1. The effects of neurotransmitter antagonists on spatially linear and non‐linear responses of Y cat retinal ganglion cells were studied. 2. The contrast sensitivity of the spatially linear receptive field centre and surround at mesopic and photopic levels of illumination was affected very little by picrotoxin, but the sensitivity of the non‐linear subunits was reduced. 3. Picrotoxin also reduced two other non‐linear effects: ‘shift responses’ and the suppression of the response to a centred test flash caused by movement of a peripheral pattern. 4. In contrast to picrotoxin, strychnine decreased the contrast sensitivity of the receptive field centre, and increased the sensitivity of the subunits. 5. The results support the idea that the non‐linear responses may all be generated by similar pathways, which are distinct from those which generate linear responses. Because both picrotoxin and strychnine affect subunit responses, cells using gamma‐aminobutyric acid (GABA) and glycine as transmitters are probably involved in subunit pathways.
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