1. The results described are for detailed analyses of fifty-four isolated LGN units, in response to monochromatic stimuli presented against achromatic, mid-mesopic backgrounds. Forty-seven were positively identified cells from the A-laminae; the remaining seven were fibres from the optic radiation.2. Cells are classified according to firing pattern. Phasic cells respond almost exclusively with a discharge transient. Tonic cells, by contrast, give a maintained component in addition. In general, tonic cells possess higher spontaneous firing frequencies than phasic cells and the antagonistic surrounds of their receptive fields are more potent. In other respects the two classes appear to be functionally similar.3. All cells within the A-laminae receive input involving both rods and 556 nm cones.4. The spatial organization of geniculate receptive fields, unlike retinal fields, is little different for cone and rod vision. In the infrequent instances where a change is apparent, it is small and can go in either direction: rod fields are then on balance slightly larger than cone fields.5. The locus of maximum sensitivity for the receptive field surround is described by a circle, concentric with the field centre; it is invariant with respect to stimulus geometry, or changeover from cone to rod vision.6. This result implies that the receptive field surround mechanism does not extend through the field centre. It supports the notion that the centre and surround of each geniculate cell receptive field are mediated by discrete retinal inputs.
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