Abstract
Stationary objects are displaced on the retina whenever the eyes or head move but the visual system attempts to correct for the effects of this self-motion to recover spatial coordinates. The accuracy of the compensation can be visualized by moving the target at the time of the saccade or head movement. For example, two dots that are presented at different locations, one before and one after a saccade, are seen to move to within 5% of world coordinates (Cavanagh & Szinte, VSS 2009). We repeat this procedure for two types of head motion: roll around and translation along the visual axis. In the first case, subjects made head rolls of approximately 90° around the visual axis in synchrony with red to green color changes of the fixation spot. Two target dots were presented sequentially, vertically aligned above fixation, one before and one after the head roll. The spatial displacement between the dots was purely vertical whereas the retinal displacement, oriented 85° away from vertical, was almost horizontal. On average, subjects reported a strong oblique component in the perceived motion. In the second case, subjects made large forward and backward translations of the head while fixating and synchronizing their head motions to the red to green color changes of the fixation spot. Two target dots were again presented sequentially above the fixation spot, one before and one after each head translation. The two target dots were displaced horizontally on the screen, but on the retina their displacement was oriented obliquely, 76° away from horizontal. In this case, only small deviations from horizontal were reported. Compensation for head translation therefore appears to be much more accurate than for head roll, implying a difference in the contribution of the otoliths and the semicircular canals to the correction of spatial locations during head movement.
This research was supported by a Chaire d'Excellence grant to PC and a French Ministère de l'Enseignement Supérieur et de la Recherche grant to MS.