Surround inhibition is modulated by task difficulty

Abstract

Objective: The aim of this study was to further characterize surround inhibition (SI) in the primary motor cortex (M1) by comparing its magnitude and time course during a simple reaction time task (SRT) and a choice reaction time task (CRT).

Methods: In both the SRT and the CRT, subjects performed the same right index finger flexion in response to an acoustic signal. For CRT, the alternative choice was a similar movement using the left index finger, as distinguished by a different tone. In both tasks, single pulse transcranial magnetic stimulation (TMS) was applied at rest, 75ms (T1) and 25ms before EMG onset (T2), and during the first peak of EMG (T3) in the right first dorsal interosseous muscle (FDI). Motor evoked potentials (MEPs) were recorded from both FDIs, which act as synergists in the task, and the right surrounding, relaxed abductor pollicis brevis muscle (APB).

Results: For right hand movement, SI started earlier and was more pronounced for CRT compared to SRT. For left hand movement in the CRT, SI was similar to that of right hand movement.

Conclusions: We conclude that SI occurs earlier and stronger with increasing task difficulty.

Significance: The timing as well as the bilateral effect of the inhibition suggests that motor areas involved in motor planning, proximate to the motor cortex, contribute to the genesis of surround inhibition.

Figure 1

The time course of the motor task. The first line shows the onset of the acoustic signal 200ms after the start of the recording. The second and third channels show the EMG of FDI, the synergist muscle, and APB, the surrounding muscle. On the bottom, the force of the index finger flexion is shown. TMS is given before the tone (100ms after the onset of the recording, rest), 75ms before the onset of the EMG in FDI (T1), 25ms before the onset of the EMG in FDI (T2) and the first peak of EMG in FDI (T3).

Figure 2

Means and standard errors of SI in APB (calculated as ratio between MEP size and MEPrest) for the three phases (T1, T2 and T3) and both tasks (SRT, black diamond; CRT, grey circle). There was significant SI for all three phases during CRT, while SI started later for SRT, as indicated by the lack of SI for T1, whereas there was significant SI for T2 and T3.

Figure 3

Means and standard errors of the ratio between MEP size and MEPrest in FDI for the three phases (T1, T2 and T3) and both tasks (SRT, black diamond; CRT, grey circle). There was significant SI for all three phases during CRT, while SI started later for SRT, as indicated by the lack of SI for T1, whereas there was significant SI for T2 and T3.

Figure 4

Means and standard errors of SI in the right APB during CRT for the three phases (T1, T2 and T3) and right or left side movement (right grey circles, left black triangle). There was significant SI for right and left hand movement and no significant difference between sides. When tested separately, SI was present for all three phases during right hand movement and the first two phases during left hand movement.