Right hemisphere damage in humans often leads to unilateral spatial neglect and/or extinction, a condition in which patients do not orient to, and may even deny the existence of, the left side of space. The syndrome has been most often associated with dorsal right posterior lesions. Consistently, when both parietal lobes are damaged (causing Balint's syndrome), external space nearly disappears entirely except for the perception of one object at any given time (simultanagnosia). Despite the often severe spatial blindness that accompanies these lesions, stimuli presented in affected areas of space can be implicitly represented (i.e., they affect performance even when undetected). A common interpretation is that the representation is intact and all that is needed is a boost above some threshold. This idea had been questioned by evidence showing that features are not integrated correctly in such cases, producing illusory conjunctions in free vision. The features that were tested were those that have been associated with specialized neural populations such as color, motion, shape and size. However, these findings only tested explicit reports. We propose a set of psychophysical studies designed to explore the extent to which features are integrated in implicit representations and what the spatial structure of these representations might be. Emphasis will be on patients who have spatial deficits due to unilateral stroke. Some questions to be addressed are: Are undetected stimuli integrated before attentional selection occurs? What is the spatial structure of implicit representations (e.g., are they 3-dimensional)? Do features of neglected stimuli contribute to first impressions of displays that are explicitly perceived? The findings should enhance knowledge of visual information that is processed before awareness in normal perception as well as how this information may influence what is explicitly seen. It will also increase knowledge about visual representations that are intact but unavailable in visual spatial disorders.
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