Effect of eye position during human visual-vestibular integration of heading perception

Visual and inertial stimuli provide heading discrimination cues. Integration of these multisensory stimuli has been demonstrated to depend on their relative reliability. However, the reference frame of visual stimuli is eye centered while inertial is head centered and it remains unclear how these are reconciled with combined stimuli. Seven human subjects completed a heading discrimination task consisting of a 2s translation with a peak velocity of 16 cm/s. Eye position was varied between 0°, and ±25° left/right. Experiments were done using inertial motion, visual motion, or a combined visual-inertial motion. Visual motion coherence varied between 35-100%. Subjects reported if their perceived heading was left or right of the midline using a forced choice task. With the inertial stimulus the eye position had an effect such that the point of subjective equality (PSE) shifted 4.6 ± 2.4° in the gaze direction. With the visual stimulus the PSE shift was 10.2 ± 2.2° opposite the gaze direction consistent with retinotopic coordinates. Thus with eccentric eye positions, the perceived inertial and visual headings were offset about 15°. During the visual-inertial conditions the PSE varied consistently with the relative reliability of these stimuli such that at low visual coherence the PSE was similar to that of the inertial stimulus and at high coherence it was closer to the visual stimulus. On average, the inertial stimulus was weighted near Bayesian ideal predictions, but there was significant deviation from ideal in individual subjects. These findings support visual and inertia cue integration occurring in independent coordinate systems.

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