Vestibular-related neuronal activity in the thalamus of the alert monkey during sinusoidal rotation in the dark.

1. In the alert monkey neuronal activity was recorded in the ventro-posterior nucleus (VP) of the thalamus in the dark during sinusoidal rotation over a frequency range from 0.01-1 Hz. 2. From 57 neurons 38 (67%) were activated with rotation to the ipsilateral side (type I) and 19 (33%) to the contralateral side (type II). The spontaneous activity was low (average 10.1 imp/sec) and irregular. No activity changes were found with eye movements. 3. At 0.2-0.1 Hz neuronal activity showed a phase lead of 10-20 degrees relative to chair velocity. At the lowest frequency (0.01 Hz) the phase lead was only slightly higher (about 30 degrees). Accordingly the decrease in gain was only moderate. 4. At lower frequencies the simultaneously recorded eye movements (nystagmus) showed an increase in phase lead comparable to the values for the neuronal activity in the thalamus. For both neuronal activity in the thalamus and nystagmus a time constant between 25-35 sec was calculated. 5. The data are compared with vestibular nerve and nuclei recordings. It is argued that the time constants of vestibular neurons in the thalamus are very similar to the time constants of neurons in the vestibular nuclei in alert animals.