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1. Micro-electrode recordings have been made from single units in the visual cortex of the cat, during stimulation by moving grating patterns generated on a cathode ray tube.

2. The responses of the units have been measured in terms of either the frequency of impulses, or the contrast sensitivity, and expressed as a function of the spatial frequency of the grating pattern.

3. The amplitude of the responses recorded from cortical cell bodies was dependent upon the orientation of the moving grating, and for these units the stimulus was always presented at the preferred orientation. The response amplitude of other units did not depend upon the orientation of the grating stimulus, and these are believed to be the terminations of geniculate fibres.

4. The high spatial frequency end of the response function measured in terms of the contrast sensitivity could be well fitted by an exponential function. Subtraction of the blank count from the impulse frequency data gave a curve fitted by the same exponential function. The low frequency end was less consistent.

5. The spatial frequency at which this exponential function had fallen by one log. unit was taken to specify the position of the unit's response in the spatial frequency spectrum. For all units these values cover a range of four octaves of spatial frequency (from 0·18 to 3·8 cycles per degree).

6. It is suggested that these neurophysiological results support psychophysical evidence for the existence in the visual system of channels, each selectively sensitive to a narrow band of spatial frequencies.

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