Figure 3.

Inhibition of the primary motor cortex reduces modulation of the long latency stretch reflex corresponding to changes in environmental stability but not changes in prepared response. (A) Visual feedback provided to subjects to ensure that a constant level of tonic activity was maintained in the biceps brachii muscle. (B) A linear motor imposed perturbations to extend the elbow joint while simulating either a stiff or compliant haptic environment. (C) Ramp-and-hold perturbations delivered by the linear actuator moved the wrist 30 mm along the –x axis, thus extending the elbow joint and stretching the biceps brachii. The actuator controller remained stiff throughout Stiff:DNI and Stiff:Resist trials and switched rapidly from compliant to stiff during Compliant:DNI trials in order to ensure consistent joint displacements; “Resist” and “DNI” correspond to how the subject was instructed to react to the perturbation in each of the haptic environments. (D) The response of the biceps brachii to stretches imposed at time zero during low-level (5% MVC) activation shows both short (SLR) and long latency reflex (LLR) responses. The amplitude of the LLR is shown to vary with both task and environment. (E) A single trial in which TMS was applied during a contraction of the biceps brachii at 5% MVC. The silent period following the excitatory motor evoked potential lasts longer than 150 ms following the TMS trigger. (F) Data from the same participant as in D shows reductions in the LLR responses obtained within a period of cortical silence in the Stiff:DNI and Compliant:DNI conditions. No reduction in LLR amplitude is evident in the Stiff:Resist condition. (G) Group means (N=8 subjects) are shown for background muscle activity (BGA), SLR and LLR responses in each experimental condition. For more details see Shemmell et al. (2009).