Effect of amphetamine on optokinetically evoked head and eye movements in the squirrel monkey

The effect of amphetamine sulfate injection on head and eye movements evoked by optokinetic stimulus was studied in squirrel monkeys. No change was found in the gain of optokinetic-oculomotor and optokinetic-caputomotor functions, even after the maximum dose (0.5 mg/kg) of amphetamine. Also, no change in eye-head coordination was found. However, amphetamine facilitated the frequency of head nystagmus, whereas that of eye nystagmus did not change. When the poststimulus responses were studied in the dark, both nystagmoid head movement and optokinetic after eye nystagmus showed first phase and second phase. A different effect was found in the duration of these two phases suggesting the existence of distinct generators. Moreover, the synchronized onsets of head and eye nystagmus in both phases, and synchronized occurrence of quick phases indicated the existence of shared neural circuitry between head and eye movements.