5. Spatial vision and motion perception Flashcards
(25 cards)
How does a Reichardt detector compute motion direction and speed?
Two spatially separated photoreceptor inputs (A and B)
Signal from A is delayed and then multiplied (“AND”ed) with the instantaneous signal from B
Coincidence only when a stimulus moves from A’s receptive field to B’s within the delay window
Why can Reichardt detectors signal apparent motion as if it were real?
They require only sequential stimulation of receptive fields in the correct order
Cannot distinguish between continuous movement and discrete jumps (e.g., frame-to-frame updates)
What does the motion aftereffect reveal about motion processing?
Prolonged adaptation to motion in one direction produces illusory motion in the opposite direction when viewing a static scene
Indicates dedicated motion detectors and population coding of direction
Tags: Adaptation, MAE
Study Reference:
Authors & Year: Addams (1834)
Method: Prolonged viewing of waterfall motion followed by static scene
Key Finding: Illusory opposite-direction motion in stationary image
How does the visual system avoid perceiving the world as moving during eye movements?
Compares retinal motion signals with an efference copy of the eye-movement command (Helmholtz’s model)
Suppresses perception of motion caused by one’s own eye movements
What is the aperture problem in local motion detection?
Within a small receptive field (“aperture”), only motion orthogonal to an edge’s orientation can be detected
True object trajectory remains ambiguous without integration across multiple apertures
How does the Intersection of Constraints (IOC) model resolve the aperture problem?
Each local motion signal defines a line of possible velocities in direction–speed space
True motion is the unique solution at the intersection of multiple constraint lines (e.g., from plaid stimuli)
Tags: Motion Integration, Adelson & Movshon 1982
Study Reference:
Authors & Year: Adelson & Movshon (1982)
Method: Psychophysical and computational modeling with plaid patterns
Key Finding: Global motion derived by combining local constraints
How do V1 and area MT/V5 differ in processing motion?
V1: Small receptive fields; encodes component (local) motion directions; susceptible to aperture problem
V5/MT: Larger fields; nearly all neurons are direction-selective; encode global motion by summing V1 inputs
Tags: Motion Pathways, V1, V5/MT
What do RDKs reveal about global motion perception?
Dots move in random directions except for a proportion (“coherence”) moving uniformly
Observers detect global motion at coherences as low as ~5–10%
Tags: Global Motion, RDK
What evidence shows V5’s role in coherence-based motion detection?
Single-unit (Britten et al., 1992): V5 neurons’ firing rates scale with dot coherence in RDKs
fMRI (Braddick et al., 2001): Greater BOLD response in V5 to coherent vs. noise motion; V1 shows no coherence effect
Tags: V5/MT, Coherence
Study Reference:
Authors & Year: Britten et al. (1992); Braddick et al. (2001)
Methods: Macaque single-unit recordings; human fMRI contrasts
Key Findings: V5 selectively encodes global motion coherence
What is “dorsal vulnerability” in motion perception development and disorders?
Global-motion sensitivity matures later than global-form sensitivity in development
Impaired in conditions like Williams syndrome, autism, dyslexia, amblyopia (Braddick et al., 2003; 2016)
What does patient LM’s case reveal about motion processing?
Bilateral V5 lesions → preserved spatial vision but almost no motion perception
Severe functional deficits (e.g., crossing streets, pouring liquids) at ~90% RDK coherence needed for detection
Tags: Neuropsychology, Akinetopsia
Study Reference:
Authors & Year: Zihl, von Cramon & Mai (1983)
Method: Case study of patient with V5 damage
Key Finding: V5 is critical and modular for motion perception
What four Fourier dimensions underlie spatial vision?
Contrast: amplitude of sinusoidal components
Phase: spatial position of wave cycles
Spatial Frequency: cycles per degree (fine vs. coarse detail)
Orientation: angle of luminance variation
What does prolonged exposure to one orientation do to subsequent perception?
Reduces sensitivity to the adapting orientation (higher threshold)
Causes repulsion of perceived tilt in dissimilar orientations (tilt aftereffect)
What is the CSF and where does it peak?
Plot of contrast sensitivity vs. spatial frequency
Peaks around ~4 cycles/degree; falls off at low and high frequencies
How does adaptation evidence support multiple SF channels?
Adaptation to one SF selectively reduces sensitivity at that SF, sparing others
What is surround suppression in spatial vision?
A central grating’s perceived contrast is reduced by an iso-orientation surround
How does simultaneous tilt contrast demonstrate contextual coding?
A target grating’s perceived orientation is repelled away from flanking gratings of different orientations
Beyond acuity, what fundamental limit affects peripheral vision?
Crowding: interference from flankers within an interference zone that grows with eccentricity
What is Bouma’s Law in crowding?
The interference zone radius ≈ 0.5× eccentricity; objects closer than this impede recognition
How does the CSF develop in infancy?
Both high-SF (acuity) and low-SF sensitivity are initially poor, improving over months (e.g., 1 mo vs. 3 mo vs. adult curves)
How does the brain prevent self-generated retinal motion from appearing as world motion?
Compares retinal motion with an efference copy of the oculomotor command (Helmholtz model)
What happens when you adapt to bidirectional motion?
A unidirectional MAE appears, indicating opponent motion channels
What do motion-energy models propose beyond Reichardt detectors?
Linear spatiotemporal filters tuned to specific motion directions and speeds, followed by half-wave rectification
What phenomenon reduces motion blur during saccades?
Transient suppression of visual sensitivity around the time of a saccade, preserving perceptual stability
