PSYB51 Detailed chapter notes (Chapter 7)

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Published on 19 Jul 2011
School
UTSC
Department
Psychology
Course
PSYB51H3
Chapter 7: Motion Perception
-Shape, location in space, color fundamental perceptual dimensions
-Motion as a low-level perceptual phenomenon cells in primary visual cortex
selectively respond to motion in one particular direction
Motion aftereffect (MAE)
-The illusion of motion of a stationary object that occurs after prolonged exposure to a
moving object
-Just as color after effects are caused by opponent processes for color vision, the
motion aftereffect is caused by opponent processes for motion detection
Computation of Visual Motion
-Page 171 look over
-Barlow and Levick: AND gate Cell X fires if and only if both its inputs (B and D)
are firing simultaneously, and it passes this message on to the motion detection cell
M.
Almost certainly not correct
Adelson and Bergen (more elaborate version): based on linear filters that delay,
sum, and then are followed by nonlinearities seems closer to the truth.
-Our motion detector is velocity sensitive in their direction sensitive
Apparent Motion
-One possible objection to this neuronal circuit is that it does not, in fact, require
continuous motion in order to fire
-Apparent motion the (illusory) impression of smooth motion resulting from the rapid
alternation of objects that appear in different locations in rapid succession
First demonstrated by Sigmund Exner in 1875
The Correspondence and Aperture Problems
-Aperture an opening that allows only a partial view of the object
-Correspondence problem (motion) the problems faced by the motion detection
system of knowing which feature in frame 2 corresponds to a particular feature in
frame 1 example on page 173 figure 7.5
-Aperture problem the fact that 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 objects
may be ambiguous.
-Every V1 cell sees the world through a small aperture therefore 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 to this problem is to have another set of neurons listen to the V1 neurons
integrate the potentially conflicting signals.
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Detection of Global Motion and Area MT
-Lesions to the magnocellular layers of the LGN (lateral geniculate nucleus) impair
perception of large, rapidly moving objects.
-Information from magnocellular neurons feeds into V1 and is then passed on to the
middle temporal lobe of the cortex, an area commonly referred to as MT.
Middle temporal lobe (MT) an area of the brain thought to be important in the
perception of motion.
-The vast majority of neurons in MT are selective for motion in one particular
direction, but they show little selectivity for form or color
-Newsome and Pare: trained monkeys to respond to correlated dot motion displays
No single dot in these displays is sufficient to determine the overall direction of
correlated motion; so to detect the correlated direction, a neuron must integrate
information from many local-motion detectors.
-Monkeys could recognize a correlated motion direction when only 2-3% of the dots
are moving in the direction. Following the surgery, the monkeys needed about 10
times as many correlated dots in order to correctly identify the direction of motion.
The monkeys ability to discriminate the orientation of stationary patterns was
unimpaired.
-Lesions may be incomplete or may influence other structures this may have led to a
third variable affecting the outcome
-Another experiment in which the researchers searched for groups of neurons that
responded to one particular direction; e.g. showed to monkey a new set of stimuli
and electrically stimulated the identified MT neurons
The monkeys showed a strong tendency to report motion in the stimulated
neurons preferred direction, even when the dots they were seeing were actually
moving in the opposite direction
This shows a strong case that MT is the site of global-motion detection neurons
in the visual system.
Motion Aftereffects Revisited
-Existence of the MAE implies an opponent-process system much like the one that
plays a role in colour vision.
-Neurons sensitive to upward motion fire at the same rate as neurons sensitive to
downward motion, so the signals cancel out and no motion is perceived
-When we look at a waterfall for a prolonged period, the detectors sensitive to
downward motion become fatigued.
-When we switch our gaze to a stationary object, the neurons sensitive to upward
motion fire faster than the downward-sensitive neurons.
-Our eyes are constantly drifting around, so there is always a small amount of retinal
motion to stimulate motion detectors at least slightly.
-Interocular transfer the transfer of an effect (e.g. adaptation) from one eye to the
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Document Summary

Shape, location in space, color  fundamental perceptual dimensions. Motion as a low-level perceptual phenomenon cells in primary visual cortex selectively respond to motion in one particular direction. The illusion of motion of a stationary object that occurs after prolonged exposure to a moving object. Just as color after effects are caused by opponent processes for color vision, the motion aftereffect is caused by opponent processes for motion detection. Barlow and levick: and gate cell x fires if and only if both its inputs (b and d) are firing simultaneously, and it passes this message on to the motion detection cell.  adelson and bergen (more elaborate version): based on linear filters that delay, sum, and then are followed by nonlinearities seems closer to the truth. Our motion detector is velocity sensitive in their direction sensitive. One possible objection to this neuronal circuit is that it does not, in fact, require.