We have recorded from 661 single neurons in the foveal and parafoveal region of area 17 of the awake trained macaque monkey. The functional properties of 538 cells were investigated in detail, with flashed and moving stimuli of varying form and colour. Irrespective of their functional properties such determined, each neuron was also tested with a 2 X 2 degrees square of various luminance and colour. This was done in order to get an idea how such a simple stimulus is represented by the activities of neurons in area 17.
Most of the neurons showed response preference for certain aspects of visual stimuli. We have distinguished the following functional groups:
1. Sustained spectrally selective neurons (21%). These cells respond with tonic discharges to light of their optimal wavelength, and their spectral selectivity corresponded to that of opponent parvocellular cells of the lateral geniculate body. 44% of these cells were excited selectively by long, 23% by middle and 33% by short wavelength light. When slowly moving the 2 X 2 degrees square of their preferred wavelength across the receptive field, discharge rate remained elevated, as long as the stimulus covered the RF and with little contour enhancement. The majority of the sustained spectrally sensitive cells responded equally well or better to large than to small (1.0 degree) stimuli, 17.5% were less activated and few of them completely suppressed by larger stimuli. Such cells were poorly orientation sensitive. Only three cells with weak double opponency could be identified (2.7% of this group).
2. Broadband contour (18%) and
3. Panchromatic contour cells (41%).
Most neurons of these two groups were strongly activated by spots (1 degree) centered on their RF. They showed a short phasic response to contrast borders and most of them responded to luminance contrasts, including contrast reversal and colour contrasts equated for luminance. The broadband contour cells showed a slight wavelength preference with only weak or without any opponent suppression, the panchromatic contour neurons did not show any wavelength selectivity. Most showed orientation or direction sensitivity, but very sharp orientation selectivity was less common in spectrally biassed than in panchromatic contour cells (see Fig. 11). They responded tonically to gratings of optimal orientation and therefore may play a role also for cortical representation of textures. 22% of a restricted sample of panchromatic contour cells (or 9% of all cells) were hypercomplex.