Thalamic interneurons and relay cells use complementary synaptic mechanisms for visual processing

Synapses made by local interneurons dominate the thalamic circuits that process signals traveling from the eye downstream. The anatomical and physiological differences between interneurons and the (relay) cells that project to cortex are vast. To explore how these differences might influence visual processing, we made intracellular recordings from both classes of cells in vivo. Macroscopically, all receptive fields were similar, made of two concentrically arranged subregions in which dark and bright stimuli elicited responses of the reverse sign. Microscopically, however, the responses of the two types of cells had opposite profiles. Excitatory stimuli drove trains of single EPSPs in relay cells but graded depolarizations in interneurons. By contrast, suppressive stimuli evoked smooth hyperpolarizations in relay cells but unitary IPSPs in interneurons. Computational analyses suggested that these complementary patterns of response help preserve information encoded within the fine timing of retinal spikes and increase the amount of information transmitted to cortex.