Chapter 8. Visual Motion Perception (237-258)
The motion aftereffect (MAE) is the illusion of motion of a stationary object that occurs after
prolonged exposure to a moving object (waterfall illusion).
Computation of Visual Motion
The motion detection cell cannot simply add up excitatory inputs from receptive field A and B.
If this were the case, then M would fire to a moving bug, but also to two stationary bugs, one
in each receptive field. To solve this problem, we need two additional components. ‘D’ receives
input from neuron A and delays transmission of this input for a short period of time, so that the
responses of A and B arrive simultaneously at ‘X’. It also has a fast adaptation rate (it fires when
cell A initially detects light, but quickly stops firing if the light remains shining on A’s receptive
field). ‘X’ is the multiplication cell: it fires only when both cells D and B are active.
By delaying A’s response and then multiplying it by receptor’s B response (in X), we can
create a mechanism that is sensitive to motion (M).
This method would be direction-selective: it would respond well to motion from left to right,
but not from right to left. It’s also tuned to velocity, because only when the bug is moving at
just the right speed, the responses of A and B occur at the same time and therefor reinforce
each other.
Apparent Motion
Apparent motion is the illusory impression of smooth motion resulting from the rapid
alternation of objects that appear in different locations in rapid succession (like the small
separate drawings in an animated cartoon).
The Correspondence Problem
The correspondence problem is the problem face by the motion detection system of knowing
which feature in frame 2 corresponds to a particular feature in frame 1. Because we have
motion detectors for all directions, it can be that you get a wrong direction of motion. The
detectors compete to determine overall perception.
The Aperture Problem
The aperture problem is 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. The aperture is the opening that allows only a partial view of an object.
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,A variety of different orientations moving at different speeds can cause identical responses in
a motion-sensitive neuron in the visual field: the motion component parallel to the grating
cannot be inferred from the visual input. 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 V1 neurons and integrate the potentially conflicting signals.
This is like the Indian parable about the blind men and the elephant (6 blind men feel 6 different
parts of the elephant: all are right, the elephant consists of a combination of all the features
they describe).
Detection of Global Motion in Area MT
Information from magnocellular neurons (in the LGN they detect large, rapidly moving objects)
feed into V1 and is then passed on to the middle temporal area (MT+/V5)
of the cortex. This is an area of the brain thought to be important in the
perception of motion and may have at least two separate maps located
on the lateral surface at the temporal-occipital boundary. The majority of
neurons in V5 are selective for motion in one particular direction, but they
show little sensitivity for form or color.
To detect a correlated direction, a neuron must integrate
information from many local-motion detectors.
Motion Aftereffects Revisited
When we look at a stationary object, the responses of neurons tuned to different directions of
motion are normally balanced (neurons sensitive to upward motion fire at the same rate as
neurons sensitive to downward motion, so they cancel out). When we look at a waterfall, the
neurons sensitive to upward motion fire faster than the ‘exhausted’ downward-sensitive
neurons, and we therefore perceive the rocks as drifting up.
Interocular transfer is the transfer of an effect (such as adaptation) from one eye to the other.
The fact that strong MAE is obtained when one eye is adapted and the other tested means that
the effect must be reflecting the activities of neurons in a part of the visual system where
information collected from the two eyes is combined.
• First-order motion is the motion of an object that is defined by changes in luminance.
Luminance-defined objects are objects that are delineated by differences in reflected
light.
• Second order motion is the motion of an object that is defined by changes in contrast
or texture. Texture-defined objects/contrast-defined objects are objects that are
defined by differences in contrast or texture.
Second-Order Motion
As in first-order apparent-motion displays, nothing actually moves in second-order motion. The
only thing that changes is that strips of dots are inverted from one frame to another. Second-
order motion proves that matching discrete objects across movie frames is not necessary for
motion perception.
2
, Double dissociation is the phenomenon in which one of two functions, such as first- and
second-order motion, can be damaged without harm to the other, and vice versa.
Why did we evolve a motion detection system like second-order motion? It turns out that
second-order-like motion does occur in the real world, especially when an object is effectively
camouflaged.
Using Motion Information
Going with the Flow: Using Motion Information to Navigate
Optic array is the collection of light rays that interact with objects in the world that are in front
of a viewer. Some of the rays strike our retinas, enabling us to see.
Gibson: when we move through our environment, we experience patterns of optic flow
that our visual systems use to determine where we’re going. Optic flow is the changing angular
positions of points in a perspective image that we experience as we move through the world.
‘Radial expansion’ is the pattern of the optic field produced by movement forward in space.
The focus of expansion is the point in the center of the horizon from which, when we’re
in motion, all points in the perspective image seem to emanate. It’s the one point that will be
stationary. The focus of expansion is one aspect of optic flow.
• Outflow is flow towards the periphery, which indicates that you are approaching a
particular destination;
• Inflow indicates retreat, assuming that your head is facing forward;
• The focus of expansion/focus of constriction if you’re looking forward while driving in
reverse, tells you where you’re coming from or going to.
As soon as gaze shifts to one side (the head/eyes move), a new ‘radial’ component is introduced
to the optic flow. If the radial shift is relatively slow, observers can compensate for simulated
eye movements just as readily as they do for real eye movements. With faster stimulated eye
movement speed, performance breaks down. This implies that the visual system can make use
of the copies of eye muscle signals when it is processing optic flow information.
Something in the Way You Move: Using Motion Information to Identify Objects
Biological motion is the pattern of movement of living beings (humans and animals). This helps
us identify both the moving objects and its actions. We are much more efficient in
discriminating biological motion of a human when two humans are acting in synchrony than
when they are out of sync. This finding suggests that when we watch two people interacting,
knowing what one is doing helps us understand the actions of the other.
Avoiding Imminent Collision: The Tao of Tau
How do we estimate the time to collision (TTC) of an approaching object? The TTC is the time
required for a moving object to hit a stationary object. It’s a distance/rate correlation. The most
direct way to estimate the TTC would be to estimate the distance and speed of the approaching
object.
There is an alternative source of information in the optic flow that could signal TTC
without the need for absolute distances or rates to be estimated: tau (τ). The ratio of the retinal
image size at any moment to the rate at which the image is expanding is tau. TTC is proportional
to tau. The advantage of using tau is that it relies solely on information available directly from
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