Interactions between ego- and allocentric neuronal representations of space

In the primate brain, visual spatial representations express distances of objects with regard to different references. In the parietal cortex, distances are thought to be represented with respect to the body (egocentric representation) and in superior temporal cortices with respect to other objects, independent of the observer (allocentric representation). However, these representations of space are interdependent, complicating such distinctions. Specifically, an object's position within a background frame strongly biases egocentric position location judgments. This bias, however, is absent for pointing movements towards that same object. More recent theories state that dorsal parietal spatial representations subserve visuomotor processing, whereas temporal lobe representations subserve memory and cognition. Therefore, it may be hypothesized that parietal egocentric representations, responsible for movement control, are not influenced by irrelevant allocentric cues, whereas ventral representations are. In an event-related functional magnetic resonance imaging study, subjects judged target bar locations relative to their body (egocentric task) or a background bar (allocentric task). Activity in the superior parietal lobule (SPL) was shown to increase during egocentric judgments, but not during allocentric judgments. The superior temporal gyros (STG) shows a negative BOLD response during allocentric judgments and no activation during egocentric judgments. During egocentric judgments, the irrelevant background influenced activity in the posterior commissure and the medial temporal gyros. SPL activity was unaffected by the irrelevant background during egocentric judgments. Sensitivity to spatial perceptual biases is apparently limited to occipito-temporal areas, subserving the observed biased cognitive reports of location, and is not found in parietal areas, subserving unbiased goal-directed actions. (c) 2005 Elsevier Inc. All rights reserved.