Surround inhibition in the motor system

Abstract

Surround inhibition is a physiological mechanism to focus neuronal activity in the central nervous system. This so-called center-surround organization is well-known in sensory systems, where central signals are facilitated and eccentric signals are inhibited in order to sharpen the contrast between them. There is evidence that this mechanism is relevant for skilled motor behaviour, and it is deficient, for example, in the affected primary motor cortex of patients with focal hand dystonia (FHD). While it is still not fully elucidated how surround inhibition is generated in healthy subjects, the process is enhanced with handedness and task difficulty indicating that it may be an important mechanism for the performance of individuated finger movements. In FHD, where involuntary over-activation of muscles interferes with precise finger movements, a loss of intracortical inhibition likely contributes to the loss of surround inhibition. Several intracortical inhibitory networks are modulated differently in FHD compared to healthy subjects, and these may contribute to the loss of surround inhibition. Surround inhibition can be observed and assessed in the primary motor cortex. It remains unclear, however, if the effects are created in the cortex or if they are derived from, or supported by, motor signals that come from the basal ganglia.

Cerebellar transcranial direct current stimulation does not alter motor surround inhibition

“…This is not seen with mSI when tested across the three sessions and quantified by the COV (Table 1). This reemphasises the importance of the deficiency of mSI seen in diseases of motor control such as focal hand dystonia and Parkinson's disease [16]. Attempting to modulate the strength of mSI, as in this study, remains an important potentially therapeutic goal in neurophysiological studies of mSI.…”

“…The mSI is defined as the functional inhibition of surround muscles seen during the movement initiation phase (and just before and during the first phase of EMG onset) [16]. The mechanisms of how and where it is generated are less well characterised.…”

“…At the synaptic level a gamma amino butyric acid (GABA)ergic mechanism for mSI has been proposed largely based on animal work [16]. In humans, proving the link between GABAergic circuits and mSI is less certain.…”

Abnormal surround inhibition does not affect asymptomatic limbs in people with cervical dystonia

“…Inspection of their data, however, revealed that their pattern of MEP amplitudes were consistent with the group data. The effects of GABAergic medications on SI, however, have not been well explored and is an important area for future research [21].…”

Biochemical mechanisms of pallidal deep brain stimulation in X-linked dystonia parkinsonism

“…In dystonia, impaired cortical and subcortical surround inhibition is thought to be one of the most prominent pathophysiological features aside from maladaptive plasticity [1]. In isolated dystonia, in-vivo positron emission tomography and magnetic resonance spectroscopy studies revealed GABA A receptor dysfunction and abnormal GABAergic processing in sensorimotor cortical and subcortical areas [2,3].…”