| Recent studies have demonstrated that the strategy an individual uses to synchronize motor behavior (e.g. finger flexions) with externally delivered, periodic stimuli depends, in part, on the stimulus presentation rate (Mates, Muller, Radil, and Poppel, 1994; Engstrom, Kelso, and Holroyd, 1995). At rates slower than approximately 0.5 Hz, subjects typically exhibit a reactive-type coordination pattern where the response follows the stimulus by an order of magnitude consistent with typical response times (i.e. 150-250 milliseconds). At faster rates, however, subjects typically anticipate the impending stimulus in order to synchronize movement with it. In the present study, scalp electroencephalographic (EEG) signals (61 channels) were recorded during a sensorimotor task designed to investigate transitions from one coordination mode to another. We found that subjects exhibited a spontaneous transition from reactive to anticipatory behavior as the stimulus presentation rate increased past some critical frequency. A spatiotemporal analysis of the EEG signals accompanying this task revealed: (1) a widespread frequency component in the EEG matching that of both the stimulus and movement; (2) peak spectral power density over central and antero-central regions in both men and women during reactive behavior; (3) an additional bilaterally distributed frontal component at the most anterior portion of the scalp in men during anticipatory behavior; (4) an additional left fronto-central component which extended posteriorly toward antero-central regions in women during anticipatory behavior; (5) fluctuation enhancement in both the EEG spectral power density and the time lag ({dollar}tau{dollar}) between the movement and stimulus accompanying the transition from reactive to anticipatory behavior; (6) that the spectral power density patterns obtained in the primary experimental condition (REACTIVE) were more similar in terms of their spatial distribution with a control condition in which subjects were asked to produce rhythmic movements without benefit of an external stimulus (MOTOR-ONLY) than with a control condition in which subjects passively watched a periodic visual stimulus (STIMULUS-ONLY); (7) that the spectral power density patterns obtained during reactive behavior in the primary experimental condition were more similar spatially to the MOTOR-ONLY condition than du ring anticipatory behavior; and finally (8) that the spectral power density patterns obtained during the experimental condition are not completely accounted for in terms of purely motor- or stimulus-related components. These results are discussed within a common framework of pattern formation instigated by dynamic instabilities in the human brain and behavior. |
