Searching for a neural correlate of the psychophysical channels for increasing and decreasing size, we quantitatively studied 56 units in area 18 of cat visual cortex. We compared responses evoked by expanding and contracting slits with the sum of spike counts evoked by individual movements of the two stimulating edges. All 56 units responded to changing-size, but 19 of these could be described as simply responding to changing light level. Thirty units fired preferentially to expansion (or contraction) independently of whether the stimulus slit was bright or dark. At first sight these 30 units looked like changing-size neurons, but 20:30 reversed their bias when stimulus location was changed. Only one cell unequivocally distinguished between expansion and contraction. On the other hand, interactions between responses to the two edges had the effect of emphasizing changing-size information in many units, not unequivocally, but with a probability greater than pure chance. Thus, although units specifically sensitive to changing-size are rare in area 18, the population behaviour of a common type of area 18 neuron is capable of signalling changing-size information. © 1979.
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