Response to movement of neurons in areas 17 and 18 of the cat: velocity sensitivity.

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Abstract
Neurons (203) in areas 17 and 18 were quantitatively tested with a multihistogram technique in paralyzed and anesthetized cats. Area 17 cells respond better to slowly moving stimuli than area 18 neurons. Area 18 neurons follow faster velocities and give more vigorous responses than area 17 neurons. According to their velocity-response (VR) curves, area 17 and 18 neurons can be classified into 4 categories: velocity low-pass, velocity broad-band, velocity tuned and velocity high-pass cells. Almost all velocity low-pass cells were found in area 17 and most velocity tuned and velocity high-pass cells in area 18. Velocity sensitivity changes with eccentricity in the visual field in both areas. Upper cutoff velocity increases with eccentricity while the response to slow movements decreases. In area 17 the proportion of low-pass cells decreases with visual eccentricity while that of velocity broad-band cells increases. In area 18 the proportion of velocity tuned cells decreases with eccentricity while that of velocity high-pass cells increases. The properties of velocity tuned and velocity high-pass cells suggest that both populations encode stimulus velocity: velocity tuned cells as discrete channels, velocity high-pass cells as a cooperative network. In area 17 velocity low-pass and velocity tuned cells were associated with small receptive fields (RF) (S and B families) and velocity broad-band cells with wide RF (A and C families). In area 18 velocity broad-band cells were associated with S family cells and velocity high-pass cells with A and C families. Velocity broad-band cells were mostly binocular in area 17 and monocular in area 18. A correlation was found between upper cutoff velocity and RF width within the population of areas 17 and 18 taken as a whole as well as among velocity tuned and velocity broad-band cells. The results are discussed in relationship to the hypothesis that areas 17 and 18 of the cat play different roles in visual perception.

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