Visual receptive field properties of excitatory neurons in the substantia nigra

The substantia nigra has been widely regarded as a structure involved in visuomotor co-ordination, but little is known about the sensory background of its function. Here we give a detailed description of the visual receptive field properties of excitatory substantia nigra neurons. The visual responses of 59 excitatory neurons were recorded in both the substantia nigra pars reticularis and the pars compacta of halothane-anesthetized, immobilized, artificially respirated cats. The substantia nigra neurons were not responsive or exhibited very low sensitivity to stationary visual stimulation. The units responded optimally to small stimuli moving at intermediate or high velocities in their extremely large receptive field. We observed no signs of retinotopical organization within the substantia nigra. A majority of the units exhibited narrow direction tuning and high direction selectivity, while a smaller proportion of them were broadly tuned and not direction-sensitive. Our results suggest that the visual properties of the excitatory substantia nigra units are quite similar to those of the superior colliculus and other extrastriatal structures that receive tectal afferents. This supports the notion that the substantia nigra processes dynamic visual information and that its excitatory visual neurons are modulated by the extrageniculate tectal visual system of the mammalian brain.