Electrocorticogram±electromyogram coherence during isometric contraction of hand muscle in human Shinji Ohara a , Takashi Nagamine a , Akio Ikeda a , Takeharu Kunieda b , Riki Matsumoto a , Waro Taki b , Nobuo Hashimoto b , Koichi Baba c , Tadahiro Mihara c , Stephan Salenius d , Hiroshi Shibasaki a,e, * a Department of Brain Pathophysiology, Kyoto University Graduate School of Medicine, 54 Kawahara-cho, Shogoin, Sakyo-ku, Kyoto, 606-8507, Japan b Department of Neurosurgery, Kyoto University Graduate School of Medicine, Shogoin, Sakyo-ku, Kyoto, 606-8507, Japan c The National Epilepsy Center, Shizuoka Higashi Hospital, Urushiyama, Shizuoka, Japan d Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, 02015HUT, Espoo, Finland e Department of Neurology, Kyoto University Graduate School of Medicine, Shogoin, Sakyo-ku, Kyoto, 606-8507, Japan Accepted 21 August 2000 Abstract Objective: To clarify how the primary sensorimotor and supplementary motor areas are involved in the generation of the rhythmicity of electromyogram (EMG) activity during continuous muscle contraction. Method: We analyzed the coherence between subdurally recorded cortical electroencephalograms (EEG) and EMGs of the contralateral wrist extensor muscle during continuous isometric contraction in 8 patients with medically intractable epilepsy. Results: In all subjects, a signi®cant coherence between the primary motor area (M1) and EMG was observed at the peak frequency of 15 ^ 3 Hz (means ^ SD). In the primary somatosensory area (S1) of 7 subjects and the supplementary motor area proper (SMA proper) of 4 subjects, signi®cant coherence with EMG was observed at 12 ^ 5 and 15 ^ 4 Hz, respectively. The time lags revealed by cross-correlogram were 10 ^ 3, 7 ^ 1 and 22 ^ 8 ms in the M1, S1 and SMA proper, respectively, with the EMG lagging in all areas. Conclusion: These ®ndings suggest that the rhythmic activity in the SMA proper, as well as in the S1 and M1, is related to the generation of the rhythmicity of EMG activity. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Electrocorticogram±electromyogram coherence; Primary sensorimotor area; Supplementary motor area proper; Electrocorticography; Isometric muscle contraction 1. Introduction Recent studies of coherence between electroencephalo- gram (EEG) or magnetoencephalogram (MEG) and electro- myogram (EMG) showed cortico-muscular oscillatory communication in humans (Conway et al., 1995; Salenius et al., 1996, 1997; Brown et al., 1998; Halliday et al., 1998; Kilner et al., 1999), as well as in monkeys (Baker et al., 1997). In humans, Salenius et al. (1997) showed signi®cant coherence in the frequency range of 15±33 Hz between MEG signals from the primary sensorimotor cortex (S1± M1) and the recti®ed EMG of the contralateral hand or foot muscles during isometric contraction of each corre- sponding muscle. Spike-triggered averaging revealed a consistent difference in the MEG±EMG time lag for hand and foot muscles. The longer time lag for foot compared with hand muscles, by about 21 ms, was in good agreement with the corresponding time difference in conduction time through the fast conducting corticospinal tract between the hand and foot. During maximal contraction, signi®cant coherence between MEG activity from the S1±M1 and the recti®ed EMG was observed in the frequency range of 30±60 Hz (Brown et al., 1998). In monkey studies, oscillations of ®eld potentials in the primary motor cortex were coherent with EMG activity of the hand muscles in the frequency range of 20±30 Hz during the hold phase of the precision grip task (Baker et al., 1997). This ®nding is in agreement with the case of moderate contraction in human studies. Those rhythmic activities over the S1±M1 have been suggested to drive spinal moto- neurons through the corticospinal tract and to play a role in the synchronization of EMG activity (Salenius et al., 1997; Kilner et al., 1999). Clinical Neurophysiology 111 (2000) 2014±2024 1388-2457/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S1388-2457(00)00448-X www.elsevier.com/locate/clinph CLINPH 2000009 * Corresponding author. Tel.: 181-75-751-3601; fax: 181-75-751-3202. E-mail address: shib@kuhp.kyoto-u.ac.jp (H. Shibasaki).