Discrimination of bilateral differences in the loci of tactile stimulation is impaired in subjects with Parkinson's disease.

We have compared the abilities of patients with idiopathic Parkinson's disease (PD), while on levodopa (L-dopa), and control subjects to discriminate bilateral differences in the loci of tactile stimulation. On each trial, one of a subject's forearms served as the reference arm and the other as the test arm. The two forearms were supinated and placed side-by-side on a table-top in front of the subject. Seven cutaneous test loci, 3 mm apart, were arrayed proximo-distally on the flexor aspect of each forearm, with the fourth (central) serving as a reference point. On each trial, two punctate tactile stimuli (of intensity well above perceptual threshold) were applied simultaneously to 1) one of the seven test loci on the test forearm and 2) the reference (central) locus on the reference arm. Subjects, with eyes closed, stated which forearm (test or reference) was stimulated more distally. Estimates of the difference limen (DL, a measure of discriminatory threshold) and the point of subjective equality (PSE, the test locus perceived to correspond to the reference locus) were derived from psychophysical functions (ogives). For each subject, one arm was defined as 'better' and the other as 'worse.' Among PD patients, this definition was in terms of the relative severity of clinical motor signs (combined scores for bradykinesia, rigidity and tremor) of the two arms, whereas among controls it was according to relative DL magnitudes (sensory/perceptual performance). The mean DL of the patients significantly exceeded that of control subjects on both the 'better' and 'worse' arms as test side, but mean PSE values did not differ significantly between groups. Thus, our PD patients were impaired in discriminating bilateral differences in the locations of tactile stimulation, while perceiving normally the relative locations of the proximo-distal centers of the two stimulus arrays. Copyright 2003 Wiley-Liss, Inc.