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Hugo Théoret

Publications and source records attributed to Hugo Théoret.

17 recordsLinked to original sources
PubMed2006

A role for the inferior colliculus in multisensory speech integration.

Multisensory integration can occur at relatively low levels within the central nervous system. Recent evidence suggests that multisensory audio-visual integration for speech may have a subcortical component, as acoustic processing in the human brainstem is influenced by lipreading during speech perception. Here, stimuli depicting the McGurk illusion (a demonstration of auditory-visual integration using speech stimuli) were presented to a 12-year-old child (FX) with a circumscribed unilateral lesion of the right inferior colliculus. When McGurk-type stimuli were presented in the contralesional hemifield, illusory perception reflecting bimodal integration was significantly reduced compared with the ipsilesional hemifield and a group of age-matched controls. These data suggest a functional role for the inferior colliculus in the audio-visual integration of speech stimuli.

Acoustic Stimulation↗
PubMed2006

Intrahemispheric dysfunction in primary motor cortex without corpus callosum: a transcranial magnetic stimulation study.

BACKGROUND: The two human cerebral hemispheres are continuously interacting, through excitatory and inhibitory influences and one critical structure subserving this interhemispheric balance is the corpus callosum. Interhemispheric neurophysiological abnormalities and intrahemispheric behavioral impairments have been reported in individuals lacking the corpus callosum. The aim of this study was to examine intrahemispheric neurophysiological function in primary motor cortex devoid of callosal projections. METHODS: Intracortical excitatory and inhibitory systems were tested in three individuals with complete agenesis of the corpus callosum and sixteen healthy individuals. These systems were assessed using transcranial magnetic stimulation (TMS) protocols: motor threshold at rest, paired-pulse curve, and cortical silent period. RESULTS: TMS revealed no difference between the patient and control groups on the motor threshold measure, as well as intracortical facilitation and intracortical inhibition systems as tested by paired stimulation. However, intrahemispheric inhibitory function was found to be abnormal in participants without callosal projections, as the cortical silent period duration was significantly increased in the patient group. CONCLUSION: These data suggest that in addition to previously reported impaired interhemispheric function, patients lacking the entire corpus callosum also display abnormal intrahemispheric excitability of the primary motor cortex.

Adult↗
PubMed2006

Autism spectrum disorder: seeing is not understanding.

Impairments in social and emotional skills are a defining feature of autism spectrum disorder. Recent research shows that structural and functional abnormalities within the neural system that matches observation and execution of actions--the mirror neuron system--may explain the social aspects of the pathophysiology of autism spectrum disorder.

Autistic Disorder↗
PubMed2006

EEG evidence for the presence of an action observation-execution matching system in children.

In the adult human brain, passive observation of actions performed by others activates some of the same cortical areas that are involved in the execution of actions, thereby contributing to action recognition. This mechanism appears to occur through activation of a population of action-coding cells known as mirror neurons (MN). In the adult motor cortex, performing actions and observing human movement reduces the magnitude of the mu (8-13 Hz) rhythm, possibly reflecting MN system activity. Despite the wealth of information available regarding the adult MN system, little is known about its existence in children. Here, we used EEG to probe mu rhythm modulation in 15 children during observation and execution of hand actions. Our data show that mu rhythm attenuation occurs in children under 11 years old during observation of hand movements. Similarly to what has been reported in adults, observation of goal/object-orientated movement produces greater modulation of the mu rhythm than intransitive movement. These data confirm the existence of an observation-execution matching system in the immature human brain and may be of clinical value in the understanding of neurodevelopmental disorders associated with a faulty MN system, such as autism spectrum disorder.

Brain Mapping↗
PubMed2006

Paradoxical facilitation of attention in healthy humans.

Transcranial magnetic stimulation (TMS)-induced virtual lesions in healthy subjects can be used to test neurofunctional models of disease. The interhemispheric rivalry model of heminglect is well suited for such investigations, as simple predictions derived from clinical data can be tested without the caveats normally associated with lesion studies. One of these predictions is that release from contralateral inhibition should lead to increased parietal responsiveness, which, in turn, would enhance spatial attention. Here, we detail studies showing TMS-induced paradoxical functional facilitation of attention in healthy individuals and highlight their contribution to the understanding and treatment of neglect syndromes.

Attention↗
PubMed2005

Modulation of motor cortex excitability during action observation in disconnected hemispheres.

The degree of involvement of the right and left hemispheres in the system matching perception and execution of actions remains unclear. Some studies have proposed that action observation involves a bilateral network of brain areas, whereas others have suggested left-hemisphere dominance. We explored hemispheric lateralization associated with action observation using transcranial magnetic stimulation over primary motor cortex devoid of callosal influences. We assessed corticospinal excitability related to observation of finger movements in one split-brain patient and six controls. Results revealed that the left hemisphere of the split-brain patient shows enhanced excitability during action observation, whereas no enhancement was found in the right hemisphere. The controls showed increased excitability in both hemispheres during action observation. Our findings suggest a left-hemisphere dominance for mirror-neuron function in the absence of a corpus callosum.

Adolescent↗
PubMed2004

Unconscious modulation of motor cortex excitability revealed with transcranial magnetic stimulation.

The neuronal effects of sensory events that do not enter conscious awareness have been reported in numerous pathological conditions and in normal subjects. In the present study, unconscious modulation of corticospinal excitability was probed in healthy volunteers with transcranial magnetic stimulation (TMS). TMS-induced motor evoked potentials (MEPs) were collected from the first dorsal interosseus muscle while subjects performed a masked semantic priming task that has been shown to elicit covert motor cortex activations. Our data show that the amplitude of the MEPs is modulated by an unseen prime, in line with temporal patterns revealed with event related potentials. These data confirm previous reports showing specific motor neural responses associated with an unseen visual stimulus and establish TMS as a valuable tool in the study of the neural correlates of consciousness.

Adult↗
PubMed2004

Intracranial measurement of current densities induced by transcranial magnetic stimulation in the human brain.

Transcranial magnetic stimulation (TMS) is a non-invasive technique that uses the principle of electromagnetic induction to generate currents in the brain via pulsed magnetic fields. The magnitude of such induced currents is unknown. In this study we measured the TMS induced current densities in a patient with implanted depth electrodes for epilepsy monitoring. A maximum current density of 12 microA/cm2 was recorded at a depth of 1 cm from scalp surface with the optimum stimulation orientation used in the experiment and an intensity of 7% of the maximal stimulator output. During TMS we recorded relative current variations under different stimulating coil orientations and at different points in the subject's brain. The results were in accordance with current theoretical models. The induced currents decayed with distance form the coil and varied with alterations in coil orientations. These results provide novel insight into the physical and neurophysiological processes of TMS.

Adult↗
PubMed2004

Modulation of right motor cortex excitability without awareness following presentation of masked self-images.

The neural substrates of self-awareness have been studied with a variety of neurophysiological and behavioral tools. In the present study, unconscious modulation of corticospinal excitability following presentation of self-images was probed with transcranial magnetic stimulation (TMS). TMS-induced motor evoked potentials (MEP) were collected from the contralateral first dorsal interosseus (FDI) muscle while subjects viewed masked pictures of their own face. MEP amplitudes were compared to those obtained when pictures of strangers were masked. Masked self-images induced a relative increase in corticospinal excitability when TMS was applied to the right primary motor cortex. These results demonstrate the utility of TMS to probe unconscious processing and support the notion of hemispheric asymmetry in the processing of self-images.

Adult↗
PubMed2003

Modulation of intracortical neuronal circuits in human hand motor area by digit stimulation.

We investigated the changes in intracortical neuronal circuits of the hand motor cortex following sensory stimulation of the fingers in 11 healthy subjects. Motor evoked potentials (MEPs) were recorded from intrinsic hand muscles (right first dorsal interosseous and abductor digiti minimi muscles). Electrical stimulation was applied to a digit near (homotopic) or distant (heterotopic stimulation) from each muscle. The right index or little finger was stimulated electrically, followed by single- or paired-pulse transcranial magnetic stimulation (TMS) at an interval of 25, 200, 600, 1,000 or 1,400 ms. Paired-pulse TMS was applied with interstimuli intervals of 2 ms or 12 ms and was expected to stimulate inhibitory or facilitatory intracortical circuits, respectively. MEPs induced by single-pulse TMS were significantly suppressed 200, 600, and 1,000 ms after heterotopic and homotopic stimuli. Intracortical facilitation was significantly enhanced only after homotopic stimuli and such enhancement was maximal 200 ms after digit stimulation. Intracortical inhibition was slightly weakened after homotopic stimulation but this effect did not reach statistical significance ( P=0.25). Our results show that sensory feedback can modify intracortical and corticospinal motor excitability and that intracortical facilitation can be enhanced in a topographic-specific way especially at long latencies. These findings suggest that indirect pathways, probably through somatosensory cortex and other areas, enhance intracortical motor excitability in a somatotopically organized manner.

Adult↗
PubMed2003

The role of motion direction selective extrastriate regions in reading: a transcranial magnetic stimulation study.

Why reading ability is correlated with motion processing ability is perplexing. Activity in motion direction processing regions (Area V5/MT+) was perturbed by means of repetitive transcranial magnetic stimulation (rTMS) to examine its effect on reading. A functional probe (significant shortening of the motion aftereffect) was used to identify Area V5/MT+. Right-handed participants (8 m, 8 f) received three 7.5 min blocks of rTMS, after which two phonological and one orthographic reading tasks were administered. Application of rTMS to Area V5/MT+ (as compared to a non-rTMS baseline) significantly decreased performance only during non-word naming. The pattern of naming errors and the absence of deficits on the second phonological task were not consistent with a role for Area V5/MT+ in phonological decoding. Instead, its role in reading may be limited to image stabilization and/or letter localization.

Adult↗
PubMed2002

Language acquisition: do as you hear.

It is not uncommon to recognize a specific action by the sound it creates. Neurons have been discovered in monkey premotor cortex that may contribute to this ability; they respond to both performing an action and hearing its action-related sound, and may be critical for communicating with others, learning gestures and even acquiring language.

Animals↗
PubMed2002

Braille character discrimination in blindfolded human subjects.

Visual deprivation may lead to enhanced performance in other sensory modalities. Whether this is the case in the tactile modality is controversial and may depend upon specific training and experience. We compared the performance of sighted subjects on a Braille character discrimination task to that of normal individuals blindfolded for a period of five days. Some participants in each group (blindfolded and sighted) received intensive Braille training to offset the effects of experience. Blindfolded subjects performed better than sighted subjects in the Braille discrimination task, irrespective of tactile training. For the left index finger, which had not been used in the formal Braille classes, blindfolding had no effect on performance while subjects who underwent tactile training outperformed non-stimulated participants. These results suggest that visual deprivation speeds up Braille learning and may be associated with behaviorally relevant neuroplastic changes.

Adolescent↗
PubMed2002

Repetitive transcranial magnetic stimulation of human area MT/V5 disrupts perception and storage of the motion aftereffect.

Following adaptation to a moving stimulus, the introduction of a stationary pattern creates the illusion of motion. This phenomenon, known as the motion aftereffect (MAE), can be delayed by placing a blank storage interval between the adapting and test stimuli. Human motion selective area MT/V5 has been proposed as the likely neural origin of MAEs. To examine the role of MT/V5 in perceiving and storing MAEs, we applied repetitive transcranial magnetic stimulation (rTMS) to this area during a 10s storage interval and while subjects perceived illusory motion. Our results show that rTMS disrupts perception of the MAE when it is delivered in the early parts of the storage period and when it is applied during the perceptual MAE itself. Stimulation of control regions corresponding to V1 or Cz did not affect the MAE. In addition, magnetic stimulation of dorsolateral prefrontal and posterior parietal cortices did not disrupt MAE perception. These data provide experimental support for the notion that MT/V5 subserves perception and storage of the motion aftereffect.

Adult↗
PubMed2001

Stereological evaluation of neurons and glia in the monkey dorsal lateral geniculate nucleus following an early cerebral hemispherectomy.

The effects of an early, unilateral cerebral hemispherectomy on the cytoarchitecture of the dorsal lateral geniculate nucleus (dLGN) were quantitatively evaluated in the green monkey. The dLGN ipsilateral to the lesion showed a 73% reduction in size, more than 99% neuronal cell loss, 50% increase in glial cell density, but a 50% reduction in the total number of glial cells. The total number of neural and glial cells estimated for the dLGN contralateral to the ablation did not differ from control values. Despite evidence for substantial degeneration of the ipsilateral dLGN, cytochrome oxidase histochemistry revealed a small population of surviving cells that exhibited features of neuronal cells. More surviving cells were found in the parvocellular than in the magnocellular layers, and surviving parvocellular cells had the same size-frequency distribution as Nissl-stained neurons in an intact animal. These findings suggest that the intrinsic geniculate circuitry may be able to sustain the residual interneurons that can, in turn, contribute to maintaining retinal and brainstem afferents. The remaining neurons in the dLGN following hemispherectomy appear to be insufficient in number to be importantly implicated in the residual visual functions that have been reported in some hemispherectomized patients.

Animals↗
PubMedDate not supplied

Behavioral and neuroplastic changes in the blind: evidence for functionally relevant cross-modal interactions.

The study of blind individuals provides insight into the brain re-organization and behavioral compensations that occur following sensory deprivation. While behavioral studies have yielded conflicting results in terms of performance levels within the remaining senses, deafferentation of visual cortical areas through peripheral blindness results in clear neuroplastic changes. Most striking is the activation of occipital cortex in response to auditory and tactile stimulation. Indeed, parts of the "unimodal" visual cortex are recruited by other sensory modalities to process sensory information in a functionally relevant manner. In addition, a larger area of the sensorimotor cortex is devoted to the representation of the reading finger in blind Braille readers. The "visual" function of the deafferented occipital cortex is also altered, where transcranial magnetic stimulation-induced phosphenes can be elicited in only 20% of blind subjects. The neural mechanisms underlying these changes remain elusive but recent data showing rapid cross-modal plasticity in blindfolded, sighted subjects argue against the establishment of new connections to explain cross-modal interactions in the blind. Rather, latent pathways that participate in multisensory percepts in sighted subjects might be unmasked and may be potentiated in the event of complete loss of visual input. These issues have important implications for the development of visual prosthesis aimed at restoring some degree of vision in the blind.

Adaptation, Psychological↗