Raclopride-induced motor consolidation impairment in primates: role of the dopamine type-2 receptor in movement chunking into integrated sequences.

Results obtained in patients with schizophrenia have shown that antipsychotic drugs may induce motor learning deficits correlated with the striatal type-2 dopamine receptors (D(2)R) occupancy. Other findings suggest that the role of the striatum in motor learning could be related to a process of "chunking" discrete movements into motor sequences. We therefore hypothesized that a D(2)R blocking substance, such as raclopride, would affect motor learning by specifically disrupting the grouping of movements into sequences. Two monkeys were first trained to perform a baseline-overlearned sequence (Seq. A) drug free. Then, a new sequence was learned (Seq. B) and the overlearned sequence was recalled OFF-drug (Seq. A recall OFF-drug). The effect of raclopride was then assessed on the learning of a third sequence (Seq. C), and on the recall of the overlearned sequence (Seq. A recall ON-drug). Results showed that performance related to the overlearned sequence remained the same in the three experimental conditions (Seq. A, Seq. A recall OFF-drug, Seq. A recall ON-drug), whether the primates received raclopride or not. On the other hand, new sequence learning was significantly affected during raclopride treatment (Seq. C), when compared with new sequence learning without the effect of any drug (Seq. B). Raclopride-induced disturbances consisted in performance fluctuations, which persisted even after many days of trials, and prevented the monkeys from reaching a stable level of performance. Further analyses also showed that these fluctuations appeared to be related to monkeys' inability to group movements into single flowing motor sequences. The results of our study suggest that dopamine is involved in the stabilization or consolidation of motor performances, and that this function would involve a chunking of movements into well-integrated sequences.

Movement-related modulation across the receptive field of neurons in the primary somatosensory cortex of the monkey.

Cutaneous signals are modulated at the various relays just preceding and during voluntary movements. In these conditions, neuronal discharge in the primary somatosensory cortex (SI) related to movement per se is still evident while discharge to air puff stimulation on the skin is diminished. This selective modulation could be explained by rapid movement-related changes in receptive field (RF) configuration. We tested this hypothesis by giving air puff stimuli at different sites within and at the edges of the RF of SI cells during rest and elbow flexions in one awake monkey. For 40 cells, analysis of the global response yielded four different types of modulation: non-modulated cells, completely gated cells, partially and uniformly modulated cells, and non-uniformly modulated cells. Cell discharge for successive 5-ms intervals was also analyzed at the different sites and showed that response uniformity across time is more robust at the RF center than at peripheral sites in the RF. While this study did not show any clear RF displacement, intra-RF excitability seems to be affected by movement in various ways at the level of the cortex. These facts could have implications for information processing during movement.

Gait problems in diabetic neuropathic patients.

OBJECTIVE: To examine whether a reduced peripheral sensibility caused by diabetic neuropathy increases the attentional demands necessary for controlling and regulating gait. DESIGN: Nonrandomized control trial. SETTING: University motor performance laboratory. SUBJECTS: Twelve diabetic patients with peripheral neuropathy and 7 control subjects, all volunteers. INTERVENTIONS: All subjects first performed a control seated reaction time task. For the walking task, auditory stimuli were randomly presented in the third, fourth, or fifth walking cycle on left foot toe off on left foot heel contact. The subject's task was to respond verbally as fast as possible to the auditory stimulus, while maintaining progression. MAIN OUTCOME MEASURES: Simple reaction times and kinematics of the gait pattern (cycle amplitude, cycle duration, cycle speed, cadence and percentage of time spent in the single support phase) were evaluated. RESULTS: For the walking task, diabetic neuropathic patients had a smaller cycle amplitude, cycle speed, and percentage of time spent in the single support phase than control subjects. Also, reaction times while walking were higher for diabetic neuropathic patients than for control subjects. CONCLUSIONS: Diabetic neuropathic patients show a less destabilizing and more conservative gait than control subjects. The increased attentional demands in gait for the diabetic neuropathic patients, along with their more conservative gait pattern, suggest that a lack of proprioception from the legs affects the control of gait. Diminished sensory information makes gait control more cognitively dependent in diabetic neuropathic persons than in control subjects.

Postural stability in diabetic polyneuropathy.

OBJECTIVE: To examine whether sensory changes in lower limbs associated with diabetic sensory polyneuropathy compromise postural stability in different visual sensory conditions. RESEARCH DESIGN AND METHODS: The presence and severity of sensory neuropathy was evaluated with a clinical scale and measures of nerve conduction velocity in the lower limbs. Balance control was evaluated by testing subjects' postural stability (with a force platform) with vision, without vision, and during a recovery period after being without vision. RESULTS: Neuropathic patients showed larger ranges of sway, a faster sway speed, and a greater dispersion of sway than control subjects in all conditions. They also exhibited similar or less stable postural performance with vision than that of control subjects without vision. There was a strong relationship between the severity of the neuropathy and the postural stability. CONCLUSIONS: This experiment highlights that even with vision, the postural stability of neuropathic patients is impaired and may put them at higher risk of falling when performing more challenging daily tasks.