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1. Responses of 122 on-centre or off-centre ganglion cells in cat to suprathreshold monochromatic stimulation have been analysed under mesopic adaptation with white light, recording from their single fibres in the optic tract at a level posterior to the chiasma. Fields described are monocularly driven, located in the right half-fields of either eye, and are all within 30° of the area centralis.

2. Retinal receptors are of two types, viz. 556 nm cones and 502 nm rods. At high mesopic adaptation (1 log cd/m²) all units receive mixed cone—rod input. Under low mesopic adaptation (0 log cd/m²) the great majority receive mixed input; a few receive pure rod input. These results are in agreement with the threshold data (Andrews & Hammond, 1970).

3. Peaks of spectral response curves of units, to suprathreshold monochromatic stimuli of different wave-length but equal quantum flux, fall primarily between 550 and 560 nm for high mesopic adaptation, and between 500 and 520 nm for low mesopic adaptation. Peak position depends on the degree of rod or cone contamination, in units treated under high or low mesopic levels respectively.

4. Units with cone—rod input to the field centre receive similar but antagonistic cone—rod input to the surround. In units with pure rod input to the field centre, only rods input to the surround.

5. Cone and rod components of on-centre discharges are identifiable in terms of colour sensitivity and latency. The cone component is primarily a short-latency, high-frequency, excitatory transient; the rod component is a longer latency, lower frequency, maintained phase of excitation.

6. Less direct evidence indicates that cone and rod input to the field surround give rise to inhibitory components of comparable latency, magnitude and time course.

7. The identified cone and rod components of responses are used in further experiments to show that cone and rod input have different spatial organization both in the receptive field centre and in the surround.

8. The boundary between the field centre and surround for rods has a diameter on average about twice as large as that for cones. This organization is such that the field centre for rods substantially overlies the cone surround.

9. Changes in receptive field organization occur within the mesopic range, associated with the changeover from cone to rod vision.

10. It is suggested that the difference between cone and rod input in the mesopic range may form the basis of the cat's ability behaviourally to discriminate between colours.