Spatial alignment of rotational and static tilt responses of vestibulospinal neurons in the cat

The responses of vestibulospinal neurons to 0.5-Hz, whole-body rotations in three-dimensional space and static tilts of whole-body position were studied in decerebrate and alert cats. The neurons' spatial properties for earth-vertical rotations were characterized by maximum and minimum sensitivity vectors (R(max) and R(min)) in the cat's horizontal plane. The orientation of a neuron's R(max) was not consistently related to the orientation of its maximum sensitivity vector for static tilts (T(max)). The angular difference between R (max) and T(max) was widely distributed between 0°and 150°, and R(max) and T(max) were aligned (i.e., within 45°of each other) for only 44% (14/32) of the neurons. The alignment of R(max) and T(max) was not correlated with the neuron's sensitivity to earth-horizontal rotations, or to the orientation of R(max) in the horizontal plane. In addition, the extent to which a neuron exhibited spatiotemporal convergent (STC) behavior in response to vertical rotations was independent of the angular difference between R(max) and T(max). This suggests that the high incidence of STC responses in our sample (56%) reflects not only canal- otolith convergence, but also the presence of static and dynamic otolith inputs with misaligned directionality. The responses of vestibulospinal neurons reflect a complex combination of static and dynamic vestibular inputs that may be required by postural reflexes that vary depending on head, trunk, and limb orientation, or on the frequency of stimulation.