Effect of click trains on duration estimates by people with Parkinson's disease.

Patients with a diagnosis of Parkinson's disease and age- and IQ-matched controls estimated the duration of short 500-Hz tones (325-1,225 ms), on trials where the tone was either preceded by 3 s of 5-Hz clicks, or presented without clicks. The click manipulation had been shown in earlier studies with student participants to make verbal estimates longer. Patients were tested both on and off their dopaminergic medication, and controls were also tested in two sessions. Verbal estimates were markedly and significantly longer on trials with clicks than on those without clicks for both the patients and the controls, but there were no significant performance differences between patients or controls, nor between the on and off medication sessions in the patients. The study shows that a manipulation of subjective time, which has had small but consistent effects in student participants, also affects timing in patients and adds to a growing body of evidence that timing in patients with Parkinson's disease may in many cases have the same characteristics as those of neurologically intact control groups.

Biases in the subjective timing of perceptual events: Libet et al. (1983) revisited.

We report two experiments in which participants had to judge the time of occurrence of a stimulus relative to a clock. The experiments were based on the control condition used by Libet, Gleason, Wright, and Pearl [Libet, B., Gleason, C. A., Wright, E. W., & Pearl, D. K. (1983). Time of conscious intention to act in relation to onset of cerebral activities (readiness-potential): The unconscious initiation of a freely voluntary act. Brain 106, 623-642] to correct for any bias in the estimation of the time at which an endogenous event, the conscious intention to perform a movement, occurred. Participants' responses were affected systematically by the sensory modality of the stimulus and by the speed of the clock. Such findings demonstrate the variability in judging the time at which an exogenous event occurs and, by extension, suggest that such variability may also apply to the judging the time of occurrence of endogenous events. The reliability of participants' estimations of when they formed the conscious intention to perform a movement in Libet et al.'s (1983) study is therefore questionable.

The effect of age on inhibition of return is independent of non-ocular response inhibition.

Inhibition of return (IOR) refers to the slowing of a response to a target stimulus presented in the same location as a previous stimulus. Increased IOR has been observed in older adults, despite a reduction in other 'inhibitory' processes. However, cue-target tasks have been used in all previous studies and because of this, IOR may have been overestimated due to non-ocular response inhibition associated with withholding a response from the cue. Could increased levels of response inhibition account for the observations of increased IOR in older adults? This confound can be circumvented by using a target-target paradigm, in which a response is made to all stimuli. We tested three groups of 24 subjects: young (mean 22.5 years), young-old (mean 61.9 years) and old-old (mean 74.8 years). Subjects completed both visual cue-target and target-target tasks with identical inter-stimulus intervals of 1400 and 1800ms. IOR magnitude increased with age in both the cue-target task and the target-target task. Furthermore, the magnitude of visual IOR was found to increase with age even when individual differences in baseline response speed were taken into account. Thus, there appears to be a genuine increase in IOR magnitude with age.

Orienting of attention and Parkinson's disease: tactile inhibition of return and response inhibition.

There is growing evidence for cognitive impairments in Parkinson's disease (PD), including in the orienting of attention and inhibition of return (IOR). IOR refers to the slowing of a response to a target stimulus presented in the same location as a previous stimulus. While some researchers have reported normal levels of visual IOR in PD patients using cue-target tasks, others have reported significant reductions in IOR in this patient group. However, the inhibitory effects observed in cue-target tasks may reflect non-ocular response inhibition associated with withholding a response from the cue stimulus, rather than attentional or oculomotor processes. Many researchers working with normal participants have circumvented this confound by using a target-target task, in which a response is made to all peripheral stimuli. Here, we compared IOR measured in cue-target and target-target tasks, using tactile rather than visual stimuli. Both the PD and the control groups exhibited significant inhibitory effects in the cue-target task, but only the control group exhibited significant IOR in the target-target task. Our results demonstrate a reduction, or elimination, of IOR in PD and this change may have been underestimated in previous studies, in which methodologically flawed cue-target tasks were used. This reduction in IOR may reflect impaired inhibitory processes or hyper-reflexive orienting in parkinsonian patients.

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.

Identification of unilateral elbow-joint position is impaired by Parkinson's disease.

We have compared the ability of patients with idiopathic Parkinson's disease (PD) with that of control subjects to identify unilateral elbow-joint position, in the absence of direct vision of the arm, by visual reference to a graduated angular scale, placed beside the elbow, across a range of test angles of 90-108 degrees. The positioning of the subject's elbow was achieved under either passive (subject relaxed, Experiment 1) or active (subject contracting, Experiment 2) conditions. PD patients' performance (while on L-dopa medication) with the elbows on the sides of "worse" and "better" motor signs was compared with that of controls with, respectively, the left and right elbows. In both experiments, (a) both the individual, overall mean unsigned (with respect to direction) error averaged across all test angles (accuracy), and the SD about this mean (precision), were significantly larger on each side among PD patients than among controls, and (b) the subjective ranges of values employed by PD patients were substantially compressed, on average, by comparison with those of controls. Within-group analyses revealed that (a) among control subjects, but not among PD subjects, individual, overall mean unsigned errors, on each side, averaged across test angles, were significantly smaller under active than under passive conditions, and (b) the subjective ranges employed by PD patients, but not by controls, under active conditions significantly exceeded those under passive conditions. We conclude that these results are generally consistent with the notion that PD impairs unilateral elbow-joint position sense.