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The cortical visuomotor grasping circuit, comprising the anterior intraparietal area (AIP), ventral premotor (PMv), and primary motor cortex (M1) allows transformation of an object's physical properties into a suitable motor command for grasp [1-9]. However, little is known about how AIP contributes to the processing of grasp-related information conveyed through the cortical grasping circuit. We addressed this by studying the consequences of AIP "virtual lesions" on physiological interactions between PMv and M1 at rest or during preparation to grasp objects with either a precision grip or a whole-hand grasp. We used a conditioning-test transcranial magnetic stimulation (TMS) paradigm to test how PMv-M1 interactions [10-12] were modified by disrupting AIP function with theta-burst TMS (cTBS) [13]. At rest, AIP virtual lesions did not modify PMv-M1 interactions. In contrast, the usual muscle-specific PMv-M1 interactions that appeared during grasp preparation were significantly reduced following AIP cTBS without directly modifying corticospinal excitability. Behaviorally, disruption of AIP was also associated with a relative loss of the grasp-specific pattern of digit muscle activity. These findings suggest that grasp-related and muscle-specific PMv-M1 interactions are driven by information about object properties provided by AIP.

Interactions between the ventral premotor (PMv) and the primary motor cortex (M1) are crucial for transforming an object's geometrical properties, such as its size and shape, into a motor command suitable for grasp of the object. Recently, we showed that PMv interacts with M1 in a specific fashion, depending on the hand posture. However, the functional connectivity between PMv and M1 during the preparation of an actual grasp is still unknown. To address this issue, PMv-M1 interactions were tested while subjects were preparing to grasp different visible objects requiring either a precision grip or a whole hand grasp. A conditioning-test transcranial magnetic stimulation (TMS) paradigm was used: a test stimulus was applied over M1 either in isolation or after a conditioning stimulus delivered, at different delays, over the ipsilateral PMv. Motor evoked potentials (MEPs) were recorded in the first dorsal interosseus and abductor digiti minimi muscles, which show highly differentiated activity according to grasp. While subjects prepared to grasp, delivering a conditioning PMv pulse 6 or 8msec before a test pulse over M1 strikingly facilitated MEPs in the specific muscles that were used in the upcoming grasp. This degree of facilitation correlated with the amount of muscle activity used later in the trial to grasp the objects. The present results demonstrate that, during grasp preparation, the PMv-M1 interactions are muscle-specific. PMv appears to process the object geometrical properties relevant for the upcoming grasp, and transmits this information to M1, which in turn generates a motor command appropriate for the grasp. We also reveal that the grasp-specific facilitation resulting from PMv-M1 interactions is differently related to the upcoming grasp muscle activity than is that from paired-pulse stimulation over M1, suggesting that these two TMS paradigms assess the excitability of cortico-cortical pathways devoted to the control of grasp at two different levels.

Chez les rongeurs et les primates non-humains ayant subi une lésion cérébrale ou spinale, il a été démontré que la réorganisation des aires motrices corticales et des projections corticospinales (CS) sous-tendait la récupération de la dextérité. Chez les patients souffrant d'hémiplégie congénitale (HC) due à une lésion cérébrale prénatale ou périnatale, l'importance de la récupération motrice couvre un large spectre, particulièrement en ce qui concerne les mouvements précis des doigts et la dextérité. Tant les méthodes d'électrophysiologie et d'imagerie fonctionnelle telles que stimulation magnétique transcrânienne (TMS), tomographie par émission de positron (PET) et imagerie par résonance magnétique fonctionnelle (fMRI) ont révélé différentes modalités de réorganisation des aires motrices et des projections CS chez les HC. Cependant, des corrélations précises entre ces modalités de réorganisations et le niveau de dextérité résiduelle doivent encore être établies. Ce travail visait deux objectifs principaux : 1) caractériser les modalités de réorganisation des aires motrices corticales et des projections CS dans l'HC au moyen de méthodes complémentaires d'imagerie fonctionnelle et d'électrophysiologie (PET, fMRI et TMS) et 2) établir des corrélations entre ces modalités de réorganisation et la dextérité résiduelle évaluée par des index quantitatifs. Contrairement au diaschisis (hypométabolisme) retrouvé chez l'adulte après un accident vasculaire cérébral sous-cortical, un hypermétabolisme de repos a été mis en évidence au moyen du PET dans le cortex moteur des deux hémisphères chez des HC avec lésion sous-corticale, suggérant un accroissement de la densité et/ou de l'activité synaptique. Chez les HC avec une lésion corticale, l'intensité de l'activation en fMRI dans l'hémisphère épargné était positivement corrélée avec la dextéritérésiduelle de la main parétique, et les mouvements de la main tant parétique que non-parétique activaient de façon prédominante l'hémisph, (SBIM 3)--UCL, 2003