Activity of ventroposterior thalamus neurons during rotation and translation in the horizontal plane in the alert squirrel monkey.

The firing behavior of 107 vestibular-sensitive neurons in the ventroposterior thalamus was studied in two alert squirrel monkeys during whole body rotation and translation in the horizontal plane. Vestibular-sensitive neurons were distributed primarily along the anterior and posterior borders of ventroposterior nuclei; three clusters of these neurons could be distinguished based on their location and inputs. Eighty-four neurons responded to rotation; 66 (78%) of them responded to rotation only and 18 (22%) to both rotation and translation. Forty-one neurons were sensitive to linear translation; 23 (56%) of them responded to translation only. The population rotational response to 0.5-Hz sinusoids with a peak velocity of 40 degrees /s showed a gain of 0.23 +/- 0.15 spike.s(-1).deg(-1).s(-1) and phase lagging behind the angular velocity by -9.3 +/- 34.1 degrees . Although rotational response amplitude increased with the stimulus velocity across the range 4-100 degrees /s, the rotational sensitivity decreased with and was inversely proportional to the stimulus velocity. The rotational response amplitude and sensitivity increased with the stimulus frequency across the range 0.2-4.0 Hz. The population response to sinusoidal translation at 0.5 Hz and 0.1 g amplitude had a gain of 111.3 +/- 53.7 spikes.s(-1).g(-1) and lagged behind stimulus acceleration by -71.9 +/- 42.6 degrees . Translational sensitivity decreased as acceleration increased and this was inversely proportional to the square root of the acceleration. Results of this study imply that changes in the discharge rate of vestibular-sensitive thalamic neurons can be approximated using power functions of the angular and linear velocity of spatial motion.