Pre-motor single unit activity in the monkey brain stem correlated with eye velocity during pursuit

Among the various classes of supra-nuclear neurons in the oculomotor system of the monkey which have been associated with the generation of saccadic or vestibularly-induced eye movements (EM), a class specifically concerned with the control of pursuit eye movements (PEM) has not yet been described3-L In this paper we describe some features of single neurons, the activity of which is correlated with eye movements specifically during PEM in a way that one would expect of neurons linking the visual to the oculomotor system. The techniques used in this study are identical with the ones described recently 2. In brief: single unit recordings were performed by means of tungsten microelectrodes in 3 monkeys (2 male Macaca fascicularis and 1 female Macaca mulatta) of 3-4 kg weight. A head gear and lucite chamber were implanted over a trephine hole in the vertex of the skull. In one animal EM were measured as horizontal and vertical DCelectro-oculograms using surface electrodes (Hellige). The other two monkeys had Ag-AgC1 electrodes (In Vivo Metric Systems, Healdsburg, California) implanted into the bone of the outer canthi of the orbits and between both eyes, so that horizontal EM of either eye could be measured independently, with very small drift (typically less than l°/min) and a gain of about 17 #V/degree. All 3 monkeys were trained to pursue a light spot with a diameter of 4 min arc in the horizontal plane at a distance of 150 cm. All relevant signals were recorded on instrumentation tape and in parallel on a jet ink writer (Siemens Mingograf 800). Both recording systems, including the pre-amplifiers, had cut-off frequencies above 1 kHz. For further analysis the impulse trains of single neurons were converted into time function of the instantaneous impulse rate IR (t). The results are based on the analysis of 42 neurons 3. They appear to form a class which has not been described so far. These neurons were clearly distinguishable from motoneurons in that their IR was typically independent of the eye position. The