Neural codes - Purduegfrancis/Classes/PSY310/L12b.pdf · 2006. 2. 3. · Prof. Greg Francis 3 PSY...
Transcript of Neural codes - Purduegfrancis/Classes/PSY310/L12b.pdf · 2006. 2. 3. · Prof. Greg Francis 3 PSY...
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Prof. Greg Francis
1PSY 310: Sensory and Perceptual Processes
Purdue University
Neural codes
PSY 310
Greg Francis
Lecture 12
Is 100 billion neurons enough?
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COC illusion The COC illusion looks like real squares because the neural
responses are similar
True squares COC squares
Ganglion cell responses
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Prof. Greg Francis
2PSY 310: Sensory and Perceptual Processes
Purdue University
Ventral stream
From visual cortex to the temporal lobe
Involved in recognizing or identifying objects
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Critical features Cells in inferotemporal cortex respond to complex features in
stimuli Not easy to measure neurophysiologically Why such bizarre patterns?
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Prof. Greg Francis
3PSY 310: Sensory and Perceptual Processes
Purdue University
Real stimuli Following the ideas about the COC stimulus, our
perceptual experience is determined by the neuralresponses to stimuli
Suppose you see this picture of a cat
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Cell responses This IT cell might respond strongly And the other cells hardly at all
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Prof. Greg Francis
4PSY 310: Sensory and Perceptual Processes
Purdue University
Cell responses One might be tempted to suggest that this cell
“codes” the perceptual experience of the cat But this would not be correct This cell would respond similarly to many different
types of stimuli But our perceptual experience is quite different!
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Neurophysiology and perception
Does this mean that our perceptual experience isnot determined by neural responses? After all similar neural responses should give rise to
similar percepts
No. Similarity between ganglion responses and
perceptual experience works for the ganglion cellsbecause of their anatomical location Everything else in the visual system is based on their
responses
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Prof. Greg Francis
5PSY 310: Sensory and Perceptual Processes
Purdue University
Neurophysiology and perception The rest of visual perception does not depend on
the responses of this IT cell
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Neurophysiology and perception The rest of visual perception does not depend on the
responses of this IT cell In fact, probably thousands of IT cells respond to this
stimulus
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Prof. Greg Francis
6PSY 310: Sensory and Perceptual Processes
Purdue University
Neurophysiology and perception The rest of visual perception does not depend on the
responses of this IT cell In fact, probably thousands of IT cells respond to this
stimulus Millions of ganglion cells
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Neurophysiology and perception The rest of visual perception does not depend on the
responses of this IT cell In fact, probably thousands of IT cells respond to this
stimulus Millions of ganglion cells Millions of orientation sensitive cells
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Prof. Greg Francis
7PSY 310: Sensory and Perceptual Processes
Purdue University
Neurophysiology and perception The rest of visual perception does not depend on the
responses of this IT cell In fact, probably thousands of IT cells respond to this
stimulus Millions of ganglion cells Millions of orientation sensitive cells Dorsal stream
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Representation of objects Objects are unlikely to be represented by a single neuron
Consider faces
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Prof. Greg Francis
8PSY 310: Sensory and Perceptual Processes
Purdue University
Representation of objects Objects are unlikely to be represented by a single neuron
Consider faces
What do you do here?
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Representation of objects Objects are unlikely to be represented by a single neuron
Consider faces
What do you do here?
What do you do withdifferent expressions?
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Prof. Greg Francis
9PSY 310: Sensory and Perceptual Processes
Purdue University
Representation of objects Objects are unlikely to be represented by a single neuron
Consider faces
What do you do here?
Since we perceive andrecognize all of thesefaces as beingdifferent. There mustbe a different neuralrepresentations.
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Distributed code Each cell codes some feature of the image
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Prof. Greg Francis
10PSY 310: Sensory and Perceptual Processes
Purdue University
Distributed code Each cell codes some feature of the image
The representationof a face is apattern across thefeatures.No two faces havethe same pattern,so not the samepercept.
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Distributed code Each cell codes some feature of the image
Change ofexpression maychange only someof the features.
Similar faces havesimilar patterns; sosimilar percepts.
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Prof. Greg Francis
11PSY 310: Sensory and Perceptual Processes
Purdue University
Features So what are the features?
A difficult question to answer
They need not be things that we would name
Nose, eye, mouth, hair
Could be Fourier components
Or something completely different
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Fourier features Selecting certain Fourier components as features might not
correspond to anything that we would name in the image
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Prof. Greg Francis
12PSY 310: Sensory and Perceptual Processes
Purdue University
Distributed code How does it all get put together?
We don’t seefeatures, we seefaces.
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Related issue We earlier discussed how we can consider the visual system to consist
of many retinotopic layers of activities from cells tuned to differentfeatures
Image
Retina
Ganglion cells
Orientationcells
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Prof. Greg Francis
13PSY 310: Sensory and Perceptual Processes
Purdue University
Related issue There are actually many layers we never got a chance to talk
about
Color, size, motion, depth, texture, many more
To an extent, they process things independently - features
Image
Retina
Ganglion cells
Orientationcells
ColorMotion
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Related issue If you see a red car go speeding by, you gets lots of responses
from different parts of the brain
Which part is the percept?
How do they get coordinated?
Image
Retina
Ganglion cells
Orientationcells
ColorMotion
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Prof. Greg Francis
14PSY 310: Sensory and Perceptual Processes
Purdue University
Related issue For that matter, if you are thinking of something else or
listening intently to something
You may not perceive the car
Even though the nervous system responds!
Image
Retina
Ganglion cells
Orientationcells
ColorMotion
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Attention It’s not entirely clear what attention is or does
But it seems to be involved in “pulling together” neuralresponses from different parts of the brain
Perhaps by synchronizing action potentials
Perhaps it selects the features that help solve a particular task
E.g., recognize a face
Attention seems to be necessary to actually perceivesomething
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Prof. Greg Francis
15PSY 310: Sensory and Perceptual Processes
Purdue University
Attentional blink
Suppose you have to identify rapidly presented (100 ms)letters
e.g., detect J and/or K in a stream of letters
MP
KR
WS
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Attentional blink
Turns out that detection of first letter tends to make detection
of the second letter very difficult
if it immediately follows the first
Attentional blink
MP
KR
JS
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Prof. Greg Francis
16PSY 310: Sensory and Perceptual Processes
Purdue University
Attentional blink Measure frequency of detection
Implies thatdetecting firstletter causes you to misssecond letter!
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Attentional blink Maybe when the J appears, some cell in IT
detects the curve
This suggests it might be a J
But it could be an O, U or S
SLNBJRKH...
J
Image
Retina
Ganglion cells
Orientationcells
ColorMotion
J
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Prof. Greg Francis
17PSY 310: Sensory and Perceptual Processes
Purdue University
Attentional blink This cell sends signals back down to other
areas to augment some features
SLNBJRKH...
J
Image
Retina
Ganglion cells
Orientationcells
ColorMotion
J
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Attentional blink This leads to strong activation of other cells
in IT
Which overall producse the pattern thatcorresponds to a J
SLNBJRKH...
J
Image
Retina
Ganglion cells
Orientationcells
ColorMotion
J
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Prof. Greg Francis
18PSY 310: Sensory and Perceptual Processes
Purdue University
Attentional blink The process takes time and may not be finished
before the J disappears
May continue even while other letters go by
Including the K
Attentional blink
SLNBJRKH...
J
Image
Retina
Ganglion cells
Orientationcells
ColorMotion
K
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Conclusions Distributed coding
Features
Neural patterns
Attention to bring it all together
Attentional blink
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Prof. Greg Francis
19PSY 310: Sensory and Perceptual Processes
Purdue University
Next time
Review for the exam
You ask questions
I answer questions
Exam 1 on Friday