Heterogeneity of biofeedback training effects in spasmodic torticollis: a single-case approach.
Data from 184 biofeedback sessions are presented, in which 10 subjects with spasmodic torticollis had been trained to reduce pathologic activity in the hypertrophied sternocleidomastoid muscle. Each session was conducted as a single case experiment with nine successive trials. Attempts were made to distinguish motor learning processes which can be elicited independently from the biofeedback-condition from the effects of specific biofeedback information. Results showed dramatic decreases of muscle activity under biofeedback. Contrary to expectation, action potentials did not decrease across a course of 14 training sessions. Instead, effects were exhibited in an all-or-none fashion early in the training. Single-case ARIMA intervention analysis has shown that in 59% of the sessions EMG decreases demonstrated under biofeedback could be elicited prior to biofeedback in a condition of instructed control. Case studies revealed complex interactions of instructed control, specific biofeedback effects, unspecific effects of the biofeedback setting, and cognitive processes. Effects obtained within experimental sessions varied highly between subjects. Results are discussed in terms of newer concepts of basal ganglia dysfunction, and conclusions for the use of biofeedback paradigms in torticollis subjects are outlined.