Spatial and temporal frequency selectivity of cells in area 21a of the cat.

1. The spatial and temporal response properties of single cells in area 21a of the anaesthetized cat were assessed using drifting sinusoidal gratings presented at the optimum orientation for each cell. 2. Responses to sinusoidal gratings were dominated by an elevation of the mean discharge, with a relatively small modulated component at the temporal frequency of grating drift. The relative modulation ratio for the majority of cells was less than 1, similar to complex cells in the striate cortex. 3. Of those cells responsive to stimulation with sinusoidal gratings, 94% displayed spatial bandpass characteristics. Values derived from spatial frequency tuning curves were: mean optimum spatial frequency, 0.26 cycles deg-1; mean spatial resolution, 0.86 cycles deg-1; mean spatial bandwidth, 1.8 octaves; and mean normalized bandwidth, 1.3. Two cells (6%) displayed spatial low-pass characteristics. 4. Approximately half our sample of cells (44%) displayed temporal low-pass tuning, while 35% displayed temporal bandpass characteristics. The mean optimum temporal frequency of bandpass cells was 3.3 Hz and the mean temporal bandwidth 1.9 octaves. The remaining cells were classified as temporal broadband (17%) and temporal high-pass (4%). 5. We conclude that the dominant functional input to cells with relatively high spatial frequency selectivity and/or temporal low-pass response properties most probably arises from area 17. The responses of the remaining cells may be explained by input from area 17 or 18.