Specific modulation of the Hoffmann reflex cutaneous facilitation during a reaction-time task

Summary The Hoffmann (H) reflex and its facilitation produced by electrical stimulation of the sural area were examined before a ballistic extension of the right foot. Modulations of the cutaneous facilitation of the H reflex (CFH) were used to assess the control exerted over the transmission of low threshold cutaneous afferents. The time-course of H and CFH changes were investigated at the end of the foreperiod and during the premotor period, i.e. between the response signal and the onset of the electromyogram (EMG) of the soleus muscle. Four stimulation conditions were set up depending on whether the H reflex was elicited on the contracting or non-contracting limb, and whether cutaneous stimulation was ipsilateral or contralateral to the reflex. During the 100 ms preceding the response signal, the inhibition of the H reflex was more marked in the contracting limb than in the noncontracting limb. At the end of the foreperiod, the CFHs had a symmetric time course: the CFHs evoked by conditioning stimulation of the contracting limb were facilitated just before the response signal, while those produced by conditioning stimulation to the non-contracting limb were depressed. It is suggested that these variations are related to postural adjustments taking place before the movement is performed. As previously reported, the H reflex of the contracting limb exhibited a marked increase in amplitude over the 50 ms preceding the EMG. Reflex facilitations showed specific variations according to the ankle stimulated and the soleus muscle tested. The CFHs produced by stimulation of the non-contracting limb regained, at the end of the premotor period, a value close to their reference level recorded in trials without movement. The CFHs, produced by conditioning stimulation of the contracting limb were modulated differently according to whether the tested soleus muscle was contracting or not: when the CFH was tested on the contracting muscle, it was found to be depressed throughout the premotor period; this contrasted significantly with the isolated depression recorded on the non-contracting muscle. Therefore, only the cutaneous afferents from the mobilized limb, modulating the H reflex of the same limb, were subject to a specific inhibition during the premotor period. Throughout the preparatory and premotor periods, negative correlations were observed between H and CFH amplitude, except just before the EMG onset in the condition where the H reflex was delivered to the contracting muscle and the cutaneous stimulation to the ankle of the non-contracting limb: in this case, CFH amplitude increase paralleled that of the reflex amplitude. The negative correlations between these two variables are discussed in terms of a central control regulating the balance between proprioceptive and cutaneous input.