Repp studied how eight people adjusted their tapping in response to 1-2 percent changes (which are very difficult to detect, and therefore considered subliminal) in a series of pulsing tones from a digital piano. Subjects tapped a key on a silent piano keyboard in or out of synchrony with the tones, as instructed, and made successful adjustments -- a process called "phase correction" -- in response to subliminal timing changes. They had not been warned that the timing might change; thus subjects stayed with the beat, even without a conscious perception of change. Thus, Repp concludes that the agent guiding corrections of this motor behavior is a brain mechanism that responds to stimulus changes below the perceptual threshold.

Repp's series of five experiments, which included various tapping instructions and pulse-change types to assess aspects of sensorimotor coordination, confirmed and extended previous research in this area. The experiments tested phase correction, the timing adjustment of a repetitive motor activity to maintain synchrony or some other intended temporal relation with an external sequence of events, and also phase resetting, a more dramatic timing adjustment that immediately restores synchrony after a large synchronization error.

Repp's findings are consistent with a theoretical two-process model of error correction -- the brain perceives as an error any deviation from the intended temporal relationship between tap and beat -- that depends on internal abilities to correct both the period (interval) and phase (time of occurrence) of a beat. Repp found that subjects adjusted their tapping whether they were instructed to go with or against (syncopating) the beat. An internal "timekeeper" is thought to govern the actual rate, or rhythm, of tapping; Repp investigated an additional mechanism that can make temporal adjustments without affecting the rate of the timekeeper.

According to Repp's findings, then, the brain at some level is much more sensitive to timing information than the results of previous psychophysical experiments suggest. This very precise timing information seems to be used in the control of actions, without awareness.

Article: "Phase Correction, Phase Resetting, and Phase Shifts After Subliminal Timing Perturbations in Sensorimotor Synchronization," Bruno H. Repp, Haskins Laboratories; Journal of Experimental Psychology: Human Perception and Performance, 2001, Vol. 27, No. 3.

Full text of the article is available from the APA Public Affairs Office and after June 13 at http://www.apa.org/journals/xhp/xhp273600.html

The American Psychological Association (APA), in Washington, DC, is the largest scientific and professional organization representing psychology in the United States and is the world's largest association of psychologists. APA's membership includes more than 155,000 researchers, educators, clinicians, consultants and students. Through its divisions in 53 subfields of psychology and affiliations with 60 state, territorial and Canadian provincial associations, APA works to advance psychology as a science, as a profession and as a means of promoting human welfare.

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