Within-object and between-object coding deficits in drawing production

We examine how perceptual grouping influenced drawing errors in two patients with constructional apraxia. The patients copied simple geometric stimuli, formed from right-angled elements of variable size arranged as the corners of a square. Grouping was systematically manipulated by altering the closure between these local parts. The patients also made perceptual judgments about similar figures. Despite relatively intact perceptual discrimination of the stimuli, both patients produced errors in drawings relative to controls. When local elements were arranged as a square, patient ECR produced some shape errors but showed a marked tendency to complete across gaps between elements. In contrast, patient RA made no such completion errors but placed the elements so that they were poorly spatially located. When grouping was reduced, ECR completed fewer gaps and RA showed improved spatial localization. These symptoms suggest impairments of perceptuomotor representations of object relations. ECR's deficit appeared to be in between-object coding - she often failed to separate the individual elements when they formed a strong perceptual whole. In contrast, RA's deficit appeared to be in within-object coding - spatial localization of elements was impaired when they shared strong grouping cues. His performance only improved when the strength of the grouping between the elements was reduced. These findings demonstrate the coding of different forms of spatial relation in the perceptual-motor domain.