VISUAL RESPONSES AND CONNECTIVITY IN THE TURTLE PRETECTUM

1. Using an isolated turtle brain preparation, we made extracellular s pike recordings in the dorsal midbrain during visual stimulation. Sing le units were isolated by their response to a slow-moving full-field v isual pattern imaged on the contralateral retina. This stimulus elicit s responses from the basal optic nucleus (BON) and the cerebellar cort ex using a similar preparation. Direction and speed tuning were then a nalyzed, as well as the size and position of the receptive field. 2. I n one brain stem region, anterior to the optic tectum and deep to the dorsal surface, all of the visually responsive neurons were direction sensitive (DS) 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 injec ted horseradish peroxidase revealed that this pretectal nucleus receiv es direct input from the contralateral eye. 3. All but 2 of the 48 cel ls of the nLM were strongly DS. The most effective stimulus was a slow ly moving complex visual pattern that drifted nasally in the contralat eral visual field. Brief flashes of spots, patterns, or diffuse light were much less effective. Receptive fields were large and usually (9 o f 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 co mpared 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 spontaneo us 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 d istributions of preferred directions. Most nLM cells preferred tempora l-to-nasal motion, whereas BON cells preferred almost any direction, a lthough few preferred the nasal direction. nLM cell responses were not affected by removal of the ventral brain stem including the BON. 5. T he 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 rotatio n, 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 ma mmalian nucleus of the optic tract, where spike activity is maximal in response to large patterns moving nasally in the contralateral visual field. The nLM is discussed as a possible site for encoding horizonta l retinal slip that leads to turtle optokinetic eye movements.