The effect of head orientation on the vestibular evoked potentials to linear acceleration impulses in rats.

Objective: To investigate the influence of linear acceleration impulses delivered when the head is held in different static head orientations, on the first wave of the short latency vestibular evoked potential (VsEP). The first wave is the compound action potential of the primary vestibular neurons synchronously activated.

Background: It has been shown previously that the VsEP elicited in response to linear acceleration is initiated mainly in the otolith organs. These organs are responsive to both dynamic and static linear forces, including gravity.

Methods: VsEPs to linear acceleration stimuli (4g) were recorded when the rats head was oriented so that a) the plane of the utricular macula was aligned with the plane of the stimulus, b) in supine position and c) with the head pitched up and down in various angles with respect to gravity (stimulus-head spatial relation remained constant) as compared to a reference position.

Results: With the stimulus aligned with the plane of the utricular macula, the amplitude of the first wave of the L-VsEPs was significantly larger than in the reference position. In the supine position, the amplitude of the first wave was significantly larger and the latency was significantly shorter. The amplitude of the first VsEP wave tended to be larger in the "head up" orientations as compared to the "head down" orientations (not statistically significant).

Conclusions: These results demonstrate the influence of head position and gravity on the VsEPs to linear acceleration impulses, which is in accordance with their otolithic origin.