to the point of executing

a task-appropriate overt response (e.g., pressing the ‘‘left’’

a button to indicate that a target was ‘‘yellow’’ when per-

forming the ‘‘colour’’ task) in order that switch costs would

be visible on a subsequent trial, or whether merely

selecting a response (i.e., deciding that ‘‘left’’ would be the

appropriate response but not actually pressing a button)

would be sufficient to trigger subsequent switch costs.

Experiment 1

Method

Participants

Twenty-seven undergraduate students (19 female, 8 male)

from the University of Aberdeen were tested in return for

course credit. The age range was 18–23 years (mean

19.2 years). The experiment (and each of the other exper-

iments presented here) was passed by the Ethics Commit-

tee of the School of Psychology, Aberdeen, and complied

with APA ethical standards.

436 Psychological Research (2013) 77:434–448

123

Apparatus and stimuli

Participants were tested using a PC running E-Prime 2.0

software (Psychology Software Tools, Inc., http://www.

pstnet.com). Participants sat at a comfortable viewing

distance from the screen. Target stimuli were single col-

oured shapes, either a square or a circle coloured blue or

yellow, displayed centrally on the screen. In addition, a

green triangle was included as the ‘‘no-target’’ stimulus.

The word ‘‘COLOUR’’ or ‘‘SHAPE’’ (the ‘‘task cue’’) was

written above each target to denote the required task. A

white tick was displayed after target stimuli on ‘‘go’’ trials;

a white cross (X) was shown on the screen following the

target stimulus on the other trials (‘‘no-go’’ and ‘‘no-tar-

get’’ trials). Before the start of every block of trials, par-

ticipants were shown a screen reminding them of the

stimulus–response mappings, i.e.: YELLOW left, BLUE

right, CIRCLE left, SQUARE right.

Design

There were three types of trials: ‘‘go’’, ‘‘no-go’’ and ‘‘no-

target’’. On ‘‘go’’ trials, the target was followed by a tick,

signalling the need for a speeded left/right button-press

response. All participants pressed left for yellow and circle,

and right for blue and square. On ‘‘no-go’’ trials, the target

was instead followed by a cross, indicating that no response

should be executed. It was important that target duration

was long enough to enable participants to process the

stimuli effectively but short enough that response times

would still be sensitive to the effects of task switching. It

was hoped that a short stimulus duration would also

motivate participants to process stimuli as rapidly as pos-

sible so that they would select a response on ‘‘no-go’’ trials.

(It was possible, for instance, that with long stimulus

durations they might adopt a strategy of delaying response

selection or even task judgement until presentation of the

‘‘go’’ stimulus.) Piloting ensured that RTs were sensitive to

task switching whilst error rates were sufficiently low.

‘‘Go’’ trials required performance of the entire task,

including execution of the task-appropriate response. On

‘‘no-go’’ trials, it was hoped that participants would make a

task judgement and select an appropriate response, but it

was required that no response was made on these trials. On

‘‘no-target’’ trials, a green triangle was presented; neither

green nor triangle had a response assignment and it was

required that no response was made on these trials. These

trials were included to represent the no-go trial type used

by Schuch and Koch (2003) and Astle et al. (2006).

The analysis would focus only on very specific trial types,

as explained in the ‘‘Introduction’’ i.e., performance on the

third trial of six particular types of three-trial sequence would

be analysed. ‘‘Repetition’’ sequences required no task

switching, with the same task being required on all three

trials (therefore with the second and third trials both being

‘‘repetition’’ trials). ‘‘Switch’’ sequences required a switch

and a switch back again (with the second and third trials both

being ‘‘switch’’ trials), such that if the middle trial did not

‘‘use’’ the task set, then effectively no task switch would be

taking place and the third trial should evince no switch cost.

The first and third trial of all sequences was always a ‘‘go’’

trial. The middle trial of the sequence was a ‘‘go’’, a ‘‘no-go’’

or a ‘‘no-target’’ trial. Thus, in all there were six sequence

types of interest: ‘‘go switch’’, ‘‘go repetition’’, ‘‘no-go

switch’’, ‘‘no-go repetition’’, ‘‘no-target switch’’ and ‘‘no-

target repetition’’.

Trial sequences for each block were initially randomised

by computer (with 50 % each of the shape and colour tasks,

50 % each of ‘‘switch’’ and ‘‘repetition’’ trials and 66 %

‘‘go’’, 17 % ‘‘no-go’’ and 17 % ‘‘no-target’’ trials). In order

to maximise the number of analysable trials within a testing

session of manageable length, these sequences were man-

ually altered, by manually swapping ‘‘go’’, ‘‘no-go’’ and

‘‘no-target’’ trials as necessary, such that around 40 of each

of the 6 critical trial sequences would be presented within a

session.

Procedure

Target stimuli were presented for 400 ms and followed by

either the tick or the cross for 200 ms, and then a blank

screen for 900 ms. Responses were made with the left and

right index fingers on the mouse buttons. A time limit of

1,000 ms was available for responses following onset of

the tick. No responses were to be made until and unless the

tick (the ‘‘go’’ signal) appeared. Participants were

instructed to be fast and accurate.

Participants first completed two ‘‘pure’’ practice blocks

of trials of 10 ‘‘go’’ trials, with only one task in a block.

Then came a practice block of 20 further ‘‘go’’ trials, with

tasks intermixed within the same block. A final 20-trial

block with intermixed tasks included ‘‘no-go’’ and ‘‘no-

target’’ as well as ‘‘go’’ trials. The experimental trials were

then presented in 15 blocks of 60 trials each. A rest break

of 10 s occurred at the end of each block, following which

a reminder of the stimulus–response mappings was shown

until the participant re-started the trials by pressing the

space bar.

Errors (too fast or absent responses as well as incorrect

left/right responses) were followed by the screen flashing

red for 500 ms.

Analysis

Mean response times (RT) and the proportion of trials on

which an error was made, for the third trial of each of the


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http://www.pstnet.com

http://www.pstnet.com

six critical trial sequences, were calculated for each par-

ticipant. Only trials preceded by two trials with correct

responses were used; in addition, for RTs, only trials with

correct responses were used. Error scores were arcsine

transformed (29 arcsine (Herrors)) for statistical analysis,

with untransformed scores being displayed in Fig. 1 and

Table 1. Data were analysed by repeated-measures

ANOVA with two factors: transition (‘‘switch’’, ‘‘repeti-

tion’’) and sequence (‘‘go’’, ‘‘no-go’’, ‘‘no-target’’).

Because the hypothesis concerned the existence of switch

costs, planned comparisons (using paired t tests) between

‘‘switch’’ and ‘‘repetition’’ trials were carried out for each

type of sequence, regardless of the significance of the

interaction term of the ANOVA. All tests were two-tailed

and based upon an alpha level of 0.05. We expected to see

significant switch costs for ‘‘go’’ sequences and none for

‘‘no-target’’ sequences. We also predicted that costs would

be present for the ‘‘no-go’’ sequences on the basis that

these would also entail the use, and therefore selection, of a

specific task. The key comparison for this prediction was

therefore between the ‘‘no-go switch’’ and ‘‘no-go repeti-

tion’’ sequences. In order to determine whether switch

costs were significantly affected by the effect of removing

just the response-execution element of a trial, we ran a

further two ANOVAs (one each for RT and arcsine-trans-

formed errors) without the ‘‘no-target’’ trials and looked

for an interaction between transition (‘‘switch’’, ‘‘repeti-

tion’’) and sequence (‘‘go’’, ‘‘no-go’’).

Results

In order that the data analysed would be a reasonable

estimate of average performance, participants’ data were

excluded where there were fewer than 15 trials available of

any of the six critical trial types (8 participants), leaving 19

participants. Data are shown in Table 1, with switch costs

in Fig. 1.

A transition (2) by sequence (3) ANOVA was performed

on the response time (RT) data. There were significant main

effects of transition [F(1, 18) = 16.02, p = 0.001], with

longer RTs on ‘‘switch’’ than ‘‘repetition’’ trials, and

sequence [F(2, 36) = 8.08, p = 0.001; RTs on ‘‘go’’

sequences were not significantly longer than those on ‘‘no-

go’’ sequences, t(18) = 1.76, p = 0.096, but those on ‘‘no-

go’’ were significantly longer than those on ‘‘no-target’’

sequences, t(18) = 2.92, p = 0.009], as well as a significant

interaction between transition and sequence [F(2,

36) = 6.34, p = 0.004). Planned comparisons showed a

significant cost of switching tasks for the ‘‘go’’ sequence

[t(18) = 2.69, p = 0.015] and the ‘‘no-go’’ sequence

[t(18) = 4.86, p \ 0.001] but none for the ‘‘no-target’’

sequence [t(18) = -0.72, p = 0.48]. In terms of the arc-

sine-transformed proportion error scores, ANOVA revealed

a significant main effect of transition only [F(1, 18)

= 14.17, p = 0.001], with more errors on ‘‘switch’’ than

‘‘repetition’’ trials. The main effect of sequence neared

significance [F(2, 36) = 0.55, p = 0.058], as did the inter-

action [F(2, 36) = 3.10, p = 0.057]. Switch costs only

reached significance for the ‘‘go’’ sequence [‘‘go’’:

t(18) = 3.75, p = 0.001; ‘‘no-go’’: t(18) = 1.78, p =

0.092; ‘‘no-target’’: t(18) = 0.27, p = 0.79].

There was no evidence of the size of costs being sig-

nificantly affected by removing the response-execution

component: with ‘‘no-target’’ trials removed, there was no

significant interaction between transition and sequence

(‘‘go’’, ‘‘no-go’’) in the analysis of either RT [F(1, 18) =

0.020, p = 0.89] or errors [F(1,18) = 2.91, p = 0.11].

Fig. 1 Switch costs (performance on switch minus repetition trials)

in Experiments 1–5. Top panel shows RT costs; bottom panel shows

error costs. Significant switch costs are evident following a ‘‘no-go’’

trial in all five experiments. Experiment 3 shows that costs are

residual, Experiment 4 shows that they are not due to cue-switching

on task-switch trials and Experiment 5 shows that they are not due to

repetition of target stimulus features across trials. X-axis values refer

to cue–target interval (CTI) in milliseconds. *Statistically significant

switch costs (p \ 0.05, two-tailed)


123

Discussion

As has previously been seen, switch costs were present fol-

lowing a ‘‘go’’ trial and absent following a ‘‘no-target’’ trial

(Astle et al., 2006; Schuch & Koch, 2003). We also observed

a significant switch cost following trials which required

completion of the intermediate task-processing stage of

response selection, but not response execution—‘‘no-go’’

trials—which was not significantly smaller than that fol-

lowing trials with execution of an appropriate overt response.

This result shows that it is not necessary to complete

processing of a task up to and including response execution

in order for switch costs to be observed on a subsequent

trial. Therefore, it suggests that the source of the switch

cost is at, or before, the response-selection stage of task

processing, supporting the findings of Verbruggen et al.

(2006) and Philipp et al. (2007). It is also in line with the

‘‘response selection’’ account of task switching proposed by

Schuch & Koch (2003), according to which the selection of

a task-appropriate response is crucial for the selection of a

particular task and the inhibition of competing tasks.

However, it does not show that response selection is actu-

ally necessary for the generation of subsequent switch

costs; it may be that an earlier task-processing stage would

be sufficient. This question was addressed in our next

experiment.

In Experiment 2, we wished to probe further back within

the sequence of task-processing operations to see whether

the source of switch costs could be located prior to that of

response selection. We aimed to terminate task processing

at the stage of making a task-specific mental judgement

about a target stimulus—e.g., deciding that a yellow circle

was specifically ‘‘yellow’’ rather than a ‘‘circle’’—but

before selecting a behavioural response. It would appear

logically that making such a decision ought to involve the

selection of one task rule over another and therefore might

be expected to trigger the type of between-task competition

which might generate switch costs. We introduced a new

procedure whereby the task rule did not specify an overt

response for any judgement (i.e., ‘‘yellow’’ was no longer

mapped to ‘‘left’’, etc.). Instead, a ‘‘response-mapping’’

screen, shown after each target stimulus, instructed par-

ticipants which response to make for each potential

judgement (e.g., if the target on a ‘‘shape’’ trial was a

yellow circle and a circle was shown on the left of the

response-mapping screen, the correct response would be

‘‘left’’). On ‘‘no-go’’ trials, the features of the target stim-

ulus were missing from the response-mapping screen (and

Table 1 RT in milliseconds (above) and untransformed proportion error (below) in Experiments 1–5 as a function of task-transition (‘‘switch’’,

‘‘repeat’’) and sequence (n - 1 trial type: ‘‘go’’, ‘‘no-go’’ or ‘‘no-target’’)

Expt 1 Expt 2 Expt 3 Expt 4 Expt 5

0 ms 0 ms 0 ms 600 ms 1,000 ms 0 ms 1,000 ms 0 ms 1,000 ms

RT

Go

Switch 502 514 511 458 447 524 449 538 470

Repetition 470 504 488 437 451 498 441 515 456

No-go

Switch 490 489 490 452 452 503 445 526 466

Repetition 457 488 492 445 444 498 439 521 465

No-target

Switch 452

Repetition 456

Errors

Go

Switch 0.116 0.201 0.115 0.131 0.123 0.143 0.120 0.199 0.143

Repetition 0.062 0.094 0.091 0.096 0.098 0.109 0.094 0.135 0.078

No-go

Switch 0.099 0.152 0.099 0.121 0.107 0.134 0.117 0.163 0.118

Repetition 0.064 0.128 0.073 0.069 0.071 0.109 0.083 0.119 0.089

No-target

Switch 0.087

Repetition 0.082

N.B. values for ‘‘task-repetition’’ in Experiment 4 are from ‘‘task-repetition/cue-switch’’ trials. In Experiments 1–3, all task-repetition trials

involved a cue-repetition; in Experiment 5, all task-repetition trials involved a cue-switch


123

alternative features were shown instead) so that no ‘‘cor-

rect’’ response could be selected.

Experiment 2

Method

Participants

Thirty-six undergraduate students (29 female, 7 male) from

the University of Aberdeen were tested in return for course

credit. The age range was 18–25 years (mean 20.4 years).


These were the same as for Experiment 1 except for the

following differences. Each target could be any one of

three coloured shapes—square, circle or triangle—in any

of three colours—blue, yellow or green (there was there-

fore nothing special about green or triangle in this exper-

iment). A response-mapping screen was shown after each

target stimulus. This screen showed two different shapes

(e.g., a square and a circle), one on the left-hand side and

one on the right-hand side of the screen. Flanking each

shape, towards the edge of the screen, was a coloured

vertical bar, the two bars being different colours (e.g., one

blue and one yellow). The combination of particular colour

and shape to the left and right of the screen was random on

each trial. The correct response on any trial could be

determined by locating which side of the screen the correct

feature for that trial was displayed upon. For instance, on a

particular trial, the target might be a yellow circle shown

together with the cue ‘‘COLOUR’’. The correct feature for

that trial would therefore be ‘‘yellow’’. The response-

mapping screen might then show, from left to right: yellow

bar, white square, white circle, blue bar. The correct

response would then be ‘‘left’’ because the yellow bar was

on the left-hand side of the screen. On ‘‘go’’ trials, the two

features of the target stimulus were both included in the

items shown on the response-mapping screen, together

with an alternative shape and an alternative colour. On

‘‘no-go’’ trials, both the colour and shape attributes of that

trial’s target were missing from the response-mapping

screen, replaced with alternative attributes. So in the case

of the trial described above, the response-mapping screen

on a ‘‘no-go’’ trial might have shown, from left to right:

green bar, white square, white triangle, blue bar.

Design, procedure and analysis

These were similar to Experiment 1. The timings were

slightly different from those in Experiment 1 because the

task was easier and we wished it to still be sensitive to switch

costs: the target and cue were displayed together for 200 ms,

the response-mapping screen was then shown for 200 ms,

and this was replaced with a blank screen for a further

1,100 ms. A time limit of 1,000 ms was available for

responding from the onset of the response-mapping screen.

The ratio of trial types was 70 % ‘‘go’’ and 30 % ‘‘no-go’’. As

there was no ‘‘no-target’’ condition in this experiment there

were only two levels of the sequence factor in the analysis.

Results

Data were excluded for the single participant for whom

there were fewer than 15 trials available in one of the

conditions, leaving 35 participants. Data are shown in

Table 1, with switch costs in Fig. 1.

A transition (2) by sequence (2) ANOVA performed on

the RTs showed a significant main effect of sequence [F(1,

34) = 14.51, p = 0.001], with faster responses following

‘‘no-go’’ than ‘‘go’’ trials, but no significant main effect of

transition [F(1, 34) = 1.88, p = 0.18], or interaction [F(1,

34) = 2.71, p = 0.11]. Planned comparisons revealed that

switch costs did not reach significance for either the ‘‘go’’

sequence [t(34) = 1.71, p = 0.10] or the ‘‘no-go’’

sequence [t(34) = 0.12, p = 0.90]. In terms of the arcsine-

transformed error data, the only significant main effect was

of transition, with more errors on ‘‘switch’’ than ‘‘repeti-

tion’’ trials [F(1, 34) = 51.50, p \ 0.001]. There was no

significant effect of sequence: [F(1, 27) = 0.34, p = 0.57],

but the interaction was significant [F(1, 34) = 17.14,

p \ 0.001], the cost of switching being significantly

reduced on ‘‘no-go’’ sequences compared with ‘‘go’’

sequences. Switch costs in the error data were, however,

significant for both types of sequence [‘‘go’’: t(34) = 7.64,

p \ 0.001; ‘‘no-go’’: t(34) = 2.57, p = 0.015].

Discussion

We observed significant switch costs in terms of errors

subsequent to a ‘‘no-go’’ trial. It seems then that neither

executing nor selecting an overt response is necessary to

trigger task-switch costs on a subsequent trial and that

making a mental judgement such as ‘‘yellow’’ or ‘‘circle’’

is sufficient to produce subsequent switch costs. Never-

theless, we did observe here that switch costs following

‘‘no-go’’ trials were significantly smaller than those fol-

lowing ‘‘go’’ trials. We did not see this in Experiment 1

where ‘‘no-go’’ trials involved response selection but not

execution. So it may be that whilst executing a selected

response (in addition to selecting that response) does not

add substantially to the size of subsequent costs, selecting a

specific response (in addition to making a mental judge-

ment) does.


123

In Experiments 1 and 2, we only used a single CTI of

0 ms so there was no opportunity for participants to pre-

pare the required task in advance of a target. This would

have meant that the task-switch costs which we observed

would include a number of elements: not only those pro-

cesses which take place after the presentation of a target

stimulus and associated with the ‘‘residual’’ switch cost, for

instance swapping between activated tasks in procedural

working memory (Rubinstein et al., 2001), but also other

elements which can take place in advance of a target, such

as switching attention between the ‘‘shape’’ and ‘‘colour’’

dimensions or swapping the rules of one task for those of

the other within declarative working memory (Rubinstein

et al., 2001). Being able to demonstrate that residual switch

costs could be triggered by mental judgements would

indicate a powerful role of covert processes—for instance,

that they were able to affect the state of activity in non-

declarative cognitive systems. To address this issue, we

used three CTIs in Experiment 3: 0 ms to replicate

Experiment 2, and 600 ms and 1,000 ms as these ought to

allow sufficient preparation to give only the residual cost

(e.g., Rogers & Monsell, 1995). Frequently, experimenters

just use one ‘‘long’’ CTI in order to target the residual cost,

but strictly speaking this cannot enable us to know that the

cost is residual because the definition of residual costs is

that it can be shown not to be reduced with a longer

preparation time. If we find evidence for a cost which is not

reduced at 1,000 ms compared with 600 ms, then we shall

be able to say that the cost is truly residual (at both inter-

vals). The different CTIs were presented to all participants

in a between-blocks design, as this should ensure that all

participants are motivated to use the long preparation

periods when they are available (Altmann, 2004).

Experiment 3

Method

Participants

Thirty-six individuals (22 female, 14 male) were tested

either as volunteers or in return for course credit. The age

range was 17–55 years (mean 21.7 years).


These were exactly the same as for Experiment 2 except that in

blocks with a CTI of more than 0 ms, a separate screen with

just the cue word (‘‘COLOUR’’ or ‘‘SHAPE’’) was presented

immediately prior to the target. The cue word appeared twice

on the screen, both above and below the target’s position, and

remained on the screen when the target appeared.

Design

There were three types of block, one for each of the dif-

ferent CTIs (0, 600 and 1,000 ms). These were presented in

a fixed order: 0, 600, 0, 1,000 ms, in order to encourage

participants to make full use of the CTI to prepare for the

upcoming task, when possible (Altmann, 2004).

Procedure

This was similar to Experiment 2. However, in this experiment

the task cue was presented in advance of the target stimulus in

some blocks. The cue was presented for 0, 600 or 1,000 ms,

followed by the target stimulus (with the cue remaining on-

screen) for 200 ms and then the mapping cue for 200 ms. A

blank screen was presented for a further 1,000 ms to allow for

responses to be made. The time limit for responding was

1,000 ms from the onset of the mapping cue. In order that total

trial length was equivalent across the different blocks, an extra

blank period of 1,000, 400 or 0 ms was then presented such

that the total trial length was 2,400 ms (excluding feedback).

If the correct response was not made, a feedback screen

(‘‘TOO SLOW’’ or ‘‘INCORRECT’’) was then presented for

500 ms, followed by an additional blank for 1,000 ms. Trials

were presented in blocks of 35 trials. Before beginning the

experimental trials, participants practised the task at each of

the different CTIs. They were aware that the CTI would vary

and that they could use that time to improve their performance

by preparing the appropriate task. Thirty-two blocks were

presented, with rest breaks between each block and the

opportunity for a longer break every eight blocks.

Analysis

Data were analysed by a three-way repeated-measures

ANOVA with the factors CTI (0, 600 and 1,000 ms),

transition (‘‘switch’’ and ‘‘repetition’’) and sequence (‘‘go’’

and ‘‘no-go’’ on trial n - 1). Because of the large number

of statistical results generated by a three-way ANOVA, we

do not report all main effects or lower-level interactions

where these are qualified by significant higher-level inter-

actions. Again, planned t tests were used to test for the

presence of significant switch costs.

Results

One participant’s data were excluded due to there being

fewer than 15 trials in a number of conditions. Data are

shown in Table 1, with task-switch costs shown in Fig. 1.

A CTI (3) by transition (2) by sequence (2) ANOVA

performed on the response time data produced a significant

three-way interaction [F(2, 68) = 4.20, p = 0.019] which

was then broken down by CTI. At 0 ms CTI, there was a


123

significant transition-by-sequence interaction [F(1, 34) =

17.40, p \ 0.001]. The switch cost was highly significant

for ‘‘go’’ sequences [t(34) = 4.99, p \ 0.001] but there

was no cost for ‘‘no-go’’ sequences [t(34) = -0.48, p =

0.63]. There was no interaction between transition and

sequence at either of the longer CTIs (F \ 2, p [ 0.2). At

600 ms CTI there was a significant switch cost for ‘‘go’’

sequences [t(34) = 3.01, p = 0.005] but not for ‘‘no-go’’

sequences [t(34) = 0.88, p = 0.38]. At 1,000 ms CTI there

was no significant switch cost for either ‘‘go’’ sequences

[t(34) = -0.50, p = 0.62] or ‘‘no-go’’ sequences [t(34) =

1.08, p = 0.29].

When arcsine-transformed error data were analysed by

repeated-measures ANOVA, there was no significant main

effect of CTI [F(2, 68) = 0.006, p = 0.99], but there were

significant main effects of both transition, with more errors

on switch than repetition trials [F(1, 34) = 27.28,

p \ 0.001], and sequence, with more errors on ‘‘go’’ than

on ‘‘no-go’’ sequences [F(1, 34) = 7.63, p = 0.009]. None

of the interactions was significant (F \ 1.5, p [ 0.3). On

‘‘go’’ sequences, switch costs were significant at 0 ms CTI

[t(34) = 2.40, p = 0.022] and approached significance at

the longer CTIs [600 ms: t(34) = 1.93, p = 0.062;

1,000 ms CTI: t(34) = 1.97, p = 0.057]. On ‘‘no-go’’

sequences, whilst switch costs were not significant at 0 ms

CTI [t(34) = 1.71, p = 0.097], they were significant at

both of the longer CTIs [600 ms: t(34) = 3.49; p = 0.001;

1,000 ms: t(34) = 3.29, p = 0.002]. In order to determine

whether the costs following ‘‘no-go’’ trials were truly

residual, we tested whether the size of the cost was smaller

at 1,000 ms compared with 600 ms. There was no signifi-

cant difference [t(34) = 0.57, p = 0.57]; therefore, we can

assume that this cost is indeed residual in nature.

Discussion

The CTI manipulation in this experiment was intended to

indicate whether any switch costs obtained following a

‘‘no-go’’ trial were residual in nature and the results show

that they were. Significant switch costs in terms of error

rate were present in the ‘‘no-go’’ sequences at both the

600 ms and 1,000 ms CTIs, and these did not decrease

significantly at 1,000 ms compared with 600 ms. There-

fore, it seems that residual task-switch costs can be trig-

gered by partial task completion which involves making a

task-based mental judgement, in the absence of any

selection or execution of an overt response.

At the 0 ms CTI there was a significant effect of removing

response selection (and execution) on the size of subsequent

switch costs. This replicates what we saw in Experiment 2.

But at the longer CTIs this was not seen; numerically, in fact,

costs were larger following ‘‘no-go’’ than ‘‘go’’ trials at

1,000 ms CTI. Perhaps, then, whatever accounts for the

residual switch cost may be determined in full by covert

mental processes taking place before response selection,

such that selecting or executing a response can add nothing

further. In contrast, it may be that part of the overall switch

cost can be triggered only by selecting an overt response,

such that switch costs increase with response selection/

execution on trials with no preparation period.

It is possible that the costs which we observed in this

and the preceding experiments were at least partly driven

by the change in cue which occurred on switch trials but

not repetition trials (Logan & Bundesen, 2003). For

instance, cue encoding would be expected to take longer on

a trial where the cue differs from the previous cue (as it

does on a standard task-switch trial) than on a trial where

the previous cue is repeated (as on a standard task-repeti-

tion trial), and this difference may well be captured by the

standard task-switch-cost measure. We addressed this issue

in Experiment 4 by using two different cues for each task:

‘‘SHAPE’’/’’FORM’’ and ‘‘COLOUR’’/’’HUE’’. Conse-

quently, a task repetition could be accompanied by either a

cue switch (e.g., SHAPE on trial n - 1 and FORM on trial

n) or a cue repetition (e.g., FORM on both trial n - 1 and

trial n), and task-switch costs which control for the cue-

switch factor (‘‘cue-controlled task-switch costs’’) could be

calculated by comparing task-switch trials with only those

task-repetition trials on which the cue also switched.

Experiment 4

Method

Participants

Thirty-six undergraduate students (27 female, 9 male) from

the University of Aberdeen were tested in return for course

credit. The age range was 18–26 years (mean 20.0 years).


These were the same as for Experiment 3 except that on a

random 50 % of trials on which the task was to attend to

colour, the cue word was HUE instead of COLOUR, and

on a random 50 % of trials on which the task was to attend

to shape, the cue word was FORM instead of SHAPE.


These were as for Experiment 3 except for the following

points. There were only two CTIs used, 0 and 1,000 ms.

Two levels of the transition factor were included in the

ANOVA: ‘‘task-repetition/cue-switch’’ and ‘‘task-switch/

cue-switch’’. A priori comparisons were carried out to test


123

for the presence of (cue-controlled) switch costs at each

combination of sequence and CTI.

Results

Data are shown in Table 1, with cue-controlled task-switch

costs shown in Fig. 1.


performed on the RT data showed significant effects of CTI

[F(1, 35) = 163.96, p \ 0.001], with faster responses at

1,000 ms than 0 ms CTI, and transition [F(1, 35) = 13.56,

p = 0.001], with faster responses on ‘‘repetition’’ than

‘‘switch’’ trials. The main effect of sequence neared sig-

nificance [F(1, 35) = 3.39, p = 0.074]; so did the transi-

tion-by-sequence interaction [F(1, 35) = 3.63, p = 0.065].

All other effects were non-significant (F \ 1.5, p [ 0.2).

Planned t tests showed that there was a significant switch

cost for ‘‘go’’ sequences at 0 ms CTI [t(35) = 3.17,

p = 0.003] but that the cost for ‘‘go’’ sequences at

1,000 ms CTI and for ‘‘no-go’’ sequences at both CTIs

were non-significant [t(35) \ 1.2, p [ 0.2].

Analysis of arcsine-transformed error data revealed

significant main effects of CTI [F(1, 35) = 5.98,

p = 0.020], with more errors at 0 ms than 1,000 ms CTI,

and transition [F(1, 35) = 25.76, p \ 0.001], with more

errors on ‘‘switch’’ than ‘‘repetition’’ sequences. All other

effects were non-significant (F \ 0.4, p [ 0.5). Planned

t tests showed that for ‘‘go’’ sequences, costs were signif-

icant at 0 ms CTI [t(35) = 3.07, p = 0.004] and almost

significant at 1,000 ms CTI [t(35) = 1.98, p = 0.055], and

that for ‘‘no-go’’ sequences, costs were significant at both

0 ms CTI [t(35) = 2.26, p = 0.030] and 1,000 ms CTI

[t(35) = 3.30, p = 0.002].

Discussion

The results from Experiment 4 demonstrate the presence of a

significant switch cost following a ‘‘no-go’’ trial when cue-

switching effects were controlled for. This cost is very likely

to be residual in nature as it was present with a long CTI of

1,000 ms, although as we did not include an additional CTI

this time we could not show definitively that it was. Impor-

tantly, the cost cannot be attributed to the difficulty of

adjusting to a change in cue from one trial to the next when

switching tasks, because the same adjustment would have

had to have been made on the ‘‘task-repetition/cue-switch’’

trials with which the ‘‘task-switch/cue-switch’’ trials were

compared. Instead, it suggests that there is a real task-

switching cost present subsequent to a trial on which the task

was used not for the selection or execution of an overt

response, but only for the making of a mental judgement.

Although there was no strong statistical evidence in

favour of an effect of removing response selection and

execution on the size of switch costs (the interaction of

transition and sequence on RTs tended towards signifi-

cance), examination of Fig. 1 suggests that the same pat-

tern is present here as was present in the preceding

experiments—i.e., response selection/execution increases

the overall switch cost but not the residual switch cost.

In the final experiment, we sought to replicate the finding

of significant switch costs subsequent to a mental judge-

ment whilst controlling for two factors which could

potentially have influenced the results presented so far.

First, instead of manipulating the order of trials within each

in order to ensure high numbers of particular three-trial

sequences, we allowed trial orders to be constructed at

random for each participant (so that switch sequences, for

example, would comprise just as many of the type BBA as

ABA). We used the standard analysis method of comparing

two-trial (n - 1 and n) sequences, regardless of the task on

trial n - 2. Whilst this may have led to costs being smaller

(because we were no longer comparing the most difficult

type of switch trial with the easiest type of repetition trial),

they would now correspond to the type of switch cost most

usually examined in the task-switching literature. Second,

we disallowed any repetitions of stimulus features from one

trial to the next. Repetition of whole stimuli, or even of

individual stimulus features, from one trial to the next can

lead to inflation of switch costs (see Hubner, Kluwe, Luna-

Rodriquez, & Peters, 2004). For example, if a yellow circle

had been shown on trial n – 1 requiring the colour task, then

the feature ‘‘yellow’’ would have been selected and the

feature ‘‘circle’’ ignored. On a subsequent switch trial to the

shape task, repetition of the feature ‘‘circle’’ would be

particularly difficult as that feature would be likely to retain

some inhibition (i.e., it would be affected by ‘‘negative

priming’’). If the subsequent trial had instead been a repe-

tition of the colour task, repetition of the feature ‘‘yellow’’

could potentially allow a ‘‘shortcut’’ to response selection,

bypassing the usual use of a stimulus–response rule (Pashler

& Baylis, 1991), making that trial particularly easy. In

Experiment 5, a target which was a yellow circle could not

be followed by a target which was either yellow or a circle.

This should ensure that switch costs are truly due to the

demands of switching between alternative task rules.

Experiment 5

Method

Participants

Thirty-six students (24 female, 12 male) from the Uni-

versity of Aberdeen were tested in return for course credit.

The age range was 18–47 years (mean 21.33 years).


123


These were as for Experiment 4 except for the following

points. The order in which trials were presented within a

block was entirely random, with 50 % of all trials being

‘‘switch’’ (and 50 % ‘‘repetition’’), and 30 % of all trials

being ‘‘no-go’’ trials (and 70 % ‘‘go’’ trials). The cue

switched on every trial, so a task-repetition necessarily

involved a cue-switch; therefore, all task-switch effects are

‘‘cue-controlled task-switch effects’’, in the terminology of

Experiment 4. In addition, there was no repetition of

individual stimulus features from one trial to the next.

Because we were primarily interested in the residual switch

cost, most of the trials used a long CTI of 1,000 ms. Within

each block of 35 trials, two short blocks of 0 ms CTI trials

(one of 5 and one of 6 trials in length) were presented,

similar to a method used previously (Astle et al., 2006;

Astle, Jackson, & Swainson, 2012). This was done to

encourage participants to prepare the appropriate task

during the preparatory period on 1,000-ms CTI trials

(Altmann, 2004). As usual, ANOVAs were performed on

RT and arcsine-transformed error data, with factors CTI,

transition and sequence, along with planned comparisons

(via t test) of switch and repetition performance for each

combination of CTI and sequence.

Results

Data are shown in Table 1 and Fig. 1.


performed on the RT data showed significant main effects

of both CTI [F(1, 35) = 127.21, p \ 0.001; faster

responses at 1,000 ms than 0 ms CTI], and transition [F(1,

35) = 22.28, p \ 0.001; faster responses on ‘‘repetition’’

than ‘‘switch’’ trials]. There was also a significant inter-

action of sequence and transition [F(1, 35) = 16.53,

p \ 0.001], with the switch cost being smaller following

‘‘no-go’’ than ‘‘go’’ trials. All other effects were non-sig-

nificant (F \ 2.5, p [ 0.1). Switch costs were significant

for ‘‘go’’ sequences at both 0 ms CTI [t(35) = 4.66,

p \ 0.001] and 1,000 ms CTI [t(35) = 4.96, p \ 0.001]

and were non-significant for ‘‘no-go’’ sequences at both

0 ms CTI (t(35) = 0.95, p = 0.35) and 1,000 ms CTI

[t(35) = 0.48, p = 0.64].

Analysis of arcsine-transformed errors revealed signifi-

cant main effects of CTI [F(1,35) = 22.64; p \ 0.001; fewer

errors at 1,000 ms than 0 ms CTI], sequence [F(1,

35) = 9.97, p = 0.003; fewer errors on ‘‘no-go’’ than ‘‘go’’

sequences] and transition [F(1,35) = 66.41, p \ 0.001;

fewer errors on ‘‘repetition’’ than ‘‘switch’’ trials]. The

interaction of CTI and sequence approached significance

[F(1, 35) = 3.37, p = 0.075] and there was a significant

interaction between sequence and transition [F(1, 35) =

10.33; p = 0.003], with costs being reduced on ‘‘no-go’’

compared with ‘‘go’’ sequences; all other effects were non-

significant (F \ 1, p [ 0.4). Switch costs were significant

for ‘‘go’’ sequences at both 0 ms CTI [t(35) = 5.15,

p \ 0.001] and 1,000 ms CTI [t(35) = 10.39, p \ 0.001];

on ‘‘no-go’’ sequences they approached significance at 0 ms

CTI [t(35) = 1.99, p = 0.054] and were highly significant at

1,000 ms CTI [t(35) = 2.84, p = 0.007].

Discussion

The results of Experiment 5 confirm that significant residual

switch costs are triggered by performing a task even if this

is completed only up to a stage of covert mental judgement,

without any selection or execution of a specific response

being involved. These data show that the results of the

preceding experiments did not reflect only an ‘‘exagger-

ated’’ form of the switch cost which could be due to the

comparison of the ABA switch sequence (involving back-

ward inhibition) with the AAA repetition sequence, because

this experiment used the standard procedure of constructing

trial orders at random and analysing two-trial sequences

(‘‘switch’’, BA, versus ‘‘repetition’’, AA). In addition, the

results cannot be due to negative and/or positive priming of

specific stimulus features across consecutive trials because

no such repetitions were allowed in this design.

Switch costs in terms of both RT and errors were reduced

in this experiment on ‘‘no-go’’ compared with ‘‘go’’

sequences. There was no interaction with CTI this time;

indeed, it is clear from Fig. 1 that the reduction was at least

as large at 1,000 ms CTI as it was at 0 ms CTI. This con-

trasts with the earlier experiments in which it had appeared

that removing the requirement to select (and execute) a

task-specific response led to a reduction in switch cost size

only with 0 ms CTI and not with either 600 or 1,000 ms

CTI. It is unclear at present why this might be. One possi-

bility is that switch costs in this design were not completely

residual, i.e., switch costs may have reduced further had we

included an additional CTI longer than 1,000 ms. This

seems unlikely given that we found no significant reduction

from 600 to 1,000 ms CTI in Experiment 3 (and therefore

costs could be classified as being residual even at 600 ms in

that experiment), but it remains a possibility. For instance, it

may be that participants tailor their speed of preparation

according to the specific CTIs experienced (c.f. Altmann,

2004); knowing that they cannot prepare on 0 ms trials, they

may become somewhat lazy about preparing in advance of a

target 1,000 ms away, whereas the possibility of a target

appearing after just 600 ms may spur them to more rapid

preparation. Alternatively, it may be that the residual, as

well as the overall, task-switch cost is contributed to by

response selection on the preceding trial. This issue remains

unresolved at present.


123

General discussion

It is well known that overt performance of a particular task

will trigger costs for switching to a different task. This

research investigated the impact of covert mental processes

upon subsequent task performance. Schuch and Koch

(2003) proposed that response selection is the critical stage

in the generation of subsequent switch costs, and that it is

at the response-selection stage that alternative task repre-

sentations compete for selection to control behaviour. If

this were the case, one would expect that selecting a

response in the absence of response execution would be

sufficient to trigger subsequent costs. In order to test this

prediction, and to investigate also whether costs could also

be generated by processes prior to response selection, we

modified the task-switching paradigm such that task-

specific processing would be terminated either at the stage

of response selection or at the earlier stage of covert mental

judgement. Our results confirmed that response selection,

in the absence of response execution, is indeed sufficient to

produce significant subsequent switch costs (Experiment

1). Moreover, they indicate that task judgement in the

absence of any response selection or execution is by itself

sufficient to trigger the presence of significant switch costs

on a subsequent trial (Experiment 2). These costs were

shown to be ‘‘residual’’ in nature as they were present with

a long preparation interval (600 ms CTI) and did not sig-

nificantly reduce at an even longer interval (1,000 ms CTI;

Experiment 3); they do not reflect a cue-switching cost as

they were present when both task-switch and task-repeti-

tion trials involved a change in cue (Experiment 4) and

they are not due to feature repetitions as they were present

when no stimulus features repeated across trials (Experi-

ment 5).

Previous studies have suggested that task-switch costs

can be triggered in the absence of overt response execution

(Philipp et al., 2007; Verbruggen et al., 2006). These

studies showed that merely selecting a task-specific

response (i.e., selecting which response would be correct

according to the task rules on that trial, but not actually

executing it) was sufficient to trigger switch costs on the

subsequent trial. Experiment 1 of our study replicates those

results. Of course, it could be the case that the costs in

those studies were actually the result of an earlier pro-

cessing stage. In Experiment 2, we developed a novel

method to terminate task processing at the stage of making

a mental judgement that required no selection of an overt

response; the results showed that a mental judgement alone

was sufficient to trigger subsequent switch costs. Impor-

tantly, neither of those previous studies had shown that

subsequent task-switch costs could be residual in nature, as

they used only short preparation intervals. Also, in both of

them cue switching was confounded with task switching,

because only on switch trials did the task cue change.

Experiments 3, 4 and 5 of the current study showed that

making a mental judgement in the absence of any pro-

cessing related to the execution or the selection of a task-

appropriate response is sufficient to generate the conditions

for significant task-switch costs on a subsequent trial, that

these costs are residual, and that they are not a result of cue

switching.

It may be noted that error rates in these experiment

appear to be relatively high, and that the principal effects

were evident in terms of error rates rather than response

times. This is most likely to be simply because there was

quite a stringent time limit for responses in all of the

experiments, so participants adopted a strategy of ensuring

that they responded relatively quickly across all conditions

whilst exerting relatively less control over the number of

errors made; this would have led to latency being a rela-

tively insensitive measure, and errors relatively sensitive,

to any factors which affected difficulty, such as task

switching.

The results reported here have implications for our

understanding of the processes involved in task switching.

In Schuch and Koch’s (2003) account, response selection

plays a special role. That account proposes that a task cue

can cause a task to be ‘‘activated’’ and that it is possible for

a number of tasks to be active concurrently, such as the

task most recently performed and the task currently cued.

But a task is not ‘‘selected’’ for the control of behaviour

until it is used to process a target stimulus for the purposes

of selecting an overt response. It is at this point that other

competing tasks are proposed to be inhibited, leading to

residual switch costs (and ‘‘backward inhibition’’) on

subsequent trials. Schuch and Koch’s study did not test

whether any intermediate task-processing stages (between

presentation of a target stimulus and execution of an overt

response) might have been capable of triggering sub-

sequent switch costs. The results reported here show that in

fact the earlier stage of making a mental judgement can

indeed trigger subsequent switch costs. We suggest modi-

fying Schuch and Koch’s proposal slightly, to be consistent

with these new results, to state: it is when a task rule is used

to process a target stimulus in a task-specific way, whether

the outcome of that processing is covert or overt, that a task

becomes selected, resulting in residual costs occurring for a

subsequent task switch.

If switch costs can be driven by making a task-based

mental judgement, are later stages of task processing then

redundant in terms of determining subsequent perfor-

mance? Philipp et al. (2007) found that executing a

response led to an increase in the size of subsequent switch

costs (not residual) compared with the situation in which a

response was selected but not executed on the trial pre-

ceding the switch; this result was attributed to the effects of


123

response monitoring and evaluation. In contrast, we found

that removing the requirement to execute a selected

response made no difference to the size of the overall

switch cost (i.e., with a 0 ms preparation interval; Exper-

iment 1), whereas removing the requirement to select an

overt response did significantly reduce the size of the

overall cost (Experiment 2). Although our results partially

contradict those of Philipp et al. (2007), both sets of results

do indicate that processes involving overt responses (i.e.,

their selection and/or execution) contribute to the size of

the overall switch cost over and above what is achieved by

making a mental judgement about a stimulus. In terms of

the residual cost, our own results were mixed: Experiments

3 and 4 suggested that making a mental judgement triggers

the entire residual cost, but in Experiment 5 costs were

increased when there was also a requirement to select and

execute an overt response. As discussed above, this may be

because the cost was not fully residual at the 1,000 ms CTI

in that experiment or because response selection and/or

execution do indeed contribute in some way to task

selection over and above what is accomplished by making

a task-specific mental judgement. Schuch and Koch (2003)

suggested that there may be different inhibition processes

relating to different stages of task processing. Because

previous studies had found that backward inhibition effects

were present in tasks with high perceptual conflict but low

response conflict, they proposed that distinct inhibitory

processes may be triggered by the perceptual and response

stages of a task. It is possible that inhibition could occur at

a number of distinct task-processing stages, potentially

including perceptual selection, mental judgement, response

selection and response execution, and that the presence and

size of any subsequent switch costs—as well as whether or

not the costs are residual—will depend on which of these

took place on the previous trial.

Our results appear to contradict the ‘‘response-based

strengthening’’ account of task-switch costs (Steinhauser &

Hubner, 2006; Steinhauser, 2010). In a series of experi-

ments examining the effect of errors on subsequent per-

formance, those authors concluded that the production of

an overt task-specific response is necessary for the pro-

duction of (residual) switch costs on a subsequent trial.

They observed that following trials on which a response

was produced which was appropriate to the wrong task,

there was a subsequent ‘‘switch benefit’’ for switching to

that same task, indicating that the response had triggered

strengthening of that task. Making an ‘‘error-signalling

response’’, thought to necessitate activation of the correct

task and identification of the correct response, did not

reverse the effect, whilst making an ‘‘error-correction

response’’ (i.e., executing the correct response) did. With a

different procedure, we came to the contrary conclusion

that an overt response is not necessary for the triggering of

subsequent switch costs. It may be that a covert judgement

made in the absence of any overt response is able to trigger

subsequent switch costs (our data) and yet not to overturn

the potentially more powerful effects of an already exe-

cuted overt response on the same trial (Steinhauser &

Hubner, 2006; Steinhauser, 2010). Indeed, there was an

indication in our data that, even for residual costs, response

execution may trigger bigger switch costs than did covert

judgement (Experiment 5), so such an argument based

upon the relative power of covert and overt responses

appears plausible. Alternatively, perhaps the difference

between the two sets of results lies in the differing nature of

the covert responses. For example, it may be that the covert

judgements that were required in our study—i.e., selection

of a particular task-specific attribute required in order to

subsequently select the correct overt response—would

have necessitated the task being used in a more precise or

complete fashion than would be the case for the type of

covert processing required in order to make a non-specific

response indicating that an error had been detected, hence

the covert responses in our experiments but not those of

Steinhauser and Hubner were able to trigger subsequent

switch costs.

It is important to state that we cannot be sure of which

stage of processing was necessary for the costs observed in

these experiments. We can say definitively that there was

no selection of a task-appropriate overt response, and so

that process cannot have been responsible; this is a novel

finding which contributes to our understanding of what

causes switch costs (c.f. Schuch & Koch, 2003). We have

assumed that an earlier process, anything up to the making

of a ‘‘mental judgement’’, was instead the key stage

responsible for triggering subsequent costs. However, this

interpretation would rely on there being no selection of any

task-appropriate response whatsoever. Whilst no overt

response was selected on no-go trials, nevertheless, argu-

ably, ‘‘no-go responses’’ were selected, and they were

selected according to the current task rule. These no-go

responses were therefore a type of ‘‘intentional non-

action’’: in other words, by not responding, the participant

was specifically indicating that a specific feature (selected

using the current task rule) was not on the screen (Kuhn,

Elsner, Prinz, & Brass, 2009; Kuhn & Brass, 2010). This

could be considered to be a type of task-appropriate

response, albeit an absent one, so possibly our costs result

from completing a stage of covert selection of absent

responses.

Another important question that remains is whether the

costs we observed actually stem from the completion of

earlier stages of task processing, and, if so, how early could

these be? The absence in previous studies of switch costs

subsequent to no-go trials in which participants were

allowed to prepare for the upcoming task in a general way


123

(i.e., with a cue such as ‘‘colour’’ but without any target

stimulus upon which the task could be performed) suggests

that task-preparation alone is not sufficient to trigger sub-

sequent costs (Astle et al., 2006; Schuch & Koch, 2003).

However, Lenartowicz et al. (2011) have recently argued

that making a no-go response on trial n - 1 would have led

to the abolition (for instance through a process of clearing

working memory) of costs which would otherwise have

been present. In that study, costs emerged following a trial

where no response was made if no target was shown (and

thus no ‘‘no-go’’ decision had to be made), such that the

cue for the current trial was simply followed by the cue for

the next. Intriguingly, that study suggests that a task need

not actually be used at all (i.e., for any task-specific pro-

cessing of a target stimulus) in order for it to be ‘‘selected’’

and subsequent costs triggered. This seems to contradict

the proposal of Schuch & Koch (2003) that a task cue

merely activates a task and does not cause the selection

between tasks which is responsible for costs. But Schuch &

Koch were referring principally to residual switch costs

(and backward inhibition costs, which were not measured

in our experiments), whereas the costs that Lenartowicz

et al. reported were only present at preparation intervals of

350 ms, and absent at intervals of 1,250 ms—i.e., they

were not ‘‘residual’’. An earlier study by Brass and von

Cramon (2004) does appear to show that residual costs can

be present following cue-only trials; that intriguing result

needs to be replicated before we can conclude whether

general task preparation alone can trigger residual switch

costs, and therefore whether it can be assumed to be

responsible for selecting between competing task repre-

sentations for the control of action.

Conclusion

The current findings support the notion that response exe-

cution is not necessary for the generation of subsequent

switch costs. Two earlier stages—the covert selection of a

response and the making of a covert mental judgement

which is not associated with either the selection or exe-

cution of any overt response—are, however, sufficient to

trigger subsequent costs. These costs are residual in nature

and cannot be attributed to cue-switching or the repetition

of target-stimulus features across trials. Selection, but not

execution, of an overt response was shown to contribute

significantly to the size of switch costs.

These data suggest that task-sets are ‘‘selected’’ at an

earlier stage of task processing than had previously been

thought. They also indicate that mental processes not

associated with any processing related to overt responses

can have a significant impact upon procedural-level task

representations.

Acknowledgments The authors would like to thank Steve Harley,

Julie Main, Sindre Henriksen, Dudu Ozlevent and Maritxu Arlegui-

Prieto for their assistance with the data collection. We would like to

thank Mike Wendt and an anonymous reviewer for their helpful

comments on the manuscript. This work was funded by an Experi-

mental Psychology Society Small Grant.

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