Auditory processing and motor systems: EEG analysis of cortical field potentials
Contemporary research has been examining potential links existing among sensory, motor and attentional systems. Previous studies using TMS have shown that the abrupt onset of sounds can both capture attention and modulate motor cortex excitability, which may reflect the potential need for a behavioral response to the attended event. TMS, however, only quantifies motor cortex excitability immediately following the deliverance of a TMS pulse. Therefore, the temporal development of how the motor cortex is modulated by sounds can’t be quantified using TMS. Thus, the purpose of the present study is to use time frequency analysis of EEG to identify the time course of cortical mechanisms underlying increased motor cortex excitability after sound onset. Subjects sat in a sound attenuated booth with their hands outstretched at 45-degree angles while frequency modulated sounds were intermittently presented from a speaker either in the left and right hemispace. Our results indicated a transient reduction in EEG power from 18-24 Hz (300-600 ms latency) and then a long lasting increase in EEG power that began at ~800 ms and continued until at least 1.7 sec. The latency of EEG power changes was shorter for sounds presented from the right speaker at both time periods. When sounds were presented from the right speaker the contralateral hemisphere over motor regions also showed greater power increases after 800 ms relative to the ipsilateral hemisphere. In addition, power increases were greater in the left-handed subjects (8-12 Hz). Results showed that sounds increased EEG power at the time of a previously observed increase in motor cortex excitability. Findings also suggest an increased attentional salience to the right hemispace in neurologically normal subjects and asymmetrical hemispheric activations in right and left-handers.