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Neuropsychological case studies suggest the existence of two functionally separate visual streams: the ventral pathway, central for object recognition; and the dorsal pathway, engaged in visually guided actions. However, a clear dissociation between the functions of the two streams has not been decisively shown in intact humans. In this study, we demonstrate dissociation between dorsal and ventral fMRI activation patterns during observation of object manipulation video clips. Parietal areas, such as anterior intraparietal sulcus (aIPS) display grasp viewing-dependent adaptation (i.e., fMR adaptation during repeated viewing of the same object-grasping movement) as well as a contralateral preference for the viewed manipulating hand. Ventral regions, such as the fusiform gyrus, show similar characteristics (i.e., adaptation, contralateral preference), but these depend on object identity. Our results support the hypothesized functional specialization in the visual system and suggest that parietal areas (such as aIPS) are engaged in action recognition, as well as in action planning.

The human primary motor cortex (M1) is robustly activated during visually guided hand movements. M1 multivoxel patterns of functional MRI activation are more correlated during repeated hand movements to the same targets than to greatly differing ones, and therefore potentially contain information about movement direction. It is unclear, however, whether direction specificity is due to the motor command, as implicitly assumed, or to the visual aspects of the task, such as the target location and the direction of the cursor's trajectory. To disambiguate the visual and motor components, different visual-to-motor transformations were applied during an fMRI scan, in which participants made visually guided hand movements in various directions. The first run was the "baseline" (i.e., visual and motor mappings were matched); in the second run ("rotation"), the cursor movement was rotated by 45° with respect to the joystick movement. As expected, positive correlations were seen between the M1 multivoxel patterns evoked by the baseline run and by the rotation run, when the two movements were matched in their movement direction but the visual aspects differed. Importantly, similar correlations were observed when the visual elements were matched but the direction of hand movement differed. This indicates that M1 is sensitive to both motor and visual components of the task. However, repeated observation of the cursor movement without concurrent joystick control did not elicit significant activation in M1 or any correlated patterns of activation. Thus, visual aspects of movement are encoded in M1 only when they are coupled with motor consequences. [ABSTRACT FROM AUTHOR]

In monkeys, neurons in the hand representation of the primary motor cortex (M1) are often tuned to the direction of hand movement, and there is evidence that these neurons are clustered according to their "preferred" direction of movement. However, this organizational principle has yet to be demonstrated in M1 of humans. We conducted a functional magnetic resonance imaging (fMRI) study in which participants used a joystick to move a cursor from a central origin to one of five equidistant targets. The fMRI signal of individual voxels was sensitive to the directional aspects of the reaching task and manifested direction-specific adaptation. Furthermore, the correlation between multivoxel patterns of responses for different movement directions depended on the angular distance between them. We conclude that M1 neurons are likely to be organized in clusters according to their preferred direction, since only such a coarse-grained representation can lead to directional selectivity of voxels, encompassing millions of neurons. A simple model that estimates cluster size suggests that the diameter of these clusters is on the order of a few hundred micrometers. [ABSTRACT FROM AUTHOR]

Mimicking hand actions made by someone facing us (that is, allocentric viewpoint) is typically performed with the opposite hand. Using functional magnetic resonance imaging (fMRI), we found a similar mirror-image representation of others' actions in the human anterior parietal cortex. Viewing egocentric-based actions elicited greater fMRI activation in the contralteral hemisphere (as in, self action), whereas observation of action seen from an allocentric viewpoint generated greater activation in the ipsilateral hemisphere. This mirror-like mapping occurs without active imitation, providing further evidence for an automatic action-simulation system in the parietal cortex. [ABSTRACT FROM AUTHOR]

The anterior part of the human intraparietal sulcus is known to be involved in visually guided grasping. This region is also active during the observation of object manipulation by others. Here, we explore the nature of action representation using functional magnetic resonance imaging (fMRI). Fourteen subjects observed video clips showing object manipulation by the right or left hand . The clips were presented in either the right or left peripheral visual field. The fMRI activation in the occipital cortex and in the caudal sections of the parietal cortex was specific to the visual-field location of the clips. In contrast, the principal factor determining the response in anterior intraparietal cortex was the identity of the observed hand. Furthermore, these "hand-specific" parietal areas also showed contralateral hand specificity during self action (i.e., object manipulation) without visual feedback. A similar selectivity for the identity of the observed hand was seen when using a region of interest analysis, focusing individually defined visuomotor voxels within the parietal cortex. This dual visuomotor grasping representation lends further evidence for the existence of a mirror system in humans and suggests that the anterior intraparietal cortex is involved in the specific motor simulation of hand actions. [ABSTRACT FROM AUTHOR]