adaptation The change in sensitivity of a neuron (or of the animal as a whole) in response to prolonged viewing of a stimulus.

bandwidth A measure of the neuron's selectivity for a property such as motion direction. A large bandwidth refers to a neuron that responds to a broad range of directions, whereas a narrow bandwidth refers to a neuron that responds to a more selective range of directions.

coherent motion A type of motion in which all the elements move in the same direction and at the same speed.

contrast A measure of the deviation of local light intensity from the average intensity.

direction selectivity A preferential response to light moving in a narrow range of directions.

gabor The profile described as the product of a sine wave grating and a Gaussian that is often used to describe the spatial sensitivity of neurons in striate cortex.

gaussian A smoothing function that has a bell-shaped profile described by a normal distribution.

grating A pattern of parallel light and dark bars.

heading direction The direction in which the observer is moving.

incoherent motion A type of motion in which the elements move in different directions or with different speeds. Each element's velocity is randomly changed over time.

mask A pattern that occurs closely in space or time to a test pattern and affects the response of the neuron (or of the observer) to the test.

motion transparency The perception of transparent sheets moving over one another produced when two sets of elements move in widely different directions or speeds.

optic flow The motion on the retina produced by observers moving relative to their environment.

preferred direction The direction of motion that evokes the strongest response in a motion-sensitive neuron.

psychometric function A function relating perceptual performance to a physical stimulus variable. The measure of perceptual performance is the proportion of perceptual decisions (e.g., right-ward/leftward judgments or correct/incorrect responses).

sine wave (sinusoidal) grating A pattern of light and dark bars in which the contrast across the bars varies as a sine wave.

spatial receptive field The region of the visual field in which a visual stimulus evokes a response in a neuron.

space-time orientation A property of a receptive field that makes it sensitive to a particular direction and speed. The receptive field of the motion-selective unit has a spatial response profile that changes with time.

Encyclopedia of the Human Brain Volume 3

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spatial frequency A measure that is inversely related to the width of the bars of a grating. The wider these are, the lower the spatial frequency. It is measured in cycles per degree of visual angle.

temporal frequency The analog of spatial frequency in time. It refers to the rate of alternation of a stimulus and is expressed in cycles per second or hertz.

threshold A criterion performance level between chance and perfect performance that is used to compare data across different conditions.

visual angle The angle that an object subtends at the eye.

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