1. Causal role of parietofrontal areas in precision grasping
When reaching for an object, one crucial step is to adjust the hand posture to the object features (e.g. size & shape). Both electrophysiological studies in monkeys and functional brain imaging in humans have shown that a cortical circuit including the anterior part of the intraparietal sulcus (AIP) connected to PMv is responsible for processing the objet visual properties. I investigated the role of AIP and PMv in humans by inducing a transient virtual lesion of either area in healthy subjects performing a standard grip-lift task (Davare et al., JNeurosci 2006 and 2007). I have shown that a bilateral lesion of AIP was necessary to alter the preshaping of either hand, suggesting that, in humans, both AIPs contribute to hand shaping, irrespective of the hand used. In contrast to AIP, a unilateral lesion of either PMv was found sufficient to impair selectively the preshaping of the right dominant hand, indicating that both PMvs are necessary to perform the visuomotor transformations related to an objectʼs intrinsic features (Davare et al., JNeurosci 2006).
Another crucial parameter that has to be controlled once the hand is positioned around the object is the fingertip force. I have shown that virtual lesions of AIP led to an increase in grip force, suggesting that the subjects overestimated the object weight. Interestingly, only a virtual lesion of the left, but not right, AIP produced such an effect (Davare et al., JNeurosci 2007 and Cereb. Cortex 2007). For a review, see Olivier, Davare et al., Curr Op Neurobiol 2007.
Another crucial parameter that has to be controlled once the hand is positioned around the object is the fingertip force. I have shown that virtual lesions of AIP led to an increase in grip force, suggesting that the subjects overestimated the object weight. Interestingly, only a virtual lesion of the left, but not right, AIP produced such an effect (Davare et al., JNeurosci 2007 and Cereb. Cortex 2007). For a review, see Olivier, Davare et al., Curr Op Neurobiol 2007.
2. Interactions between areas of the cortical grasping network
Previous studies provided a picture of distinct cortical areas that are distributed within the parieto-frontal network, and necessary to control precise hand movements. However, the nature and timing of the interactions between these areas, namely AIP, PMv and the primary motor cortex (M1), during visually-guided grasping movements are critical issues that still remained unanswered. Investigating the connectivity between these areas is important because it allows us to shed light on how visual information about the object properties is transferred from one area to another and finally transformed into a precise motor command.
A series of experiments (Davare et al., J. Physiol 2008; Cortex 2009; Current Biology 2010) aimed at testing the causal role of AIP in mediating the interactions between PMv and M1. I tested PMv-M1 connectivity before and after inducing an AIP virtual lesion using rTMS. 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 rTMS without directly modifying corticospinal excitability. Behaviourally, disruption of AIP was also associated with a relative loss of the grasp-specific pattern of digit muscle activity. These findings suggested that grasp-related and muscle-specific PMv-M1 interactions are driven by information about object properties provided by AIP. For a review, see Davare et al., Curr Op Neurobiol 2011.
A series of experiments (Davare et al., J. Physiol 2008; Cortex 2009; Current Biology 2010) aimed at testing the causal role of AIP in mediating the interactions between PMv and M1. I tested PMv-M1 connectivity before and after inducing an AIP virtual lesion using rTMS. 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 rTMS without directly modifying corticospinal excitability. Behaviourally, disruption of AIP was also associated with a relative loss of the grasp-specific pattern of digit muscle activity. These findings suggested that grasp-related and muscle-specific PMv-M1 interactions are driven by information about object properties provided by AIP. For a review, see Davare et al., Curr Op Neurobiol 2011.
3. Interactions between visual inputs and internal object representations
Besides studying how the brain processes visual information about an object, I am also interested in understanding how internal object representations, so-called internal models, are recruited by M1. This is important because when no visual information is available, the weight of the object has to be predicted based on information learned from previous grasps. Thus, the scaling of fingertip forces when lifting up an object critically relies on rapidly retrieving a memory of the object properties. Internal models related to object weight are learned through previous motor experience and allow prediction of the actual grip force appropriate for the object load. In a recent study (Loh et al., JNeurosci 2011), I investigated whether changes in corticospinal excitability (CSE) and grip force scaling depend on the presence of visual cues and the weight of previously lifted objects. In conditions in which visual information was not provided, CSE was higher when the previous object was heavy compared to light. Interestingly, the previous lift also affected CSE when visual cues about object weight were available but only in the period immediately after object presentation. In a second experiment (Loh et al., JNeurosci 2011), we demonstrated that these CSE changes are used by the motor system to scale grip force. This suggests that the corticospinal system stores a ʻsensorimotor memoryʼ of the grasp of different objects and relies on this memory when no visual cues are present. Moreover, visual information about weight interacts with this stored representation and allows the corticospinal system to switch rapidly to a different model of predictive grasp control.
