Soleus H-reflex operant conditioning changes the H-reflex recruitment curve

Abstract

Introduction: Operant conditioning can gradually change the human soleus H-reflex. The protocol conditions the reflex near M-wave threshold. In this study we examine its impact on the reflexes at other stimulus strengths.

Methods: H-reflex recruitment curves were obtained before and after a 24-session exposure to an up-conditioning (HRup) or a down-conditioning (HRdown) protocol and were compared.

Results: In both HRup and HRdown subjects, conditioning affected the entire H-reflex recruitment curve. In 5 of 6 HRup and 3 of 6 HRdown subjects, conditioning elevated (HRup) or depressed (HRdown), respectively, the entire curve. In the other HRup subject or the other 3 HRdown subjects, the curve was shifted to the left or to the right, respectively.

Conclusions: H-reflex conditioning does not simply change the H-reflex to a stimulus of particular strength; it also changes the H-reflexes to stimuli of different strengths. Thus, it is likely to affect many actions in which this pathway participates.

Copyright © 2012 Wiley Periodicals, Inc.

Figures

Figure 1

Average H-reflex recruitment curves for the 6 baseline sessions (o) and the last 6 conditioning sessions (x) of 2 HRup subjects. H-reflexes are plotted against the sizes of the accompanying M-waves. Second-order polynomial curves are fitted from the M-wave threshold to 50–70% Hmax of the down slope so that Hmax can be calculated for each curve (vertical lines). The arrow indicates the stimulus level used by the conditioning protocol (i.e., the M-wave size targeted by the protocol). In the subject in A (as in 5 of the 6 HRup subjects), the recruitment curve is broad, the entire curve is elevated by conditioning, and the stimulus level that produces Hmax does not change. In contrast, in the remaining HRup subject (B), the recruitment curve is narrow, the stimulus level that produces Hmax falls with conditioning, and the curve shifts to the left. In both subjects, the H-reflex produced by the stimulus level used in the conditioning protocol increases.

Figure 2

Average H-reflex recruitment curves for the 6 baseline sessions (o) and the last 6 conditioning sessions (x) of 2 HRdown subjects. H-reflexes are plotted against the sizes of the accompanying M-waves. Second-order polynomial curves are fitted from the M-wave threshold to 50–70% Hmax of the down slope so that Hmax can be calculated for each curve (vertical lines). The arrow indicates the stimulus level used by the conditioning protocol (i.e., the M-wave size targeted by the protocol). In the subject in A (as in 3 of the 6 HRdown subjects), the recruitment curve is broad, the curve is depressed by conditioning, and the stimulus level that produces Hmax does not change. In contrast, in the subject in B (as in the other 3 of the 6 HRdown subjects). the curve is narrow, the stimulus level that produces Hmax rises, and the curve shifts to the right. In both subjects, the H-reflex produced by the stimulus level used in the conditioning protocol decreases.