Neurons within the sensorimotor cortex of monkeys and humans are known to have synchronous oscillatory activity in the 15-30 Hz range. Furthermore, this oscillatory activity is known to influence descending motor commands to the contralateral hand muscles (Conway et al. 1995; Baker et al. 1997). Such coherence in the 15-30 Hz range between sensorimotor cortex and contralateral muscle EMG and between different hand muscles exhibits task-dependent modulations; the coupling is abolished during finger movements and strongest during steady hold periods just following movement (Kilner et al. 2000). Previously we have speculated that levels of coherence might reflect important changes in sensorimotor state encompassing alterations in both grip force and digit position, and hypothesised that the level of coherence in the 15-30 Hz range is modulated by sensory afferent inputs from the hand (Kilner et al. 2000). The current study tested this hypothesis.

We measured the activity of four hand muscles in ten healthy human subjects and in a somatosencory deafferented patient, GL, whilst they performed a precision grip task. All subjects gave informed consent and the study had local ethical committee approval. The coupling between the muscles as a function of the task was subsequently estimated using coherence analysis.

The study demonstrated a significant difference (P < 0.05 Crawford modified t test) in the degree of muscle-muscle coherence during the second hold period: healthy subjects all showed robust 15-30 Hz coherence, while this was at a very low level in the deafferented subject. We have previously argued that oscillatory synchrony may characterise a low-level control system which engages and then maintains the particular level of activity in the large number of synergistic muscles that are needed to exert efficient grip between the digits. Such a control system would be highly sensitive to changes in finger position signalled by cutaneous, joint and muscle afferents.

The present results suggest that this oscillatory control system cannot exist in the complete absence of somatosensory afferent inputs. These afferent inputs may normally serve to modulate or even be involved in the generation of 15-30 Hz oscillations in the motor system.

This work was supported by the Wellcome Trust and the Medical Research Council (UK). The authors would like to thank Prof. J Paillard, Dr. A Schmied and Dr. J-P Vedel.