Functional organization of the superior vestibular nucleus of the squirrel monkey.

The response to angular acceleration of units in the superior vestibular nucleus (SVN) of barbiturate-anesthetized, cerebellectomized squirrel monkeys was used to study the distribution of semicircualr-canal inputs to the nucleus. Some so-called intact animals had 6 active semicircular canals. In other animals, the 3 canals on one side were rendered nonresponsive by plugging. In plugged animals, superior, posterior, and horizontal-canal units were encountered on both the plugged and unplugged sides, showing that all 6 canals influence the nucleus. Most units responded bilaterally to labyrinthine polarization; 92.5% of units in intact animals responded to angular acceleration, and this incidence was not decreased in plugged animals. These results suggest that most units in the superior nucleus receive bilateral canal inputs. Convergence of influences arising in orthogonally related canals was detected in less than 10% of units, so the bilateral ampullary influences must arise in parallel canals. Most SVN canal units on the plugged and unplugged sides gave type I responses, indicating that the contralateral canal influence is carried by a crossed inhibitory pathway. Most units influenced by the ipsilateral superior canal were located in the lateral half of the SVN. Posterior-canal units were in the medial half. There was no clear localization of the relatively few horizontal-canal units which were encountered.