Pigs in space¹: How we recognize rotated objects

As organisms that move freely in space, we are adept at visually recognising objects regardless of their orientations. This facility probably does not depend on a correction mechanism, such as mental rotation, that might render an object in some canonical orientation. Rather, it is likely that an orientation-free description is extracted, probably by the ventral visual system. This suggests further that we can recognize the identity of an object before we can determine its orientation in space. This may depend in turn on the integration of shape information extracted by the ventral system with information about the space-occupying property of the object extracted by the dorsal system. The dissociation between identity and orientation may explain cases of orientation agnosia, in which the patient can recognize common objects but cannot determine their orientations. Orientation-free descriptions are nevertheless relatively crude. In order to distinguish between shapes that differ in more subtle ways, such as individual faces, or between shapes that are mirror-images of one another, a correction may be necessary, through either physical or mental rotation to the upright.