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1. Responses of single geniculate cells to moving light and dark bars and light/dark edges were studied in cats anaesthetized with nitrous oxide/oxygen (70%/30%).

2. Over 95% (230 out of 241) of geniculate cells had antagonistic centre-surround receptive fields. Their responses could be characterized as centre-activated or centre-suppressed depending on the receptive field type (ON- or OFF-centre) and the contrast between stimulus and the background (brighter or darker than the background). Moving light and dark edges evoked responses which were very similar to the responses evoked by these stimuli in simple cells of striate cortex.

3. A number of cells (45) with antagonistic centre-surround receptive fields were classified according to their X/Y (sustained/transient) properties. Units with sustained properties (X-cells) did not increase their firing rate with an increase of stimulus velocity and some of them showed a clear-cut preference for slow movement (around 1-2°/sec). On the other hand, units with transient properties (Y-cells) showed a clear-cut preference for fast-moving stimuli (50-100°/sec.)

4. Elongation of the stimulus beyond the antagonistic surround in both X- and Y-cells produced a clear-cut reduction of amplitude of both centre and surround components of the response. Thus the existence of a suppressive field component beyond the antagonistic surround is confirmed.

5. About 5% of cells had receptive fields which did not have an antagonistic centre-surround organization but gave a mixed ON-OFF discharge from the central region of the field. Around the central region there was a silent suppressive zone. These units were not directionally selective, responded preferentially to fast-moving stimuli (25-100°/sec) and had a substantial (spontaneous) maintained activity.