The prefrontal corticotectal projection in the monkey; An anterograde and retrograde horseradish peroxidase study

Abstract

The prefrontal corticotectal projection was studied in macaque and cebus monkeys using the horseradish peroxidase technique. Following stereotaxic horseradish peroxidase injections into the superior colliculus, large numbers of retrogradely-labelled corticotectal neurons were observed in the prearcuate frontal eye field cortex, dorsal and ventral convexity and sulcus principalis cortex, as well as on the medial aspect of the prefrontal cortex. These experiments, taken together with anterograde data derived from other cases in which horseradish peroxidase gel implants were made in various subregions of the granular prefrontal cortex, suggested a topographic organization of prefrontal projections to the tectum. Area 8 (frontal eye field) primarily projected to the stratum intermedium. The dorsal sulcus principalis cortex projected most heavily to the deeper stratum intermedium, stratum profundum and central gray matter, and gave rise to a characteristic line of puffs in the dorsal stratum intermedium. The rostral dorsal convexity and medial prefrontal cortex projected primarily to the stratum profundum and central gray matter. The orbitofrontal cortex lacked projections to the superior colliculus and only contained retrogradely-labelled neurons when the injection spread into the central gray matter. It was also observed that prefrontal corticotectal fibers followed 2 divergent routes to the superior colliculus: the classically-described ‘pedunculo-tegmental’ route, and a ‘transthalamic’ pathway. This more detailed description of a major cortical input to the tectum not only contributes to our understanding of the functional organization of both the granular frontal cortex and the colliculus, but suggests anatomical connectivity which may underlie their roles in oculomotor mechanisms.