Abstract The subthalamic nucleus (STN) is a key structure within the basal ganglia, inactivation of which is a current strategy for treating parkinsonism. We have previously shown that bilateral lesions of the STN or pharmacological inactivation of this structure in the rat induce multiple deficits in serial reaction time tasks. The aim of the present study was to investigate further a possible role for the STN in response preparatory processes by using simple (SRT) and choice (CRT) reaction time tasks. In contrast to the CRT procedure, the information related to the location of where the response had to be made was given in advance in the SRT procedure. Accurate performance on these tasks requires not only the selection of the correct response (i.e. which response), but also preparation in order to perform when required. A comparison between the two tasks allows assessment of whether STN lesions affect which response (‘which’) or when to perform it (‘when’). As previously observed in these procedures, the responses were faster as a function of the variable foreperiod preceding the trigger stimulus. This well‐known effect, termed ‘motor readiness’, was maintained after STN lesions, suggesting that STN lesions did not affect the ‘when’ phase of action preparation. However, while performance on the SRT was faster than on the CRT task preoperatively, STN lesions slowed RTs and abolished the beneficial effect of advance information, suggesting a deficit in the selection (‘which’) phase of response preparation. This deficit in the selection phase was further supported by deficits in accuracy of responding after STN lesions, as well as increases in mislocated premature responding in the SRT condition. Together, these results suggest that the STN plays an important role in response preparatory processes, including response selection and inhibitory control processes.
Research has shown that individuals who have sustained mild head injury demonstrate a slowed speed of processing that is exacerbated by fatigue/stress. We administered the Paced Auditory Serial Addition Test (PASAT) at the beginning and at the end of a 4-h experimental protocol to determine whether fatigue or a stressor would result in poorer scores for individuals who had previously sustained mild head injury. A significant improvement was found between the first and second administration for both head-injured and control subjects, but difference scores revealed a significant between-groups difference for the first of the four trials, with the head-injured participants performing worse than controls. Apparently, head-injured participants were slower to develop, as well as slower to regain, a means of efficiently processing rapidly presented information.
Vierordt [(1868). Der zeitsinn nach versuchen. Tübingen: Laupp] observed that participants over-reproduce short durations and under-reproduce long durations within a test range of sample durations (Vierordt’s law). Similar phenomena have been reported for sensory and other processing (e.g. [Helson (1964 Helson, H. (1964). Adaptation-level theory. New York, NY: Harper & Row. [Google Scholar]). Adaptation-level theory. New York: Harper & Row; Stevens, and Greenbaum (1966). Regression effect in psychophysical judgment. Perception & Psychophysics, 1, 439–446]. Performance feedback (knowledge of results) does not correct this performance distortion although it does correct other errors. In Experiment 1, I tested the hypothesis that a random presentation order of the standard durations makes feedback on one trial inappropriate for the next trial preventing correction of the Vierordt effect. In Experiment 2, participants performed two consecutive reproductions after each standard duration. Having the opportunity to immediately utilize the feedback from the first reproduction did not eliminate the Vierordt effect, although participants did attempt to correct the error on the first reproduction. In Experiment 3, the Vierordt effect was reduced in a blocked presentation design. Feedback produced more veridical performance. I conclude that the resistance of the Vierordt effect to correction by feedback may result, in part, from feedback on a given trial being misapplied to correcting performance on the next trial. Ironically, the Vierordt effect, which produced the differing directions and magnitudes of performance errors reported by the feedback, may be what prevents feedback from correcting for the Vierordt effect.
Abstract Lesions of the subthalamic nucleus can restore some imbalances in motor output of the basal ganglia induced by nigrostriatal dopamine depletion, and have been proposed as a potential therapy for Parkinson's disease. Although there is substantial supporting evidence from experimental studies in both rats and primates, there is less information on the effects of subthalamic lesions alone. In order to characterize potential side effects, the present study evaluates the behavioural effects of unilateral excitotoxic lesions of the subthalamic nucleus in rats that have previously received either unilateral saline or 6‐hydroxydopamine injections into the nigrostriatal bundle on the same side. The 6‐hydroxydopamine lesions induced ipsilateral orientation asymmetries in head position and body axis bias, rotational asymmetries following injections of direct or indirect dopamine agonists, neglect of contralateral stimuli, and a reduction in the numbers of pellets retrieved with the contralateral paw in a skilled reaching task. Subsequent excitotoxic lesions of the subthalamic nucleus reduced (but did not abolish) rotational asymmetries, had no effects on the measures of neglect and skilled paw‐reaching, and produced contralateral orientation biases in head turning and body axis curling. Rats that received subthalamic lesions alone exhibited de novo impairments comprising contralateral biases in the orientation tests. These results support a neuromodulatory role of the subthalamic nucleus in regulating motor outputs of the basal ganglia, and caution that there may be distinct side effects of the lesion by itself. Whereas some impairments attributable to dopamine depletion may be alleviated by subthalamic manipulations, other symptoms are not, or may even be aggravated.
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