Creating Equivalence between Bodies

The observation that patients with autotopagnosia, finger agnosia, and defective imitation of meaningless gestures commit errors when trying to replicate the demonstrated body configuration on an external model indicates that their difficulties concern the perception and representation of human bodies in general rather than of only their own bodies. This generality is not surprising if one considers that imitation per se requires recognition of equivalence between one's own body and that of another person. It would be surprising if a representation of body that applies to two arbitrarily paired persons will not be valid for any other instances of human bodies as well.

A feasible way to create equivalence between human bodies would be to code their configurations with reference to conceptual knowledge classifying significant body parts and specifying the boundaries that define them. Application of this knowledge reduces the multiple visual features of a demonstrated gesture to simple relationships between a limited number of significant body parts. Coding a gesture's visual appearance by classification of body parts produces an equivalence between demonstration and imitation that is independent of the particular angle of view under which the demonstration is perceived and also independent of accidental minor differences of the exact shapes of demonstrated and imitated body configurations.

The requirements for body part coding of the demonstrated gesture may be different for proximal body parts and the fingers. There are a considerable number and large diversity of proximal body parts, such as forehead, eyebrows, eyes, nose, cheeks, lips, and chin on the face, or shoulder, upper arm, elbow, lower arm, wrist, back, and palm of the hand on the upper extremity. Knowledge about the classification and boundaries of many of these parts is needed for conceptual mediation of proximally defined body configurations. In contrast, the fingers are a uniform set of only five body parts, and the conceptual distinction between them can be reduced to an appreciation of their serial position. Discrimination of proximal body parts may therefore tax conceptual knowledge about the structure of the human body more than discrimination of fingers. Selection of fingers may pose particular difficulties for visuospatial analysis preceding body part coding. The shapes of fingers two, three, and four are nearly identical, and their identity is mainly determined by their spatial position with respect to the other fingers. A careful analysis of their spatial position is necessary for discrimination. A distinction between the index and the middle finger is likely to put higher demand on visuospatial exploration than a distinction between the lips and the chin.

Left and right brain damage may interfere with imitation at different levels: Right brain damage impairs visual exploration and visual analysis of spatial relationships. It affects the discrimination of the spatial position of fingers but not the recognition of perceptually salient proximal body parts. Left inferior parietal damage abolishes body part coding. Selection of fingers can be spared because it puts lower demands on knowledge about body parts and because a direct mapping between the visuospatial configurations of the model's and one's own fingers can partially circumvent body part coding.

Imitation of body configurations is thus subserved by representations that apply equally to one's own and to other persons' bodies and that are not restricted to only one half of the body. Nonetheless, there is no unique neural substrate for all aspects of body configurations. Although the left hemisphere affords classification of body parts, a right hemisphere contribution is needed for spatial discrimination of perceptually similar body parts. A complete represen tation of the configuration of all parts of the human body requires both aspects and hence involves at least two anatomically separated parts of the brain.

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