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Saccade-generating burst neurons (BN) are inhibited by omnipause neurons (OPN), except during saccades. OPN activity pauses before saccade onset and resumes at the saccade end. Microstimulation of OPN stops saccades in mid-flight, which shows that OPN can end saccades. However, OPN pause duration does not correlate well with saccade duration, and saccades are normometric after OPN lesions. We tested whether OPN were responsible for stopping saccades both in late-onset Tay-Sachs, which causes premature saccadic termination, and in individuals with cerebellar hypermetria. We studied gaze shifts between two targets at different distances aligned on one eye, which consist of a disjunctive saccade followed by vergence. High-frequency conjugate oscillations during the vergence movements that followed saccades were present in all subjects studied, indicating OPN silence. Thus, mechanisms other than OPN discharge (e.g., cerebellar caudal fastigial nucleus-promoting inhibitory BN discharge) must contribute to saccade termination., (© 2011 New York Academy of Sciences.)

The discharge of neurons in the deeper layers of the superior colliculus (SC) was studied while monkeys performed two visual discrimination tasks that required different amounts of cognitive processing. In a search paradigm the animal's task was to saccade to the location of an odd-colored stimulus located in an array of distractors of uniform color (pop-out visual search). The visual stimuli remained on the screen as the discrimination process distinguished target from distractors. In a choice response task the color of a central cue signaled which stimulus from a previously presented array of colored stimuli was to be the target of a saccade. The stimulus array was turned off well before the central cue was presented. Most neurons showed activity aligned on both the visual input and the motor response in single-target tasks. Many of these same neurons showed additional discharge that was correlated with the required higher-level decision processes in both of these more natural visual tasks. In the case of pop-out search the SC has been shown to be functionally involved in the decision processes. The cue-aligned activity in SC in the choice response task is surprising because no transient visual stimulus appeared in the response field of the neuron.

A long-standing belief holds that the saccadic and smooth-pursuit eye movement systems are composed of largely separate premotor circuits, at least in the brainstem. One crucial prediction predicated on this belief is that the tonic discharge of omnipause neurons (OPNs), which are thought to be part of only the saccadic system, should not be modulated during pursuit eye movements. This report shows that the discharge of OPNs, in contradiction, is modulated downward during pursuit movements. In contrast to their behavior during saccades, where they pause completely for the duration of the movement, the downward modulation during pursuit did not totally silence OPNs. The depth of the downward modulation was correlated with the speed of the ongoing pursuit movement. Another type of cell, which we have named saccade/pursuit neurons, was recorded in the paramedian pontine reticular formation near the location of OPNs. This subpopulation of burst cells discharged a cascade of spikes for saccades in a preferred direction. They also displayed a lower-frequency sustained discharge of spikes for the duration of pursuit in the same preferred direction. These data suggest a new type of combined model for the organization of the brainstem saccade/pursuit system. In this new combined model, the OPNs form a common inhibitory mechanism for both types of movements, and the saccade/pursuit neurons participate in the eye-velocity modulation of OPN discharge or membrane polarization during either type of movement.

Neural studies of oculomotor function in the past have been conducted with the use of very simple visual stimuli. More recently there has been a new emphasis on using more natural stimuli to extend our knowledge of oculomotor organization. Visual search paradigms are an example of the use of these more natural visual surrounds. In search a subject must locate and saccade to a target that appears simultaneously with an array of distractors. When monkeys are used in this paradigm, it is possible to record from neurons located in various central structures in the brain while the initial visual response, subsequent discrimination processes and final saccadic movement unfold. In the present study we used an array of four visual stimuli, and the target was distinguished by its odd color from three distractors of uniform color. Location of the target within the array and its color were randomly selected on each trial. Neurons located in the deeper layers of the superior colliculus (SC) were recorded by standard methods in blocks of search trials. We found several new features in the discharge of SC neurons using this search paradigm that have not previously been reported in studies using single-target visual displays. (1) In contrast to the "winner take all" behavior previously reported for the SC, we found evidence of concurrent processing of alternative movement vectors. When incorrect movements were made to distractor locations, this concurrent activity was associated with significantly shorter intersaccadic intervals. (2) The discharge profile of the visual response in many units was modified by the appearance of a second prominent peak which followed the initial phasic visual response, but which was clearly differentiated from a third burst in activity associated with a saccade into the cell's response field. In some neurons, the activity in this second peak was discriminatory for the impending saccade vector. That is, it was larger when the target appeared in the response field of the cell than when it contained a distractor. This target selection signal was thus distinct from the burst normally associated with saccades into the movement fields of SC neurons. (3) Some saccades in search had a curved trajectory bowing toward the location of a distractor. These saccades were accompanied by an elevated discharge of neurons coding that distractor.