Learning Modulates Mirror Neuron Activity Depending on Stimulus Modality: An EEG Study
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The mirror neuron system (also called the FPMC) is active when one performs a goal directed action, as well as when an individual observes someone else perform the same or similar action. The traditional mirror neuron hypothesis suggests that the FPMC should only be active while an individual observes goal-directed biological behavior, whereas some studies have challenged that assertion, suggesting the FPMC may be modulated by sensorimotor experience, regardless of the stimulus. We used EEG to test the two competing theories of the mirror neuron system (the traditional mirror neuron hypothesis, and the Associative Learning Model). We looked at mu-event related desynchronization (ERD) over the left and right motor clusters while participants engaged in a novel motor learning task over three sessions. During the first session (pre-learning), participants were naïve to any stimulus-motor response associations while they observed three stimuli: (1) a pair of hands executing key-press sequences (Biological Movement), (2) dot animations executing key-press sequences (Abstract Movement), and (3) static abstract images (colored pound signs and asterisks; Abstract Static). During Session 2 (learning), participants were taught correct responses to the stimuli from Session 1, and were instructed to observe the stimuli, plan the appropriate responses, and execute their responses at the correct time. During Session 3 (post-learning), participants observed the same stimuli, as well as an additional control condition (a white ampersand) for which participants had no stimulus motor response association. During Sessions 1 and 3, mu-ERD was significant relative to baseline for both Biological and Abstract Movement. During Session 1 (pre-learning), mu-ERD for Biological and Abstract Static was significantly greater than Abstract Static; however, during Session 3 (post-learning), mu-ERD for Abstract Static was now significant relative to baseline. During Session 3, there was no difference between the three conditions; however, mu-ERD for all three conditions was significantly greater than the control condition which failed to exceed mu- levels greater than baseline. These findings appear to support the Associative Learning Model, suggesting that the FPMC is not exclusively activated by biological behavior, but can be active in response to abstract stimuli that have become associated with a motor behavior.