Responses of neurons of the nucleus of the optic tract and the dorsal terminal nucleus of the accessory
optic tract in the awake monkey
U. J. Ilg & K.-P. Hoffmann
European Journal of Neuroscience, 8:92-105, 1996
- The nucleus of the optic tract (NOT) and the dorsal terminal nucleus of the accessory optic tract (DTN) are
essential nuclei for the generation of slow-phase eye movements during horizontal optokinetic nystagmus. We recorded
from 101 neurons (all directionally selective) in four NOT/DTN of three trained and behaving rhesus monkeys. Neuronal
activity increased when stimuli moved ipsiversively with respect to the recording site and decreased below spontaneous
activity when stimuli moved contraversively. While the monkey fixated a small spot, some NOT/DTN neurons did not
respond at all to the retinal image slip of a whole-field random dot pattern; others showed a monotonic increase
of activity to increasing velocities of that stimulus. The velocity range tested was up to 100 degrees/s. During
the execution of optokinetic nystagmus, 39 of 73 cells tested showed a velocity-tuned response with an average
optimum at 21 degrees/s retinal image slip. Following saccades during optokinetic nystagmus (quick phases), the
NOT/DTN neuronal activity briefly attained the level of spontaneous activity, as predicted from the velocity selectivity
during optokinetic nystagmus. Immediately upon cessation of optokinetic stimulation in the preferred direction,
NOT/DTN activity returned to the spontaneous level and did not reflect the ongoing optokinetic afternystagmus in
darkness. Most NOT/DTN neurons displayed direction selectivity also during smooth pursuit. Twenty-one of 50 cells
tested (42%) always responded to the retinal slip of the target (target velocity cells), 16 cells (32%) responded
to the retinal slip of the background (background velocity cells), and 13 cells (26%) did not respond at all during
smooth pursuit. We conclude from our results that the NOT/DTN is an essential structure for the processing of the
direction and speed of retinal image slip. This information is then used for the generation and maintenance of
slow eye movements, preferentially during horizontal optokinetic nystagmus but also during pursuit eye movements.
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