Visuospatial Memory Computations During Whole-Body Rotations in Roll

1 Nijmegen Institute for Cognition and Information, 2 Department of Biophysics, and 3 FC Donders Centre for Cognitive Neuroimaging, Radboud University Nijmegen, Nijmegen, The Netherlands Submitted 7 January 2005; accepted in final form 22 April 2005 We used a memory-saccade task to test whether the location of a target, briefly presented before a whole-body rotation in roll, is stored in egocentric or in allocentric coordinates. To make this distinction, we exploited the fact that subjects, when tilted sideways in darkness, make systematic errors when indicating the direction of gravity (an allocentric task) even though they have a veridical percept of their self-orientation in space. We hypothesized that if spatial memory is coded allocentrically, these distortions affect the coding of remembered targets and their readout after a body rotation. Alternatively, if coding is egocentric, updating for body rotation becomes essential and errors in performance should be related to the amount of intervening rotation. Subjects ( n = 6) were tested making saccades to remembered world-fixed targets after passive body tilts. Initial and final tilt angle ranged between –120° CCW and 120° CW. The results showed that subjects made large systematic directional errors in their saccades (up to 90°). These errors did not occur in the absence of intervening body rotation, ruling out a memory degradation effect. Regression analysis showed that the errors were closely related to the amount of subjective allocentric distortion at both the initial and final tilt angle, rather than to the amount of intervening rotation. We conclude that the brain uses an allocentric reference frame, possibly gravity-based, to code visuospatial memories during whole-body tilts. This supports the notion that the brain can define information in multiple frames of reference, depending on sensory inputs and task demands. Address for reprint requests and other correspondence: S. Van Pelt, Nijmegen Institute for Cognition and Information, Radboud University Nijmegen, PO Box 9104, 6500 HE Nijmegen, The Netherlands (E-mail: s.vanpelt{at}nici.ru.nl )