Week 8 Flashcards
What is motion?
The change of position (displacement) over time
- speed=distance/time
In terms of ‘gratings’
- speed = temporal frequency / spatial frequency
- spatial frequency = number of oscillations in a grating pattern per unit of space
- temporal frequency = number of luminance oscillations in a grating pattern per unit of time
How do we sense motion?
Motion is sensed DIRECTLY (we don’t judge position and time separately)
An example of this specialised mechanism is the MOTION AFTEREFFECT (MAE)
How do motion detectors work?
A unit that could signal motion in a part direction?
Delay and compare: The ‘and’ unit fires only when its received a simultaneous input from both neurons, the first receptor’s output experiences a delay.
This is unidirectional
BUT
motion detectors for opposite directions could be combined (allowing to detect motion with the same speed in either direction using the same 2 receptors)
What is stroboscopic (apparent) motion?
Motion on TV is illusory
- there is not motion, just several discrete positions over time
- given the right distance moved and framerate, it looks like motion
- looks so real because apparent motion stimulates motion detectors in the same way real motion (i.e. Reichardt detectors don’t care what happens in between the two receptors)
What is the wagon wheel effect? Why does it happen?
Car wheels appearing to move backwards / stay stationary in movies.
For TV, no motion, only separate frames featuring image displacements.
If the clockwise displacement is further than the anticlockwise displacement then the wheels will appear as if they are moving backwards (because the 4 spokes are identical)
We tend to perceive the motion corresponding to the shorter displacement, which is why fast motion can appear backwards.
What happens if there are 2 correctly timed/spaced double flashes setting off a 2-directional reichardt detector?
Motion is signalled in both directions - but we won’t see motion – problem for the Reichardt model.
Thus we need a comparator unit that compares responses from each direction-selective motion detecting unit (AND cell)
- allows the larger directional signal to win
- if activity is equal in each AND unit, the comparator signals no motion
What are some examples of adaptations?
Adaptation: prolonged exposure to a particular moving stimulus
- increased (elevated) detection thresholds for similar stimuli
–> (as seen with adapting to low spatial frequency gratings makes it more difficult to detect low contrast gratings with the same spatial frequency) - reduction of perceived intensity of stimulus
–> mid-contrast gratings with same spatial frequency seem to be low contrast - biased (often opposite) percept
–> mid spatial frequency gratings now look higher in spatial frequency
What is the motion aftereffect (MAE)
AKA the waterfall illusion
Adaptation involves a biased percept
MAE is a biased percept as a result of adapting to motion stimulus
- look at motion in one direction, stationary things appear to drift in the opposite direction
Explain the Motion Aftereffect using the Ratio Model
Direction-selective units tuned to opposite directions are connected to a comparator. Direction firing most is the perceived direction.
a) stationary stimulus: both up and down units respond at baseline, comparator registers no difference between the two (no motion)
b) downward motion stimulus: up detector at baseline, down detector firing rigorously, comparator registers more downward activity (downward motion)
–> ADAPTION: as motion continues, activity drops for down detector
c) stationary stimulus: up detector continues at baseline, adapted down detector activity is now lower than baseline therefore registering more activity in up than down (upward motion)
Describe the physiology of motion perception: magnocellular pathways
Major forward connections - the magnocellular pathway is selective to motion:
Retina (specialised magnocellular cells receive input from photoreceptors)
LGN:
- 6 major layers, M1-M2 is magnocellular
- magno: low spatial resolution (large receptive fields), tuned to transient stimuli (high temporal frequency)
V1: Primary Visual Cortex
- direction tuning (key to motion selectivity)
- cell fires vigorously to one direction of motion, but remains at spontaneous rate when stimulated in opposite direction
- small response to similar directions (tuning)
- applies to some simple, complex and hypercomplex cells
MT+ (AKA V5)
- many motion selective cells
- large receptive fields
- preferred directions represented in columns
- cells respond to relative motion
What is the aperture problem?
Simple/complex cells in V1 are sensitive to 1D featuers (edges, bars, gratings)
They respond to stimuli within a fixed receptive field (aperture)
We tend to see the direction perpendicular to the 1D feature but we can’t really know the actual directions
- example = the barberpole illusion
How do you solve the aperture problem?
A) Identify 2D features at the edge of the object –> intrinsic terminators
- ambiguous signals in the centre of the pole, unambiguous signals at the edges (2D features / terminators)
Terminators are not all equal:
- edge of an object (intrinsic terminators) TRUE OBJECT DIRECTION
- occlusion of an object by another closer object (extrinsic terminators) NOT REAL
B) integrate information across 1D features at different orientations –> intersection of constraints
- each oriented line has its own line of constraint
- line of constraints intersect at a point representing the velocity of the object
What is the plaid fad?
- combinations of 2 gratings at different orientations
- coherent = checkerboard pattern
- transparent: two transparent layers sliding over eachother
Coherence is determined by simlarity of contrast and spatial frequency
Compare the inflow vs outflow theory:
Inflow theory: eye muscle proprietor sense muscle stretch and send signal to comparator –> eye movement signal from your brain to your eye muscles to tell you how much to move, your eye muscles send information to a comparator to compare the amount of movement to the amount of motion
Outflow theory: eye movement command from brain goes to eye muscle and to comparator (efference copy / corollory discharge) –> when the brain sends a command to the eye muscles, it sends a copy of that straight to the comparator
Who wins? Inflow or Outflow theory?
Tests
1. Move eyes around with an afterimage = motion is perceived
2. externally generated eye movements with an isolated after image = motion is incorrectly perceived in inflow (because eye muscle signal sends to comparator), but outflow correctly perceives no motion
3. externally generated eye movements with non-stabilised eye = no motion is perceived in inflow, outflow perceives motion
4. attempt to move a paralysed eye (wedge) = no motion perceived from inflow theory, but motion is correctly perceived in outflow theory
