Optokinetic Responsiveness In The Macaque Parieto-insular Vestibular Cortex (PIVC)

The parieto-insular vestibular cortex (PIVC) has been suggested to "monitor the movement of the head in space" based on convergence of vestibular, visual (optokinetic) and other signals. However, previous studies have shown that PIVC neurons did not respond to optic flow stimulation. To examine the optokinetic-vestibular convergence in PIVC, we recorded from parieto-insular vestibular cortex (PIVC) neurons during sinusoidal three-dimensional (3D) vestibular and optokinetic stimulation (OKS). Sinusoidal 3D motion was delivered using a six degree of freedom motion platform. The optokinetic stimuli simulated yaw, pitch, and roll rotation inside a sphere whose inner surface consisted of spots of varying sizes.The majority of neurons (83.6%) in PIVC responded during both rotational and translational motion. PIVC neurons were also tested during OKS while monkey suppressed their optokinetic nystagmus by fixating a head-fixed target. Only a handful of cells (13.1%) modulated during 0.5-Hz OKS suppression, but the number of responsive cells (23.6%) increased during 0.5-Hz OKS (no suppresion), the number of responsive cells (31.5%) increased furthur during 0.02-Hz OKS. Meanwhile, we compared the nystagmus elicited and the responses of PIVC neurons during the different OKS.Besides, we examined the preferred direction of PIVC neurons to rotation vestibular and optokinetic stimulation(OKS). Preferred directions for rotation and OKS were not matched on individual neurons, and OKS gains were smaller than the respective gains during rotation. We conclude that optokinetic-vestibular convergence might not be as prevalent as suggested.