Lec 7 Motion Perception

What is motion

a change in position over time

What is the basic idea behind a neural motion detector

compare signals from adjacent receptors over time using a delay

What is the Reichardt motion detector

a model of motion detection using delayed and direct signals compared at a coincidence detector

What is the key mechanism in the Reichardt detector

delay-and-compare computation

When does the Reichardt detector fire

when signals from two detectors arrive simultaneously at the comparator

What directions is a Reichardt detector tuned to

specific directions such as rightward or leftward motion

What happens if motion is too fast or too slow

signals arrive at different times so no response

What is a motion opponency system

two detectors tuned to opposite directions whose outputs are subtracted

Why use opponent motion detectors

to distinguish leftward vs rightward motion more clearly

What is apparent motion

illusion of smooth motion from rapidly presented static images

Who first demonstrated apparent motion

Sigmund Exner (1875)

What is SOA

stimulus onset asynchrony (time between stimulus onsets)

What is ISI

interstimulus interval (blank time between stimuli)

What is displacement

shift in position between frame 1 and frame 2

When is there no motion perception

when ISI or SOA is too short or too long

When is motion perception strongest

when ISI and SOA are balanced

What is perceptual phase space

different motion percepts depending on ISI and displacement

What is the correspondence problem

figuring out which object in frame 2 matches which object in frame 1

What helps solve the correspondence problem

Gestalt cues like proximity color and shape

Why can motion be ambiguous in dot displays

multiple valid matches exist between frames

What bias does proximity create

tendency to perceive closer elements as corresponding

Can color or shape override proximity

yes but they are weaker and attention-dependent

What is the Ternus effect

perception of motion changes depending on timing and grouping

What happens with no blank interval

one element appears to jump over another

What happens with a blank interval

elements are perceived as moving together as a group

What is motion entrainment

surrounding motion cues influence perception of central motion

What is first-order motion

motion defined by luminance changes

What is second-order motion

motion defined by contrast or texture not luminance

Is anything physically moving in second-order motion

no motion is computed by the visual system

What is a texture-defined object

object defined by contrast or texture differences

Which system processes first-order motion

a different neural system than second-order motion

Which motion allows larger displacement

first-order motion

Which motion can have longer ISI

first-order motion up to about 500 ms

Which motion produces aftereffects

first-order motion

What is an aperture

a limited viewing window of a moving object

What is the aperture problem

motion direction is ambiguous when viewing only part of a moving object

Why does ambiguity occur

only local motion is visible not full object trajectory

What brain area is important for motion perception

middle temporal area (MT)

What is special about MT neurons

most are selective for motion direction

What pathway supports fast motion perception

magnocellular pathway (LGN layers)

What is the main challenge of motion perception

combining local motion signals into a global motion percept

What did Newsome and Pare (1988) study

monkey motion perception using correlated dot displays

How few correlated dots can monkeys detect motion with

as little as 2–3% correlated dots

What happens when area MT is lesioned

monkeys need about 10× more signal dots to detect motion

What does MT lesion evidence suggest

MT is critical for global motion perception

What happens when MT neurons are electrically stimulated

monkeys perceive motion in the neuron’s preferred direction

What does MT stimulation demonstrate

MT activity directly influences motion perception

What is the conclusion from MT stimulation studies

MT is a causal global motion detector

What is a limitation of lesion studies

they are invasive and may affect multiple brain areas

What is a modern alternative to lesions

transcranial magnetic stimulation (TMS)

What is the motion aftereffect

illusion of motion in a stationary object after viewing motion

What is the waterfall illusion

stationary rocks appear to move upward after viewing falling water

What does MAE suggest

the visual system uses opponent processes for motion

What is interocular transfer

transfer of adaptation effects from one eye to the other

What does interocular transfer imply

motion adaptation occurs in cortical areas not retina

Which areas combine input from both eyes

V1 and higher areas like MT

Why are eye movements important for motion perception

they create retinal motion even when objects are stationary

What happens to the retina when you move your eyes

the image shifts even if the world is stable

Why don’t we perceive the world as constantly moving

the brain compensates for eye movements

What is a saccade

fast eye movement shifting gaze between targets

What is smooth pursuit

eye movement that tracks a moving object

What is vergence

inward or outward eye movement for depth alignment

What are microsaccades

tiny involuntary eye movements during fixation

What brain structure initiates eye movements

superior colliculus

What does the frontal eye field (FEF) do

plans and targets eye movements

What does the lateral intraparietal area (LIP) do

helps plan and guide eye movements

What is optokinetic nystagmus (OKN)

eye follows moving scene then resets

What is the vestibulo-ocular reflex (VOR)

stabilizes gaze during head movement

What is angular VOR

eyes move opposite head rotation

How fast are saccades

up to about 1000°/second

How many saccades do we make per day

about 172

Why do saccades occur

to shift gaze toward areas of interest

Why don’t we perceive blur during eye movements

saccadic suppression reduces visual sensitivity

What is saccadic suppression

reduced vision during saccades to prevent motion blur