Velocity sensitivity and direction selectivity of neurons in areas V1 and V2 of the monkey: influence of eccentricity.

One hundred and forty two neurons in V1 and V2 were quantitatively tested using a multihistogram technique in paralyzed and anesthetized macaque monkeys. V1 neurons with receptive fields within 2 degrees from the fixation point (central V1 sample) and V1 neurons with eccentric receptive fields (15-25 degrees eccentricity, peripheral V1 sample) were compared to assess changes in velocity sensitivity and direction selectivity with eccentricity. The central V1 sample was compared with V2 neurons with receptive fields in the same part of the visual field (central V2 sample) to compare the involvement of both areas in the analysis of motion. Velocity sensitivity of V1 neurons shifts to faster velocities with increasing eccentricity. V1 and V2 neurons subserving central vision have similar preference for slow movements. All neurons could be classified into three categories according to their velocity-response curves: velocity low pass, velocity broad band, and velocity tuned. Most cells in parts of V1 and V2 subserving central vision are velocity low pass. As eccentricity increases in V1, velocity low-pass cells give way to velocity broad-band cells. There is a significant correlation between velocity upper cutoff and receptive field width among V1 neurons. The change in upper cutoff velocity with eccentricity depends both on temporal and spatial factors. Direction selectivity depends on stimulus velocity in most V1 cells. Neurons in the central V1 sample retain their direction selectivity at lower speeds than do neurons in the peripheral V1 sample. The proportion of direction-selective cells is low in both V1 and V2. In V1, direction selectivity decreases with eccentricity. In V1, both velocity upper cutoff and direction selectivity correlate more with laminar position than with receptive field type. The similarity between V1 of the monkey and area 17 of the cat, and the dissimilarity between V2 of the monkey and area 18 of the cat, are discussed.