TILT PERCEPTION DURING DYNAMIC LINEAR ACCELERATION

Head tilt is a rotation of the head relative to gravity, as exemplifie d by head roll or pitch from the natural upright orientation. Tilt sti mulates both the otolith organs, owing to shifts in gravitational orie ntation, and the semicircular canals in response to head rotation, whi ch in turn drive a variety of behavioral and perceptual responses. Stu dies of tilt perception typically have not adequately isolated otolith and canal inputs or their dynamic contributions. True tilt cannot rea dily dissociate otolith from canal influences. Alternatively, centrifu gation generates centripetal accelerations that simulate tilt, but sti ll entails a rotatory (canal) stimulus during important periods of the stimulus profiles. We reevaluated the perception of head tilt in huma ns, but limited the stimulus to linear forces alone, thus isolating th e influence of otolith inputs. This was accomplished by employing a ce ntrifugation technique with a variable-radius spinning sled. This allo wed us to accelerate the sled to a constant angular velocity (128 degr ees/s), with the subject centered, and then apply dynamic centripetal accelerations after all rotatory perceptions were extinguished. These stimuli were presented in the subjects' naso-occipital axis by transla ting the subjects 50 cm eccentrically either forward or backward. Cent ripetal accelerations were thus induced (0.25 g), which combined with gravity to yield a dynamically shifting gravito-inertial force simulat ing pitch-tilt, but without actually rotating the head. A magnitude-es timation task was employed to characterize the dynamic perception of p itch-tilt. Tilt perception responded sluggishly to linear acceleration , typically reaching a peak after 10-30 s. Tilt perception also displa yed an adaptation phenomenon. Adaptation was manifested as a per-stimu lus decline in perceived tilt during prolonged stimulation and a rever sal aftereffect upon return to zero acceleration (i.e., recentering th e subject). We conclude that otolith inputs can produce tilt perceptio n in the absence of canal stimulation, and that this perception is sub ject to an adaptation phenomenon and low-pass filtering of its otolith input.