The binocular input to cells in the feline dorsal lateral geniculate nucleus (dLGN).

1. Cells in the A laminae of the dorsal lateral geniculate nucleus receive their primary innervation from either the contralateral (A) or ipsilateral (A1) eye. This paper provides evidence concerning the responses they give to visual stimulation of what is commonly regarded as the ineffective or non-dominant eye. It also examines the contribution of the corticofugal input to these responses. 2. Cells were identified and classified according to their responses to stimulation of the dominant eye receptive field. This was then occluded and the corresponding location in the non-dominant eye field stimulated by a moving bar. Out of fifty-seven cells examined forty-three (75%) gave a response to stimulation of the non-dominant eye. There was no obvious difference between the effects on X and Y cells in these experiments. 3. In most cases (thirty-seven) the response involved an inhibition of the resting discharge level, but three cells gave a mixed excitatory and inhibitory response and three a pure excitatory response. All the responses were weak and only revealed by prolonged periods of averaging (20-100 trials). 4. Ionophoretic application of the GABA antagonist N-methyl-bicuculline (NMB) blocked the visually elicited inhibitory effects and in most cases (twenty-seven out of thirty-two tested) revealed an excitatory response. Out of a further eight cells previously unresponsive to the non-dominant eye, NMB application revealed excitatory responses in three. 5. Increasing background discharge levels and cell excitability by ionophoretic application of either acetylcholine or the excitatory amino acid, quisqualate, did not eliminate inhibitory responses and did not reveal excitatory responses. We suggest that the visually driven non-dominant eye suppression of the background discharge involves a GABA-mediated inhibitory input which masks an underlying excitatory input. 6. An excitatory non-dominant eye response could potentially derive from the influence of the corticofugal projection. However, removal of the corticofugal input by aspiration of areas 17 and 18 did not reduce either the excitatory or the inhibitory components of the response. 7. In the absence of corticofugal input all cells tested (fourteen) exhibited a non-dominant eye response and all studied during NMB application (eleven) gave an excitatory response. The primary effect of removing the corticofugal input appeared to involve the loss of a 'damping' influence on the excitatory and inhibitory responses, such that they were more easily revealed. The significance of these findings is discussed.