Structure-function relationships in the primate superior colliculus. II. Morphological identity of presaccadic neurons.

1. Microelectrodes filled with horseradish peroxidase (HRP) were inserted in the superior colliculus (SC) of alert squirrel monkeys. Spontaneous eye movements were monitored in the dark during recording and intraaxonal injection of fibers carrying presaccadic signals. 2. Analysis of the relationship between neuronal activity and saccadic parameters indicates that saccade-related neurons can be functionally classified into: 1) vectorial long-lead burst neurons (n = 31), and 2) directional long-lead burst neurons. 3. Vectorial long-lead burst neurons have little if any spontaneous activity and burst intensely before spontaneous saccades within their movement fields with a latency of approximately 20 ms. Their cell bodies were recovered mostly (4/5) in the stratum opticum of the SC. The mediolateral and anteroposterior location of these tectal long-lead burst neurons (TLLBs) together with their movement fields are consistent with existing descriptions of the motor map of the deeper tectal layers. Due to their somatodendritic morphology and pattern of axonal trajectories, TLLBs belong to the T group of tectal efferent neurons that was described in our companion report. Through its branched axonal system each TLLB can relay a signal coding intended eye displacement to reticular targets of the predorsal bundle (PDB), contralateral tectum, ipsilateral mesencephalic reticular formation (MRF), and rostrally located ipsilateral targets of the SC, besides participating in intratectal information processing. 4. Recovered tectal neurons (n = 4) with activity not related to spontaneous saccades participate in the predorsal and ventral ascending tectofugal bundles as well as the projection to the ipsilateral mesencephalic reticular formation. They do not participate in the commissural projection of the SC and need not have recurrent collaterals. Due to their somatodendritic morphology and pattern of axonal trajectories, these cells belong to the X group of tectal efferent neurons that was described in the preceding paper. 5. Recovered cells of origin of directional long-lead burst fibers recorded in the SC (n = 5) are located in the tectorecipient portion of the MRF and their axonal terminals are entirely contained within the SC. The high-frequency portion of the discharge of these reticulotectal long-lead burst neurons (RTLLBs) precedes most contraversive saccades by approximately 19 ms.(ABSTRACT TRUNCATED AT 400 WORDS)