Electroencephalography and Clinical Neurophx'siolog3. 1974. 36:191 199 Else~'ier Scientific Publishing Compan.,,. Amsterdam Printed in The Netherlands

191

H U M A N A U D I T O R Y E V O K E D P O T E N T I A L S . I I : E F F E C T S O F A T T E N T I O N ~

T. W . PiCTON AND S. A. HII.I.YARD

Department of Neuroxcie.~u'es, ! 'nh'ersitv ol Colilon~ia, San Die~lo, La ,hdla, Calif. 92037 ¢ U.S..4. )

tAccepted for publication:August 22, 1973~

Auditory attention involves the selective per- ception of a particular auditory message and the relative suppression of competing sensory infor- mation. Neurophysiological theories of auditory attention have ranged l¥om selective peripheral gating prior to sensory analysis (e.,q,, Hernandez- Peon 1966), to a more central selective response to tho,~ stimuli evaluat~xt as signilicant after sen- soD' analysis (e,,q., Sokolov 1963). One approach to determining the neural mechanism of this sti- mulus-selection process is to study the influence of different attentive strategies upon the compo- nents of the human auditory evoked potential. The "'early" (0-8 msec) and "'middle" (8 50 msec) latency components have been found to be stable during fluctuations in subjective arousal and environmental conditions {Mendel and Goldstein 1969, 197 ! : Jewett and Williston 197 ! ; Amadeo and Shagass 1973). but have not been examined during changes in the direction of selective attention. Directing attention toward an auditory stimulus has consistently been reported to enhance the N~ (90 msec) P_, (170 msec) components ofthe evoked response (Davis 1964: Gross et al. 1965; Satterfield 1965; Spong et ai. 1965; Wilkinson and Morlock 1967; Hirsch 1971; Keating and Ruhm 1971: Picton et al. 1971). In certain conditions attended task relevant auditory stimuli may also elicit a late positive component or P3 wave (Sutton et al. 1965. 1967: Wilkinson and Morlock 1967; Ritter and Vaughan 1969: Sheatz and Chapman 1969; Smith et al. 1970; Hillyard et al. 1971:

t This investigation was supported by NASA Grant No. 05-009-198 and NIH Grant No USPHS NS 10482-01 awar- ded to Professor Galambos. the Medical Research Council of Canada. and the Sloan Foundation

K arlin et ai, 1971 ; Picton et al. 1973). The present study examines the effects of alteration upon the human auditory evoked potentials recorded concurrently from all levels of the auditory sys- tem so as to delineate those levels at which dif- ferent stimulus selection processes t..ke place.

MI~rll('II)S

Paid students, age 20-30, most of whom were quite familiar with the techniques and require- ments of evoked potential recording, served as subjects for this experiment. They sat in a com- fortable chair in a sound attenuated room, and adjusted their posture so that little or no muscle activity was observed in the EEG monitored on an oscilloscope. 60 dB SL 50 #sec "'standard" clicks were presented to the subject's right ear through earphones at a rate of once a second. Every 5-30 sec the intensity of a single click ( " s igna l" ) was lowered by a fixed amount, which was between 1 and 5 dB for different subjects. During the "'attend" condition, the subject was asked to detect and count the signals in a block of ! 100 stimuli. The signal intensity was adjusted so that the subject could detect between 80 and 950. of the signals. During the "'ignore" condi- tion the subject read a book and was instructed to disregard as much as possible the ongoing auditory stimuli. Attend and ignore conditions were presented in a balanced manner so as to rule out any possible order effects. One subject underwent il paired attend-ignore conditions, another subject 6, four subjects underwent 4 pairs, and two subjects 2 pairs. The experiment

.took several hours to complete and most sub- jects spread the recording sessions over 2 or 3 days.

192 1. %V. PICTON AND S. A. HILLYARD

There were two modifications to this basic experimental paradigm. For two subjects the experiment was repeated using more frequent signal clicks (every 1-10 stimuli, on the average 1 in 5). This allowed more detailed examination of the evoked response to the signal. In a final experiment the signal stimulus was changed from a lower intensity click to a complete absence of any stimulus. This allowed examination of the evoked response to a detected omitted stimulus.

The evoked potential over the first 50 rasec after the standard st imulus was recorded between vertex and right mastoid electrodes, amplified on a Grass P 15 preamplifier (30-3 kc/sec) and a Tektronix FMI22 amplifier {8-10 kc/sec)and averaged online on a Fabritek 1052 Signal Averager. EEG activity from Pz. Cz. Fz elec- trodes referred to a left mastoid reference and a vertical electrooculogram (EOG) were amplified on a Grass IVlodel 6 Polygraph (frequency ban@ass 1+70 c/see) and recorded on FM tape for later off-line analysis o1" tlae longer latency components of the evoked potential to either standard or signal stimuli. Examination of the early components of the evoked potential to the signal stimulus was done by recording all EEG activity on high speed FM tape (0-2.5 kc/sec) and averaging off-line using different computer analysis periods. The scalp distribution of the evoked response to a detected stimulus omission was evaluated in seven subjects using the 16- channel averaging technique described in the preceding paper.

RESULTS

Ez ,okedpo ten I h7 l~" to s tat ~cks:'d c l icks Fifteen distinct components o1" the auditory

evoked potential could be consistently recog- nized in all subjects. The identification and nomenclature of these components have been reviewed in the previous paper. A baseline was determined from the first 0.5 msec of the respon- se and extended through the succeeding evoked potential components. If such a baseline was too difficult to evaluate because of stimulus artifact contamination, an arbitrary baseline was drawn at the midpoint between the troughs following components I and II. The baseline to peak ampli- tude and the latency of each component was

measured for each.of the attend and ignore conditions.

The amplitude measurements were cbp, x~crted to percentages of the mean ignore condition for each subject prior to statistical analysis in order to remove intersubject variability. If the resul- tant distributions were approximately normal a t-test comparison was performed. Otherwise (i.e., for Waves VI, No, Po), a Wilcoxon signed rank analysis was used. The results are delineated in Table I. All significance levels below 0.05 are shown, although since fifteen measurements are being compared only significance levels below 0.0033 should be considered as meaningful {Hays 1963, p. 489).

TABLEI

Effect of attention on components of evoked potential.

Amplitude {ttV } Latency ...............................

Component (msec) Attend Ignore Significance

1 1.5 0 . 2 ~ tt._'+, + r '~ _ ,)._ _+0.2~ tt I.', N.~

I! 2.6 0.21 tJ,24 + 0.1 +_014 +0.15 Ns

lit 3.8 0.41 0.38 +_ 0.3 +0.23 +0.14 Ns

IV 5.0 0.47 0,40 ± 0.5 +0.28 +0.19 Ns

V 5,8 0.52 0.46 _ 0,3 +_0 .29 _+0,23 Ns

V! 7,4 0,21 0,18 _+ 0,4 +0~11 +0,09 Ns

N,~ 8.9 0.21 0.19 _+ 0.7 +0.15 +0.17 Ns

Po 12 0.08 0.08 + ! +0,09 +_0.07 Ns

N~ 16 0,34 0,29 • -+ I ~ 0 . 1 7 +_0 .19 P<0,05

P,, 25 0,62 0,59 • _+ 2 _+0.36 +_0.37 Ns

Nh 36 0.49 0,49 +- 3 _+0.30 -+0.38 Ns

Pn 50 0.60 0.67 +- 4 _+0.41 _+0.54 Ns

N n 83 [..58 0.97 -+ 7 +-0,89 _+0.85 P<0,002

P., 161 3.58 2.65, +- 17 +- 1.07 -+0.97 P<'O.O001

N_, 290 2.02 1.51 + 47 +1.09 _+0.88 Ns

All measurements are means and standard deviations.

HUMAN AUDITORY ATTENTION 193

I G NOR E ATTEND IGNORE ATTEND

- • * i j ~ O m s

• - . G S s . . . . . . - . _ . , ~ - - . O , S s

Fig. !, Effects of attention on the auditory evoked potential. 60 dBSL click stimuli were presented every second and the sub- ject asked to "attend" to the clicks and detect occasional faint click signals or to "'ignore" the clicks and read a book. Each tracing represents the average of 1024 responses obtained from vertex-mastoid electrodes. Three separate time bases were used during averaging so that all components of the evoked potential could be examined. Figure shows comparison between "'attend" and "ignore" conditions for two subjects H.K. and S.V.

As can be readily seen the only components susceptible to attention were N1 and P2 both of which were substantially increased during the "attend" condition. These changes are illustrated in Fig. 1 which shows the results from two dif- ferent subjects.

There was no significant change in the peak latency of any component between the "attend" and "ignore" conditions. Wave V, for example, the latency of which is extremely susceptible to the intensity of the signal (Lev and Sohmer 1972; Galambos et al. 1973), occurred at 5.8_+0.3 msec during "attend" and 5.8_+0.2 during "ignore". The N I component had a latency of 82_+9 msec during the attend conditio.i!, and 83+8 msec during the ignore condition.'::The Pz component measurements were 163 _+ 19 and 158_+ 19 msec respectively.

Evoked potential to signal clicks The effect of attention on the late components

of the evoked response to the occasional fainter stimulus was examined in six of the subjects. As with the standard stimuli, there was a marked increase in the N I-P2 components with atten- tion. Moreover, a large positive wave peaking near 450 msec was evoked to the attended signal. These results are detailed in Table II and illus- trated in Fig. 2. Since these signal evoked poten- tials were averaged over far fewer stimuli than the standard stimulus evoked potential, they tended to be more variable due to unaveraged random noise.

The P3 component of the evoked response was distributed more posteriorly on the scalp than the N I-P2 components. This is readily apparent in Fig. 2 and detailed numerically in

194 ,. ; V . PICTON AND S. A. HILLYARD

Table Ill. The distribution of the components P~ through N, were generally similar for signal and standard responses, although the N t corn-

TABLE 111

Scalp distribution of evoked potentials expressed as ",, of Cz amplitude {means and s.d.}.

TABLE II Non signal Signal

Fz Pz E~oked potentials to detected signal stimuli.

Amplitude ( l t v ~ Latency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(reset) Attend Ignore Significance Component

Pt 47 0.7 1.0 + 6 +0.3 +0.8 Ns

N t 85 1.6 1.0 _+ 5 _+ 1.5 +0.5 Ns

P, 166 3.3 2.0 .+ II _+1.5 _ + 1 . 4 P < I 0 -"

N. 268 2.8 2.5 ± 22 -+ I.! -+0.8 Ns

P.~ 450 8.0 1.0 ! ' < I0 "* ~: 26 +4,9 ±0.6

All nleastlrenlellts are nleans and standard deviations.

IGNORE ATTEND

5.0,uV

. . . . . . . . . 0.8s Fig. 2 The evoked response to detected signal stimuli. The late components of the evoked potential to the occasional faint clicks while tile subject detected and counted them, and while he read a book and disregarded all auditory stimuli. Attending to the signals resulted in both a marked increase in N t and P, and the occurrence of a large posterior P3 com- ponent. Each tracing represents the average of 32 responses, Evoked potentials are shown for three different scalp loca- tions - frontal, central and parietal - to show the different distributions of the components, Subject S.V.

Component Fz Pz

Pt 125 61 98 107 + 33 +24 +_ 19 +_ 50

N l 80 43 104 60 +_ 19 419 _+ 48 + 30

P, 65 70 53 73 + 15 4-11 + 22 + 29

N, 112 42 118 46 + 15 +24 _+ 36 _+ 32

P~ 54 165 _+ 12 _+ 27

NON-SIGNAL SIGNAL

'~0ms

• ; ' , : ' , • ; , ; : " . . ,

. , ;

- ' ' 0 . 6s , . - -

IGNORE . . . . . . . . . A T T E N D

Fig. 3. Evoked potentials during attention. The evoked potentials to both signals and nonsignal stimuli each plotted on three different time bases, during " ' a t t end" (dotted line) and " ' ignore" (continuous line) conditions. The tracings for the signal response represent the average of 768 responses, and the tracings for the nonsignal response represent the average of 3072 responses. Subject R.H.

HUMAN AUDITORY ATTENTION 195

ponent was more prominent at the vertex in the evoked response to the standard stimulus.

In order to evaluate the earlier components of the sig,aal evoked potential, the paradigm was altered such that the signals occurred more fie- quently, i in 5 of the stimuli being sign Is. This allowed sufficient signal stimuli for the separate analysis of the entire series of components in the evoked potential to both signal and nonsignai. Fig. 3 shows the results from one of two subjects investigated in this manner. No significant change was seen in any component of the evoked potential prior to N ~. The N m and P_, components evoked by all stimuli were enlarged with atten- tion. and in the response to signal there was a definite P3 component with attention. This P3 wave was smaller in this paradigm than in tile prior experiments with less frequent signal sti- muli,

Eroked imtenthdx to omitted sthmdi In this part ol'the experiment the subject was

asked to detect and count the number or" omitted stimuli occurring irregularly cver x 5 311 seconds

in a regular ! sec train of clicks. Under these conditions, a compicx of ~va,,es with a large positix~e component at about 41111 msec was re- corded following the absent stimulus. The laten- cy. shape, and scalp distribution of this compo- nent (Fig. 4) were simihtr to those parameters of the P3 wave evoked b.x the detected signal sti- mulus in the original paradigm.

1he scalp distribution of th,- response to ;t detected omitted stimuhts was stuqied using 16- channel averaging techniques. Fig. 5. ,4 shows the average distribution for seven subjects and Fig. 5. B gives an ilhtstrative example. The wide-

A OMITT STIMU

I

I

I

I

i 15"OjJV 1

I

a 200ms t marking

CL ICK O H I T T E D CLICK CLICK

Fig. 4. Evoked potentials to omitted stimuli. (:licks were presented regularly every I. I sec and occasionally a click was omitted; the subject was asked to count the number of omissions. The averaging computer was triggered by the click immediately precedmg the omission. Evoked potentials are shown l\~t three different scalp positions. Each tracing represents the average of 64 responses. Subject T.P.

• 8 . 1 p V

N : 7 3 4 4 m s

OMITTED .... S T I M U L U S ; • "~'L"" "

\ \

I

\ /

S . A . H . " - - . . . . " 0 " 2 s

t-Q. 5. Scalp distribution of the omitted stmmlus re,,ponse. .-I ' iones were presented regularl.,, ever.v second and occasio- nail> one of lhe stimuli v, as omitled: the subject was asked to rich?el and ¢Ollnl the omissions. A%er;.lgillg COlllpll|~.'r re- corded the polenliais folloxving the omissioli at various ,,calp locations using a chest ret'crence electrode. Distribution Map represents the averaged data from seven subjects. B' illustralixe example of mapping records. Each tracing re- presents the average of 30 responses..Subject S.tt.

196 T . W . PICTON AND S. A. HILLYARD

SENSORY EVOKED

RESPONSE

,rPERCEPTU/~L + [DECISION

COMPLEX DETECTED !

: SIGNAL RESPONSE

y ! t / STIM ULUS 0 [CISI ON SIGNAl. __J

200~,s Fig. 6. Diagrammatic conceptualization of the components of the evoked response to a detected signal.

spread scalp distribution contrasted with the more circumscribed distributions of the N I and P2 components detailed in the preceding paper. The evidence suggested to us that the evoked response to a detected signal was composed of two distinct parts: a sensory evoked response, and a perceptual decision complex that did not require a definite stimulus, but could be associat- ed with the decision that a stimulus had not occurred. This concept is illustrated in Fig. 6.

DISCUSSION

We have shown that there is no significant change in the auditory evoked potential prior to NI-P2 components when attention is directed towards auditory stimuli in order to perform a difficult loudness discrimination. This supple- ments our earlier report (Picton et al. 197 l) that cochlear nerve potentials were unchanged during an identical attention task. We have, therefore, again obtailied no evidence for peripheral gating in the auditory system (Hernandez-Peon 1966) as a mechanism for auditory attention. Our results also indicate that if there is a subtle peripheral attenuation mechanism underlying attention ('l'reisman 1964) rather than a definite gating, it is not substantially reflected in the evoked potential. Rather, our observations sug- gest that attention most directly involves changes in responsiveness of association cortex which is the presumed major generator of the N~-P2 components (see preceding paper).

The generality of these observations must be qualified by the limitations of both the recording technique and the experimental paradigm. During the intensity discrimination task evoked

potee'ials were recorded only from a vertex- mastoid derivation; evoked potential changes having dipole fields orthogonal to the recording derivation, or having electrical fields too smali to be detected at the scalp, could have been missed. The evoked potential itself represents in part the spatial average of the activity from a large number of cells and might, therefore, fail to detect subtle changes in the individual firing pattern of these neurons. It is also possible that the attentional processes activated during an intensity discrimination task of this type might differ in some manner from the attentive mecha- nisms involved with such other auditory tasks as binaural listening or forced choice signal detec- tion. These different attentional processes all have common elements, however, in that atten- tion is "focused" or "directed" towards a rele- vant auditory stimulus and to another stimulus in the control condition. Considering all such qualifications, our experimental results still de- monstrate that there is no substantial effect of attentional manipulation upon auditory process- ing in the nervous system prior to the generation of the N t-P2 components.

There is some controversy in the literature as to whether these N I-Pz evoked potential effects are signs of the actual selective information pro- cess or correlates of concurrent changes in nonspecif: arousal (Nii~it~inen 1967; Karlin 1970). In the present study, however, there were no obvious differences reported by the subjects in arousal or alertness between conditions. EEG evidence of changes in arousal are difficult to evaluate: a slight increase in alpha activity with auditory attention has been reported (Walter et al. 1967), but tilis is probably more indicative of selective visual inattention than of any change in nonspecific arousal level. Striking changes in the N component of the evoked potential, as we have shown in the preceding paper, occur with decreasing levels of arousal. Thus the fact that the N I-P2 changes with attention occurred without significant change in the N2 component is further suggestive evidence that such effects are indicative more of selective attention than of arousal or alertness. Attention-related changes similar to those reported here have also been noted in binaural listening tasks. In such tasks the N~ component evoked by stimuli in an

HUMAN AUDITORY ATTENTION 197

attended ear is enhanced relative to that evoked by concurrent stimuli presented to the opposite ear (Picton e t al. 1971 ~ Hillyard et ai. 1973). Such reciprocal changes in evoked potentials within conditions cannot be explained by any differences in nonspecific arousal level between conditions.

The N r P " , process is dependent in both amplitude (Davis and Zerlin 1966, Rapin e t al. 1966) and distribution (Ruhm 1971 ) on stimulus parameters. There is also a high dependence in amplitude (see introduction) and possibly distri- bution (Wood et al. 1971 : Matsumiya et al, 1972) upon attentional manipulation, One striking example of this is the recording of an N~-P2 complex (prior to a succeeding positivity) in frontal lobe cortex in response to an expected but absent stimulus (Weinberg et al. 1970), Our omitted stimulus paradigm did not allow as good a time-locking of response to expectation as that of Weinberg et al. and any N t-P2 event preceding the positive wave was probably lost in the time jitter, This evidence suggests that the N~-P: complex may represent the activation of neural assemblies involved with the analysis ofincoming auditory information, the extent and nature of which is determined both by the stimulus and by the nature of the attentional process required. N~-P2 seems to reflect higher order analysis rather than differential recognition of signals. since the percentage change with attention was approximately the same for both the signal and the standard stimuli.

The most striking effect of attention noted in the evoked potential was the appearance of the large P3 component in the response to the detect- ed signals. Such a late positive wave has been recorded in response to a wide variety of stimuli which deliver task-relevant information (see recent review by Squires et al. 1973a). Several hypotheses have been advanced as to the psy- chological nature of this P3 wave. It might represent the resolution of the uncertainty (Sut- ton et al. 1967), an orienting response to an un- expected stimulus (Ritter et al. 1968), the percep- tual decision that an expected signal has occurred (Hillyard 1969; Smith et al. 1970~ Picton et al. 1973) or a nonspecific reactive change in arousal following such a decision (Karlin 1970).

Several observations seem pertinent to the

evaluation of this P3 process. It is quite distinct in scalp topography from either N~ or P2 com- ponents, the P3 being much more generalized and somewhat more posteriorly distributed. The scalp distribution of the positive component of the omitted stimulus response reported here is similar to the distribution of the P3 wave evoked by an infrequent auditory stimulus (Vaughan and Ritter 1970), by a discriminated auditory signal (Ritter et al. 1972) and by other task-rele- vant auditory stimuli (Hillyard et al. 1974). It seems to be independent of the nature of the trig- gering stimulus and can readily be recorded in response to detected omissions in trains of visual (Barlow 1969), somatosensory {Klinke et al. 1968) and auditory (Picton et ai. 1973) stimuli. It is closely correlated with the parameters of perceptual decision making: signal probability (Teuting et ai. 1970)~ Squires et al. 1973a)payoff values and decision criteria (Paul and Sutton 1972) and confidence levels (Squires et al. 1973b). However, reaction time studies suggest that the major portion of the P3 wave actually follows in time the perceptual decision since the motor response precedes the P~ peak latency by several tens of milliseconds (Ritter et ai. 1972: Picton et ai. 1973). A reasonable evaluation would then be that the P3 complex indexes the percepto- motor sequelae of the decision that a certain signal has occurred. Such sequelae could involve the registration of pertinent sensory informa- tion in memory, the resetting of perceptual ana- lyzers or an appropriate behavioral response.

It has recently been proposed that two funda- mental modes of selective attention can be distinguished: stimulus set and response set (Broadbent 1970, 1972). Stimulus set attention involves the selection of a particular input chan- nel to be examined in preference to others. Response set involves the comparison of inputs in attended channels to "memory" or "template" units in order to recognize selected target signals which require sp¢cial response. Our experimen- tal paradigm entailed a stimulus set attention directed to the auditory channel (clicks) in one condition and the visual channel (book) in the other. Response set was employed to recognize the faint click signals when the auditory channel was attended. Thus stimulus set attention appears to be reflected electrophysiologically in the

198 I. W. PI(TON AND S. A. HILLYARD

enhanced N t P2 response to all stimuli in the attended channel. Response set attention is indexed by the generation of a P3 complex to the target signal (Hillyard eta/. 1973 ).

In conclusion we should like to propose a tentative synthesis of all such data into an elementary model for the physiological basis of human auditory attention. The stability of the early components of the evoked response would seem to indicate that auditory information is analyzed in the lemniscal or primary auditory system in much the same manner regardless of whether it is attended or not. A secondary audi- tory system, imperfectly defined but probably compri:dng reticular forlnatim~, medial thalamus and association cortex, is involved when further evaluation of the significance of this auditory information is required. Stimulus set directs the preferential input to this secondary system from that part of the lemniscal system involved in processing the attended sensory channel. Stimu- lus set is evidenced, therefore, by the increased N t P, response of the frontal association cortex. The secondary system functions to compare input from the primary auditory system with memory models or templates of expected or significant stimulus alternatives. Once a relevant or sig- nificant signal has thereby been recognized, this decision is tbllowed by appropriate percepto- motor sequelae. These sequelae, reflecting the contingencies of a response set mode of attention are associated with the generation of the wide- spread P3 complex recorded from the scalp.

SUMMARY

Attention directed toward auditory stimuli, in order to detect an occasional fainter "'signal" stimulus, caused a substantial increase in the N t (83 msec} and P2 (161 msec} components of the auditory evoked potential without any change in preceding components. This evidence shows that human auditory attention is not mediated by a peripheral gating mechanism. The evoked response to the detected signal stimulus also contained a large P3 (450 reset) wave that was topographically distinct from the preceding components. This late positive wave could also be recorded in response to a detected omitted stimulus in a regular train and therefore seemed

to index a stimulus-independent perceptual deci- sion process.

RESUME

P()II-NIiELS EVOQUES AUDITIFS CItEZ L'HOMME.

I1: EFFETS Dli L'ArTENTION

Le fait de diriger l'attention vers des stimuli auditifs, afin de d6tecter un stimulus signal occa- sionnel plus faible, provoque une augmentation substantieile des composantes N t (83 msec} et P, (161 msec)du potentiel auditif ~voqu& sans aucune modification des composantes ant~rieu- res. Cette donn6e montre que l'attention auditive chez l'homme n'est pas transmise par un m&:a- nisme d'ouverture p6ripherique. La r6ponse ~vo- qu6e au stimulus signal d6tect~ contient ~gale- ment une grande onde P3 (450 msec) qui est topographiquement distincte des composantes pr6c6dentes. Cette onde positive tardive peut ~galement &re enregist r~e en r~ponse a l'omission d'tm stimulus, d~tectge a I'int~rieur d'un train r6gulier, et semble ainsi constituer le t6moin d'un processus de d~cision perceptuelle ind~pendant du stimulus.

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