Vestibular response kinematics in posterior parietal cortex neurons of macaque monkeys.

Perception of extrapersonal space is a fundamental requirement for accurate interaction with the environment and moving in it. Parietal cortical areas are thought to play an important role in this function. A significant sensory input to this area arrives from the vestibular system. We quantified neuronal responses in the ventral intraparietal area and the medial intraparietal area of awake head-fixed macaque monkeys during classical vestibular sinusoidal stimulation protocols and with a newly developed random vestibular testing paradigm. The goal was to study more specifically the signal content of parietal vestibular neurons with respect to head movement kinematics. Traditional sinusoidal stimulation analysis revealed that about one-third of the neurons responded in phase with either head position or head acceleration, besides classical head velocity tuning. Random vestibular stimulation revealed more complex signal profiles in the majority of neurons, although quantification of the kinematic variables that drove the neurons most effectively led to similar results to phase shift analysis. Thus, a majority of cells was principally driven by head velocity, and a minority by either acceleration or position. Nevertheless, random stimulation also revealed the simultaneous presence of all three kinematic response parameters (i.e. velocity, position and acceleration) in a majority of neurons. A minority of cells coded only two kinematic variables, i.e. head velocity coupled with either acceleration or position. Neurons coding only one kinematic variable were not found. We hereby demonstrate for the first time that central vestibular neurons carry several head movement kinematic variables simultaneously.