Operant conditioning of the primate H-reflex: factors affecting the magnitude of change.

Primates can gradually increase or decrease H-reflex amplitude in one leg when reward depends on that amplitude. The magnitude of change varies greatly from animal to animal. This study sought to define the factors that control this magnitude. It evaluated the influence of animal age, muscle size (absolute and relative), background electromyographic activity (EMG) level, M response amplitude, initial H-reflex amplitude, performance intensity, and behavior of the contralateral leg. Fifty-four animals (Macaca nemestrina) underwent operant conditioning of the triceps surae H-reflex in one leg (the trained leg). Twenty-eight were rewarded for larger H-reflexes (HRup animals), and 26 were rewarded for smaller H-reflexes (HRdown animals). In the HRup animals, H-reflex amplitude in the trained leg rose to an average final value of 177% of its initial amplitude. Magnitude of increase varied widely across animals. Nine animals rose to 120–140%, 11 to 160–240%, three to 300% or more, and five remained within 20% of initial amplitude. In the HRdown animals, H-reflex amplitude in the trained leg decreased to an average of 69% of initial amplitude. Magnitude of decrease varied widely. Five animals decreased to 20–40%, seven to 40–60%, six to 60–80%, and eight remained within 20% of initial amplitude. Animal age, as assessed by weight, markedly affected HRdown conditioning, but not HRup conditioning. Heavy HRdown animals (≥6 kg) were more successful than light HRdown animals (< 6kg). Thirteen of 14 heavy animals and only five of 12 light animals decreased to less than 80% of initial amplitude. One heavy animal and seven light animals remained within 20% of initial amplitude. Established correlations between weight and age indicate that heavy animals were young adults, while many light animals were adolescents. This striking difference in HRdown performance was not attributable to weight-related differences in other factors. Initial H-reflex amplitude varied considerably across animals and affected the magnitude of change. In HRup animals, H-reflex amplitude in the trained leg tended to increase more if initial H-reflex amplitude was small, while in HRdown animals it decreased more if initial amplitude was large. The inter-animal variation in initial H-reflex amplitude was probably largely attributable to variation in Ia afferent excitation by the stimulating electrode pairs and to variation in motoneuron recruitment. Performance intensity, measured as trials per day, had no significant effect on the magnitude of change in either HRup or HRdown animals. Together with available human data, this finding suggests either that the chronic descending influence responsible for the gradual H-reflex change need only be present for a relatively brief period each day, or that it persists between periods of task performance. Final H-reflex amplitude in the control leg varied greatly across animals. It averaged 131% of its initial amplitude in HRup animals and 108% in HRdown animals. Within each group, final control leg amplitude did not correlate with the magnitude of change in the trained leg. Its wide variation and lack of correlation with final amplitude in the trained leg is consistent with evidence that operant conditioning of the H-reflex produces plasticity at multiple spinal and supraspinal sites both ipsilateral and contralateral to the trained leg.