A neuropsychological and functional brain imaging study of visuo-imitative apraxia

We describe the case of a 58-years-old right-handed women suffering from an occipital-parietal lesion. The administration of a cognitively based assessment tool for limb praxis (Batterie d'Evaluation des Praxies, B.E.P., Peigneux and Van der Linden, 1998) demonstrated bilateral visuo-imitative apraxia. Gesture production was mainly characterised by spatial, errors, and imitation of meaningful gestures was worse than their pantomime on verbal command. Moreover, the imitation of meaningless gestures and their reproduction on a manikin were worse than imitation of their matched meaningful gestures. In a cognitive perspective, adapted from the Rothi et al. (1997) and Goldenberg (1995) contributions to our understanding of limb praxis, this configuration of performance suggests deficits occurring at multiple levels. On one hand, it suggests either access difficulties or alteration of the output praxicon, i.e., the lexicon for visuo-kinesthetic engrams of meaningful gestures. On the other hand, the simultaneous deficit for meaningless gesture reproduction on the subject's own body and on a manikin favors an alteration of the structural descriptions of the human body (i.e., human body knowledge), underlying the mental transposition processes occurring between the visual analysis of a meaningless gestural configuration and its effective reproduction on oneself or on a manikin, thus contradicting the classic view of a direct pathway linking visual analysis and motor planning in meaningless gesture imitation. Finally, due to the output praxicon deficit, imitation of meaningful gestures is partly processed in the same way as meaningless gestures (also impaired in this case), leading to an interference effect between both degraded memory-based and visually-transposed traces, which account for imitation of meaningful gestures being worse than their pantomime on verbal command. We also assess regional cerebral metabolism using positron emission tomography (PET). Comparison with 41 healthy subjects (SPM96) demonstrated a statistically significant hypometabolism in the left intraparietal sulcus and superior parietal lobule, and in the right dorsal prestriate cortex. These results, together with a review of the other studies of visuo-imitative apraxia, suggest that the left intraparietal sulcus may be associated with access or integration of information from the output praxicon. The left superior parietal and the right dorsal prestriate deficits functionally impaired a bilateral dorsal network implied in the mental transformations of the body, thus suggesting that these mental transformations are underlined by knowledge of the human body, which may subsequently explain the deficit for the reproduction of meaningless and meaningful configurations.Nous rapportons le cas d'une patiente présentant une apraxie visuo-imitative à la suite d'une lésion occipito-pariétale bilatérale. La production de gestes était caractérisée par des erreurs de transposition spatiale du mouvement, avec une dégradation des performances entre l'exécution sur commande verbale et l'imitation des mêmes gestes significatifs. Une altération supplémentaire a été observée lors de l'imitation de mouvements appariés sans signification et de leur reproduction sur un mannequin, et la reconnaissance de gestes était normale. Dans une perspective cognitive adaptée des apports de Rothi et al. (1997) et Goldenberg (1995), ce profil de performances suggère l'altération (ou un accès défectueux) du lexique de sortie des gestes significatifs, en parallèle à une atteinte des descriptions structurales du corps humain nécessaires à la reproduction de configurations gestuelles sans signification. Une analyse du métabolisme régional cérébral évalué par tomographie à émission de positons suggère que les difficultés d'activation du lexique de sortie des gestes pourraient être liées à l'atteinte du sillon intrapariétal dans l'hémisphère gauche, tandis que les déficits d'imitation de gestes seraient sous-tendus par une lésion touchant bilatéralement le système pariétal dorsal en charge des transformations du geste reposant sur des descriptions structurales du corps humain