We have investigated the notion that directional responses of cells in the visual cortex depend on the type of stimulus used to drive the cell. Specifically, we have asked if sinusoidal gratings provide a different estimate of direction selectivity than bars that are brighter or darker than the background.

Using standard techniques, we recorded from 176 cells in the visual cortex of nine cats. For each cell, bright bars, dark bars, and sinusoidal gratings were presented in a randomly interleaved fashion. Complex cells exhibited around twice as many direction-selective as nondirection-selective responses. Estimates of direction selectivity were nearly identical for bright and dark bars and for gratings. For simple cells, a similar ratio of direction-selective to nondirection-selective responses was observed for gratings. However, a larger proportion of simple cells were classified as direction selective when bars were used for stimulation.

A simple cell that exhibited direction selectivity to a grating behaved in a similar manner when stimulated with bright or dark bars. However, in contrast to complex cells, some simple cells classed as directionally nonselective on the basis of their responses to gratings, displayed directionally selective behavior to bars. In addition, the preferred directions for dark and bright bars sometimes differed. These results demonstrate that the classification of a simple cell as directionally selective or nonselective can depend critically on the visual stimulus used.

The study of the gaussian parameter ω—the approximate width of a peak at half height—of the intensity curves obtained by means of the Unidirectional Laser Oblique Illumination technique employed in the optical microscopy of materials permits introduction of the concept of variation of a surface pattern at different magnifications. This concept establishes how the surface pattern or topography varies among different areas when different magnifications are used. Diffraction gratings and metallic surfaces which have been subjected to different metallographic procedures are analyzed, using objectives of 8×, 16×, and 32×.