Unit analysis of the pretectal nuclear group in the rat

Abstract

Unit responses to visual stimuli in the pretectal region of the rat were investigated with steel microelectrodes. Most units could be classified into two broad categories; "phasic" (brief on and off discharges) and "tonic-on" (sustained discharge during the visual stimulus). The responsive region appears to be coextensive with the n. praetectalis anterior (Pta) which can be divided into a dorsal portion containing "phasic" units and a ventral portion containing "tonicon" units. The projection of the contralateral visual field is a mirror reversal of the field projection onto the superior colliculus. "Tonic-on" units may subserve total luminous flux detection necessary f o r the pupillary reflex. The topographical organization of units within Pta, as well as their diverse properties may account for residual visual function in animals with extensive lesions of the main visual pathways.The pretectal region has often been implicated in vision by virtue of its proximity to the lateral geniculate body and the overlying superior colliculus, evidence of optic tract fibers terminating in the principal pretectal nucleus (Polyak, '57, Hayhow et al., '60; Giolli, '65) and experiments suggesting this region is involved in the pupillary light reflex (Magoun, '35; Magoun and Ranson, '35). The present study is a continuation of a study of the organization of the visual field projection onto the superior colliculus of the rat (Siminoff et al., '66a) in which a distinct topography and some unusual unit properties were encountered below the anterior portion of the colliculus. Evidence is presented indicating that some portions of the pretectal nuclear group of the thalamus display distinctive unit properties in response to visual stimuli. A preliminary account of some of these findings has been presented earlier (Siminoff et al., '66b). METHODSFourteen pigmented Long-Evans rats were employed in the present experiments.The animals were anesthetized by intraperitoneal injection of 1.3 g/kg urethane, followed by one supplementary dose of 0.4 g/kg. The caudal portion of the right cerebral hemisphere was exposed and aspirated, thus uncovering the dorsal J. COMP. NEUR., 130: 329-342.surface of the superior colliculus as well as the caudal diencephalon.The skull was fixed to a metal plate which rigidly suspended the animal from above without obscuring the left visual field, and centering of the eye and location of the optic disc was accomplished in the manner described in the previous paper (Siminoff et al., '66a).Luminous stimuli were applied at a hemispheric surface, and several methods were employed for mapping the receptive fields of units. In most cases a flexible fiber optics bundle was used with a circular aperture subtending one degree of visual angle. All animals were dark adapted. The intensity of this "spot" of light was controlled by a neutral density wedge and monitored by a photocell. A number of units failed to respond to this small stimulus. In these cases (these units invariably had relatively large receptive fie...

Topography of Retinal Efferent Connections in Sciurids; pp. 362–375

“…Yet Siminoff et al [1967b] have apparently described receptive field characteristics of single cells in the pretectal complex of the rat after removing part of the striate cortex. Since the cortex probably sends a topographically organized projection to the region, it would be of interest to know what the cortex may contribute to the very characteristics they were describing.…”

“…By contrast, Siminoff et al [1967b] identified sev eral classes of single units in the pretectal complex of the rat; one type of unit was characterized by very large receptive fields and a tonic discharge that did not habituate, and Sim in o ff et al suggest that this population may mediate the pupillary light reflex. More important, in both species a majority of the units in the pretectal complex respond maximally to mov ing stimuli and many are directionally selective.…”

The Topography of the Visual Projection to the Mesencephalon: A Comparative Survey

“…In the course of our investigations of the retinotopic projection to the rat colli culus, we were surprised to find a discordant pattern in electrode penetra tions through the rostral pole. A more detailed analysis [Siminoff, Schwassmann and Kruger, 1967] revealed a crude but distinct topo graphic pattern ( fig. 4) within the principal or anterior pretectal nucleus, and that the physiological properties of neural elements in this nucleus were different from the features disclosed for other visual structures.…”

Receptive Field Properties of Single Units From the Visual Projection to the Ipsilateral Tectum in the Frog

“…Humphrey [1968], however, found no evidence of directional selectivity in units recorded from the superior colliculus of the rat or monkey although Simonoff, Schwassman and Kruger [1967] reported directional units in the rat pre-tectum. We have found no directionally selective units in the optic nerve, contralateral tectum or ipsilateral tectum of the frog.…”

“…In the superior colliculus of many vertebrate species a proportion of the visually activated units are directionally selective (goldfish: Cronly-Dillon [1964], Jacobson and Gaze [1964]; pigeon: Maturana and Frenk [1963]; rabbit: Hill [1966]; cat: Marchiafava and Pepeu [1966], Straschill and Taghavy [1967], Mcllwain and Buser [1968], Sterling and Wickelgren [1969]; squirrel: Michael [1967]). Humphrey [1968], however, found no evidence of directional selectivity in units recorded from the superior colliculus of the rat or monkey although Simonoff, Schwassman and Kruger [1967] reported directional units in the rat pre-tectum. We have found no directionally selective units in the optic nerve, contralateral tectum or ipsilateral tectum of the frog.…”