Anatomical and functional organization of pathway from superior colliculus to lateral posterior nucleus in hamster.

A series of anatomical (autoradiographic and horseradish peroxidase, HRP) and electrophysiological experiments were carried out to determine the organization of the pathway from the superior colliculus (SC) to the lateral posterior nucleus (LP) in the hamster. Small, electrophoretic HRP deposits restricted to LP labeled numerous cells in both the ipsilateral and contralateral colliculus. Over 95% of the labeled cells were located in the lower one-half of the stratum griseum superficiale (SGS) and the upper stratum opticum (SO). A number of different morphological cell types contributed axons to the tecto-LP pathway. The receptive-field properties of antidromically activated tecto-LP neurons were delineated using extracellular single-unit recording techniques. Ninety-eight percent of the tecto-LP cells recorded were isolated in the SGS and SO. All tecto-LP cells responded more vigorously to moving than to flashed stimuli, one-third were directionally selective, and one-third exhibited some degree of speed selectivity. The responses of tecto-LP neurons did not differ appreciably from those of superficial layer collicular cells that could not be antidromically activated by LP shocks. Small pressure injections or electrophoretic deposits of [3H]leucine into sites with known retinotopy in the superficial collicular laminae were used to determine whether or not the tecto-LP projection in hamster was topographically organized. Injections anywhere in the SGS and SO yielded dense label in almost all of the caudal (LPc) and rostrolateral (LPrl) subnuclei of LP, ipsilaterally, and sparser labeling in these same subnuclei, contralaterally. No injection produced significant labeling in the rostromedial (LPrm) subnucleus. Our autoradiographic data gave no indication of any topographic order in the tecto-LP projection. Electrophysiological methods were also used to map the tecto-LP projection. Multiple stimulating microelectrodes were positioned at physiologically defined sites in the SGS, and single cells were recorded in LP, ipsilaterally. Threshold currents for activation of LP cells from different collicular sites were then compared with the angular separation of SC and LP receptive-field centers. No significant correlation between these two variables was noted, again indicating a lack of topographic organization in the tecto-LP projection. The receptive-field properties of individual LP neurons (n = 211) were also assessed and correlated with subnuclear location and responsivity to SC shocks.(ABSTRACT TRUNCATED AT 400 WORDS)