Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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We investigated whether human attentional processes influence the activity of intracortical inhibitory and excitatory circuits--short-interval intracortical inhibition (SICI), long-interval intracortical inhibition (LICI), and the intracortical facilitation (ICF)--elicited by paired-pulse transcranial magnetic stimulation (TMS) in healthy subjects. In eight healthy subjects we tested SICI, LICI and ICF under different attention-demanding conditions: "relaxed", "target hand" and "non-target hand". To compare the effects of attentional levels on SICI, LICI and ICF with those produced on the MEPs elicited by repetitive TMS (rTMS), in the same subjects we also delivered supra-threshold 5-Hz rTMS under the same three experimental conditions. ⋯ Attentional levels had no influence on SICI, ICF and LICI activated by paired-pulse TMS, but increased the MEP facilitation elicited by 5-Hz rTMS. Varying the attention level left the findings from 1-Hz rTMS unchanged. The finding that attention leaves the activity of intracortical inhibitory and excitatory circuits elicited by paired-pulse TMS unchanged but influences the MEP facilitation elicited by 5-Hz rTMS suggests that attention operates only when the stimulation entrains neural circuits made up of a large number of cortical cells with plasticity properties.
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The aim of the current study was to examine the influence of visual feedback on compensatory variations in force within and between trials. In Experiment 1, the task was to maintain a constant force of 5 N for 15 s using both index fingers. In Experiment 2, the task was to produce discrete force pulses such that the peak value of the sum of the two finger forces was 5 N. ⋯ Similarly, between-trial correlations showed that the amount of error compensation decreased when feedback was removed, indicating that participants produced a smaller set of goal equivalent solutions. It appears that feedback has a dual role in coordination-removing the constraints on the degrees of freedom within trials, and facilitating the utilization of redundancy between trials. The distinction between these two classes of variation is central to understanding the redundancy problem in motor control.
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Comparative Study
Attenuation of N2 amplitude of laser-evoked potentials by theta burst stimulation of primary somatosensory cortex.
Theta burst stimulation (TBS) is a special repetitive transcranial magnetic stimulation (rTMS) paradigm, where bursts of low-intensity stimuli are applied in the theta frequency. The aim of this study was to investigate the effect of neuronavigated TBS over primary somatosensory cortex (SI) on laser-evoked potentials (LEPs) and acute pain perception induced with Tm : YAG laser stimulation. The amplitude changes of the N1, N2, and P2 components of LEPs and related subjective pain rating scores of 12 healthy subjects were analyzed prior to and following continuous TBS (cTBS), intermittent TBS (iTBS), intermediate TBS (imTBS), and sham stimulation. ⋯ The subjective pain perception also decreased during the experimental sessions, but did not differ significantly from the sham stimulation condition. The main finding of our study is that TBS over SI diminished the amplitude of the N2 component evoked from the contralateral side without any significant analgesic effects. Furthermore, imTBS produced responses similar to those observed by other forms of TBS induced excitability changes in the SI.
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Comparative Study
Contralesional paired associative stimulation increases paretic lower limb motor excitability post-stroke.
Following stroke, an abnormally high interhemispheric inhibitory drive from the contralesional to the ipsilesional primary motor cortex (M1) is evident during voluntary movement. Down-regulating motor excitability of the contralesional M1 using inhibitory neuromodulatory protocols has demonstrated a correlation between balanced interhemispheric interactions and increased motor recovery. In 2005, our laboratory first reported bidirectional modulation of healthy subjects' tibialis anterior (TA) motor excitability during walking, using a stimulation paradigm known as paired associative stimulation (PAS). ⋯ In healthy subjects, MEPs in response to TMS revealed that mean MEP amplitude from the stimulated TA decreased to 87% and the mean MEP amplitude from the non-stimulated TA increased to 126%. This is the first study to demonstrate that inhibitory PAS applied to the contralesional lower limb motor system of stroke survivors increases motor excitability of the paretic lower limb assessed during walking. This finding suggests that inhibitory PAS may be a useful tool to study how the human lower limb motor cortex recovers after neural injury, and that PAS may be a candidate adjuvant therapy for patients with neurological walking impairments.
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Comparative Study
Audiovisual temporal adaptation of speech: temporal order versus simultaneity judgments.
The temporal perception of simple auditory and visual stimuli can be modulated by exposure to asynchronous audiovisual speech. For instance, research using the temporal order judgment (TOJ) task has shown that exposure to temporally misaligned audiovisual speech signals can induce temporal adaptation that will influence the TOJs of other (simpler) audiovisual events (Navarra et al. (2005) Cognit Brain Res 25:499-507). Given that TOJ and simultaneity judgment (SJ) tasks appear to reflect different underlying mechanisms, we investigated whether adaptation to asynchronous speech inputs would also influence SJ task performance. ⋯ While performing these tasks, participants monitored a continuous speech stream for target words that were either presented in synchrony, or with the audio channel lagging 300 ms behind the video channel. We found that the sensitivity of participant's TOJ and SJ responses was reduced when the background speech stream was desynchronized. A significant modulation of the point of subjective simultaneity (PSS) was also observed in the SJ task but, interestingly, not in the TOJ task, thus supporting previous claims that TOJ and SJ tasks may tap somewhat different aspects of temporal perception.