Inhibitory interaction between X and Y units in the cat lateral geniculate nucleus

Intracellular and extracellular recordings were obtained from X and Y type relay neurons in the cat lateral geniculate nucleus (LGN). In both cell populations, unit responses to visual and electrical stimuli were studied. Stimulation electrodes were placed in the optic chiasm (OX), the optic radiation (OR), the homolateral superior colliculus (SC) and the homolateral mesencephalic reticular formation (MRF).X neurons have a different response to certain visual stimuli than Y neurons; when the visual stimulus exceeds the receptive field center (RFC) X neurons show a delayed response more often than do Y neurons. This differential delay between X and Y responses is partly because of short latency postsynaptic inhibition in the LGN. For fast, moving visual stimuli, Y cells respond with a brisk burst at stimulus velocities of 300°/sec; X cells are inhibited by such stimuli.The EPSP threshold for OX stimuli is higher in X cells than in Y cells. At stimulus intensities close to the EPSP threshold, X cells show a near maximal IPSP whereas in Y cells, the IPSP is minimal. The IPSP latency to OX stimulation is between 1.8 and 2.5 msec in X and Y cells; because the EPSP latency in X cells is significantly longer than in Y cells, X cell EPSPs frequently start at the same time as the IPSP.75% of the Y cells could be orthodromically driven from both the OX and SC stimuli. Analysis of the EPSP shape and correlation of the latencies suggests that the activation from SC is mediated via bifurcating optic tract fibers which project to both SC and LGN. X cells were never activated from SC; however, there was an IPSP in X and Y cells after stimulation of the SC. IPSP amplitude increased with the eccentricity of the neuron's RFC and was comparable to the IPSP that followed an OX stimulus set at the estimated threshold of the Y fibers.These results suggest an inhibitory convergence of the X and Y system in the LGN. The analysis of EPSPs shows that quite frequently several OT fibers converge onto a single X or Y relay cell; often, however, one fiber appears to provide the dominant input. With a few exceptions, the converging fibers have similar conduction velocities.