Hemispheric asymmetry and somatotopy of afferent inhibition in healthy humans.
ABSTRACT A conditioning electrical stimulus to a digital nerve can inhibit the motor-evoked potentials (MEPs) in adjacent hand muscles elicited by transcranial magnetic stimulation (TMS) to the contralateral primary motor cortex (M1) when given 25-50 ms before the TMS pulse. This is referred to as short-latency afferent inhibition (SAI). We studied inter-hemispheric differences (Experiment 1) and within-limb somatotopy (Experiment 2) of SAI in healthy right-handers. In Experiment 1, conditioning electrical pulses were applied to the right or left index finger (D2) and MEPs were recorded from relaxed first dorsal interosseus (FDI) and abductor digiti minimi (ADM) muscles ipsilateral to the conditioning stimulus. We found that SAI was more pronounced in right hand muscles. In Experiment 2, electrical stimulation was applied to the right D2 and MEPs were recorded from ipsilateral FDI, extensor digitorum communis (EDC) and biceps brachii (BB) muscles. The amount of SAI did not differ between FDI, EDC and BB muscles. These data demonstrate inter-hemispheric differences in the processing of cutaneous input from the hand, with stronger SAI in the dominant left hemisphere. We also found that SAI occurred not only in hand muscles adjacent to electrical digital stimulation, but also in distant hand and forearm and also proximal arm muscles. This suggests that SAI induced by electrical D2 stimulation is not focal and somatotopically specific, but a more widespread inhibitory phenomenon.
SourceAvailable from: Luigi Tamè[Show abstract] [Hide abstract]
ABSTRACT: Moving and interacting with the world requires that the sensory and motor systems share information, but while some information about tactile events is preserved during sensorimotor transfer the spatial specificity of this information is unknown. Afferent inhibition (AI) studies, in which corticospinal excitability (CSE) is inhibited when a single tactile stimulus is presented before a transcranial magnetic stimulation pulse over the motor cortex, offer contradictory results regarding the sensory-to-motor transfer of spatial information. Here, we combined the techniques of AI and tactile repetition suppression (the decreased neurophysiological response following double stimulation of the same vs. different fingers) to investigate whether topographic information is preserved in the sensory-to-motor transfer in humans. We developed a double AI paradigm to examine both spatial (same vs. different finger) and temporal (short vs. long delay) aspects of sensorimotor interactions. Two consecutive electrocutaneous stimuli (separated by either 30 or 125 ms) were delivered to either the same or different fingers on the left hand (i.e. index finger stimulated twice or middle finger stimulated before index finger). Information about which fingers were stimulated was reflected in the size of the motor responses in a time-constrained manner: CSE was modulated differently by same and different finger stimulation only when the two stimuli were separated by the short delay (P = 0.004). We demonstrate that the well-known response of the somatosensory cortices following repetitive stimulation is mirrored in the motor cortex and that CSE is modulated as a function of the temporal and spatial relationship between afferent stimuli. © 2015 Federation of European Neuroscience Societies and John Wiley & Sons Ltd.European Journal of Neuroscience 03/2015; DOI:10.1111/ejn.12890 · 3.67 Impact Factor
Brain Stimulation 01/2014; 7(2). DOI:10.1016/j.brs.2013.12.015 · 5.43 Impact Factor
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ABSTRACT: BACKGROUND: Mild cognitive impairment in Parkinson's disease (PD) is common and predicts those at risk of dementia. Cholinergic dysfunction may contribute to its pathophysiology and can be assessed using short latency afferent inhibition. METHODS: Twenty-two patients with PD (11 cognitively normal; 11 with mild cognitive impairment) and 22 controls participated. Short latency afferent inhibition was measured by conditioning motor evoked potentials, which were elicited by transcranial magnetic stimulation of the motor cortex with electrical stimuli delivered to the contralateral median nerve at varying interstimulus intervals. RESULTS: There was no significant difference between cognitively normal PD and controls for short latency afferent inhibition (62.8±30.3% vs. 55.7±21.7%; P=0.447). The PD-mild cognitive impairment group had significantly less inhibition (88.4±25.8%) than both cognitively normal PD (P=0.021) and controls (P=0.01). CONCLUSIONS: Cholinergic dysfunction occurs early in those with PD -mild cognitive impairment. Short latency afferent inhibition may be a useful biomarker of increased risk of dementia in PD patients. © 2013 Movement Disorder Society.Movement Disorders 08/2013; 28(9). DOI:10.1002/mds.25360 · 5.63 Impact Factor