Differential coding of head rotation by lateral-vertical canal convergent central vestibular neurons

Convergent inputs from the lateral and vertical semicircular canals (LC and VC) to 31 central vestibular-only (VO) and vestibular-plus-saccade (VPS) neurons were determined by oscillating monkeys about a spatial vertical axis while the head was tilted forward and backward up to 90°. Activity of each neuron varied as a function of head tilt. Seven neurons had maximal activation when the head was tilted ≈30° forward (spatial phase), indicating convergent inputs from the LC, while peak activation of 10 units occurred with the head tilted back ≈50°, indicating VC input. Fourteen neurons had spatial phases that deviated more that 15° from the LC and VC planes, indicating convergent inputs from LC and VC. Seven of these neurons had a spatial phase less than 15° forward and 35° back, indicating canal inputs from both sides. Seven other neurons had spatial phases more that 45° forward and 65° back, indicating inputs from canals located on the same side. Thus, there are two groups of central vestibular neurons: one group responds maximally when the head is rotated about a spatial vertical axis in an upright position, declining as the head is tilted away from this position. Another group responds minimally to rotation in an upright head orientation, increasing as the head is tilted away from the upright. A majority of the cells also had convergent otolith input. The otolith and canal inputs superposed when the animals were rotated about roll and pitch axes from an upright position. This insured that these neurons would respond over a broad frequency range from very low to high frequencies.