Transcallosal Excitability Research Articles

Altered motor system function in post-concussion syndrome as assessed via transcranial magnetic stimulation

ObjectiveIt is unclear why specific individuals incur chronic symptoms following a concussion. This exploratory research aims to identify and characterize any neurophysiological differences that may exist in motor cortex function in post-concussion syndrome (PCS). MethodsFifteen adults with PCS and 13 healthy, non-injured adults were tested. All participants completed symptom questionnaires, and transcranial magnetic stimulation (TMS) was used to measure intracortical and transcallosal excitability and inhibition in the dominant motor cortex. ResultsCortical silent period (p = 0.02, g = 0.96) and ipsilateral silent period (p = 0.04, g = 0.78) were shorter in the PCS group compared to the control group which may reflect reduced GABA-mediated inhibition in PCS. Furthermore, increased corticomotor excitability was observed in the left hemisphere but not the right hemisphere. ConclusionsThese data suggest that persistent neurophysiological differences are present in those with PCS. The exact contributing factors to such changes remain to be investigated by future studies. SignificanceThis study provides novel evidence of lasting neurophysiological changes in PCS.

Expression of praxis induction on cortical excitability in juvenile myoclonic epilepsy.

This study aimed to evaluate the effects of praxis induction on sensorimotor cortical and transcallosal excitability in juvenile myoclonic epilepsy (JME). A total of 36 subjects (18-62years) were included. The JME group was screened by video-electroencephalography neuropsychological protocol and divided into JME without praxis induction [JME-WI (n=12)], JME with praxis-induced seizures or epileptiform discharges [JME-PI (n=10)], and healthy controls (n=14). Motor and somatosensory cortical excitability and transcallosal pathways were evaluated through single-pulse transcranial magnetic stimulation (sTMS) and somatosensory evoked potentials (SEPs). Motor and transcallosal excitabilities tested with sTMS were not different in the motor-dominant or non-dominant hemisphere among groups. Significant differences were found in cortical SEP amplitudes in the P27 component of the non-dominant hemisphere (p=0.03, Cohen's d=0.98), N35 in the dominant hemisphere (p=0.04, Cohen's d=0.96), and P27-35 interpeak amplitude in both somatosensory cortices of the JME-PI group (p=0.03, Cohen's d=0.96; p=0.02, Cohen's d=1.05) when compared with healthy controls. Giant SEPs were observed in two (16.7%) and five (50%) patients of the JME-WI and JME-PI groups, respectively. Cortical latencies did not reveal differences. Praxis induction was associated with enhanced excitability in the somatosensory cortex of JME patients. These findings may help clarifying the less favorable therapeutic response in the JME-PI group and indicate identifying praxis induction as an important determinant in differentiating between JME patients.

Neurophysiological and behavioural effects of dual-hemisphere transcranial direct current stimulation on the proximal upper limb.

Dual-hemisphere transcranial direct current stimulation over the primary motor cortex (M1-M1 tDCS) is assumed to modulate neural excitability in a polarity-dependent manner and improve motor performance of the hand. In the proximal upper limb, the neurophysiological and behavioural after-effects of M1-M1 tDCS are not well known. This study investigated the after-effects of M1-M1 tDCS on contralateral, ipsilateral and transcallosal excitability to the proximal upper limb muscle biceps brachii (BB). Circle tracing was used to assess motor performance before and after tDCS as this task requires coordination of proximal and distal musculature. Sixteen healthy right-handed adults participated in the study, each receiving M1-M1 tDCS (1mA, 15min) or sham tDCS in separate sessions. The anode was positioned over right M1 and cathode over left M1. M1-M1 tDCS suppressed transcallosal inhibition from the M1 under the cathode (P<0.045). No other neurophysiologic or behavioural effects were observed (P>0.6). The study provides important information regarding inconsistent neurophysiological and behavioural changes following tDCS that have implications for future tDCS research on the motor system.

An age-related change in the ipsilateral silent period of a small hand muscle

ObjectiveTo establish the presence or absence of an age effect on the ipsilateral silent period (iSP) for the abductor pollicis brevis (APB) muscle in healthy subjects. MethodsTwenty young adults (10 men, 10 women; age range: 20–40) and 20 older adults (10 men, 10 women; age range: 50–70) were matched by age (+30years), gender and height (±5cm). All were right-handed. We investigated the iSP for the APB by applying transcranial magnetic stimulation (TMS) and recording surface electromyograms. The contralateral motor-evoked potential (MEP) onset latency, the iSP onset and end latency (iSPOL and iSPEL) were measured and the iSP duration (iSPD) and transcallosal conduction time (TCT) were calculated. We evaluated the correlation between age and iSP, the latter’s intra- and intersession reproducibility and potential influencing factors. ResultsMean iSPOL, iSPEL and TCT values were significantly greater in older adults (both men and women) than in young adults. Intra- and intersession reproducibility was good. The mean left-side iSPEL and iSPD were longer than the right-side mean values in young adults but not in older adults. In both age groups, women displayed shorter latencies than men. ConclusionsThere is a strong effect of age on iSP parameters. SignificanceOur iSP results may evidence a decrease in transcallosal excitability with age, rather than slowing of the transcallosal interneuron conduction velocity.

P8.5 An age-related change in the ipsilateral silent period of a small hand muscle

Abstract Objective To establish the presence or absence of an age effect on the ipsilateral silent period (iSP) for the abductor pollicis brevis (APB) muscle in healthy subjects. Methods Twenty young adults (10 men, 10 women; age range: 20–40) and 20 older adults (10 men, 10 women; age range: 50–70) were matched by age (+30 years), gender and height (±5 cm). All were right-handed. We investigated the iSP for the APB by applying transcranial magnetic stimulation (TMS) and recording surface electromyograms. The contralateral motor-evoked potential (MEP) onset latency, the iSP onset and end latency (iSPOL and iSPEL) were measured and the iSP duration (iSPD) and transcallosal conduction time (TCT) were calculated. We evaluated the correlation between age and iSP, the latter’s intra- and intersession reproducibility and potential influencing factors. Results Mean iSPOL, iSPEL and TCT values were significantly greater in older adults (both men and women) than in young adults. Intra- and intersession reproducibility was good. The mean left-side iSPEL and iSPD were longer than the right-side mean values in young adults but not in older adults. In both age groups, women displayed shorter latencies than men. Conclusions There is a strong effect of age on iSP parameters. Significance Our iSP results may evidence a decrease in transcallosal excitability with age, rather than slowing of the transcallosal interneuron conduction velocity.

Bilateral Cortical Interaction: Modulation of Delay-Tuned Neurons in the Contralateral Auditory Cortex

Transcallosal excitation and inhibition have been theorized based on the effect of callosotomy on intractable epilepsy and dichotic listening research, respectively. We studied bilateral interaction of cortical auditory neurons and found that this interaction consisted of focused facilitation and widespread lateral inhibition. The frequency modulated (FM)-FM area of the auditory cortex of the mustached bat is composed of delay-tuned neurons tuned to the combination of the emitted biosonar pulse and its echo with a specific echo delay [best delay (BD)] and consists of three subdivisions in terms of the combination sensitivity of neurons. We found that focal electric stimulation of one of these three subdivisions evoked BD shifts of delay-tuned neurons in all three subdivisions of the contralateral FM-FM area, presumably via the corpus callosum. The effect of electric stimulation of the delay-tuned neurons on the contralateral delay-tuned neurons was different depending on whether the BD of a recorded neuron was matched or unmatched in BD with that of the stimulated neurons. BD-matched neurons did not change their BDs and increased the responses at their BDs, whereas BD-unmatched neurons shifted their BDs away from the BD of the stimulated neurons and reduced their responses. The ipsilateral and contralateral BD shifts evoked by the electric stimulation were identical to each other. The contralateral modulation, in addition to the ipsilateral modulation, increases the contrast in the neural representation of the echo delay to which the stimulated neurons are tuned.

Effects of transcranial direct current stimulation over the human motor cortex on corticospinal and transcallosal excitability.

Weak transcranial direct current stimulation (tDCS) can induce long lasting changes in cortical excitability. In the present study we asked whether tDCS applied to the left primary motor cortex (M1) also produces aftereffects distant from the site of the stimulating electrodes. We therefore tested corticospinal excitability in the left and the right M1 and transcallosal excitability between the two cortices using transcranial magnetic stimulation (TMS) before and after applying tDCS. Eight healthy subjects received 10 min of anodal or cathodal tDCS (1 mA) to the left M1. We examined the amplitude of contralateral motor evoked potentials (MEPs) and the onset latency and duration of transcallosal inhibition with single pulse TMS. MEPs evoked from the tDCS stimulated (left) M1 were increased by 32% after anodal and decreased by 27% after cathodal tDCS, while transcallosal inhibition evoked from the left M1 remained unchanged. The effect on MEPs evoked from the left M1 lasted longer for cathodal than for anodal tDCS. MEPs evoked from the right M1 were unchanged whilst the duration of transcallosal inhibition evoked from the right M1 was shortened after cathodal tDCS and prolonged after anodal tDCS. The duration of transcallosal inhibition returned to control values before the effect on the MEPs from the left M1 had recovered. These findings are compatible with the idea that tDCS-induced aftereffects in the cortical motor system are limited to the stimulated hemisphere, and that tDCS not only affects corticospinal circuits involved in producing MEPs but also inhibitory interneurons mediating transcallosal inhibition from the contralateral hemisphere.

Motor cortical disinhibition in the unaffected hemisphere after unilateral cortical stroke.

Following a hemispheric stroke, various degrees of neuronal reorganization around the lesion occur immediately after disease onset and thereafter up to several months. These include transcallosal excitability, changes of the intact motor cortex and ipsilateral motor responses after transcranial magnetic stimulation (TMS) on the intact hemisphere. To elucidate the relationship between lesion localization and motor cortex excitability (intracortical inhibition; ICI) in the intact hemisphere, we applied a paired conditioning-test TMS paradigm in 12 patients with unilateral cortical stroke (cortical group) and nine patients with subcortical stroke caudal to the corpus callosum (subcortical group), with interstimulus intervals varying from 1 to 10 ms. All patients exhibited unilateral complete hand palsy. ICI was significantly less in the cortical group than in age-matched healthy control subjects. It was especially more marked in the cortical group patients with a disease duration of less than 4 months after onset. Patients in the cortical group with a duration longer than 4 months showed a tendency for ICI to be normalized, and there was a significant correlation between ICI and disease duration. Patients in the subcortical group showed normal excitability curves. All patients in the cortical group showed no transcallosal inhibition (TCI) in the active unaffected hand muscle after TMS of the affected motor cortex, whereas all the subcortical patients showed some TCI. No ipsilateral motor responses were elicited in the paretic hand in any of the patients. The reduced ICI in the cortical group might have been a result of disruption of TCI. The normalization of ICI in the patients with longer disease duration and the normal ICI in the subcortical group patients do not support the functional significance of motor cortex hyperexcitability in the unaffected hemisphere, at least in a patient population with poor motor recovery.

The callosal dilemma: Explaining diaschisis in the context of hemispheric rivalry via a neural network model

It is often suggested that a major factor in diaschisis is the loss of transcallosal excitation to the intact hemisphere from the lesioned one. However, there is long-standing disagreement in the broader experimental literature about whether transcallosal interhemispheric influences in the human brain are primarily excitatory or inhibitory. Some experimental data are apparently better explained by assuming inhibitory callosal influences. Past neural network models attempting to explore this issue have encountered the same dilemma: in intact models, inhibitory callosal influences best explain strong cerebral lateralization like that occurring with language, but in lesioned models, excitatory callosal influences best explain experimentally observed hemispheric activation patterns following brain damage. We have now developed a single neural network model that can account for both types of data, i.e., both diaschisis and strong hemisphere specialization in the normal brain, by combining excitatory callosal influences with subcortical cross-midline inhibitory interactions. The results suggest that subcortical competitive processes may be a more important factor in cerebral specialization than is generally recognized. [Neurol Res 2001; 23: 465-471]

Subject's own speech reduces reactivity of the human auditory cortex

Previous studies on monkeys have shown that uttering-related cortical areas exert an inhibitory effect on the auditory cortex, and cerebral blood-flow analyses on humans have revealed modulation of the activity of the auditory cortex during own speech. To study this modulation on a millisecond time scale, we recorded neuromagnetic evoked responses to short 1-kHz tones while the subjects were reading silently and aloud. The 100-ms response (M100) of the auditory cortex was delayed by 10–21 ms and its amplitude was dampened by 44–71% during reading aloud compared with reading silently. This effect was more prominent in responses to ipsilateral than contralateral tones, possibly due to a sum effect of diminished ipsilateral input to the cortex and decreased transcallosal excitation.

New evidence on the callosal system.

In the present paper experimental data on the principles of functional organization of the animal callosal system and the roles of symmetry and asymmetry in interhemispheric interaction are considered. A conclusion has been made that the cerebral hemispheres function not according to homotopicity principle but according to the principle of heterotopicity. A utilization of both homotopical callosal projections and intracortical pathways takes place. In interhemispheric relations the major role is played by the principle of the common pathway. Of great importance in asymmetry organization is the physiological dominant, which is formed and reinforced by excitation spreading through the corpus callosum and along intracortical pathways. Transcallosal excitation, interacting with thalamocortical and extracallosal ones, contributes to the creation and modulation of functional interhemispheric asymmetry.