Localization of motor cortex by MEG using a tremorometer

Abstract

Abstract We demonstrate the utility of a tremorometer to determine where the continuously recorded MEG data should be segmented to create epochs for averaging. The averaged MEG data was used to localize motor cortex. Seven normal controls and five patients (1 with localization related epilepsy, 3 with Parkinson's disease, and 1 with a thigh muscle twitch) were monitored with a 148-channel MEG while performing motor tasks. Each was asked to lift a finger, or entire arm, in response to hearing the word “on”. Using the middle finger, each subject performed the task twice with each hand. A tremorometer (tri-axial accelerometer, FlexAble Systems, Fountain Hills, AZ) was taped to the middle finger while each subject lay supine in the shielded room. The z direction of the tremorometer was oriented vertically, with x direction in the right–left plane, and y direction in the forward–backward plane. In the one subject with a thigh muscle twitch, the tremorometer was taped over the active muscle with z direction oriented in a plane normal to the subject. Analogue outputs from all three directions were displayed simultaneously with the MEG data. Data were visually inspected to create epochs by selecting the peaks of the tremorometer signal in the z direction. Epochs were from 500 ms before to 500 ms after a peak. The peak amplitude of the tremorometer output indicated extent of movement. Forty trials were averaged and bandpass filtered 1–50 Hz. MR-FOCUSS was used to generate MEG cortical localization maps. The tremorometer signal indicated the onset of movement of the body part to which it was attached. In all subjects the motor cortex for third digit or arm movement was identified on the motor strip during voluntary movement, at approximately 120 ms prior to the maximum movement flexion as recorded by the tremorometer. For the subject with thigh muscle twitch the motor cortex associated with the upper leg was activated at 45 ms prior to the involuntary muscle twitch. The tremorometer has provided a useful means to assess the motor cortex from voluntary (finger and arm) movements and, in particular, from involuntary movements such as a muscle twitch in one patient.