Visual Responses and Connectivity in the Turtle Pretectum. Tian-Xing, Fan, Amy E. Weber, Gary E. Pickard, Kevin M. Faber, and Michael Ariel. Department of Anatomy and Neurobiology, Saint Louis University, Saint Louis, MO 63104; Department of Behavioral Neuroscience, University of Pittsburgh, Pittsburgh, PA 15260; Departments of Psychiatry and Neuroscience, University of Pennsylvania, PA 19104.
APStracts 2:0065N, 1995.
SUMMARY AND CONCLUSIONS
1. Using an isolated turtle brain preparation, extracellular spike recordings were made in dorsal midbrain during visual stimulation. Single units were isolated by their response to a slow moving full field visual pattern imaged on the contralateral retina. This stimulus elicits responses from the basal optic nucleus (BON) and the cerebellar cortex using a similar preparation. Direction and speed tuning were then analyzed as well as the size and position of the receptive field. 2. In one brainstem region, anterior to the optic tectum and deep to the dorsal surface, all of the visually responsive neurons were direction-sensitive to contralateral retinal stimulation. The location and properties of these cells indicate that they are in the mesencephalic lentiform nucleus (nLM). Anterograde transport of intravitreally injected horseradish peroxidase revealed that this pretectal nucleus receives direct input from the contralateral eye. 3. All but two of the 48 cells of the nLM were strongly direction-sensitive (DS). The most effective stimulus was a slowly moving complex visual pattern that drifted nasally in the contralateral visual field. Brief flashes of spots, patterns or diffuse light were much less effective. Receptive fields were large and usually (9/13 cells) centered in the superior visual field near the horizon and nasal to the blind spot. 4. The visual responses of nLM cells were compared to those of cells in the superficial layers of the optic tectum. In contrast to nLM, the responses of tectal cells were heterogeneous and frequently not DS. Neither tectum or nLM cells had much spontaneous spike activity during darkness or stationary patterns. On the other hand, visual responses of nLM cells were very similar to those of the BON, where neurons also had low spontaneous activity, preferred slow moving patterns and were DS. However, nLM and BON exhibit different distributions of preferred directions. Most nLM cells preferred temporal-to-nasal motion, while BON cells preferred almost any direction, though few preferred the nasal direction. nLM cell responses were not affected by removal of the ventral brainstem including the BON. 5. The visual properties of nLM cells recorded in vitro were very similar to those that were recorded in intact turtles. They were also similar to those from brain preparations that included the temporal bones. In such preparations, nLM cells lack responses to horizontal head rotation, a vestibular stimulus that excited neurons in the vestibular nuclei of the same brains. Responses of turtle nLM also resembled responses of cells in homologous structures of other vertebrates, such as the mammalian nucleus of the optic tract, where spike activity is maximal in response to large patterns moving nasally in the contralateral visual field. The nLM will be discussed as a possible site for encoding horizontal retinal slip that leads to turtle optokinetic eye movements. 

Received 18 October 1994; accepted in final form 14 February 1995.
APS Manuscript Number J645-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  3 April 1995.