Differential AIP activation for precision and whole-hand visually-guided grasping

Background Recent neuroimaging evidence suggests that, in humans, special mechanisms are engaged in the planning and execution of grasping movements. Grasping requires a visuomotor transformation of object properties into a specific motor program, giving the hand a shape suitable to grasp the object: a fundamental role in this process is played by the anterior intraparietal area (AIP), whose selective involvement in grasping but not reaching actions has been demonstrated by several studies (Binkofski, 1998; Culham, 2004). Thus, experimental evidence seems to converge on the hypothesis of a key role of AIP in the "hand-shaping" process. However, the extent to which different types of grasp are represented in the same or different cortical loci remains matter of debate. To date, no studies have considered the influence of object dimensions - and thus, the type of prehension required- on AIP involvement. Methods A functional magnetic resonance imaging study has been conducted. Spherical 3D plastic objects (3 and 6 cm) were presented, requiring either precision grip -thumb-index finger opposition- or whole-hand prehension: participants had to reach or grasp them. Hypotheses We hypothesize that the subtraction of the BOLD signal (Blood Oxygenation Level Dependent) detected for reaching trials from that detected for grasping trials should isolate brain areas responsible for the "hand-shaping" process.Vice versa, the subtraction of grasping-related activation from that detected for reaching trials should highlight areas engaged in inhibition of the act of prehension, presumably elicited by the vision of graspable objects (Handy, 2003). Results Results indicate that the two kinds of grasping actions are supported by different patterns of brain activations. AIP contribution seems to be significant only for actions requiring high level of precision: while for whole-hand prehension significant BOLD increase is restricted to primary motor cortex (BA 6) and cerebellum, precision grip involves also parietal areas, as the inferior parietal lobule (BA 40) and the post-central gyrus (BA 3). The opposite contrast (reaching - grasping) underlines contribution of the right inferior frontal gyrus (BA 44, 45) and of the left superior frontal gyrus (BA 6). Increase of activity in these brain regions was detected only for actions performed toward the small object. Conclusions Our results are not in a complete agreement with the hypothesis emerging from the literature suggesting AIP as the "grasping area" in humans: its involvement seems to be necessary only when action goal requires high level of precision. Also prehension inhibition seems to be more demanding when occurring toward a small object than toward a bigger object Taken together, our results suggest the hypothesis of a modulatory role of target dimension on brain activity supporting hand-object interactions.