Spatial properties of cells in the rabbit's striate cortex.

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The rabbit's striate cortex contains a variety of receptive-field types, including concentric, uniform, simple and complex types. The spatial summation of these receptive fields was studied. Two types of linearity tests were performed: null tests and drifting grating tests. For the null tests, stimuli were sine-wave gratings, temporally modulated by either a 1 Hz sine-wave or by a 1 Hz on-off square wave. For the drifting grating tests, the stimuli were sine-wave gratings moved across the cells' receptive fields at 1-3 Hz. Fourier analyses were performed on the averaged data from both the null tests (sine-wave modulation) and the drifting grating tests. Thirty-one concentric cells were studied. Of these, nine were classified as X and fourteen as Y based upon their responses to ratings. The X cells exhibited linear spatial summation; their responses were mainly at the fundamental modulation frequency. The Y cells exhibited non-linear spatial summation. Their responses contained, in addition to a fundamental component, a second harmonic component whose relative strength increased as the spatial frequency of the display was raised. Eight other concentric cells responded poorly to gratings and were classified as sluggish. Other properties of concentric cells were examined, particularly their responses to standing contrast. The X cells' responses were generally sustained, the Y cells' responses generally transient. Sluggish cells were either sustained or transient, but their responses to standing contrast could frequently be distinguished from those of X and Y cells. All five uniform cells studied exhibited non-linear spatial summation. Their behaviour on the linearity tests was indistinguishable from that of Y cells. Of the seventeen simple cells studied, seven behaved like X cells (linear simple) and ten like Y cells (non-linear simple). However, near their optimum spatial frequencies the responses of both linear and non-linear simple cells were dominated by the fundamental component. Two complex cells behaved like complex cells in the cat's striate cortex (Movshon, Thompson & Tolhurst, 1978b). Their responses were qualitatively like those of non-linear simple cells, but were quantitatively more non-linear. Near their optimum spatial frequencies, harmonic components dominated their responses. The behaviour of nine other complex cells was difficult to describe. Generally, they were unresponsive to gratings. When they could be made to respond to gratings, they gave phase-sensitive responses like those of simple cells.

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