The interstitial nucleus of Cajal (NIC) is the largest and most prominent of the cell groups of the medial longitudinal fasciculus (MLF). It contains at least two distinct cell classes, namely large pyramidal or multipolar neurons and small- to medium-size pyramidal, fusiform, or round cells (Zuk et al. 1982). Several lines of evidence have implicated the NIC in oculomotor control and in particular in the process of velocity to position integration in the vertical plane (for a review, see Moschovakis 1997). First, lesions of the NIC prevent monkeys and cats from holding eccentric gaze and impair their vertical vestibuloocular responses (Anderson et al. 1979; Crawford et al. 1991; Fukushima et al. 1992; Helmchen et al. 1998). Second, upward and downward medium lead burst neurons terminate profusely within the NIC (Moschovakis et al. 1990, 1991a,b). Finally, the discharge of NIC neurons often encodes the vertical position of the eyes (Fukushima et al. 1990; King and Leigh 1982; King et al. 1981).
Three projection systems are known to arise from the NIC (Kokkoroyannis et al. 1996): one directed through the posterior commissure that deploys dense terminal fields in the contralateral NIC, the oculomotor nucleus, and the trochlear nucleus, a descending system that deploys terminal fields in ipsilateral pontine and medullary nuclei and the ventral horn of cervical spinal segments, and an ascending system that deploys terminal fields in ipsilateral mesencephalic and diencephalic structures. The integrity of the first is crucial for normal velocity to position integration in the vertical plane because after lesions of the posterior commissure, the eyes can no longer be held at eccentric vertical eye positions (up or down) and the gain of the vertical vestibuloocular response (VOR) is reduced and its phase advanced, particularly at lower stimulation frequencies (Partsalis et al. 1994). If the vertical eye-position-related NIC signals are sent to the oculomotor complex via the posterior commissure, it should be possible to demonstrate the existence of fibers that originate in the NIC, course through the posterior commissure, and discharge in relation to vertical eye position. To test this, we studied the signals carried by oculomotor-related fibers in and near the posterior commissure of alert behaving squirrel monkeys and then injected these same fibers with a tracer to establish the location of their cell bodies and the targets of their axons. Here we describe the signals that were carried by such fibers. Preliminary versions of some of our results have appeared before (Moschovakis 1995; Moschovakis et al. 1997).