neuro vs. cognitive psychology:
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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Neuro vs. Cognitive Psychology:
A case study
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Outline
• What is activation?- The view from fMRI
• The logic of subtraction
• Imaging orthographic similarity
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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
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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
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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
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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
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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)
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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.
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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
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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
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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!
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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
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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
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ON by FREQUENCY
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
0 1 2 4 8 16 32 64 128 256 512
Frequency
2
8
14
20
Uncommon Common
Large ON
Small ON
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ON by FREQUENCY
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
0 1 2 4 8 16 32 64 128 256 512
Frequency
2
8
14
20
Uncommon Common
Large ON
Small ON
Almost all words are uncommon.
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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
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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.
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Behavioral data: RTs
Psych Lab Scanner
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Behavioral data: Errors
Psych Lab Scanner
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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
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Words vs NWs
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Words - NWs
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Words - NWs
i.) Almost exclusively LH
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Words - NWs
ii.) Dorsal + inferior medial prefrontal activity
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Semantic decision - phonological decision
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Semantic decision - phonological decision
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Words - NWs
iii.) Angular gyrus activity
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Semantic decision - phonological decision
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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
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Words - NWs
iv.) Extensive midline activity
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Words - NWs
iv.) Extensive midline activity
QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.
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High versus low ON
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NW
W
Small ON [hard] - Large ON [easy]
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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
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ii.) Bilateral midline activity• The midline is not normally associated
with lexical processing
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ii.) Bilateral midline activity• The midline is not normally associated
with lexical processing– But we saw some in the W - NW contrasts:
QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.
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ii.) Bilateral midline activity• The midline is not normally associated
with lexical processing– But we saw some in the W - NW contrasts:
– And it was mirrored in the semantic tasks:
QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.
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iii.) Words >> NWs
• There is almost no activity for the high - low ON condition for NWs
NW
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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
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ON vs [Semantics - phonology]
W
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ON vs Semantics
W
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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
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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
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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
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fin.