Differential Contributions of Viewer-centred and Environment-centred Representations in Unilateral Neglect

Unilateral neglect refers to the failure, due to unilateral brain damage, in exploring portion of space controlateral to the side of lesion. It is a largely shared opinion that the deficit could be described with reference to specific spatial co-ordinate systems. Where neglect is referring to the mid-sagittal plane of the patient’s body, it has been defined viewer–centered (or egocentric) and within it a variety of components have been distinguished, such as retinotopic-, head-, limb- and trunk-centered (Vallar, 1998). Whereas were patients show failures in representing stimuli within the extra-personal domain such as the surrounding ‘environment’ and the hand-reach visual space disorder has been specifically defined “allocentric” neglect. Several studies have compared the specific contribution of viewer and allocentric frames of reference on neglect (Calvanio, Petrone, Levine, 1987; Ladavas, 1987; Mennemeir, Chatterjee and Heilman, 1994; Farah, Brunn, Wong, Wallace and Carpenter, 1994, Chatterjee, 1994; Behrmann and Moscovitch, 1994). Some evidence suggests a major contribution of the environment-centered system on neglect (Mennemeir, Chatterjee and Heilman, 1994). Other data support a viewer-centered predominance when the environmental frame is derived exclusively from gravitational information without using visual cues (Karnath, Fetter and Niemeier, 1998). With the current study we wished to learn how neglect allocation is affected by the differential intervention of viewer, environment and array centered spatial representations. Fifteen left-neglect patients were asked to search for visually presented targets. They performed the searching either in an upright position or lying on their right side, in a position orthogonal to the environment. This latter condition was meant to dissociate viewer-centered and environment-centered frames. The visual display, within which stimuli were presented, could be either aligned with the patient’s body or rotated of 90 degrees counterclockwise. Finally, we analyzed the effects of stimulus content on neglect severity. Numbers, letters, and drawings of objects or animals were used as stimuli. Patients’ neglect was found to be allocated mainly with reference to the viewer’s body and, to a lesser degree, to the environment. Moreover, the amount of neglect centered on the environment was found to be enhanced by the alignment of the display with its vertical. We interpreted these data as an evidence that environment-centered neglect may be partially due to a mental rotation of viewer-based representations. This phenomenon would very likely be mediated by environmental visual cues as well as the visual display within which patients are searching for targets. Regarding stimulus material, patients manifested more severe neglect with figures, improved with letters and did even better with numbers. This pattern of results is in line with Weintraub and Mesulam’s (1988) finding of a less severe neglect with ‘verbal’ than with ‘non verbal’ stimuli.