Directional tuning of cells in area 18 of the feline visual cortex for visual noise, bar and spot stimuli: a comparison with area 17.

Directional tuning for visual noise, bar and single spot stimuli was compared over a wide range of velocities in cells from areas 17 and 18 of the visual cortex in lightly-anaesthetized cats. In each area, S-cells were predominantly insensitive to motion of a field of visual noise. C-cells were more sensitive to noise motion than B-cells, but showed heterogeneity in noise sensitivity, which was associated with other response properties: strongly noise-sensitive C-cells had relatively high spontaneous activity and broad directional tuning, and were predominantly direction-selective and binocularly-driven. Frequently, directional tuning for noise was unimodal at low velocity, but became progressively more bimodal as velocity was increased: a trough of depressed response corresponding to the peak in tuning for the bar separated two progressively more widely disparate preferred directions. In area 18, cells with velocity tuned (VT) functions for bar motion developed bimodal tuning for noise well below the optimum velocity for bar or for noise motion, while velocity high-pass (VHP) cells became progressively more bimodally tuned for noise over a wide range of velocities, in parallel with a steep increase in response to bar and noise motion. A high proportion of VT and VHP cells was bimodally tuned for noise at all velocities, one VHP cell showing two discrete lobes of tuning for noise below the threshold velocity for bar motion. Among cells which remained unimodally tuned for noise, VT and VHP cells in area 18 had radically dissimilar preferred directions for noise and bar motion at all velocities. With the exception of VHP cells, velocity bandpass was higher for noise than for bar motion. These results, together with other novel observations on the modality of tuning for noise in preferred and opposite directions of motion, demonstrate that bimodality of tuning for noise cannot simply be an effect of upper cut-off velocity for bar motion (Movshon et al. 1980; Orban 1984). It is argued that the trough between the lobes of tuning arises through laterally-directed inhibitory convergence from superficial- and deep-layer, large basket cells. In 40% of noise-sensitive cells, tuning for bar motion was broader on the flank closest to the preferred direction for noise and for a moving sport, while some 25% of cells showed variations in tuning for bar motion with velocity, which were associated with velocity-dependent changes in tuning for noise.(ABSTRACT TRUNCATED AT 400 WORDS)