Identifying objects in conventional and contorted poses: contributions of hemisphere-specific mechanisms

Three experiments were designed to rest the hypothesis that different mecha nisms are used to encode objects seen in unfamiliar contortions than are us ed to encode objects seen in conventional poses. When a familiar non-rigid form (e.g. an animal) is seen in a contorted pose, we hypothesize that obje ct identification may be achieved by (1) encoding the object's parts separa tely, (2) encoding the spatial relations among the parts, and (3) matching these encodings to a stored structural description. However, once this form has become familiar, its global shape can be directly matched to informati on stored in memory. Based on the idea that 'categorical' spatial relations are encoded better by the left cerebral hemisphere and are used in structu ral descriptions, we predicted a left-hemisphere advantage when one first e ncodes contorted poses; in contrast, based on the idea that overall shapes are encoded better by the right hemisphere, we predicted a right-hemisphere advantage for encoding the same shapes after they are familiar. Three expe riments confirmed these predictions, which supports the hypotheses that dif ferent visual mechanisms operate in the recognition of familiar and unfamil iar views of known non-rigid objects. Moreover, correlational analyses betw een visual-field differences in several perceptual tasks (matching whole pi ctures to names, body parts to the whole body, and judging categorical spat ial relations) revealed that the degree and lateralization of categorical s patial encoding predicts the left hemisphere's initial advantage in the ide ntification of contorted shapes. (C) 1999 Elsevier Science B.V. All rights reserved.