Motion%20Perception

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Motion Perception
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• Motion perception has many functions -
• it plays a role in segregating figure from
  ground
• in encoding depth
• in helping us avoid collisions with objects and
  to control our locomotion and posture

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Motion Perception is Important
She had difficulty, for example, in pouring tea
or coffee into a cup because the fluid appeared
to be frozen, like a glacier. In addition, she
could not stop pouring at the right time since
she was unable to perceive the movement in the
cup (or a pot) when the fluid rose. . . . In a
room where more than two other people were
walking she felt very insecure and unwell, and
usually left the room immediately, because
"people were suddenly here or there but I have
not seen them moving." . . . She could not cross
the street because of her inability to judge the
speed of a car, but she could identify the car
itself without difficulty. "When I'm looking at
the car first, it seems far away. But then, when
I want to cross the road, suddenly the car is
very near. (Zihl, von Cramon, and Mai, 1983)
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When do we perceive movement 2 general
cases 1) - something moves in the world- image
moves across our retina (However, movement on the
retina cant be the whole story - when we move
our eyes and the world is stationary there is
movement on the retina, but we dont perceive
movement) 2) - something move and we track it -
doesnt move across our retina
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To perceive motion, the nervous system begins by
extracting from the retinal image the spatial
displacements of features over time.
This defines motion as a spatio-temporal event.
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Basic Motion Sensitive Mechanism (Direction and
Speed Selective)
Reichardt detector
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Motion or Movement Aftereffect
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Addams - 1834 - Edinburgh Philosophical Magazine
and Journal of Science, 5, 373-374 "Having
steadfastly looked for a few seconds at a
particular part of the cascade, admiring the
confluence and decussation of the currents
forming the liquid drapery of waters, and then
suddenly directed my eyes to the left, to observe
the face of the sombre age-worn rocks immediately
contiguous to the water-fall, I saw the rocky
surface as if in motion upwards, and with an
apparent velocity equal to that of the descending
water, which the moment before had prepared my
eyes to behold that singular deception."
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Linear Motion Aftereffect
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Phi Phenomenon - Apparent Movement
A spot against a background is exposed briefly
and then is followed a short time later by
exposure of a second spot a short distance
away We see the spot as moving from one spot to
another - apparent motion
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2.5.1
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Random Dot Kinematograms
Random dot kinematograms are a motion analogue to
the random-dot stereograms - sterograms appear to
be a random collection of dots unless viewed
properly with both eyes. Similarly, random dot
kinematograms appear to be a random array of dots
unless they are viewed properly - that is, moving.
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Random Dot Kinematogram
2.7.2
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Neural Mechanisms of Motion Perception
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The Aperture Problem
A
B
Moving a square behind a small, circular aperture
creates an an ambiguous perception of
movement Although the squares are moving in
different directions in A and B, through the
aperture the square appears to move horizontally
to the right
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The aperture problem occurs because each basic
direction selective unit sees what is
happening within its own receptive field.
The aperture problem tells us that perceived
motion is not determined solely by individual
responses to the stimulus velocity generated
within isolated, local regions of the field.
Instead, similar local measurements of velocity
are integrated, producing an overall response.
This integration points to some place in the
visual system at which the local velocity-related
signals from Area V1 are brought together.

One area that may be involved in solving the
aperture problem is the mediotemporal area (Area
MT) or V5.
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Area MT may help in solving the aperture problem
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Receptive fields in area MT are considerably
larger in size than those in Area V1, implying
that an array of spatially distributed V1 neurons
contribute to the synthesis of individual MT
receptive fields. Also, in Area V1 only a
minority of neurons exhibit direction-selectivity,
but in Area MT nearly every neuron is
directionally-selective. Some neurons in MT
receive input from V1 neurons with different
preferred directions of motion. The result? The
directional-selectivity of these MT neurons
differs qualitatively from that of their
predecessors in V1. And these differences enhance
the importance of MTs contribution to the
perception of motion.
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When two gratings that are moving in different
directions are superimposed, the resulting plaid
pattern appears to be moving as a whole and in a
direction different from the movement of each
grating when seen alone
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A
B
Response of component (V1) cells and pattern (MT)
cells to a plaid. Both cells respond to movement
to the right In A. the component cell fires to
horizontal movement of one of the gratings and
the pattern cell does not fire In B. the
component cell does not fire, but the pattern
cell fires to rightward movement of the pattern.
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Converging lines of evidence implicate area MT in
motion perception. For example, damage to MT
impairs motion perception. The motion-blind
patient D.L. whose symptoms were described
earlier in the lecture has brain damage centered
in the area where MT is located. We also know
that motion perception in normal individuals can
be transiently impaired by a brief, strong pulse
of magnetic energy applied to the region of the
scalp overlying MT.
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The role of Area MT in motion perception has been
fruitfully studied using one particular class of
animation sequences computer-generated displays
consisting of moving dots. Such displays make it
possible to control precisely the strength of
motion signals present in animation sequences.
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100 correlation
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0 correlation
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20 correlation
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10 correlation
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MTs key role in motion perception was most
dramatically shown in a study done by Salzman,
Murasugi, Britten and Newsome (1992). Used tiny,
localized electrical currents to induce a
temporary change in the responses of selected MT
units. These currents boosted the response of
particular directionally-selective neurons and
thereby altered perception of visual motion. The
study began by establishing the direction
preferences of various MT neurons. This purely
physiological phase of the study was followed by
a phase that was purely behavioral while they
viewed random dot displays, monkeys moved their
eyes in one direction or another to signal the
perceived direction of those displays. Finally,
Salzman and his colleagues combined behavior and
physiology, eliciting perceptual judgments (i)
while electrical stimulation was being applied to
particular MT neurons, or (ii) without
accompanying electrical stimulation. Electrical
stimulation of neurons that share a direction
preference biased the monkeys perceptual
judgments of random dot displays.
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Motion Sensitive Mechanisms Beyond MT
Normal motion perception depends upon activity
distributed over many areas of the brain, each
extracting somewhat different information from
the retinal image. To emphasize this point,
consider the computations carried out in another
region of the cortex, Area MST (Medial Superior
Temporal)
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Cells in MST respond to Optic Flow
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Motion Perception and Eye Movements
When our eyes track a moving object - there is no
movement of the image on the retina but we see
movement. Also, what about the case in which the
world is stationary but we move our eyes - there
is image movement on the retina but the world
appears to be stationary.
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Over the years, people have proposed two
different mechanisms by which eye movements could
be monitored. According to one view, the visual
system tracks actual changes in the extraocular
muscles (Inflow theory) according to an
alternative view, the visual system tracks the
command signals that go to the extraocular
muscles (Outflow Theory).
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M motor signals from brain S sensory
movement of the image on the retina C
corollary discharge (efferent command signal)
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2 Predictions 1) If eye moves but no C, then S
will not be cancelled and the scene should appear
to move. 2) If there is C but no S to cancel it
scene should to move - no sensory input from
retina, but movement signal sent from brain.
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