Visual System

Abstract

Rats show low visual acuity compared to that of diurnal primates and carnivores, but the behavioral sensitivity threshold of rats is comparable with that of dark-adapted humans. The rats’ laterally positioned eyes provide a panoramic view of the world with a thin binocular visual field centered above the snout. Most of the output cells of the rat retina (retinal ganglion cells, RGCs) have center-surround receptive fields. Unlike in primates and carnivores, virtually all rat RGCs project to the midbrain (superior colliculus, SC) rather than the dorsal thalamus (dorsal lateral geniculate nucleus, DLG). The RGCs innervate DLG relay cells which project to primary visual cortex (V1). Additional to retinal input, the DLG receives strong “feedback” input from layer 6 of V1 and from GABAergic neurons in the thalamic reticular nucleus, as well as norepinophrinergic, dopaminergic, serotonergic and cholinergic inputs from brain stem nuclei. Principal inputs to superficial layers of SC originate from the retina and layer 5 of the ipsilateral visual cortex. The SC is reciprocally connected with the (cholinergic) parabigeminal nucleus, and receives GABAergic and serotonergic inputs from several brain stem nuclei. The DLG input to rat’s V1 is visuotopically organized and most V1 neurons show orientation selectivity and some direction selectivity. Although ipsilaterally projecting RGCs constitute a very small (1.5–3%) proportion of RGCs, about 80% of V1 neurons are binocular. Area V1 has reciprocal connections with other visuotopically organized cortical areas and receives substantial direct inputs from non-visuotopically organized visual areas, and from limbic and somatosensory cortices and frontal eye field.