Direction selectivity of cells in the cat's striate cortex
Casanova, C., Nordmann, J.P., Ohzawa, I., and Freeman, R.D. (1992)
Direction selectivity of cells in the cat's striate cortex: differences between bar and grating stimuli.
Visual Neurosci. 9: 505-513.
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