Lateral geniculate neurons of cat: Retinal inputs and physiology

The number of retinal axons which made functionally significant excitatory synapses on a lateral geniculate neuron ranged from one to about six. About 8 per cent of geniculate neurons were driven by only a single input fiber from the retina. Much more commonly, two or three inputs accounted for most of the impulses from the geniculate neuron. It has long been known that the threshold of a geniculate neuron rises more rapidly that that of its retinal input as the diameter of a stimulus spot is increased. This effect has been traced to a new component of the receptive field of the geniculate neuron: the suppressive field. This appeared in uncomplicated form as an annular zonebeyond the limits of the conventional antagonistic surround. Stimulation of the suppressive field produced only inhibitory effects on responses elicited by simultaneous stimulation of center or surround. A rather uncommon class of geniculate neurons (5 per cent) having on-off receptive fields was also found. Their responses to electrical stimulation of area 17 suggested they were interneurons. A model is proposed which accounts for the suppressive field of geniculate neurons in terms of a recurrent loop through recurrent collaterals of principal cells and the presumed interneurons.