ELSEVIER Electroencephalographyand clinical Neurophysiology 95 (1995) 444-452 Bilateral activation of the human somatomotor cortex by distal hand movements R. Salmelin a,*, N. Forss a, j. Knuutila b, R. Hari a Low Temperature Laboratory, Helsinki University of Technology, 02150 Espoo, Finland b Neuromag Ltd., 00510 Helsinki, Finland Accepted for publication: 14 August 1995 Abstract We recorded cortical magnetic signals, simultaneously over the whole scalp, from 6 healthy subjects during 3 motor tasks to track the varying proportion of contra- vs. ipsilateral activation. The subjects performed self-paced index finger flexions, simultaneous flexion of 4 fingers, and a sequence of rapid digit movements in different sessions. Index finger and 4-finger movements were associated with phasic bilateral dampening of spontaneous 10 and 20 Hz rhythms along the central sulcus, starting approximately 1 sec before the movement in the contralateral hemisphere. A rebound occurred within 1 sec after the index finger and 4-finger flexions; the rapid finger movements resulted in a persistent blocking of the rhythms. Averaging with respect to movement onset showed a slow bilateral frontal readiness field starting about 0.5 sec prior to motion onset. It was followed, within 200 msec after movement onset, by phasic movement-evoked fields (MEFs) which were bilateral during the tasks involving several fingers. The contra- vs. ipsilateral MEF amplitude ratio C/I decreased from 4.0 during index finger movements to 0.6 during rapid finger flexions, reflecting the enhanced activation of the ipsilateral primary somatomotor cortex with increasing complexity of movement. Keywords: Evoked responses; Cortical rhythms; Temporal spectral evolution; Human; Source localization; Magnetoencephalography I. Introduction The relative timing of cortical areas involved in volun- tary movements can be assessed in electroencephalo- graphic (EEG) or magnetoencephalographic (MEG) recordings. The electric readiness potential (RP) and the magnetic readiness field (RF; for a review, see Weinberg et al., 1990) start approximately 1 sec prior to voluntary movements. Before unilateral movements, these slow shifts are generally contralaterally dominant but may also be bilateral. Direct intracranial electric measurements have shown bilateral activation of the hand primary somatomo- tor cortex SmI prior to voluntary finger movement, and additional bilateral activation in the supplementary motor area, SMA (Neshige et al., 1988; Ikeda et al., 1993; Rektor et al., 1994). The movement-evoked potentials (MEPs) or fields (MEFs; Cheyne and Weinberg, 1989; Cheyne et al., 1991; Chiarenza et al., 1991), occurring within 200 msec * Corresponding author. Tel.: +358-0-451 2950; Fax: +358-0-451 2969; E-mail: riitta@neuro.hut.fi. of motion onset, have so far been observed strictly con- tralaterally for unilateral finger and toe movements, both with MEG and with scalp and intracranial EEG records. We employed whole-head MEG to focus on the se- quence of cortical activation and hemispheric balance dur- ing 3 motor tasks: index finger flexions, simultaneous flexion of 4 fingers, and a sequence of rapid digit move- ments. The results imply that MEFs are strongly task dependent. In particular, an early ipsilateral MEF appeared in 4-finger flexion and increased further in the rapid digit sequence. Markedly, the contralateral dominance of MEF amplitudes during index finger and 4-finger flexions was reversed during the rapid sequence. In addition, the sponta- neous rolandic 10 and 20 Hz brain rhythms showed bilat- eral dampening, starting about 1 sec prior to movement and followed by a transient rebound within 1 sec after finger flexion. Our results thus support the involvement of the ipsilateral SmI in complex unilateral finger move- ments, both prior to and during movement execution. A preliminary report of this work has been presented in abstract form (Hari et al., 1993b). 0013-4694/95/$09.50 © 1995 Elsevier Science Ireland Ltd. All fights reserved SSD! 0013-4694(95)00193-X EEG 94729