Psychophysics of muscle tension in psychiatric inpatients.

Twelve psychiatric inpatients and 16 control subjects each took part in a psychophysics experiment in which the method of production was used to study the perception of tension in the frontalis and forearm extensor muscles. Subjects tensed each muscle between 0% and 50% maximum effort, with 25% effort repeated every third trial, and used as a reference stimulus. Patients showed significantly lower correlations between frontalis EMG and percent effort than the control subjects, but no between-groups differences were found for forearm. Correlations were higher for differences between successive trials than for absolute values because of apparent baseline shifts in perception and/or production of muscle tension. The performance among the patients was not correlated with severity of psychiatric symptoms, antipsychotic medication, abnormal movements, or parkinsonian symptoms, although the n was small. Analysis of the regression of EMG on percent effort revealed approximately equal descriptive strength for three functions: a linear relationship, Stevens' power function, and Fechner's law. The implications of these findings for self-regulation therapies are discussed.