Temporal dynamics of ipsilateral and contralateral motor activity during voluntary finger movement

Abstract

The role of motor activity ipsilateral to movement remains a matter of debate, due in part to discrepancies among studies in the localization of this activity, when observed, and uncertainty about its time course. The present study used magnetoencephalography (MEG) to investigate the spatial localization and temporal dynamics of contralateral and ipsilateral motor activity during the preparation of unilateral finger movements. Eight right-handed normal subjects carried out self-paced finger-lifting movements with either their dominant or nondominant hand during MEG recordings. The Multi-Start Spatial Temporal multi-dipole method was used to analyze MEG responses recorded during the movement preparation and early execution stage (-800 msec to +30 msec) of movement. Three sources were localized consistently, including a source in the contralateral primary motor area (M1) and in the supplementary motor area (SMA). A third source ipsilateral to movement was located significantly anterior, inferior, and lateral to M1, in the premotor area (PMA) (Brodmann area [BA] 6). Peak latency of the SMA and the ipsilateral PMA sources significantly preceded the peak latency of the contralateral M1 source by 60 msec and 52 msec, respectively. Peak dipole strengths of both the SMA and ipsilateral PMA sources were significantly weaker than was the contralateral M1 source, but did not differ from each other. Altogether, the results indicated that the ipsilateral motor activity was associated with premotor function, rather than activity in M1. The time courses of activation in SMA and ipsilateral PMA were consistent with their purported roles in planning movements.

Figure 1

MSST analysis of the MEG recording during a right finger‐lifting response from Subject 1. A: In a six‐dipole fit, 3,000 sets of starting locations were selected randomly within a search volume. The MEG pick‐up coils are also shown. Each set contains six starting dipole locations indicated by six different colors. B: The 15 best‐fitting six‐dipole MSST solutions (indicated by asterisks) with similar reduced‐χ² values form six clusters in space (x, anterior [+] posterior [−] coordinate; y, left [+] right (−) coordinate; z, superior [+] inferior [−] coordinate). Each vertical line represents the centroid of averaged dipole location for the cluster. C: Measured magnetic field waveforms from 122 MEG channels are superimposed. D: Predicted magnetic fields based on the dipole locations modeled in B. E: Residual magnetic fields, namely the difference between C and D.

Figure 2

Six dipolar sources (yellow clusters) from Subject 1's right index finger‐lifting MEG response, localized using MSST, and superimposed on the subject's MRI. Three hundred Monte‐Carlo analyses were carried out to obtain the uncertainty of the dipole locations, defined as two standard deviations from the mean of the clustered locations. A: View from front‐right, the right (ipsilateral) hemisphere premotor area (PMA) source. B: View from front‐left, the left (contralateral) hemispheric primary motor area (M1) source and the left prefrontal source. C: View from top, the left M1 and right PMA sources, supplementary motor area (SMA), and left superior parietal source. D: Sagittal view through the interhemispheric fissure, SMA, and posterior cingulate sources.

Figure 3

Contralateral primary motor (M1), ipsilateral premotor area (PMA), and supplementary motor area (SMA) sources across all eight subjects evoked by unilateral MEG index finger‐lifting were superimposed on individual subjects' MRI. Neurologic convention was adopted in the MR images. Blue clusters represent activations due to left index finger‐lifting, whereas the yellow clusters are activations due to right index finger‐lifting. The results from 300 Monte‐Carlo analyses are plotted.

Figure 4

The time courses and dipole strengths of the ipsilateral premotor area (PMA), contralateral primary motor area (M1), and SMA due to finger lifting using the non‐dominant left hand (A–C) and the dominant right hand (D–F) for eight subjects. Time courses are color‐coded for each subject. Peak latencies of the sources for each subject are indicated by the vertical lines. Time courses of the contralateral M1 source (B and E) showed another visible weak peak (arrow), which was earlier than were the strongest peaks.

Figure 5

Peak latency (A) and peak dipole strength (B) of the ipsilateral PMA and contralateral MI sources in eight left finger‐lifting (circles) and seven right finger‐lifting (asterisks) studies for all eight subjects.