Spatial processing in Bálint syndrome and prosopagnosia: a study of three patients.

BACKGROUND

Spatial analysis may be subdivided into between-object and within-object spatial coding. We investigated the contribution of various visual cues to grouping processes that might determine whether single or multiple objects were perceived and therefore which type of spatial coding would be used for a stimulus.

METHODS

We asked three patients to make shape judgments with a series of displays showing triangular arrangements, moving from more implicit triangles defined by separate objects at the apices (between-object spatial coding) to more explicit triangles with line edges or surface texture (within-object spatial coding).

RESULTS

In two patients with prosopagnosia, within-object spatial judgments were impaired, whereas between-object spatial judgments were normal. In a patient with Bálint syndrome, the reverse pattern was obtained. Surface texture but not outline closure led to mandatory within-object coding in the prosopagnosic patients, whereas outline or surface texture was sufficient to support intact within-object spatial judgments in the patient with Bálint syndrome. Illusory contours were ineffective in promoting within-object coding in either condition.

CONCLUSIONS

These findings support the existence of parallel representations of space for within-object and between-object processing and reveal the efficacy of different cues in determining which representation is potentially accessible.