Spatial and temporal selectivity in the suprasylvian visual cortex of the cat

We recorded from single units in the medial and lateral banks of the posterolateral suprasylvian visual cortex (PMLS/PLLS) of the cat. The responses to drifting high-contrast gratings of optimum orientation and direction of motion, but varying in spatial and temporal frequency, were examined quantitatively for a sample of cells, whose receptive fields covered a wide range of eccentricities. The optimum spatial frequencies (average about 0.2 cycles/deg) were low compared to the values reported for striate cortex but similar to those for area 18. The mean spatial bandwidth (about 2 octaves) was slightly broader than that of cells in other cortical visual areas. The cut-off spatial frequencies ("acuities") covered a wide range, from 0.05 to 2.1 cycles/deg, similar to those of cells in area 18. Responses to drifting sinusoidal gratings were usually dominated by an unmodulated elevation of discharge, although some modulation occurred at the temporal frequency of drift, especially at low spatial frequencies. Modulated responses were relatively stronger in PMLS than in PLLS. For those cells that responded to flashed stimuli, stationary, contrast-modulated gratings presented at different spatial positions typically evoked small responses at the fundamental frequency (dependent on spatial phase) and a larger component at the second harmonic of temporal frequency, with no overall "null-position." The optimum spatial frequency was usually higher than would be predicted by simple summation within the dimensions of the receptive field. Thus, neurons in PMLS and PLLS, like complex cells in areas 17 and 18, behave nonlinearly and their spatial selectivity is determined by "subunits" smaller than their receptive fields. The range of preferred temporal frequencies ranged from less than 2.5 Hz to more than 10 Hz. In their temporal selectivity neurons in PMLS resembled cells in area 17, with little attenuation at low temporal frequencies, whereas there was a tendency for cells in PLLS to prefer higher temporal frequencies, as is common in area 18.