Littérature scientifique sur le sujet « Somatosensory temporal discrimination »

Background and objective Somatosensory temporal discrimination was recently reported as prolonged during migraine attacks, which is consistent with disrupted sensorial perception in migraine. However, knowledge about central sensory processing in tension-type headache is still lacking. This prospective, controlled study aimed to investigate somatosensory temporal discrimination thresholds in tension-type headache. Methods The study included 10 tension-type headache patients, 10 migraine patients and 10 healthy volunteers without headache. Somatosensory temporal discrimination thresholds were evaluated during the headache attacks of tension-type headache and migraine patients. Results Somatosensory temporal discrimination thresholds of tension-type headache patients (39.0 ± 5.5 ms for the right hand and 40.6 ± 4.6 ms for the left hand) were significantly lower than those of episodic migraine patients (137.1 ± 35.8 ms for the right hand and 118.4 ± 34.3 ms for the left hand, p < 0.0001 and p < 0.0001 respectively), and comparable to those of healthy volunteers (38.6 ± 5.3 ms for the right hand and 38.3 ± 7.2 ms for the left hand, p = 0.79 and p = 0.45 respectively). Conclusion Central sensory processing, as tested by somatosensory temporal discrimination, was remarkably disrupted during the headache attacks in migraineurs, whereas it remained intact in the tension-type headache patients.

The somatosensory temporal discrimination threshold (STDT) has been used in recent years to investigate time processing of sensory information, but little is known about the physiological correlates of somatosensory temporal discrimination. The objective of this study was to investigate whether the time interval required to discriminate between two stimuli varies according to the number of stimuli in the task. We used the third-stimulus temporal discrimination threshold (ThirdDT), defined as the shortest time interval at which an individual distinguishes a third stimulus following a pair of stimuli delivered at the STDT. The STDT and ThirdDT were assessed in 31 healthy subjects. In a subgroup of 10 subjects, we evaluated the effects of the stimuli intensity on the ThirdDT. In a subgroup of 16 subjects, we evaluated the effects of S1 continuous theta-burst stimulation (S1-cTBS) on the STDT and ThirdDT. Results show that ThirdDT is shorter than STDT. We found a positive correlation between STDT and ThirdDT values. As long as the stimulus intensity was within the perceivable and painless range, it did not affect ThirdDT values. S1-cTBS significantly affected both STDT and ThirdDT, although the latter was affected to a greater extent and for a longer period of time. We conclude that the interval needed to discriminate between time-separated tactile stimuli is related to the number of stimuli used in the task. STDT and ThirdDT are encoded in S1, probably by a shared tactile temporal encoding mechanism whose performance rapidly changes during the perception process. ThirdDT is a new method to measure somatosensory temporal discrimination. NEW & NOTEWORTHY To investigate whether the time interval required to discriminate between stimuli varies according to changes in the stimulation pattern, we used the third-stimulus temporal discrimination threshold (ThirdDT). We found that the somatosensory temporal discrimination acuity varies according to the number of stimuli in the task. The ThirdDT is a new method to measure somatosensory temporal discrimination and a possible index of inhibitory activity at the S1 level.

Recent neuroimaging studies have revealed that tactile perceptual learning can lead to substantial reorganizational changes of the brain. We report here for the first time that combining high-frequency (15 Hz) repetitive transcranial magnetic stimulation (rTMS) over the primary somatosensory cortex (SI) with tactile discrimination training is capable of facilitating operant perceptual learning. Most notably, increasing the excitability of SI by 15-Hz rTMS improved perceptual learning in spatial, but not in temporal, discrimination tasks. These findings give causal support to recent correlative data obtained by functional magnetic resonance imaging studies indicating a differential role of SI in spatial and temporal discrimination learning. The introduced combination of rTMS and tactile discrimination training may provide new therapeutical potentials in facilitating neuropsychological rehabilitation of functional deficits after lesions of the somatosensory cortex.

This thesis describes a series of studies involving both healthy subjects and patients with dystonia, in which the mechanisms of inhibitory plasticity have been explored with the use of a novel non-invasive brain stimulation technique, namely High-Frequency Repetitive Sensory Stimulation (HF-RSS), to understand how inhibitory mechanisms contribute to the pathogenesis of dystonia. To this aim, several “preliminary” and parallel experiments have been conducted to fully characterize the neurophysiological abnormalities in dystonia and the physiological changes induced by HF-RSS in healthy subjects. Thus, I have explored: 1. The neurophysiological correlates of abnormal somatosensory temporal discrimination in cervical dystonia, linking this behavioural abnormality with defective inhibitory mechanisms within the sensory cortex; 2. The behavioural consequences of HF-RSS in healthy subjects in terms of somatosensory temporal discrimination, showing that this technique can be in fact used as a novel non-invasive brain stimulation protocol in order to reversibly improve somatosensory temporal discrimination; 3. The neurophysiological mechanisms by which the observed behavioural improvement occurs after HF-RSS in healthy subjects. Thus, the improvement of somatosensory temporal discrimination is mostly driven by an enhancement of inhibitory processes occurring within the primary sensory cortex, a phenomenon known as inhibitory plasticity; 4. Whether HF-RSS could ameliorate inhibitory processes in cervical dystonia and, in turn, lead to an improvement of somatosensory temporal discrimination. It is here shown that patients showed a paradoxical response to such a stimulation protocol, suggestive of defective inhibitory plasticity as one of the main mechanisms contributing to the pathogenesis of dystonia. These results contribute to the understanding of the pathophysiology of dystonia, opening a novel window for future research and possibly novel treatments. Moreover, these results widened the understanding relative to this novel type of non-invasive brain stimulation that can be theoretically used for the study of other disorders where central inhibitory processes are thought to be defective.

Animals need to assess when to initiate actions based on uncertain sensory evidence. To formulate a response, decision making systems must prioritize extraction of neuronal signals that represent ecologically relevant events from signals that are behaviorally less relevant. This is commonly known as selective attention. The current thesis aims to investigate two simple forms of attention in rodents: sensory prioritization to a specific modality and temporal cueing. The rat whisker system is functionally efficient, and anatomically well characterized. We therefore utilize the whisker touch as a model sensory system to investigate the neuronal and behavioral correlates of attention in rats. We begin this thesis by designing a novel simple detection task that investigated whether rats dedicate attentional resources to the sensory modality in which a near-threshold event is more likely to occur. Detection of low-amplitude events is critical to survival, and to formulate a response, animals must extract minute neuronal signals from the sensory modality that is more likely to provide key information. We manipulated attention by controlling the likelihood with which a stimulus was presented from one of two modalities. In a whisker session, 80% of trials contained a brief vibration stimulus applied to whiskers and the remaining 20% of trials contained a brief change of luminance. These likelihoods were reversed in a visual session. When a stimulus was presented in the high-likelihood context, detection performance increased and was faster compared with the same stimulus presented in the low-likelihood context. Sensory prioritization was also reflected in neuronal activity in the vibrissal area of primary somatosensory cortex: single units responded differentially to a whisker vibration stimulus when presented with higher probability compared to the same stimulus when presented with lower probability. Neuronal activity in the vibrissal cortex displayed signatures of multiplicative gain control and enhanced response to vibration stimuli during the whisker session. In Chapter 3, we replicated these findings in a forced choice paradigm and extended the investigation from somatosensory/visual to the somatosensory/auditory. Attention was similarly manipulated by controlling likelihoods of stimulus presentation. Again, we observed improvements in detection performance and reaction time, as well as improvements in discrimination performance for stimuli presented in a high-likelihood context. The behavioral consequences of a forced choice compared to simple detection task are discussed. Finally, we developed a novel task that investigated whether rats were able to dedicate attentional resources in time. Operating with some finite quantity of attentional resources, by direct these resources at the expected time, animals would benefit from prioritizing processing based on temporal cues. We manipulated temporal cueing by presenting an auditory cue that preceded a target vibration stimulus in a subset of trials. On another subset, no auditory cue was presented. Presentations of these trials were of equal probability. Critically in this paradigm, the auditory cue provided temporal information but did not provide any spatial information about the location of the vibration stimulus. The auditory cue increased detection and discrimination performances and resulted in faster responses compared to trials in which the cue was absent. We observed neuronal signatures of temporal cuing in the vibrissal area of the primary somatosensory cortex. Single units showed enhanced response to the vibration stimulus during trials in which the stimulus was temporally expected. However, we did not observe signatures of multiplicative gain control in this paradigm. Instead, a decrease in baseline activity was observed that was phase locked to the onset of the auditory cue. In summary, this thesis presents two novel paradigms to study selective attention in rats in the form of sensory prioritization and temporal cueing. In addition, we investigate the neuronal correlates of selective attention in the vibrissal area of the primary somatosensory cortex. These series of experiments establish the rat as an alternative model organism to primates for studying attention.

Introduction: Cannabis is widely popular recreational drug of choice in the US. The drug is known to alter the subjective experience of time. However, its effects on time estimation at a brain level are still largely unexplored. Our goal was to investigate acute effects of cannabis on an fMRI time estimation task by evaluating brain activation differences between cannabis and placebo conditions. We hypothesized that participants’ time estimation accuracy and corresponding BOLD response would be altered during the cannabis condition in a dose-related manner, compared to placebo. Methods: In this placebo-controlled, double-blind randomized trial, a total of N=44 participants had 3 dose visits, at each of which they received either high-dose cannabis (0.5 gm of ~12.5% THC flower), low dose cannabis (0.5 gm of ~5.7% flower) or 0.5 gm placebo, using paced inhalation from a volcano via vaporizer. Drug material was supplied by NIDA/RTI. For the current study we analyzed fMRI data from the first of placebo and high dose fMRI sessions throughout each dosing day in which participants performed a time estimation task. Participants were asked to respond with a mouse click as to which box of two boxes displayed for different intervals was displayed on the screen longer. Both sub-second and supra-second temporal intervals were tested, with a range of easy to hard discriminations. We used the Human Connectome Project processing pipeline to prepare fMRI data for GLM modeling of activation using the FSL FEAT toolbox. This model estimated the unique effect sub-second (short) and supra-second (long) interval discrimination, their average effect, and their difference. From these contrasts, the mean activation amplitudes within 387 brain parcels from the Human Connectome cortical atlas were extracted. Robust statistics in R software estimated a paired t test equivalent using the bootdpci function to assess the difference between placebo and the high dose drug conditions for each contrast. Results: Only premotor cortex survived False Discovery Rate corrections for searching all 387 parcels across the entire brain for the average of short and long temporal estimation conditions. Numerous other brain regions differed between placebo and high doses at p<.05 uncorrected for various task contrasts: Short duration stimuli: Premotor cortex, posterior cingulate cortex, medial temporal cortex, visual area, somatosensory cortex, anterior cingulate and medial prefrontal cortex, paracentral and mid-cingular cortex, inferior frontal cortex. Long duration stimuli: Premotor cortex, visual areas, somatosensory motor cortex, paracentral and mid- cingulate cortex, the tempo-parieto-occipital junction, dorsolateral-prefrontal cortex, posterior opercular cortex, medial temporal cortex, posterior cingulate cortex, orbito-frontal cortex. Average of short and long duration stimuli: Premotor cortex, somatosensory and motor cortex, posterior cingulate cortex, visual are, medial temporal cortex, paracentral and midcingulate cortex, anterior cingulate and medial prefrontal cortex, inferior frontal cortex, tempo-parieto-occipital junction, premotor cortex, somatosensory motor cortex, posterior cingulate cortex, medial temporal cortex, orbital and polar frontal cortex, hippocampus. Difference of short and long duration stimuli: Anterior cingulate and medial prefrontal cortex, ventral stream visual cortex, dorsal stream visual cortex, early visual cortex. Conclusions: The current study elicited multiple brain activation differences for the initial, acute high-dose cannabis vs. placebo condition, but only premotor cortex region survived as significant following multiple comparison correction for short and long duration stimuli contrast. A post hoc power analysis showed that adding 10 additional subjects to this sample would achieve significance with multiple comparison correction for medium effect sizes at alpha=0.05. Future studies on a larger sample can help identify such significant activation differences, and examining all doses and tasks would elucidate unfolding of effects longitudinally post-dose, and dose-dependence of effects.