Master of Science
Grahn, Jessica A.
Humans perceive and synchronize to regularity in auditory temporal sequences. Auditory regularity activates motor areas, but how the timing of motor responses relates to the regularity is unclear. Thus, we examined whether motor excitability, an index of motor activity, fluctuated to an isochronous sequence and characterized the timing of these fluctuations. Participants heard isochronous tones followed by a short silence, during which they imagined the tones continuing. Using single pulse transcranial magnetic stimulation (TMS), we indexed excitability throughout the sequence. Cosine models were fit to constructed excitability timecourses to quantify periodicity of the excitability fluctuations. Motor excitability did not fluctuate at the stimulus frequency during either listening or silent portions. Thus, the study does not provide evidence for motor excitability fluctuations during isochronous tone perception or generation. Future work may reduce measurement noise by acquiring more samples over a shorter time or using a more engaging stimulus.
Summary for Lay Audience
Humans can perceive and synchronize their movements with regularly repeating patterns in sound. For instance, people can spontaneously tap their feet or bob their heads to the beat in Western music. In recent decades, researchers have tried to determine how the human brain accomplishes this feat. Studies using brain scanning technology show that motor areas of the brain (areas involved in generating and coordinating movement) are active when people listen to rhythms containing a beat, even while they remain still in the scanner. Other studies suggest that motor excitability (i.e., the readiness of neurons in the motor regions to ‘fire’ or activate) fluctuates at the same rate or ‘tempo’ as the regularly repeating sounds. However, no study so far has directly monitored motor excitability over the course of a rhythm. So, in the present study, we set out to address this gap in research and determine whether motor excitability does indeed fluctuate at the same rate as a regularly repeating sound sequence. In this study, we used a 10-tone isochronous sequence (a simple sequence in which tones are equally spaced apart in time, similar to a metronome). While participants were listening to the sequence, we stimulated their brains using a non-invasive technology known as transcranial magnetic stimulation, or TMS. When performed over a specific region of the brain known as the primary motor cortex, this stimulation causes a muscle twitch in the participants’ hands, which can be quantified and used to assess the degree of motor excitability. By stimulating at many time points throughout the tone sequence, we were able to observe how excitability changes as people listen to the tone sequence. We found that motor excitability does not fluctuate at the same rate as the isochronous sequence. Although we did not get the results we expected, these findings still help clarify how exactly the motor regions of the brain are involved in perceiving regularity. Future studies can now build on these findings and continue to explore the intersection between regularity and movement.
Raza, Syed Z., "Is Motor Excitability Modulated by Isochronous Rhythms?" (2021). Electronic Thesis and Dissertation Repository. 7756.
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