Does the world rock when the eyes roll?

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Abstract
When the head is inclined sideways, the eyes are counter-rotated with respect to the head (ocular-counterroll, OCR). In man, the gain of OCR in static body tilts is limited to about 10% of the angle of roll tilt, which suggests that its function is vestigial. However, it is still unclear how the residual OCR is related to the perceived orientation of visual stimuli. Wade and Curthoys (1997) claim that the brain does not “take into account” the OCR, so that the eye position directly interferes with perception of visual orientation. Alternately, it has been argued that OCR is partly compensated by an extraretinal eye-position information such as, e.g., an efference copy ( Haustein, 1992 ; Haustein & Mittelstaedt, 1990 ). The two experiments reported in this study are targeted towards critically examining this inter-relation between OCR and perceived visual orientation. The latter was assessed via the subjective visual vertical, SVV, which is determined when a subject judges the orientation of an indicator (e.g., a short line segment) as apparently vertical. The OCR was measured by using a video-oculographic system. In Experiment 1, a human centrifuge was used to test the effect of an increase of the gravito-inertial force (GIF) on SVV and OCR. Experiment 2 was inspired by the fact that OCR can also be elicited during “barbecue rotation”. Again, it was the aim to compare OCR and SVV in different body positions, such as pure roll and barbecue rotated tilts. The present study provides convincing experimental evidence that SVV is widely uninfluenced by the course of OCR. Increasing the GIF in Experiment 1 had a divergent effect on SVV and OCR; the gain of OCR increases whereas the SVV changed differently, at obtuse tilt angles even in the opposite direction. OCR and SVV were again found to dissociate in Experiment 2, which emphasizes the fact that the SVV and OCR are not controlled by the same neural mechanism, but rather use different spatial reference information.