visual motion perception Flashcards
Motion
A change in a position of an image over time
One of the most basic aspects of vision
Evolutionary important
First-order motion
the motion of an object that is
defined by changes in luminance
luminance defined objects
luminance-defined objects
an object that is delineated by difference in reflected light
Second-order motion
the motion of an object that is
defined by changes in contrast or texture, but not by
luminance
texture defined objects
Texture-defined (contrast-defined) object
an object that is defined by changes in contrast or texture, but not by luminance
Significance of second order motion
Proves that matching discrete objects in time is not
necessary for motion perception
Just as random dot stereograms prove that matching discrete objects across the two eyes is not necessary for stereoscopic depth perception
Nothing is actually moving
Second-order motion in the real world: breaking of camouflage
Double dissociation
When one of the two functions like 1st and 2nd motion can be damaged without harm to the other
Perceptual Organization By Motion
Camouflage (second-order motion, common fate)
Point-light (biological motion)
Kinetic depth effect (structure from motion)
Perceptual system takes characteristics of the environment, as well as knowledge, expectations and memories, into account to make sense of stimuli
Optic array
the collection of light rays that interact with objects in the world in front of a viewer
rays that strike the retina allow us to see
Optic flow
the changing angular position of points in a perspective image that we experience as we
move through the world
visual system uses to determine where we are going.
mere presence dictates locomotion, and lack of flow is a signal you stationary
a clean optic flow only occurs if the head is stationary.
inflow and outflow
focus of expansion
the point where the center of the horizon is when we are in motion (ie/driving on the highway)
all points in the perspective seem to spread out
an aspect of optic flow
outflow
flow toward periphery indicates approaching.
inflow
Indicates retreat
Motion-induced blindness
If you carefully fixate a central target, stationary targets in your periphery will disappear
Time to collision (TTC)
the time required for a moving
object to hit a stationary object
TTC = distance/rate
Tau
the ratio of the retinal image size at Any moment to the rate at which the image is expanding
As ball approaches nose the image of the ball on retina grows larger.
Computation of Visual Motion
What are the components of a motion detector?
Two adjacent receptors
Register the change in position
Incorporate a delay
Accounts for change in time
high acuity falls rapidly w/ eccentricity (distance between retinal image and fovea).— we must constantly more our eyes to fixate on the object.
Saccade
Given that motion within any single aperture (or receptive field) is ambiguous, how might the visual system correctly perceive the overall motion of objects?
Motion information from several local apertures (or receptive fields) can be combined to determine the
global motion of the object)
There are several directions of motion within each aperture that are compatible with the stimulation the receptor is receiving
Whichever possible motion direction is the same in all apertures is the true global motion direction of the object
Apparent motion
the illusory impression of smooth
motion resulting from the rapid alternation of objects that appear in different locations in rapid succession
Highly dependent on:
1. The physical separation of the two stimuli
2. The alternation rate of the two images
a series of images played one another at a fast rate
Correspondence problem (motion)
the problem faced by the motion detection system of knowing which feature in frame 2 corresponds to which feature in frame 1
Problem because the can be multiple interpretations of how something moved
How do we know which circles in frame I correspond to frame 2–to determine how they move
How do we determine which features or elements in two different frames are the same object in motion.
Aperture
an opening that allows only a partial view of an object
Small window
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 of part of an object may be ambiguous
VI cells need to combine their responses
Closely related to motion correspondence problem
When only a limited part of a moving object is visiable it is impossible to determine the true direction of its movement
Motion aftereffect (MAE)
the illusion of motion of a stationary object that occurs after prolonged exposure to a moving object
Existence of MAE implies an opponent process system, like that of color vision
After viewing motion in a constant direction for a sustained period of time (at least 15 sec) we see any stationary objects that we view as moving in the other direction.
Interocular transfer
the transfer of an effect (such as
adaptation) from one eye to another
MAE exhibits interocular transfer
Therefore, MAE, must occur in neurons that respond to both eyes
Input from both eyes is first combined in area V1, so MAE must be in V1 or later
MAE likely occurs in MT
MAE happens in middle temporal area (MT or V5)
Perception of Motion
How do we perceive motion?
Movement of an image across the retina? NO
Perceive movement despite no actual movement
on the retina:
Autokinetic effect
Induced motion
Apparent motion
Movement aftereffect
Tracking (smooth pursuit)
Perceive NO movement despite actual movement on the retina:
Scanning (position and direction constancy)
