Suppression Timing Stimuli in V1

We used two dynamic stimuli to test the timing of iso-orientation surround suppression and cross-orientation inhibition . Each stimulus consisted of four possible states, examples of which are shown in Figure 6-2. We used drifting gratings which moved in either the preferred direction for the cell or an orthogonal direction (chosen to produce the smallest response). Each state lasted for one cycle of drift, typically 80 or 160 ms (range, 80-320 ms). The temporal frequency of the grating was sometimes increased from the optimum (if the optimum was less than 4 Hz and if doing so did not cause firing rate to drop below 90% of optimal) because this allowed us to collect more repeats of each state transition. We used temporal frequencies ranging from 3.1 to 25 Hz, but most cells were tested at either 6.25 Hz or 12.5 Hz. For all cells in which we ran both stimuli, the same temporal frequency was used. The phase of the surround was set to match that of the center.

The center-surround stimulus used the CRF and surround apertures that were previously defined. The center and surround gratings could each be oriented either parallel or orthogonal to the preferred orientation of the neuron. Each trial lasted about 30 s and was interleaved with a center-alone and surround-alone stimulus. If the surround-alone stimulus produced a substantial response to the transition from orthogonal to parallel, we increased the inner diameter of the annulus to eliminate the response. The cross-orientation stimulus was presented entirely within the CRF. It consisted of two gratings, one parallel and one orthogonal to the preferred orientation, each presented at 50% contrast. This produced four possible states: a blank screen, the preferred grating, an orthogonal grating, or both gratings added to produce a 100% contrast cross-orientation masking stimulus. For simple cells, whose modulation to the drifting grating made their responses difficult to analyze, we used a static version of the cross-orientation masking stimulus. This stimulus was analogous to the drifting version, with the exception that each state lasted for a duration set for each cell based on the transience of its response (usually 80 ms). The phase for the preferred grating was assessed quantitatively to be optimal for the cell, while the phase for the orthogonal grating was always set to an arbitrary value.