CONTRAST MODULATED STEADY-STATE VISUAL-EVOKED POTENTIALS (CMSS VEPS) - RECORDING EVOKED-POTENTIALS AND RELATED SINGLE-CELL RESPONSES IN AREA-17 OF THE CAT

(1) The functional characteristics of the neuronal substrate, respondi ng to the CMSS VEP stimulus, were studied by recording CMSS VEPs and r elated single unit activity in area 17 of the anaesthetised and paraly sed cat. CMSS VEPs use an 8 Hz phase reversing (i.e., 16 reversals/sec ) grating stimulus with rapid contrast sweeping and allow the contrast thresholds and lag values to be measured as a function of the spatial frequency. (2) The CMSS VEPs of the anaesthetised cat have a wave for m similar to those of humans but are shifted to lower spatial frequenc ies, higher contrast thresholds and longer lag values. (3) The cellula r response to a sinusoidal grating, phase reversing at 8 Hz, was studi ed in order to identify the neuronal substrate generating the CMSS VEP s. Sixty percent of the area 17 cells respond to this stimulus. Cells responding at 8 Hz reversal comprise a distinct subpopulation of visua l cortical cells selective for higher velocities and lower spatial fre quencies. (4) Although the CMSS VEPs contain almost exclusively energy at 16 Hz, the temporal response pattern of striate cells is quite dis parate, including first and second harmonic response patterns as well as an intermediate type. (5) There is a near-perfect correlation betwe en the contrast thresholds of single cells, obtained with the contrast swept stimulus and those obtained with a static contrast test, valida ting the technique of rapid linear contrast sweeping. (6) The influenc e of the temporal parameters of the contrast sweeping on the calculate d contrast threshold was investigated at the neuronal level. These par ameters only marginally influence the responses. (7) CMSS VEP contrast thresholds and neuronal thresholds were compared. The sensitivity of VEPs corresponds to that of the most sensitive neuronal generators. CM SS VEP lag values are longer than the values for individual neurones.