Single-unit responses to visual stimuli were recorded in the transitional region between optic tectum and thalamus; recording sites, marked by Prussian blue, were located in the nucleus of the optic tract, the deep pretectal nuclei, the pretectal area, and the nuclei of the posterior commissure. Most units were activated primarily by moving objects and a considerable proportion reacted in a directionally selective manner. Some units strongly preferred objects moving in a midsagittal plane towards the animal (Type I) or away from it (Type II). When tested with continuously increasing or decreasing sizes of stationary bright discs or shadows, Type I was primarily activated during increase, Type II during decrease of stimulus area. Increase or diminuition of stimulus intensity were ineffective. In another group of units visual angles of effective stimuli had to exceed 7 deg in small sized objects; threshold of visual angles increased with increasing object size. We suggest that these units require a minimum light flux to be excited. Selectivity for relatively low stimulus background contrasts was found in some units. Most neurons responded to stimulation of either eye. A considerable proportion did not react to diffuse illumination of their receptive field. Light spots flashed into the field center evoked in most cases phasic on-off reactions. Phasic-tonic responses occurred only in the nucleus of the optic tract. Receptive field sizes ranged from 2-120 deg. Only in the nucleus of the optic tract and in the pretectal area evidence of a retinotopic organization was obtained. Some neurons responded reliably to visual and tactile or visual and acoustic stimuli (multisensory convergence). Incidence of response properties varied with anatomical localization. Units of the nucleus of the optic tract were directionally unselective, frequently responsive to diffuse light and mostly excitable by one eye. The nucleus of the posterior commissure contained neurons which were directionally selective for vertical movements or for sagittal approach movements (Type I). Type II neurons were more laterally localized. Comparison of the present results with those of single unit studies in the optic tectum demonstrates many similarities but also definite differences. © 1969.
