wk 5 motion perception

Why is motion perception important?

Motion perception is important for navigation and survival.

What does motion provide information on?

1. Attracting attention

2. Three-dimensional form

3. Figure and ground (camouflage)

4. Interaction with the environment

Attracting attention

Movement in the periphery attracts attention

Three-dimensional form

Movement of an object relative to an observer provides information about form

Figure and ground (camouflage)

Movement segregates the figure from the background

Interaction with the environment (eg. Walking and driving)

Movement perception allows us to avoid colliding with hazards and keeping on course

What the 5 types of motion

1. Real motion

2. Apparent motion

3. Induced motion

4. Autokinetic motion

5. Motion After-effects

Real motion (brief definition)

• physical movement of an object

• Must consider eye and head motion

(Physically moves)

Apparent motion (brief definition)

• motion perceived when objects are presented sequentially and separated in space

• Inter-stimulus interval is important

(Perceived motion when there is no actual movement)

Induced motion (brief definition)

• Perceived motion of a stationary object due to motion of surrounding objects

Autokinetic motion (brief definition)

• Perceived motion of a small stationary object in a dark room

Motion after-effects (brief definition)

• Illusory motion following adaptation to motion

(Waterfall illusion)

Physiology of motion

1. Retinal image motion

2. Visual cortex

3. Extrastriate areas involved in motion

Visual cortex in physiology of motion

• complex cells

• Area V1

• Directional tuning curve that displays the relationship between the orientation of a moving bar and impulses per presentation (cell response peaks at 15-20 degrees)

Extrastriate areas involved in motion (physiology of motion)

• medial temporal area (MT)

• Medial superior temporal area, dorsal part (MSTd)

• Magnocellular system mediates input to area MT - good temporal and contrast sensitivity (dorsal system: “where system”

Corollary discharge theory in real motion

• compares retinal and eye-head movement systems

• Outside visual cortex

• Eye and head movements must be taken into account in determining whether an object has moved.

1. Motor area sends corollary discharge signal to comparator (CDS)

2. Motor area also sends motor signal (MS) that moves eye muscles Which generates an image movement signal (IMS) to the comparator

3. If both reach comparator they cancel each other out = no movement perceived

When do we perceive real motion

When:

1. Just the sensory motion signal is sent to the comparator

2. Just the corollary discharge is sent to the comparator

No movement is perceived if both reach the comparator

Thresholds of real motion

• motion thresholds are the smallest amount of motion that can be detected

• Approximately 10-20 min arc per second

• Displacement thresholds are the smallest change in position that can be detected

Factors affecting real motion perception (8)

1. Retinal eccentricity

2. Stimulus size

3. Reference stimuli

4. Optical blur

5. Luminance duration

6. Eye movements

7. Age

How does retinal eccentricity affect real motion perception

• motion sensitivity is poorer in the periphery but optimum at fovea

How does stimulus size affect real motion perception?

Motion perception better for smaller objects.

• a larger object has to move faster than a smaller object to be perceived as moving at the same speed

how does reference stimuli affect motion perception?

• Motion perception is better if other stationary objects are located near the target

• increase motion sensitivity (10x)

How does optical blur affect motion perception?

• correction of refractive error/aberrations improves peripheral motion sensitivity

How does luminance affect motion perception?

Increasing luminance improves motion sensitivity

How does duration affect motion perception?

Longer duration of moving target improves motion sensitivity

How does eye movements affect motion perception?

Aubert-Fleishel paradox where objects appear to move more slowly when tracked.

How does age affect motion perception?

Reduction in motion sensitivity (poorer performance) as age increases.

• extent of reduction depends on target configuration, duration, speed and central/peripheral

• Unlikely due to age-related changes in pupil size or ocular media

• Involves neural factors including decreases in spatial summation, visual attention, reduced sampling efficiency, cortical inhibition and increased neural noise.

When is apparent motion perceived?

Apparent motion is perceived when objects present sequentially and separated in space. Seen in tv, movies and cartoons.

Inter-stimulus interval (ISI) is important in apparent motion

1. Simultaneous flickering

2. Disembodied motion (phi motion)

3. Smooth optimal motion (beta motion)

4. Sequential alternation

Simultaneous flickering (apparent motion)

ISI < 30ms

No motion

Disembodied motion - phi motion (apparent motion)

30 ms < ISI < 60 ms

• Motion is perceived to ‘jump’ from one light to another

• No perception of intermediate positions

Smooth optimal motion - beta motion (apparent motion)

60ms < ISI < 200-300ms

• continuous motion perceived across positions between lights

Sequential alternation (apparent motion)

200-300ms < ISI

• alternate flashing of two distinct lights

• No motion

How is induced motion perceived?

Perceived motion of a stationary object is due to motion of surrounding objects

• examples: moon and clouds, walking pigeon

• Movement tends to be assigned to smaller object

• site for this illusion is the posterior parietal association cortex

How is Autokinetic effect perceived?

Autokinetic effect of a small stationary object is perceived in a dark room.

• cause may be due to misinterpretation of involuntary eye movements

How is motion after-effects perceived?

Illusory motion following adaptation to motion where adapting motion may be transverse, looming or spiralling.

• example: moving belt and waterfall.

Physiological mechanism of motion after-effects

When viewing stationary object, responses of neurons tuned to different directions.

• neurons sensitive to upwards and downwards motion fire at the same rate so signals cancel out and no motion is perceived

• Neuron’s sensitivity to downwards motion become fatigued when viewing waterfall

• Neurons sensitive to upwards motion fire faster than the fatigued downwards sensitive neurons and so perceive upwards motion of adjacent rocks.

Kinetic depth effect

Construction of 3D effect from a 2D projection (structure from motion (SFM))

• example: rotating wire cube projected onto a screen

Biological motion

Is a class of structure from motion (SFM) event.

Pattern of motion of living creatures is very different to that of inanimate objects. Hence small lights are placed at strategic locations on the limbs where the moving lights are quickly perceived as a moving human.

• Allows judgement of gender and size

• Superior temporal sulcus (STS) and fusiform face area (FFA) = areas specialised for perception of biological motion

Locomotion (movement/motion)

Motion perception is important for:

1. Maintaining balance

2. Moving around in the environment

3. Avoiding obstacles

Balance - swinging room experiment

Visual cues provide a powerful input to balance:

• optic flow patterns provide cues regarding forwards to backwards movement

• When room swings forward, produce optic flow patterns

• Results in infant swaying forwards to compensate

Balance - force plate experiments

Sway (measured using force plates) is increased when the eyes are closed (right trace) compared to open (left trace)

Balance

Vision is important for balance with age.

• compensates for age-related deterioration in somatosensory and vestibular systems

Movement and perception

Motion aids distance and shape perception:

• 3D info - structure from motion (SFM)

• Motion parallax

• Deletion and accretion

Important for balance control and moving around safely in your environment

Motion parallax

Characteristic motion gradients provide accurate cues to distance

• nearer objects appear to move faster and to a greater extent than distant objects.

• Distant objects move relative to near objects

• Objects nearer than fixation point move against (opposite direction) to observer’s movement

Deletion and accretion

• occurs when an observer moves in a direction not perpendicular to two surfaces that are at different depths

• Deletion: object at back is covered up

• Accretion: object at back is uncovered

Navigation

Patterns of optic flow (imaginary lines) provides cues for navigation

• radial expansion: move forwards in space, optic array expands outwards in a pattern

• Focus of expansion (FOE): point we are heading towards which is always at centre (FOE is always at centre)

• Focus of constriction: move backwards in space, optic array constricts

Optic flow patterns inform us regarding heading direction as well as assist in maintaining balance

Obstacle avoidance

• Angular expansion (looming) provides info on time to collision

• motion cues (eg. Motion parallax) provides info on how far away an obstacle is from the observer which assists the observer in avoiding obstacles in the pathway.