Visual information about object size and object position are retained differently in the visual brain: Evidence from grasping studies.

摘要

Many recent studies have focused on the question of whether or not visually-guided and memory-guided hand movements rely on dissociable visual representations that are processed in anatomically different brain areas (dorsal vs. ventral). However, little attention has been paid to the issue of which aspects of the visual information decay over time and how decay functions differ for relevant visual features. In three experiments, we investigated whether the representation of object position (guiding hand transport) decays more rapidly than the representation of object size (guiding grasp pre-shaping). Using a mirror setup, we dissociated information about object position and size. In Experiment 1, we presented objects at different distances to the observer. After varying pre-response delays that increased memory demands successively, participants pointed to the remembered object position. We found that distance errors increased exponentially with increasing memory demands, indicating that visual information about object position decays rapidly as soon as vision is occluded. In Experiment 2, we investigated grasping movements to differently sized objects while providing information about their position (using a landmark) in the same vision conditions. We found that grasp pre-shaping remained constant and well-adjusted to object size even after long delays, suggesting that object size is encoded in a relatively stable manner. This was confirmed in Experiment 3 in which we measured pre-liftoff fingertip force rates as an indicator of remembered object size that is unaffected by object position. Fingertip force rates were similar across all delay conditions, further highlighting the robustness of the object size representation. Our findings suggest that visual information about object positon and object size are stored independently, and that larger grip apertures typically seen in delayed grasping tasks are likely to reflect an increased uncertainty about object position rather than object size (larger safety margin compensating for increased reaching errors).