W5: Motion Perception

Motion Perception

  • Basic Aspect of Perception

    • Information about motion is fundamental to perception.

    • Detecting motion is crucial for navigation and survival in the environment.

    • Recommended textbooks on motion perception:

      • Sensation and Perception, Goldstein, EB, 2007; 2010.

      • Sensation and Perception, Wolfe, JM et al, 2009.

  • Significance of Motion

    • Provides essential information for:

      • Attracting attention (movement in periphery draws focus)

      • Identifying three-dimensional form (how objects relate to an observer) - if something moves in the environment or if we walk around an object.

      • Supporting figure-ground segregation (movement helps distinguish an object from its background)

      • Interaction with the environment (e.g., walking aids in avoiding collisions and maintaining course)

Types of Motion

  • Real Motion

    • Involves physical movement of an object.

    • Must also account for eye and head motion.

  • Apparent Motion

    • Motion perceived when objects are sequentially presented and separated in space.

    • Inter-stimulus interval (ISI) is crucial in determining perceived motion.

  • Induced Motion

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

    • Example: Moon appearing to move against clouds.

  • Autokinetic Motion

    • Perceived movement of a small stationary object in a dark room, often misinterpreted due to involuntary eye movements.

  • Motion After-effects

    • Illusory motion experienced after adaptation to actual motion.

Physiology of Motion

Retinal Image Motion: Reichardt’s Model

  • Differentiates between the motion of small and large objects using a simple neural circuit that reflects motion direction and velocity.

    • E.g., need to differentiate between input from motion of a small ladybug moving from L to R (1) and that from one large stationary ladybug (2)

      • can be achieved by a simple circuit that includes 2 additional components.

Two components in circuit (3):

  • D delays transmission from A to M

    • Fires when A is initially stimulated but stops if light continues to shine on A.

    • This behavior accounts for cases such as the large stationary ladybug (2).

  • X, a multiplication cell

    • Only fires when both B and D are active, making it sensitive to motion.

  • Direction-Selective Circuit

    • The circuit responds well to motion from left to right (L to R) but not from right to left (R to L).

    • It is tuned to velocity, firing when a delayed response at A and a direct response from B occur at the same time.

    • additional receptors would enable detection of longer duration / extend of motion.

This forms the basis of Reichardt’s Model of how flies detect motion and also the basis of the most theoretical human models of motion detection.


Visual Cortex Involvement

  • Involves complex cells in area V1 and

    • Extrastriate areas involved in 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 and Magnocellular Systems

    • The dorsal system is responsible for 'where' processing, while the magnocellular system contributes to high temporal and contrast sensitivity.

    • magnocellular lesions - can’t detect movement. Can’t relate movement and time.

Real Motion and Corollary Discharge Theory

  • retinal motion detectors cannot fully explain our perception of motion.

    • need to know if eyes and head are moving.

  • 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.


Corollary Discharge Theory

  • Compares input from retinal and eye-head movement systems.

  • outside visual cortex

  • Efferent copy (corollary discharge signal) sent to the comparator for distinguishing whether movement is perceived based on eye/head motion.

  • Retinal image motion is combined with this corollary discharge. If both reach the comparator they cancel each other out

  • Perception of Movement

    • Just the sensory motion signal is sent to the comparator

    • Just the corollary discharge is sent to the comparator

    • No movement is perceived if both reach the comparator

Examples:

  • Eyes tracking a moving object: stationary retinal image, eyes moving so CDS sent to comparator.

  • Moving Object but eyes remain stationary: Moving retinal image (IDS) sent to comparator but as eyes are stationary, no CDS is sent to comparator.

  • Person scanning a stationary seen: moving retinal image (IDS), eyes moving (CDS) - both signals reach the comparator, and no motion is perceived.

  • pushing on eyeball: stimulates eye muscles but retinal image doesn’t change. Therefore will see motion



Factors Affecting Motion Perception

  • Retinal eccentricity: poorer sensitivity in peripheral vision, peak sensitivity at fovea.

  • Stimulus Size: motion perception is better (higher sensitivity) for smaller objects.

    • the larger circle must move faster than the smaller circle for them to be perceived to be moving at the same speed. This is known as Velocity transposition

  • Reference Stimuli: Motion perception better if other stationary objects are located near to the target

    • Increase motion sensitivity (10x), eg, 10-20 min arc per second down to 1 min arc per second

  • Optical Blur: Correction of refractive error / aberrations improve peripheral motion sensitivity.

  • Luminance: Increasing luminance improves motion sensitivity

  • Duration: longer duration of moving target improves motion sensitivity

  • Eye movements: Aubert-Fleishel Paradox - objects appear to move more slowly when tracked.

  • Age: Reduction in motion sensitivity (poorer performance) with increased age

    • Extent depends on target configuration, duration, speed and central or peripheral

    • Unlikely to be 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

    • implications for moving in environment such as driving.

Types of Motion

Apparent Motion

  • Motion Perception

    • Motion is perceived when objects are presented sequentially and separated in space.

    • As the speed of the two images are appearing/ replaced at different locations, the images appear to be moving.

  • Media Applications

    • Apparent motion underlies our perception of motion in television, movies, and cartoons.

Inter-Stimulus Interval (ISI)

  • Simultaneous Flickering

    • ISI < 30 ms:

      • No motion perceived

  • Disembodied motion: Phi Motion

    • 30 ms < ISI < 60ms

    • Motion is perceived to ‘jump’ from one light to the other;

    • No perception of intermediate positions.

  • Smooth Optimal Motion (Beta Motion)

    • 60 ms < ISI < 200–300 ms:

    • Continuous motion perceived across positions between lights

  • Sequential Alteration:

    • 200–300 ms < ISI:

    • Alternate flashing of two distinct lights

    • Apparent Motion & Induced Motion

Induced Motion

  • perceived motion of a stationary object due to motion of surrounding objects.

    • moon racing against clouds

    • building falling over

    • walking pigeon

  • movement tends to be assigned to the smaller object

  • possible site for this illusion is the posterior parietal association cortex.

  • pigeon walk moves it head forward, then moves body forwards whilst head is stationary but gives the illusion that the head is moving backwards. 

Autokinetic effect

  • perceived movement of a small stationary object in a dark room.

  • Cause is not well understood - probably due to misinterpretation of involuntary eye movement.

Motion After-Effects

  • Illusory motion following adaptation to motion

  • the adapting motion might be transverse, looming or spiralling.

Physiological Mechanism

  • when viewing a stationary object the responses of neurons are tuned into different directions.

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

    • Neurons’ sensitivity to downwards motion becomes fatigued when viewing waterfalls.

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


Movement and Perception

  • motion aids distance and shape perception

    • 3D information - Structure From Motion (SFM)

    • Motion parallax

    • deletion and accretion

  • Also important for balance control and moving around safely in your environment

Kinetic Depth Effect

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

  • E.g., rotating wire cube projected onto a screen.

Biological Motion

  • 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.

  • Evident in babies and does not deteriorate with age.

Biological motion experiment

  • small lights are placed at strategic locations on the limbs

  • the moving lights are quickly perceived as a moving human

  • The effect allows judgement of gender, size.

Physiological part of biological motion

  • Networks of areas - superior temporal Sulcus (STS) and fusiform face area (FFA) - specialised for perception of biological motion.

Locomotion

  • motion perception is important for:

    • maintaining balance

    • moving around in the environment

    • avoiding obstacles

Balance

  • Visual cues provide strong input to balance

    • Force plate experiments: Sway (as measured using force plate) is increased when the eyes are closed (right trace) compared to open (left trace)

  • vision becomes more important for balance with age.

    • compensated for age-related deterioration in somatosensory and vestibular systems.

Factors of balance:

  1. Vision: reduced vision leads to reduced balance

  2. Vestibular Neurons: kinaesthetic cues

  3. Somatosensory: muscle function and reflexes

Swinging room experiment:

  • indicates that visual cues provide a powerful input to balance.

  • false ceiling and walls, mounted on a pulley system - makes room move backwards or forwards

  • optic flow patterns provide cues regarding forward (a) or backwards (b) movement.

  • When room swings forwards, produce optic flow patterns of a backwards movement (b)

    • results in infant swaying forwards to compensate the feeling of going backwards.

Optic Flow Fields

  • pattern of movement of the environment.

    • As we move forwards, it expands

    • As we move backwards, it constricts.

  • imaginary patterns that the visual system is receiving and using to interpret patterns of movement.

Movement and Perception

Motion Parallax

  • characteristic motion gradients provide accurate cues to distance

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

    • objects nearer than fixation point moves 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

  • Gives relative position in space.

  • Deletion: object at the back is covered up

  • Accretion: Object at the back is uncovered

Ecological approach to perception

  • information from environment including surfaces, contours and textures creates an “optical array”

  • Collection of light rays that interact with objects in the world in front of the viewer

  • Disturbances in the optical array provide information

  • As you walk through a room, there are flow patterns on the retina, but you properly judge that the objects are not moving.

Navigation

  • patterns of optic flow provide cues for navigation

    • radial expansion: move fowards in space, optic array expands outwards in a pattern known as “radial expansion”

    • focus of expansion (FOE): point we are heading towards which is always at the centre. Always Stationary

    • Focus of constriction: move backwards in space, the optic ray constricts.

  • optic flow patterns inform us regarding direction as well as assist in maintaining balance.

Obstacle Avoidance:

  • angular expansion (looming) provides information on time to collision.

  • Motion cues (e.g., motion parallax) provide information on how far away an obstacle is from the observer.

    • Assists the observer in avoiding obstacles in the pathway.