Inter- and intra-laminar distribution of tectospinal neurons in 23 mammals.

Based on retrograde labeling from the high cervical spinal cord, the inter- and intra-laminar distributions of tectospinal tract (TST) somata within the tectum of 23 mammals and one reptile are described. The results show that TST somata are found only in the intermediate and deep layers. Although more TST somata are usually found in the intermediate layer, there are no useful relationships for predicting the number in one layer given the number in the other. The ratio of numbers of TST somata in the intermediate relative to the deep layer varies widely, from 0:1 (in rabbits) to over 8:1 (in marmosets). Within both layers the majority of TST somata (> 80%) are found in the lateral half of the tectum--the area subversing the lower visual field. In contrast, the variation between temporal and nasal visual fields is adequately accounted for by the animal's 'visual axis'--the azimuth of its field of best vision. In general, the present results uphold the idea that the significance of the TST somata, and perhaps of the tectospinal tract itself, is to be found in directing the head so that the retinal area of best vision can be brought to bear on stimuli either almost outside, or about to pass outside, of the area of best vision. The larger and possibly universal predominance of TST somata subserving the lower visual field suggests that the tectospinal tract may be primarily concerned with adjusting the step dimensions of the forelegs to accommodate obstacles to normal progression.