Parietal cortex involvement in visually guided, non-visually guided, observed, and imagined reacing, compared to saccades
Flavia Filimon, Jonathan D. Nelson, and Martin I. Sereno
In macaques, visuomotor neurons in and around MIP and V6A have been found to respond more during planned reaches than during planned saccades to visual targets. Recent fMRI studies have attempted to find a human homologue of this area. However, in those studies, (1) pointing, instead of reaching, was compared to saccades, (2) delayed (planned), rather than immediately executed hand movements were used, and (3) the effect of seeing the hand was not investigated. It remains unclear whether naturalistic reaching activates human medial parietal areas more than saccades, whether that activation is present during execution of reaching, and whether the activation depends on whether the reaching hand is visible. Also, it is unknown if observation and motor imagery of reaching activate those medial parietal areas. Here, we addressed these questions with fMRI and cortical-surface-based methods. In experiment 1, subjects either fixated centrally, or reached or saccaded to peripheral visual targets. In experiment 2, subjects reached on a visually hidden horizontal plate by moving their hand from the center to peripheral locations corresponding to visual target locations (e.g. targets at "12 o'clock" corresponded to an upward movement on the plate) and made saccades as before. Both visually guided and non-visually guided reaching activated a medial parietal area more strongly than saccades. Saccade activations partially overlapped with the medial region activated by reaching, but were generally less strong. In experiment 3, subjects either observed, imagined, or executed reaching to targets presented on a screen. Interestingly, observed and imagined reaching activated the same medial parietal region identified above by comparing reaching with saccades. These findings suggest the presence of a medial parietal human homologue to monkey MIP/V6A that is activated more by reaching than by saccades, and that may contain neurons involved in execution, mental simulation, and observation of action.