Transcript Slides from Discussion section VI 11/15/2004 (Elissa

Visual categorization shapes feature
selectivity in the primate temporal
cortex
Natasha Sigala &
Nikos K. Logothetis
In: Nature, vol 415, 17 Jan 2002
Questions:
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Generally: How do we categorize objects? What
does the Inferior Temporal (IT) cortex do?
Which object features are represented in neurons in
the inferior temporal cortex of the macaque?
How are these representations affected by
categorization training?
Specifically: Do Inferior Temporal neurons fire
differentially in response to object features that are
experimentally deemed to be significant for success
on a categorization task?
Alternatives:
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Neurons in the IT cortex will differentially
respond to specific features after
categorization training
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Neurons in the IT cortex will not differentially
fire in response to specific features after
categorization training
Logic:
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Experience effects our perception of a given
object
Neurons in IT are known to be part of the
ventral stream-- “what” processing system
If experience effects the way we perceive
objects, neuronal firing will show it—that is,
experience of attention to specific features
will change the firing patterns for objects that
we see subsequently
Methods:
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Two [happy] macaque monkeys
Stimuli: “parameterized” line drawings of
faces and fish (stimuli had measurable
features)
Diagnostic and non-diagnostic features
Training
Testing
Methods: Stimuli
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Faces:
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EH, eye height
ES, eye separation
NL, nose length
MH, mouth height
EH, ES—separable; diagnostic
NL, MH not separable; non-diagnostic
Methods: Stimuli
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Fish
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DF, shape of dorsal fin
T, tail
VF, ventral fins
M, mouth
DF, T separable; diagnostic
VF, M not separable; non-diagnostic
Stimuli
Methods
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Categorization task
Monkeys trained to pull a different lever
depending on if are shown stimulus from
category 1 or 2
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Trained until they reached 98% accuracy in about
½ second (416 ms, 530ms)
Tested—34 face, 34 fish, 10 repetitions
Methods
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Recorded from 150 single neurons from
anterior IT—in different experimental
sessions
Of these 150, 96 responded to visual stimuli
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96 tested with faces stimuli
65 tested with fish stimuli
Results:
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44 of 96 (45.8%) neurons responded
statistically significantly to one or more
values of the face stimuli features
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32 of these 44 (72.7%) responded selectively to
either one or both diagnostic features of the face
stimuli (but did not respond for the non-diagnostic
features)
Results
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28 of 65 (43.1%) neurons responded
statistically significantly to one or more
values of the fish stimuli features
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21 of these 28 (75%) responded selectively to
either one or both diagnostic features of the fish
stimuli (and not for the non-diagnostic features.)
Results: Definitions
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There were three possible values for each stimulus
feature (eg: large, medium or small eye width)
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Best feature, defined as the stimulus value that
provoked the largest number of spikes per second
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Worst feature, defined as the stimulus value that
provoked the least number of spikes per second
Results: Figure 4
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For both faces and fish, more
neurons fired in response to the
diagnostic features
Red: neurons that fired
statistically significantly
selectively for diagnostic
features only
Blue: neurons that fired
statistically significantly
selectively for diagnostic and
non-diagnostic features
Black: no significant selectivity
Interpretation:
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This study supports the notion that
perception of visual category information is
processed in the Inferior Temporal cortex
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Inferior Temporal neurons do fire differentially
in response to object features that are
experimentally deemed to be significant for
success on a categorization task.
Limitations:
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Before and After snapshot missing: It would have been an even
more exciting study if the authors had shown the patterns of
neuronal firing before the training and compared that to posttraining firing rates
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Or alternatively, they could have trained other monkeys on the
“non-diagnostic” features in order to provide more evidence that it
was actually the training that caused the selective firing
Having shown such an interesting result, it would have been
even more theoretically important if they had correlated the
monkeys’ category discriminations (behavior) with the patterns
of neuronal firing.
According to visual system expert (Pascal) the stimuli were a bit
small for the monkeys to see