Motion Flashcards
Motion is a _______ event
Spatiotemporal (a change in position over time)
Motion helps distinguish …
objects from their background : figure ground segregation
True or false : we perceive the world as successive still frames
False, everything seems smooth
Akinetopsia
A rare neuropsychological disorder in which the affected individual has no perception of motion.
- causes streams of multiple, frozen images trailing in the wake of moving objects (due to sensory memory not suppressing them)
Cause of akinetopsia
Lesion of areas MT/MST (middle temporal gyrus)
Apparent motion
The illusory impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession.
Perceived movement is always ______ motion
Apparent (our eye gets snapshot in time of objects, and if the objects change position, we will perceive it as movement )
Relation between movement perception and distance between objects
High distance, low speed = smooth movement
Low distance, high speed = smooth movement
What happens if there is no delay neuron in the motion detection circuit (“Reichardt detectors” )
A neuron between A and B registers a change in position
But …
-> would also respond to 2 still cars
-> would also respond to a car moving backwards, not sensitive to direction of movement
Perceiving motion with delay neurons
Rightward motion tuned neuron :
1. neuron A (left) gets activated, sends the information to D delay neuron
2. Delay neuron on neuron A waits for a couple of milliseconds before sending the information to middle neuron M at the same time B (right neuron) sends the information to M
- If the car moves from right to left , no activity in neuron M because information from B will arrive before information from neurons at A&D
Same concept for leftward movement, but with delay neuron on neuron B (right)
Movement after-effect
Neurons in V1 get habituated, then movement is perceived in other direction
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 an object may be ambiguous.
Correspondence problem
The problem faced by the motion detection system of knowing which feature in frame 2 corresponds to which feature in frame 1.
Solution to the aperture problem
- There are several directions of motion within each aperture (receptive field) that are compatible with the stimulation the receptor is receiving.
- The local motion detectors (V1) are connected to the apertures of a receptive field
- Whichever possible motion direction is the same in all apertures is the true global motion direction of the object that the local motion detectors (V1) send to the global motion detector neuron
Why do our eyes constantly move while looking at a visual scene ?
We do not see well outside central vision
Smooth pursuit
Voluntary eye movement in which the eyes move smoothly to follow a moving object.
Saccade
A type of eye movement, made both voluntarily and involuntarily, in which the eyes rapidly change fixation from one object or location to another (3-4 seconds every second).
Vergence
A type of eye movement, both voluntary and involuntary, in which the two eyes move in opposite directions.
Convergent eye movements turn the eyes ______
Inward
Divergent eye movements turn the eyes ______
Outward
Reflexive eye movements
automatic and involuntary eye movements. —for example, when the eyes move to compensate for head and body movement while maintaining fixation on a particular target.
Microsaccade
an involuntary small jerklike eye movement.
- prevent visual fading
- allows us to see behind the blood vessels in our eye
- improve visibility of sharp details
- compensate for the sudden loss of acuity outside of the fovea
Saccadic suppression
Temporary reduction of visual sensitivity that occurs during rapid eye movements (saccades) to prevent motion blur and maintain visual stability
Consequence of injection of muscle paralysis on eyes
Visual fading : difficult remembering colors : visual system forgets the color, so you interpret the colors as uniform gray
Feeling movement in opposite direction : before initiating eye movement, a copy of the movement is sent to the brain, and as you do it, your visual system is going to correct the image for you
Movement illusions stop
