Time-frequency analysis of tremors

Time-frequency analysis methods were applied to surface EMG records of pati ents with tremor. Variation in tremor frequency over time and between muscl es was measured in subjects with Parkinson's disease (n = 20), essential tr emor(n = 8) and psychogenic tremor(n = 7), The effect of externally paced v oluntary contractions on tremor frequency was also characterized. Psychogen ic tremor involved fewer limbs and fewer limb segments than Parkinson's dis ease rest tremor and essential tremor, and its frequency was less consisten t. In all subject groups, muscles within a single extremity generally had i dentical instantaneous frequencies. Frequency dissociation, used here to de scribe a modal contemporaneous frequency difference of more than 0.1 Hz bet ween two extremities, was demonstrated for symptomatic tremors in 17 subjec ts with Parkinson's disease, in four subjects with essential tremor and in none of the subjects with psychogenic tremor. Dissociation between tapping and tremor limbs was demonstrated in an additional two subjects with Parkin son's disease and in all four remaining essential tremor subjects but in no ne of the psychogenic tremor subjects. Tremor maintained a different freque ncy from the tapping limb in Parkinson's disease and essential tremor, and its frequency in many cases shifted by at least 0.3 Hz compared with the no n-tapping condition. For example, arm and leg tremors at 5.2 and 3.8 Hz, re spectively, shifted to a common frequency of 4.6 Hz in one Parkinson's dise ase patient while using the contralateral arm to perform a tapping movement in time with a metronome at 2 Hz. These observations suggest the existence of distinct oscillator systems projecting to each tremoring limb, which ca n be linked to a variable degree, and which can be modulated by voluntary a ctivation of another limb. Psychogenic tremor was not maintained while tapp ing with the contralateral arm: tremor either dissipated or shifted to the metronome's frequency. The latter response was also seen in normal voluntee rs mimicking tremor, but not in Parkinson's disease or essential tremor. We suggest that maintenance of phasic contraction in psychogenic tremor is no t due to intrinsic instability of the motor system and that muscle activati on in involved limbs may instead be synchronized to a common oscillator. As in voluntary movements, only a single rhythm may be easily followed at a t ime. Coexistence of muscle groups phasically contracting at consistently di fferent instantaneous frequencies is evidence against a psychogenic aetiolo gy of tremor.