Selective enhancement of motoneurone short-term synchrony during an attention-demanding task

Abstract

Recent experiments on monkeys suggest that attention-related changes in the synchronous activity of neurones occur in the motor cortex. In humans, the clinical data available suggest that the weak synchronization of the motor unit firing observed during voluntary contraction might reflect the activity of the motoneurone cortico-spinal afferents. The present study was therefore designed to investigate how the synchronous motor-unit activity might depend on the attention required in performing a motor task. Twenty-five motor unit pairs were tested in the right extensor carpi radialis muscles of five right-handed subjects, who were instructed to keep both motor units firing tonically while trying to maintain the extension force as constant as possible using visual feedback set either at low or high gain. Fifteen motor unit pairs (60%) showed a greater amount of synchronous activity, without any consistent changes in the motor unit firing rates, when the subjects were monitoring the force at high gain. In addition, the mean value and the steadiness of the force did not differ significantly between the two tasks. The amplitude of the motor units' contractile force extracted by spike-triggered averaging did not change consistently despite the slight increase in the synchronous impulse probability observed under the high-gain conditions. Changes in synchrony affected slowly contracting, low-threshold motor units as well as fast-contracting, high-threshold motor units. The most noteworthy finding was that these changes actually focused on a narrow component of the synchronous activity (within a period of less than 4 ms) in keeping with the short-term synchronization process. This suggests that common inputs, possibly of cortical origin, may have contributed more greatly to controlling the motoneurone firing pattern in the motor task which required the subjects to pay more attention.

Sensitivity of EMG-EMG coherence to detect the common oscillatory drive to hand muscles in young and older adults

“…Second, coherence between EEG and EMG during a finger flexion task increased in the 15-30 Hz frequency range when the visual feedback and precision demands of the pressing task were altered to require more precision (Kristeva-Feige et al 2002). This observation is consistent with the finding that attentiondemanding behavioral tasks augment motor unit synchronization (time-domain correlation between motor unit discharge times) in humans (Schmied et al 2000) and oscillatory activity in the motor cortex of monkeys (Murthy and Fetz 1996;Sanes and Donoghue 1993). Third, EMG-EMG coherence is sensitive to the task being performed, as shown with decreased EMG-EMG coherence between hand muscles during dynamic tasks relative to static isometric grip force tasks (Baker et al 2001;Kilner et al 1999).…”

“…In the present study, precision demands were varied by manipulating the gain of the visual feedback presented to the subjects while asking them to maintain their target force as steadily as possible in both conditions. This slightly different approach was used because increasing the gain of the visual feedback signal had previously been shown to effectively alter motor unit synchronization (Schmied et al 2000) and to increase the coefficient of variation in force magnitude in older adults relative to young adults (Sosnoff and Newell 2006). As the common modulation of input received by active motor units has been identified to play a significant role in force fluctuations Kakuda et al 1999), it was expected that including a high visual gain condition in older adults would further increase EMG-EMG coherence.…”

“…Visual gains varied from 180.0 to 1,800 pixels/N for the 2-N force level and from 102.9 to 1,029.4 pixels/N for the 3.5-N force level. These gains were selected to match the 10-fold increase in visual gain reported by Schmied et al (2000) to increase motor unit synchronization.…”

Adaptations in biceps brachii motor unit activity after repeated bouts of eccentric exercise in elbow flexor muscles

“…As such, variations in the amount of motor unit synchronization are interpreted as a change in the CNS strategy that is used to perform the task. For example, motor unit synchronization is increased during the performance of lengthening contractions (Semmler et al 2002) and is greatest during attention-demanding contractions (Schmied et al 2000), suggesting increased common neural input during these tasks. Several chronic adaptations in motor unit synchronization have also been observed, such as greater motor unit synchronization in strength-trained weightlifters and reduced synchronization in skill-trained musicians (Semmler and Nordstrom 1998;Fling et al 2009).…”

Neural correlates of task-related changes in physiological tremor

“…This may be relevant for the interpretation of studies that relied on single measures of MU synchrony. For example, Schmied et al (2000) showed that common input strength increases in a target-guided force holding task when the task is made more difficult by manipulating the visual gain of force feedback. Although our study also introduced higher demands on accuracy and error correction, we found no increase in the overall strength of common input (integrated coherence).…”