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Chapter 8

PSYB57H3 Chapter 8: PSYB51-Chapter 8 Notes


Department
Psychology
Course Code
PSYB57H3
Professor
George Cree
Chapter
8

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Chapter 8: Motion Perception
Our visual system distinguishes the bug by a number of features, such as its
shape, its location in space, and its color. Previous chapters have established
these features as fundamental perceptual dimensions: characteristics of
visual stimuli that are directly encoded by neurons fairly early in the visual
system.
Motion aftereffect (MAE): The illusion of motion of stationary object that
occurs after prolonged exposure to a moving object.
Just as colour aftereffects are caused by opponent processes for colour
vision, MAEs are caused by opponent processes for motion detection.
Computation of Visual Motion
Because motion involves a change in position over time, a logical place to
start is with two adjacent receptors (call them neurons A and B) separated by
fixed distance. A bug (or a spot of light) moving from left to right would first
pass through neuron A’s receptive field, and then a short time later it would
enter neuron B’s receptive field. )n theory, a third cell that listens to
neurons A and B should be able to detect this movement.
However, out motion detection cell (call it M) cannot simply add up
excitatory inputs from A and B. Given such a neutral circuit, M would fire in
response to the moving bug, but it would also respond to a large, stationary
bug that covered both receptive fields. To solve this problem, we need two
additional components in our neural circuit, as shown in Figure 8.3c. The
first new cell, labeled D in the figure, receives input from neuron A and
delays transmission of this input for a short period of time. Cell D also has a
fast adaptation rate. That is, it fires when cell A initially detects light, but
quickly stops firing if the light remains shining on A’s receptive field. Cell B
and D are then connected to neurons X, a multiplication cell. This
multiplication cell will fire only when both cells B and D are active. By
delaying receptor A’s response D and then multiplying it by receptor B’s
response (X), we can crate a mechanism that is sensitive to motion.
This mechanism would be directed-selective: it would respond well to
motion from left to right, but not from right to left. The mechanism would
also be tuned to velocity because when the bug is moving at just the right
speed, the delayed response from receptor A and the direct response from
receptor B occur at the same time and therefore reinforce each other.
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Now that we’ve constructed our neutral circuit, consider what happens when
our bug moves into cell A’s receptive field and then into cell B’s receptive
field. Cell A will activate cell D, which after a short delay, will communicate
its response to cell X. If the bug is flying at just the right speed, it will move
into cell B’s receptive field just as cell D begins to fire. The signals from B and
D will be multiplied by X, and X will drive cell M, the motion detector, to fire.
Note the M will not respond to our large sedentary bug, because of the fast
adaptation of the D cell. Furthermore, M will respond to motion in only one
particular direction left to right. A bug moving from cell B’s receptive field
into cell A’s receptive field would cause B and D to fire in the wrong order, so
X would not receive its two inputs simultaneously, and M would not fire. Out
motion detector is also velocity-sensitive: if the bug moves too fast or too
slow, the outputs from B and D will again be out of sync.
Apparent Motion
Apparent motion: The illusory impression of smooth motion resulting from
the rapid alternation of objects that appear in different locations in rapid
succession.
Apparent motion was first demonstrated by Sigmund Exner in 1875. Exner
set up a contraption that would generate electrical sparks separated from
each other by a very short distance in space and a very short period of time.
Even though there were two separate sparks that is, two different
perceptual objects observers swore that they saw a single spark moving
from one position to another.
The Correspondence Problem
Each movie has two frames that alternate back and forth. The only difference
between the two frames is that the only object in the movie, the red squares
studded with small circles, has been shifted diagonally by a short distance.
The square moves down and to the right, and then back up and to the left,
and detectors sensitive to these directions pick up and signal this movement.
Now consider movie 2 (8.5b), where we cover most of the squares with a
black mask leaving three of the circles viewable through a small window.
Beneath the mask, the square moves exactly as before, but it you view the
movie you will quite clearly perceive up-and-down, not diagonal, motion.
Corresponding problem: In motion detection, the problem faced by the
motion detection system of knowing which feature in frame 2 corresponds to
a particular feature in frame 1.
The difficulty for our motion
Detection system is this: how
Does it know which circle in frame
2 corresponds to which circles
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in frame 1? Because we have motion detectors for all directions, one detector
will sense the diagonal motion implied by matching the circle labeled A in
Figure 8.5c with the circle labeled C. But another detector will sense the
vertical motion implied by matching circle A with circle B. These detectors
compete to determine our overall perception.
The Aperture Problem
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 object may be ambiguous.
Aperture: An opening that allows only a partial view of an object.
A variety of contours of different orientations moving at different speeds can
cause identical responses in a motion-sensitive neuron in the visual system.
Without the aperture, there’s no ambiguity and no problem. But when we
view the grating through the aperture, the system appears to impose some
kind of shortest-distance constraint, and thus the vertical-motion detector
wins.
Every V1 cell sees the world through a small aperture.
The solution to this problem is to have another set of neurons listen to the VI
neurons and integrate the potentially conflicting signals.
Only one direction down and to the right - is consistent with what all four
V1 cells are seeing here, and this is the direction we perceive when we see
the object as a whole.
Detection of Global Motion in Area MT
Lesions to the magnocellular layer of the lateral geniculate nucleus (LGN)
impair the perception of large, rapidly moving objects. Information from
magnocellular neurons feeds into V1 and is then passed on to (among other
places) the middle temporal area of the cortex, an area commonly referred to
as MT in nonhuman primates. The human equivalent of MT has been
localized using functional MRI (fMRI), and variously labeled as MT+ or V5.
Recent work suggests that this motion-sensitive area may have at least two
separate maps located on the lateral surface at the temporal occipital (TO)
boundary.
The vast majority of neurons in the MT are selective for motion in one
particular direction, but they show little selectivity for form or colour.
Motion Aftereffect Revisited:
You might wonder why any motion detectors would fire in response to a
stationary rock, but as we will see in a bit, our eyes are constantly drifting
around, so there is always a small amount of retinal motion to stimulate
motion detectors at least slightly.
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