EEG correlates of action awareness in the parietal cortex

It is assumed that while performing a goal-oriented arm movement we anticipate feeling changes in our arm configuration that should correspond to the intention and to the motor command that produced the movement. The function of matching the predicted with the actual sensory consequences of a movement may not only help to adjust a given motor program but may also contribute to the feeling of our self as the agent of our own actions, that is the “sense of agency”. Current opinion is that this mismatch gains access to consciousness when discrepancies between the intended action and its perceived consequences, either visual or kinesthetic, reach a certain threshold. Beyond this limit, awareness of action discrepancy or a perturbed sense of agency appears. Various brain areas, especially the prefrontal and parietal, have been involved in such conscious monitoring by neuroimaging or inactivation studies, which, however, did not allow us to elucidate the operating mode of the engaged neural network/s nor the time course of the process. To evaluate at the level of brain rhythms the neural correlates of action awareness, we submitted six normal subjects to a 64 channel EEG recording during the execution of cued reaching movements from a starting point to a circumference under a variable degree of perturbation of the visual feedback. At this aim the output of an electromagnetic motion-tracking system, whose sensor was located on the subject right finger, was processed by a computer and projected on a mirror where the subjects saw their virtual finger as a cursor, having their hand hidden by the mirror. Computer processing used an algorithm for adding a linear directional bias in clockwise/counterclockwise direction of varying amplitudes or for producing a randomly-generated distortion (d). Thus, five experimental conditions were presented according to apseudorandom sequence: perfect (0 ° d) or no correspondence (randomly-generated distortion) between the actual and the seen movement, visual displacement of 7.5 ° or 18 ° which was respectively under or above the threshold for conscious detection and, finally, displacement at the threshold value (12.5 ° d) such that elicited conscious perception of distortion in approximately half movements. After each movement subjects gave their judgment whether visual feedback was congruent (C), distorted (D) or independent (other's, O) with respect to their actual movement. As expected, a discrepancy was detected in 4 ± 3%, 23 ± 15%, 52 ± 19% and 79 ± 11% of the 0 ° , 7.5 ° , 12.5 ° and 18 ° d movements, respectively. In all cases in which a randomly-generated distortion was applied, the movement was refused as self. The time-frequency EEG analysis demonstrated that at parietal sites movement-related β1 desynchronization was significantly higher in 18 ° d as compared to 0 ° and 7.5 ° d conditions. Accordingly, desynchronization was higher for movements scored as D as compared to C. In the case of O judgment, desynchronization in α3 and β1 frequency bands at parietal sites was not different from that associated with D judgment within the first 500 ms since the exit from the starting point, but, in contrast to D, it partially and sharply recovered towards pre-movement values in the following 1 s period though movement was still ongoing. Analysis of independent components is in progress in the attempt to localize the source/s of α/β desynchronization. Our data indicate that conscious monitoring of the discrepancies between predicted (and kinesthetically detected) and visually perceived consequences of an arm movement is associated with an increase of movement-related low beta desynchronization in the parietal cortex. We may speculate that in trials in which subjects become aware that visual feedback is not related to their own but instead to another's movement, the computational cost needed to incorporate/select afferent signals such as the incongruent visual and the veridical proprioceptive signals in the monitoring/matching process may decrease and this may lead to a partial recovery of beta rhythm.