Projections from cortex to tectum in the tree shrew, Tupaia glis.

Sensory neocortex of the tree shrew was divided into three main areas: the visual field, the auditory field, and the somatic field which includes motor cortex. Cortical cells which project to the tectum were identified by injecting HRP into superficial or deep layers of the superior colliculus and into various parts of the inferior colliculus. The main result is that these descending projections are well organized according to their origin in the three main sensory fields of the cortex. (1) Auditory field: labeled cells are found only in the core or auditory koniocortex, after injections of HRP in the pericentral area of the inferior colliculus; labeled cells are found in auditory belt areas after injections in posterior parts of the intermediate and deep layers of the superior colliculus, adjacent to the inferior colliculus. (2) Somatic field: labeled cells are also found in the somatic field after injections in the intermediate and deep layers of the superior colliculus, so that auditory and somatic fields probably overlap to some extent. The results do not exclude the possibility that somatic koniocortex has an exclusive target in the intermediate or deep layers of the superior colliculus. (3) Visual field: labeled cells are found only in the core or striate cortex after injections in the superficial layers of the superior colliculus. Labeled cells are found in the visual belt after injections in the rostral parts of the intermediate layers of the superior colliculus. When these results are related to ascending sensory pathways a picture emerges of a series of circuits or loops which interconnect parallel sensory pathways. These loops eventually reach the deep layers of the superior colliculus which of course have indirect access to motor neurons.