Motion Perception: Uses and Measuring of Motion Information, Study notes of Psychology

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Motion Perception

Uses of motion information: Detection Grouping Distance Shape Heading

Measuring motion information: Motion measurement mechanisms Correspondence problem Aperture problem Brain areas

Detection

The image of an object must move on the retina or it will disappear. Examples: stabilized images, retinal vasculature.

Grouping

Image features that move together tend to be grouped together (Gestalt principle of common fate).

Motion cues to distance. When fixating the horizon while translating in a car or train, the nearer an object the faster it moves across the retina. When fixating a closer object while translating, the further an object from the fixated object the faster it moves across the retina.

Demonstration of shape from motion. If small lights are attached at each joint in the human body, the static pattern of lights is nearly incomprehensible. However, as soon as the person moves the scene becomes easy interpret. For example, here are the static patterns for two people dancing. A second demonstration in class showed random dot movies of a rotating objects.

The upper figure shows optic flow similar to the example in the previous slide. The lower figure shows the optic flow vectors when the person is fixating at a nearby point while continuing to translate toward the point on the horizon. Notice that the flow pattern changes a great deal. Humans still correctly interpret this flow pattern. The optic flow pattern when approaching a nearby by surface (e.g., a wall) can be used to judge the time until contact with the surface, even if the observer does not know how fast he/she is moving.

Motion Perception

Uses of motion information: Detection Grouping Distance Shape Heading

Measuring motion information: Visual latency Motion measurement mechanisms Correspondence problem Aperture problem Brain areas

Changes in response latency for 12 different neurons in primary visual cortex (V1) as a function of stimulus contrast. Response latency decreases an average of 20- ms as the contrast is increased. (Time shift = change in response latency).

Luminance (light intensity) also affects latency. One demonstration of this is the Pulfrich effect. A filter (which reduces luminance) causes a longer latency of neural response in the left eye. Thus, the moving green bar can appear at location P in the left eye but at another location in the right eye. The result is an effective disparity which causes the bar to appear to be at location Q (a different depth). The bar appears to be at a greater distance than P when motion is left to right.

Mechanisms for Motion Measurement

Delayed summation

Feature tracking

Motion blur

Combining motion components

Motion is extremely important to measure accurately. The brain probably uses several different kinds of mechanism to measure motion.

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Schematic of simple delayed summation circuit for creating a direction selective simple cell. A stimulus moving right to left will result in signals arriving at the summation site at different times and so no spikes will be generated at the output.