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In situ hybridization histochemistry and RNA blots were used to study the expression of glutamic acid decarboxylase (GAD) mRNA in rats with or without a unilateral lesion of midbrain dopamine neurons. Two populations of GAD mRNA positive neurons were found in the intact caudate-putamen, substantia nigra and fronto-parietal cortex. In caudate-putamen, only one out of ten of the GAD mRNA positive neurons expressed high levels, while in substantia nigra every second of the positive neurons expressed high levels of GAD mRNA. Relatively few, but intensively labelled neurons were found in the intact fronto-parietal cerebral cortex. In addition, one out of six of the GAD mRNA positive neurons in the fronto-parietal cortex showed a low labeling. On the ipsilateral side, the forebrain dopamine deafferentation induced an increase in the number of neurons expressing high levels of GAD mRNA in caudateputamen, and a decrease in fronto-parietal cortex. A smaller decrease was also seen in substantia nigra. However, the total number of GAD mRNA positive neurons were not significantly changed in any of these brain regions. The changes in the levels of GAD mRNA after the dopamine lesion were confirmed by RNA blot analysis. Hence, midbrain dopamine neurons appear to control neuronal expression of GAD mRNA by a tonic down-regulation in a fraction of GAD mRNA positive neurons in caudate-putamen, and a tonic up-regulation in a fraction of GAD mRNA positive neurons in fronto-parietal cortex and substantia nigra.

In the current investigation, we modified the high Go, low No-Go Sustained Attention to Response Task (SART). Some researchers argue a commission error, an inappropriate response to a No-Go stimulus, in the SART is due to the participant being inattentive, or perceptually decoupled, during stimulus onset. Response delays in the SART reduce commission errors. A response delay may therefore enable a participant who is initially inattentive to recouple their attention in time to appropriately perceive the stimulus and withhold a response to a No-Go stimulus. However, shortening stimulus display duration in the SART should limit the possibility of the participant identifying the stimulus later, if they are initially not attending the stimulus. A response delay should not reduce commission errors if stimulus duration is kept to the minimum duration enabling stimulus recognition. In two experiments, we shortened stimulus onset to offset duration and added response delays of varying lengths. In both experiments, even when stimulus duration was shortened, response delays notably reduced commission errors if the delay was greater than 250 ms. In addition, using the Signal Detection Theory perspective in which errors of commission in the SART are due to a lenient response bias–trigger happiness, we predicted that response delays would result in a shift to a more conservative response bias in both experiments. These predictions were verified. The errors of commission in the SART may not be a measures of conscious awareness per se, but instead indicative of the level of participant trigger happiness—a lenient response bias.

This investigation aimed at assessing the extent to which memory from practice in a specific condition of target displacement modulates temporal errors and movement timing of interceptive movements. We compared two groups practicing with certainty of future target velocity either in unchanged target velocity or in target velocity decrease. Following practice, both experimental groups were probed in the situations of unchanged target velocity and target velocity decrease either under the context of certainty or uncertainty about target velocity. Results from practice showed similar improvement of temporal accuracy between groups, revealing that target velocity decrease did not disturb temporal movement organization when fully predictable. Analysis of temporal errors in the probing trials indicated that both groups had higher timing accuracy in velocity decrease in comparison with unchanged velocity. Effect of practice was detected by increased temporal accuracy of the velocity decrease group in situations of decreased velocity; a trend consistent with the expected effect of practice was observed for temporal errors in the unchanged velocity group and in movement initiation at a descriptive level. An additional point of theoretical interest was the fast adaptation in both groups to a target velocity pattern different from that practiced. These points are discussed under the perspective of integration of vision and motor control by means of an internal forward model of external motion.

Direct and indirect corticospinal pathways to finger muscles may play a different role in control of the upper extremity. We used transcranial magnetic stimulation (TMS) and coherence analysis to characterize the corticospinal drive to the first dorsal interosseous (FDI) and abductor pollicis brevis (APB) when active during a precision and power grip task. In experiment 1, single motor units were recorded during precision grip and power grip in 20 adults (25.2 ± 7.1 years). Post-stimulus time histograms (PSTH) were obtained following TMS. In experiment 2, coherence and cross-correlation analysis of the FDI and APB surface EMG were used to investigate the temporal organization of corticospinal drive during precision grip and power grip in 15 adults (27.4 ± 8.1 years). We found no significant differences in PSTH peak onset (26.6 ± 1.9 vs. 26.7 ± 2.0 ms, p = 0.75), maximal peak (27.4 ± 1.9 vs. 27.4 ± 1.9 ms, p = 1.0) or peak duration (2.3 ± 1.1 vs. 2.3 ± 1.0 ms, p = 0.75) for the 11 recovered motor units during precision grip and power grip. Also, no significant difference in coherence or the width of the synchronization peaks during precision grip (7.2 ± 3.7 ms) and power grip (7.9 ± 3.1 ms) could be observed (p = 0.59). The short duration of peaks elicited in the PSTH of single motor units following TMS and central synchronization peaks of voluntarily activated motor units during precision and power grip suggests that the direct corticospinal pathway (the corticomotoneuronal system) is equally involved in the control of both tasks. The data do not support that indirect pathways would make a larger contribution to power grip.

Saccadic latencies measured in response to a step-wise displacement of the target may be substantially reduced if a gap separates the offset of the initial fixation point and the onset of the peripheral target. According to Fischer and Ramsperger (1984) this paradigm provokes a bimodal latency distribution which consists of a peak of very fast saccadic responses (express saccades) at about 110 ms and another peak arising from somewhat slower saccades (regular saccades). Using again the gap paradigm, we investigated the effect of an additional go/no-go (i.e. target trial/catch trial) decision on saccadic latencies. The experiments yielded the following results: (i) the distribution between the peaks of express and regular saccades strongly depends on the proportion of catch trials introduced into the trial sequence, which suggests the existence of different modes of operation of the decision processes for express and regular saccades. (ii) The catch trial effect on saccadic latency proved to be a local phenomenon in time: saccades which follow catch trials tend to be slower than those following target trials.

Motor imagery (M.I.) is a mental state in which real movements are evoked without overt actions. There is some behavioural evidence that M.I. declines with ageing. The neurofunctional correlates of these changes have been investigated only in two studies, but none of the these studies has measured explicit correlations between behavioural variables and the brain response, nor the correlation of M.I. and motor execution (M.E.) of the same acts in ageing. In this paper, we report a behavioural and functional magnetic resonance imaging (fMRI) experiment that aimed to address this issue. Twenty-four young subjects (27 ± 5.6 years) and twenty-four elderly subjects (60 ± 4.6 years) performed two block-design fMRI tasks requiring actual movement (M.E.) or the mental rehearsal (M.I.) of finger movements. Participants also underwent a behavioural mental chronometry test in which the temporal correlations between M.I. and M.E. were measured. We found significant neurofunctional and behavioural differences between the elderly subjects and the young subjects during the M.E. and the M.I. tasks: for the M.E. task, the elderly subjects showed increased activation in frontal and prefrontal (pre-SMA) cortices as if M.E. had become more cognitively demanding; during the M.I. task, the elderly over-recruited occipito-temporo-parietal areas, suggesting that they may also use a visual imagery strategy. We also found between-group behavioural differences in the mental chronometry task: M.I. and M.E. were highly correlated in the young participants but not in the elderly participants. The temporal discrepancy between M.I. and M.E. in the elderly subjects correlated with the brain regions that showed increased activation in the occipital lobe in the fMRI. The same index was correlated with the premotor regions in the younger subjects. These observations show that healthy elderly individuals have decreased or qualitatively different M.I. compared to younger subjects.

To better understand how arm weight support (WS) can be used to alleviate upper limb impairment after stroke, we investigated the effects of WS on muscle activity, muscle synergy expression, and corticomotor excitability (CME) in 13 chronic stroke patients and 6 age-similar healthy controls. For patients, lesion location and corticospinal tract integrity were assessed using magnetic resonance imaging. Upper limb impairment was assessed using the Fugl-Meyer upper extremity assessment with patients categorised as either mild or moderate–severe. Three levels of WS were examined: low = 0, medium = 50 and high = 100% of full support. Surface EMG was recorded from 8 upper limb muscles, and muscle synergies were decomposed using non-negative matrix factorisation from data obtained during reaching movements to an array of 14 targets using the paretic or dominant arm. Interactions between impairment level and WS were found for the number of targets hit, and EMG measures. Overall, greater WS resulted in lower EMG levels, although the degree of modulation between WS levels was less for patients with moderate–severe compared to mild impairment. Healthy controls expressed more synergies than patients with moderate–severe impairment. Healthy controls and patients with mild impairment showed more synergies with high compared to low weight support. Transcranial magnetic stimulation was used to elicit motor-evoked potentials (MEPs) to which stimulus–response curves were fitted as a measure of corticomotor excitability (CME). The effect of WS on CME varied between muscles and across impairment level. These preliminary findings demonstrate that WS has direct and indirect effects on muscle activity, synergies, and CME and warrants further study in order to reduce upper limb impairment after stroke.