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1. Extracellular recordings were made from directionally selective ganglion cell units in the isolated frog retina and decapitated Necturus preparation.

2. Intracellular recordings were made from individual photoreceptor cells in the frog and Necturus retinae while stimuli which had evoked directionally selective responses at the ganglion cell level were presented. No evidence for inhibition of photoreceptors for any direction of movement of the light stimulus was found. This appeared to rule out a mechanism for directional selectivity involving inhibition of photoreceptor potentials.

3. Intracellular recordings were made from the nuclear layer between photoreceptors and ganglion cells in Necturus. The responses were of two types: either transitory or sustained.

4. The sustained type responses could be divided into two classes depending on their receptive field organization. One type of sustained potential had a large receptive field without any evidence for a centre-surround antagonism and corresponded to the luminosity type S-potential recorded in fish. The other type had a smaller receptive field and showed a difference in sign of response between centre and surround if the centre was flooded with a steady light. This is very similar to what has been described for a type of on-centre, off-surround ganglion cell.

5. The transitory type of responses showed some centre-surround antagonistic organization. Some of these transitory units also appeared to show some discrimination in response as a function of the distribution of light on the retina.

6. No specific directional selectivity was found from units at the inner nuclear layer. This further excluded any mechanism of directional sensitivity which involves selectivity at the photoreceptor level.

7. It was concluded that although inner nuclear layer units may play a role in the mechanism of directional selectivity, no specific directionality was found at the first synaptic level of the retina.

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