Velocity selectivity in the cat visual system. I. Responses of LGN cells to moving bar stimuli: a comparison with cortical areas 17 and 18.

Velocity selectivity of 92 LGN cells was measured quantitatively using long, narrow light or dark bars of high contrast in N2O-anesthetized and paralyzed cats. The optimal velocities of the main responses to a moving light bar, representing center responses (i.e., due to entering the ON center or leaving the OFF center) were significantly lower for X-cells than for Y-cells. The velocity upper cutoffs were significantly higher for Y-cells than for X-cells, whereas responses to slow movement were significantly stronger in X-cells than in Y-cells. The velocity range over which secondary responses were found was significantly lower for X-cells than for Y-cells. The velocity characteristics of LGN cells were compared with those measured under precisely the same experimental conditions in areas 17 and 18. Overall, the LGN cells were sensitive to much faster velocities than cortical cells. The differences between these cortical areas were found to be much larger than the differences in velocity selectivity observed in the LGN between X- and Y-cells or within the X and Y classes. In particular, the ubiquitous presence of cells responding only to very low velocities (less than 10 degrees/s) in area 17 subserving central vision cannot directly reflect LGN velocity selectivity, since such extreme preference for low velocities was not found in the LGN sample. Changes in eccentricity had much less effect on the velocity characteristics in the LGN than in the cortex. The latency of responses to a moving light bar as estimated using a spatial lag-velocity method was on average 46 and 37 ms for X-ON and Y-ON cells as opposed to 75 and 68 ms for X-OFF and Y-OFF cells, respectively. These latencies were slightly shorter than the ON and OFF latencies (time to peak) measured with stationary presentations of the same light bar (averages 61 and 53 ms for X-ON and Y-ON, 113 and 93 ms for X-OFF and Y-OFF). For a moving dark bar the average latency was 35 and 29 ms for X-OFF and Y-OFF cells, respectively, whereas it was 47 and 54 ms for X-ON and Y-ON cells. There were no significant differences in response strength between ON and OFF cells nor between X- and Y-cells. Many Y-OFF cells had nonlinear spatial lag-velocity relationships. This indicates a shift in response origin from distal to more proximal parts of the receptive field when going from low to high velocities.(ABSTRACT TRUNCATED AT 400 WORDS)