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Chapter 7: Perceiving Motion (eye movements and the perception of motion…
Chapter 7: Perceiving Motion
perceptual organization from motion
perceptual organization is the process whereby the visual system identifies those portions of the retinal image that belong to one object or another
motion perception involved in various aspects of perceptual organization, including perceptual grouping, figure-ground organization, and the perception of moving biological objects
perceptual grouping based on real and apparent motion
by principle of perceptual grouping, principle of common motion leads us to group together multiple elements in scene if they move together in same direction
perceptual system must have way of determining that an object now seen in one location is the same as the object seen before in a different location
also has a strong tendency to group successive perceptions of moving objects in just the way
apparent motion:
a visual illusion in which two stimuli separated in time and location are perceived as a single stimulus moving between the two locations
perception of single moving object when object flashed briefly in two different positions is so powerful that if different shapes are flashed, observer will perceive a single moving object morphing from one shape into another as it moves
apparent motion quartet:
a display in which four symmetrically placed stimuli presented at alternating moments in time are perceived as two stimuli in apparent motion
two dots shown in upper left and lower right corner in frame 1 and then lower left and upper right in frame 2
perception tends to flip between vertical and horizontal motion every 10-20 seconds
if display contains several apparent motion quartets, all of them will be perceived as moving in the same direction
shows that different quartets in the display are grouped together, with the principle of common motion leading the visual system to perceive an identical direction of apparent motion in all quartets
powerful factor in determining the perceived direction of motion in the apparent motion quartet is the relative distances between the dots
when it can, the visual system interprets apparent motion in a way that minimizes the distance over which the stimuli appear to move
figure-ground organization
abrupt discontinuity in brightness or color is powerful cue to boundary between two surfaces, often signaling the edge of an object against its background
just as powerful is a discontinuity in the speed of visual elements such as textures as an object moves across a stationary background
evidence that motion discontinuities alone (without changes in brightness, color, depth, texture) enable the visual system to distinguish a shape from its background
random dot kinematogram:
a display in which a grid is filled with tiny, randomly placed black and white square dots and in which the dots in a region of the grid are then moved rigidly together as a group; the shape of the region is visible when the dots move but not when they’re still
shape is defined only by relative motions at its edges—when it’s not moving, the shape doesn’t exist
sensitivity to biological motion
motion can be extremely powerful cue to the 3-D form of an organism
point-light walker:
a display in which biological motion is made visible by attaching small lights at critical locations on an organism’s body (e.g., at a person’s arm, leg, and hip joints) and then shooting a video of the organism in motion in darkness
no explicit information about the shape of the object; any one frame of video looks like randomly scattered dots
as soon as person moves, shape becomes apparent—visual system is so attuned to the perception of biological motion that it almost immediately understands the complex pattern of moving dots as the motion of a person engaging in some specific activity
ability to make sense out of a point-light display gets better as number of points increases, and points on joints are more helpful than points elsewhere
recognizability of point-light displays depends most crucially on the coordinated timing of the point motions
body of moving person is not rigid overall, but it is ‘piecewise rigid’, as the person walks, the distance between the elbow and the wrist remains constant, as does the distance between the knee and ankle, while distance between wrist and ankle changes dramatically
even piecewise rigid distance not perfect since distance appears to change depending on tilt
in conjunction with knowledge about the structure of the human body, the visual system is able to pick up on these regularities to correctly perceive the invisible person in a point-light walker display
perception of biological motion associated with specific region of brain—posterior superior temporal sulcus (STSp) in the temporal lobe
transcranial magnetic stimulation where use magnetic pulse to render STSp inactive leads to difficulty in perceiving biological motion
also seem especially sensitive to biological motion in other respects
motion of animate being is animate motion while motion of inanimate object is called inanimate motion
motion and speed of animate object is purposive, can decide where and how fast it wants to go while motion of inanimate objects is responsive to obstacles/environment
did experiment where showed four dots moving across screen, three showing inanimate motion and one showing animate motion where participants had to respond to disappearance of one dot by pressing button
faster to detect the disappearance of spot that had exhibited animate motion
as magnitude of animate direction change increased, time needed to detect the disappearance decreased
suggest two conclusions
animate motion captured participants’ attention, presumably because quick reaction to the presence of animals is important for survival
large chances in direction are more likely to be associated with an animal than an inanimate object
eye movements and the perception of motion and stability
frequent movement of eyes is necessary because field of view is limited
can move eyes by moving body or by moving eyes themselves using extraocular muscles; often tighter
two different types of eye movements
saccadic eye movements (or saccades):
brief, rapid eye movements that change the focus of gaze from one location to another
occur about three times per second, each lasts less than 1/20th of a second
smooth pursuit eye movements:
eye movements made to track a moving object or to track a stationary object while the head is moving
eye movements don’t produce blurry vision because visual system essentially shuts down
saccadic suppression:
the visual system’s suppression of neural signals from the retina during saccadic eye movements
stationary scenes don’t appear to move when we move our eyes
when eye remains stationary while viewing moving object, image of moving object moves on retina and image of nearby stationary object is stationary on retina
when eye moves to follow movement of moving object, image of moving object is stationary on retina while image of nearby stationary object moves on retina
still see moving object as moving and stationary object as stationary
means perception of motion and stability can’t just depend on motion or lack of motion in retinal image, also has to take into account extraretinal information (information that doesn’t originate in the retinal image)
must also be based on information about the position and movement of eyes
study showing responses of single neurons in area V3A in visual cortex, known to respond to motion
in first condition, fixated on stationary point while bar of light moved across receptive field of V3A neuron; produces strong response
in second condition, fixation point itself moved and eye tracked it, causing receptive field to move across bar of light, which was stationary
stimulated receptive field in same way, but neurons response was much weaker, presumably because movement of bar of light across receptive field resulted entirely from the eye movement
real-motion cells:
neurons that signal actual movement of an object in the environment, based on combining retinal information with information about eye movements
two types of information about position and movement of eyes that might be available to real-motion cells
information from the extraocular muscles
sensors in extraocular muscles provide information about how stretched or relaxed the muscles are, position and movement of eyes
brain could combine this information with information from the retinal image to construct an accurate perception of movement and stability in visual scene
information from the superior colliculus, the part of the brain that controls eye movements by sending motor commands to the extraocular muscles
superior colliculus sends eye-movement commands to the extraocular muscles and a copy of the commands to some region of the brain that combines the information in the CDS with the information in the signals from from the retina to indicate motion
corollary discharge signal (CDS):
a copy of an eye-movement command from the superior colliculus to the extraocular muscles, sent to the brain to inform the visual system about upcoming eye movements; used to ensure a stable visual experience even during eye movements
information from CDS would be more useful than information from extraocular muscles because CDS would be sent before the eyes actually move
evidence points to mechanism using CDS
some visual neurons change their receptive field locations just before a saccadic eye movement; find that neuron will respond to flash of light in position that it is about to focus on rather than position is is currently focussed on when flash shown just before movement
anticipatory mapping of neuron’s receptive field could minimize any disruption in visual perception resulting from the eye movement
fact that remapping begins before eye starts to move strongly implies that part of brain that controls for remapping of neuron’s receptive field has received information about intended eye movement
suggests that we maintain accurate perception of position and motion of objects in visual scene despite frequent eye movements via mechanism by which the information in a CDS is combined with the information in signals from the retina
believed that similar mechanism operates to compensate for head movements
neural basis of motion perception in area V1 and area MT
a simple neural circuit that responds to motion
in order to detect/represent motion of some feature in retinal image, a neural circuit must monitor at least two different retinal locations and must register the order in which these locations were stimulated and how far apart in time they were stimulated
for feature moving in straight line at constant speed, direction is specific by line connecting two retinal locations and order in which the locations are stimulated and speed is specified by distance between those locations and the time between the stimulation of one location and the next
in circuit in which signals from neurons 1 and 2 have equal transmission times to Neuron M, wouldn’t be tuned to to represent a particular direction and speed
neuron M wouldn’t respond strongly since signal from neuron 1 and signal from neuron 2 wouldn’t arrive at same time
neuron M will respond selectively to the direction and speed of motion if the circuit includes a delay in the signals from either neuron 1 or neuron 2
Neuron M would respond strongly if tuned to motion left-to-right at specific speed that causes signal from neuron 1 and signal from neuron 2 to arrive at neuron M at same time
means neurons are tuned to specific speeds and directions
delay in transmission of neural signals could be based on various mechanisms
could have longer axon or might be unmyelinated
direction tuning is determined by whether the delay is built into the transmission of signals from neuron 1 or neuron 2 and speed tuning is determined by length of delay
can account for phenomenon of apparent motion
direction and speed tuning are not all-or-none, but have gradual falloff from the preferred direction or speed to nearby directions or speeds because will partially stimulate receptive fields and produce moderate increase in firing rate of M
area V1 is first part of visual pathway where neurons like Neuron M have been found
many neurons in V1 are tuned for both direction and speed of motion in addition to being tuned for orientation (like simple cells) or disparity (like bipolar cells)
the motion aftereffect
motion aftereffect (MAE):
a visual illusion in which a stationary element of the visual scene appears to be moving in a direction opposite to the direction of motion experienced during the immediately preceding time interval
neural circuit accounting for MAE contains two subunits, each tuned to represent motion in opposite directions
subunit with motion-selective neuron ML, leftward motion in which RF2 is stimulated first
subunit with motion-selective neuron MR, rightward motion in which RF1 is stimulated first
neuron ML has excitatory effect on D while MR has inhibitory on D and D’s response represents the difference in responses of ML and MR
before motion, MR, ML, and D all firing at baseline rate
during leftward motion, ML strongly activated and MR at baseline, so D fires above baseline
if leftward motion continues for long enough, ML becomes fatigued and when motion stops, ML produces very few spontaneous action potentials (below baseline) while MR continues firing at baseline
baseline inhibitory MR is not stronger than below-baseline excitatory ML, meaning D gets slightly inhibitory signal and signals rightward motion
suggests that motion is represented in brain by output of an opponent circuit that compared opposite directions of motion; opponent motion circuits represent motion contrast, or differences in direction and speed of motion
motion represented by difference in responses of two oppositely tuned motion-selective neurons because responses of ML and MR depend on many things, including direction and speed, object’s orientation, contrast against background, degree of binocular disparity, and other factors
if ML and MR are tuned to opposite directions but similar speeds, orientations, contrasts, disparities, a low-contrast stimulus moving in the preferred direction would produce different response than a high-contrast stimulus moving in a nonpreferred direction
area MT
MT neurons respond selectively to motion
almost all neurons in area MT are tuned for direction and receptive fields of MT neurons are 5-10 times larger than those in V1; neurons in MT well-suited for representing large-scale motions
neurons in other areas of higher levels of visual pathway are tuned to other things, such as curvature, shape, color, but not motion
MT responds the same to objects that differ in other features
unlike V1, MT much more active when looking at moving stimulus than at stationary one
patterns of activity in MT differs according to attended direction of motion
activity of MT neurons causes directionally selective motion perception
possible that activity in MT when viewing motion is simply ‘side-effect’ of motion perception; activity of neurons elsewhere in brain could produce motion perception while also causing MT neurons to increase activity
monkeys trained to press bar to indicate the direction of motion of dots presented on a computer screen
monkeys responded as if they had seen motion in specific direction when specific neurons in MT stimulated with microstimulation
clear, quantitative relationship between the neural activity and monkey’s judgments about direction of motion; supports idea that activity of MT neurons plays causal role in directionally selective motion perception
shown array of dots hon computer screen, each array covering receptive field of neuron being recorded; some dots moving in specific direction while rest moving randomly
proportion moving in same direction called coherence of motion
at 12.8% coherence, when monkey’s responses more than 90% correct, clear differences in number of spikes in response to tuned direction and opposite direction; neuron’s response about 80 sps when in preferred direction and about 30 when not
at 8% coherence, when money’s responses just more than 50% correct, firing rate of specific neuron about the same whether moving in tuned direction or opposite
in terms of decision criterion, noise, and signal detection theory
set criterion of 50 spikes per trial; if neuron produced more, monkey would respond that motion was in neuron’s preferred direction and if fewer, would respond that motion was in opposite of preferred direction
at 12.8% coherence, this rule almost always gives correct answer- a hit or correct rejection
the few trials in which this rule yields incorrect response is due to noise- miss or false alarm
when coherence at about .8%, neurons’ response about same for motion in either direction, average 40sps; any rule based on number of spikes will lead to incorrect answer in about 50% trials
when coherence is close to 0%, monkey’s rate of correct answers is around 50%
as coherence rises, so does accuracy, with monkey making no errors once coherence level around 30%
motion coherence threshold defined as level of coherence at which performance was 82% correct
neuron’s threshold somewhat lower (better) than monkey’s; neuron required slightly lower level of coherence than did the monkey to correctly judge the direction of motion 82% of the time
overall found that performance of individual neurons wasn’t significantly different from that of the monkey across all levels of coherence; suggests that monkey’s perceptual judgements about direction of motion are about what you’d expect if they were based on the responses of this population of MT neurons
disruption of area MT impairs motion perception
when area MT is damaged or neural activity is disrupted in some way, lose virtually all ability to perceive motion visually
deliberate temporary deactivation of MT (by transcranial magnetic stimulation) if done at just right moment relative to presentation of moving-dot stimulus dramatically imparts perception of motion
many neurons in area V1 are tuned to direction and speed of motion, but many other neurons in V1 aren’t; V1 neurons also have very small receptive fields
V1 neurons can’t represent motion of large objects moving over extended distances
area MT (middle temporal area) aka V5 receives signals from V1 and V2 as well as from superior colliculus (which controls eye movements)
the aperture problem: perceiving the motion of objects
challenge in direction-tined neurons in V1
since V1 neurons monitor just small portion of retinal image, multiple movements can look the same in receptive fields of V1 neurons
aperture problem:
the impossibility of determining the actual direction of motion of a stimulus by the response of a single neuron that “sees” the stimulus only through a small “aperture” (the neuron’s receptive field) and “sees” only the component of motion in the neuron’s preferred direction
any given direction-tuned V1 neuron will often fail to produce an accurate representation of the direction of motion of an object with a retinal image larger than the neuron’s receptive field
needs to combine information contained in the signals from multiple V1 neurons in order to assess motion over a larger area of the retina than that seen by individual V1 neurons
process carried out by neurons in MT, which have much larger receptive fields, meaning they receive signals from multiple V1 neurons
many MT neurons respond to only one direction; those called component cells
about 1/3 neurons in MT responded best to combined motion; those called pattern cells
if retina of observer is absolutely stationary, then motion of visual feature such as spot of light produces an exactly corresponding change over time in corresponding part of retinal image
if wanted to adjust behavior according to motion, would need to know its position, its direction of motion, and its speed
visual system uses retinotopic mapping to represent the positions of features in the retinal image