neuro vs. cognitive psychology:

Neuro vs. Cognitive Psychology:

A case study

Outline

• What is activation?- The view from fMRI

• The logic of subtraction

• Imaging orthographic similarity

Signs of activation

• Cellular activity in the brain is accompanied by:– Increased blood flow (and so temperature) in the

activated area – Increased oxygen uptake in the activated area– Increased glucose use (during oxidative metabolism)

– IF we can detect changes in blood flow or oxygen uptake or glucose metabolism or temperature, then we can deduce where cellular activity differences occur during any given task

Functional magnetic resonance imaging (fMRI)

• Measure ‘blood oxygen level dependent’ (BOLD) signal- increased local CBF during activity leads to excessive oxygenated hemoglobin (oxyhemoglobin) in that region (Anyone know why?)- Oxygenated and deoxygenated hemoglobin have different magnetic properties, the latter being magnetically charged- We can detect two different relaxation times: T1 (spin lattice relaxation time) and T2 (spin-spin relaxation time)- it is the latter that is used for functional imaging

• T2* is induced by local magnetic field homogeneity in the slice under current study

• fMRI resolution is about 1 X 1 x 3-4 mm.; temporal resolution is several seconds for whole brain

Subtraction logic

• Due to Donders, 1868

• Let’s say you are interested in A

• Devise a task A+B, which incorporates A

• Ask subjects to do A+B and B alone– Then (A+B) - B = Time to do A– i.e. Color discrimination of lights - RT to lights

= Color discrimination time

Subtraction logic• Subtraction logic makes many assumptions, some

of which are debatable• The subtraction method necessarily or implicitly

assumes:– that cognitive processing is serial– that cognitive processing is hierarchically organized– that cognitive processing unfolds in an exclusively

forward fashion– that structures participate in an all or nothing fashion in

a cognitive process

Subtraction logic in fMRI

• The subtraction method necessarily or implicitly assumes:– that peak CBF or glucose uptake or oxygen use corresponds

to one single cognitive component of the task

– that the same cognitive component of a task is always performed by the same brain region (and thus, implicitly, that the brain is not redundantly organized), even if that component is shared between different tasks

– that subjects perform all and only the requested task (or that other tasks are are associated with random or perfectly consistent activity)

Subtraction logic• None of the assumptions seems terribly

likely, and several fly in the face of current theory about brain organization.

• What can we do to overcome doubt?Gather converging evidence.

Subtraction logic

• Subtraction logic is almost always used in imaging experiments– Recently, some have started using auto-correlation

instead, but this is not wide-spread

• The nature and ‘purity’ of the subtraction is vital to interpretation of the imaging results

• For this reason, imaging results and experimental design are intimately and necessarily yoked

Language studies• Language access is very fast & very complex,

with multiple ‘micro-functional’ constraints– Experimental psycholinguistics has identified an over-

whelming number of variables (several dozen) with demonstrable behavioral impact on lexical access = multiple ‘functional constraints’ in play

• There was a dissociation between early language imaging and psycholinguistic understanding, with stimuli in imaging studies failing to meet the rigourous control demands of psycholinguistic understanding

Micro-functional dissection

• Since even the simplest lexical access task is a multi-dimensional conglomeration of functionality, the key is to use very simple tasks, with very highly-controlled stimuli– In this way we try to ‘trap’ an automatic

function of interest, well below conscious awareness

– And we pray that it is fine-grained enough to be informative!

J.R. Binder, K.A. McKiernan, M.E. Parsons, C.F. Westbury, E.T. Possing, J.N. Kaufman, L. Buchanan (in press) Neural Systems Underlying Lexical Access During Word Recognition, Journal of Cognitive Neuroscience.

ON

ON• Coltheart’s Orthographic N [ON]: The number of words

that are one-letter different from the target word-i.e. DOG ---> HOG, DOE, DOT, DIG etc.

• Many experiments manipulating ON have found a frequency-modulated neighborhood size effect. • Uncommon words with large ON are recognized as words more rapidly than low-frequency words with small neighborhoods • This effect disappears with common words• This is among the bigger effects, with freq and ON together accounting for > 30% of variance in behavioral measures

ON by FREQUENCY

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Uncommon Common

Large ON

Small ON

ON by FREQUENCY

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Small ON

Almost all words are uncommon.

The trap

• Task is lexical decision: decide whether or not a presented string is a word

• 50 high/low ON concrete nouns & nonwords (100 each in all) matched one-by-one on frequency, length, bigram frequency, and phonological neighborhood size, and (between wordness) on ON= the NWs are highly word-like, & the two real word sets

are very similar to each other except for the manipulated ON

Hypotheses

(i) Words should produce stronger activation than word-like nonwords in many of the brain regions previously identified in studies comparing semantic to non-semantic tasks, and

(ii) A subset of these regions should show stronger responses to items with many lexical neighbors, indicating activation of pre-semantic word codes.

Behavioral data: RTs

Psych Lab Scanner

Behavioral data: Errors

Psych Lab Scanner

fMRI parameters• GE Signa 1.5 Tesla scanner • T1-weighted anatomical reference images: 124

contiguous sagittal slices (.9375 x .9375 x 1.2 mm) • Functional imaging: 19 contiguous (7 - 7.5 mm) sagittal

slice locations covering the entire brain x 3.75 x 3.75 mm• 136 whole-brain image volumes collected from each

subject at 2-sec intervals• Each image was yoked to a behavioral decision (event-

activated fMRI), allowing separate imaging of high/low ON x W/NW

Words vs NWs

Words - NWs

Words - NWs

i.) Almost exclusively LH

Words - NWs

ii.) Dorsal + inferior medial prefrontal activity

Semantic decision - phonological decision

Words - NWs

iii.) Angular gyrus activity

Semantic decision - phonological decision

Transcortical sensory aphasia

X

-Damage to the ‘long route’ between Broca’s & Wernicke’s area-Main feature is a deficit in accessing (thinking about or remembering) the meanings of words

- Comprehension is therefore severely impaired

- The patient can neither read nor write and has major difficulty in word finding

Lichtheim, 1885

Words - NWs

iv.) Extensive midline activity

Words - NWs

iv.) Extensive midline activity

QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.

High versus low ON

NW

W

Small ON [hard] - Large ON [easy]

i.) Small ON activation > Large ON activation

• We had (perhaps foolishly) hypothesized the opposite

• Although small ON is ‘harder’ by evidence of RT and error rates, high ON seems to coordinate a wider variety of information

• However: Greater constraints = Easier computation – Think of 20 questions after 19 questions have been

asked

ii.) Bilateral midline activity• The midline is not normally associated

with lexical processing


with lexical processing– But we saw some in the W - NW contrasts:



with lexical processing– But we saw some in the W - NW contrasts:

– And it was mirrored in the semantic tasks:


iii.) Words >> NWs

• There is almost no activity for the high - low ON condition for NWs

NW

iii.) Words >> NWs

• There is almost no activity for the high - low ON condition for NWs

• What differentiates words from NWs?– Semantics!– By evidence of activation, ON manipulations are sensitive to

semantics

NW

ON vs [Semantics - phonology]

W

ON vs Semantics

W

Semantics as a ‘final push’

• Small ON words seem to require more extensive semantic processing – Why? To compensate for the fact that these items are less

orthographically word-like.

• High ON biases the subject toward a positive response (increases the tendency towards ‘yes’)– Relatively little semantic activation is then needed to complete

the response selection task: hence [low ON - high ON] looks like [semantics - phonology]

• This explains why low ON > high ON, and why we don’t see the effects for NWs, which take the same semantic processing in both ON conditions

Why are high ON NWs slow and error-prone?

• Presumably high ON NWs are rejected more slowly and more likely to be accepted because of their greater resemblance to words = harder to reject

• However, the semantics interpretation fails: no NWs have any semantics

• And we cannot explain why there are (almost) no imagable effects of this very reliable behavioral difference, save some puzzling midline activity

Cognitive + Neuro psychology

• We probably would not (did not) have a view that ascribed semantic effects to ON sensitivity without imaging: Experimentation fails

• However, the only evidence we have (right now) of ON effects in NWs are robust experimental effects: Imaging fails

• It is a good thing that cognitive neuropsychology embraces both behavior and the brain

neuro vs. cognitive psychology:

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