of visual angle. We then proceeded with experiments under
computer control.
We first determined the neuron's preferred orientation or direction, spatial frequency, temporal frequency, size and position by hand. We then quantitatively characterized the cell's response properties to gratings in this order: (1) orientation and direction tuning; (2) spatial frequency tuning; (3) temporal frequency tuning; and (4) size tuning. We chose a small patch of optimized grating and adjusted the vertical and horizontal position by hand to obtain the maximal response. This patch was taken to be centered in the receptive field. We classified cells as simple or complex using the standard F1:DC ratio (Movshon et al., 1978a; Skottun et al., 1991), where DC is the mean firing rate (minus baseline) and F1 is the amplitude of the Fourier component at the fundamental frequency of the response to an optimized drifting grating. Units were classified as simple if the F1:DC ratio of their spatial frequency tuning curves was greater than one, while all other units were classified as complex. These experiments consisted of multiple blocks of stimuli, each composed of a randomly ordered group of all the stimuli in a set. All stimuli within a block were equal in duration, and were separated by presentation of a uniform mean gray background for about 1.5 s. Each stimulus was typically repeated three or more times (but no less than two).
In determining size tuning for a cell, we collected responses to increasing the diameter of a patch of optimized drifting grating and also to increasing the annular inner diameter. We defined the classical receptive field (CRF) to be the smallest circular patch which gave the maximum response. We defined the surround region to be the annulus with the smallest inner diameter that evoked no response above the spontaneous level.