Chapter 7 - Study notes

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19 Dec 2010

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-motion is a low level perceptual phenomenon. Many cells in V1 selectively respond to
motion in one particular direction
Motion aftereffect (MAE) = illusion of motion of a stationary object that occurs after
prolonged exposure to a moving object. Similar to color aftereffect, just as it is caused by
opponent processes for color vision. MAE is caused by opponent processes for motion
detection. If fixate on falling object then watch stationary object that object seems to be
moving up
Motion = change in distance over time. To build a motion detector start with 2 adjacent
receptors pt A and B, neuron A and B, separated by a fixed distance. A bug moving from left
to right would first pass through neuron As receptive field then a short time later would
enter receptor Bs receptive field. Thus a 3rd cell (M-motion detection cell) that listens to
neurons A and B should be able to detect this movement
M cant just add up excitatory inputs form A and B. M would fire in response to the moving
bug, but it would also respond to a large stationary bug that covered both receptive fields.
Thus we have 2 additional components in neural circuit. First new cell = D receives input
form A and delays transmission of this input for a short period of time. D also has a first
adaptation rate, it fires when cell A initially detects light, but quickly stops firing if the
light remains shining on As receptive field. B and D are then connected to neuronX, a
multiplication cell. This multiplication cell will fire only when both cells B and D are active.
By delaying receptors As response (D) and then multiplying it by receptor Bs response X,
we can create a mechanism that is sensitive to motion. This mechanism would be direction-
selective. It respond well to motion from left to right but not from right to left. This
mechanism also tuned to velocity cuz when bug is moving at just right speed (if bug moves
to fast it not work), delayed response from receptor A and direct response from receptor B
occur at the same time and thus reinforce each other
AND gate cell X fires if and only if BOTH itz inputs (B and D) are firing simultaneously,
and it passes this message on to the motion detection cell M. This concept is not correct. A
better concept is based on linear filters that delay, sum and then are followed by
Neural circuit not require continuous motion in order to fire. Apparent motion = illusory,
impression of smooth motion resulting from the rapid alternation of objects that appear in
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diff locations in rapid succession, so even though there are say 2 separate sparks, ie 2 diff
perceptual objects, observers swore that they saw a single spark moving from one position
to another. Ex tv motion, movie, computer ie animated cartoon is really a series of still
drawings. We perceive these positions, still drawings, changes over time as motion
Aperture = opening that allows only a partial view of object
Correspondence problem (motion) = problem faced by motion detection system of
knowing which feature in frame 2 corresponds to a particular feature in frame 1
Aperture problem = fact tat when a moving object is viewed through an aperture, or a
receptive field, the direction of motion of a local feature or part of the object may be
Every neuron in V1 has a limited receptive field, thus every V1 cell sees the world through
a small aperture, none of the V1 cells can tell with certainty which visual elements
correspond to one another when an object moves, even when no mask is present. Solution:
have another set of neuron listen tot eh V1 neurons and integrate the potentially conflicting
-middle temporal lobe (MT) an area of brain thought to be important in the perception of
motion. Lesions to the magnocellular layers of LGN impair perception of large, rapidly
moving object. Info from magnoecllular neurons feeds into V1 and is then passed on to MT,
vast majority of neurons in MT are selective for motion in one direction, but they show little
selectivity for form or color
Trained monkeys to recognize correlated motion directions, created lesion in monkeys MT
areas. Monkeys then needed more correlated dots in order to correctly identify direction of
motion. However monkeys ability to discriminate orientation of stationary patterns was
generally unimpaired. Monkeys performance in correlated dot motion task improved weeks
later following lesion, thus they start to use other brain areas to discriminate motion
The study above about creating lesion may accidently damage other parts of the brain and
thus did a study where monkeys trained to discriminate correlated motion directions, then
poked around in the monkeys MT areas to find groups of neurons that responded to one
particular direction. Once they found a group of neurons that responded to say rightward
motion, they showed monkey a new set of stimuli and electrically stimulated identified MT
neurons. But monkeys showed a strong tendency to report motion in the stimulated
neurons preferred direction, even when dots they were seeing were actually moving in
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