Paired-Pulse Parietal-Motor Stimulation Differentially Modulates Corticospinal Excitability across Hemispheres When Combined with Prism Adaptation

Dual-site TMS as a tool to probe effective interactions within the motor network: a review

Dual-site transcranial magnetic stimulation (ds-TMS) is well suited to investigate the causal effect of distant brain regions on the primary motor cortex, both at rest and during motor performance and learning. However, given the broad set of stimulation parameters, clarity about which parameters are most effective for identifying particular interactions is lacking. Here, evidence describing inter- and intra-hemispheric interactions during rest and in the context of motor tasks is reviewed. Our aims are threefold: (1) provide a detailed overview of ds-TMS literature regarding inter- and intra-hemispheric connectivity; (2) describe the applicability and contributions of these interactions to motor control, and; (3) discuss the practical implications and future directions. Of the 3659 studies screened, 109 were included and discussed. Overall, there is remarkable variability in the experimental context for assessing ds-TMS interactions, as well as in the use and reporting of stimulation parameters, hindering a quantitative comparison of results across studies. Further studies examining ds-TMS interactions in a systematic manner, and in which all critical parameters are carefully reported, are needed.

Effective connectivity underlying neural and behavioral components of prism adaptation

Prism adaptation (PA) is a form of visuomotor training that produces both sensorimotor and cognitive aftereffects depending on the direction of the visual displacement. Recently, a neural framework explaining both types of PA-induced aftereffects has been proposed, but direct evidence for it is lacking. We employed Structural Equation Modeling (SEM), a form of effective connectivity analysis, to establish directionality among connected nodes of the brain network thought to subserve PA. The findings reveal two distinct network branches: (1) a loop involving connections from the parietal cortices to the right parahippocampal gyrus, and (2) a branch linking the lateral premotor cortex to the parahippocampal gyrus via the cerebellum. Like the sensorimotor aftereffects, the first branch exhibited qualitatively different modulations for left versus right PA, and critically, changes in these connections were correlated with the magnitude of the sensorimotor aftereffects. Like the cognitive aftereffects, changes in the second branch were qualitatively similar for left and right PA, with greater change for left PA and a trend correlation with cognitive aftereffects. These results provide direct evidence that PA is supported by two functionally distinct subnetworks, a parietal–temporal network responsible for sensorimotor aftereffects and a fronto-cerebellar network responsible for cognitive aftereffects.

Choosing Sides: Impact of Prismatic Adaptation on the Lateralization of the Attentional System

Seminal studies revealed differences between the effect of adaptation to left- vs. right-deviating prisms (L-PA, R-PA) in normal subjects. Whereas L-PA leads to neglect-like shift in attention, demonstrated in numerous visuo-spatial and cognitive tasks, R-PA has only minor effects in specific aspects of a few tasks. The paucity of R-PA effects in normal subjects contrasts with the striking alleviation of neglect symptoms in patients with right hemispheric lesions. Current evidence from activation studies in normal subjects highlights the contribution of regions involved in visuo-motor control during prism exposure and a reorganization of spatial representations within the ventral attentional network (VAN) after the adaptation. The latter depends on the orientation of prisms used. R-PA leads to enhancement of the ipsilateral visual and auditory space within the left inferior parietal lobule (IPL), switching thus the dominance of VAN from the right to the left hemisphere. L-PA leads to enhancement of the ipsilateral space in right IPL, emphasizing thus the right hemispheric dominance of VAN. Similar reshaping has been demonstrated in patients. We propose here a model, which offers a parsimonious explanation of the effect of L-PA and R-PA both in normal subjects and in patients with hemispheric lesions. The model posits that prismatic adaptation induces instability in the synaptic organization of the visuo-motor system, which spreads to the VAN. The effect is lateralized, depending on the side of prism deviation. Successful pointing with prisms implies reaching into the space contralateral, and not ipsilateral, to the direction of prism deviation. Thus, in the hemisphere contralateral to prism deviation, reach-related neural activity decreases, leading to instability of the synaptic organization, which induces a reshuffling of spatial representations in IPL. Although reshuffled spatial representations in IPL may be functionally relevant, they are most likely less efficient than regular representations and may thus cause partial dysfunction. The former explains, e.g., the alleviation of neglect symptoms after R-PA in patients with right hemispheric lesions, the latter the occurrence of neglect-like symptoms in normal subjects after L-PA. Thus, opting for R- vs. L-PA means choosing the side of major IPL reshuffling, which leads to its partial dysfunction in normal subjects and to recruitment of alternative or enhanced spatial representations in patients with hemispheric lesions.

Improvement of phonemic fluency following leftward prism adaptation

Anatomo functional studies of prism adaptation (PA) have been shown to modulate a brain frontal-parieto-temporal network, increasing activation of this network in the hemisphere ipsilateral to the side of prism deviation. This effect raises the hypothesis that left prism adaptation, modulating frontal areas of the left hemisphere, could modify subjects’ performance on linguistic tasks that map on those areas. To test this hypothesis, 51 healthy subjects participated in experiments in which leftward or rightward prism adaptation were applied before the execution of a phonemic fluency task, i.e., a task with strict left hemispheric lateralization onto frontal areas. Results showed that leftward PA significantly increased the number of words produced whereas rightward PA did not significantly modulate phonemic fluency. The present findings document modulation of a language ability following prism adaptation. The results could have a huge clinical impact in neurological populations, opening new strategies of intervention for language and executive dysfunctions.

Modifying auditory perception with prisms? Aftereffects of prism adaptation on a wide auditory spectrum in musicians and nonmusicians

Prism adaptation consists of pointing to visual targets while wearing prisms that shift the visual field laterally. The aftereffects are not restricted to sensorimotor level but extend to spatial cognition. There is a link between spatial representation and auditory frequency, with an association of low frequencies on the left side and high frequencies on the right side of space. The present study aimed first at evaluating the representation of auditory frequencies on a wide range of frequencies in musicians and nonmusicians. We used the 'auditory interval bisection judgment' within three auditory intervals. The results showed a pseudoneglect behavior in pretest in musicians and nonmusicians for high frequency intervals, reflecting a perceptual bias of the subjective interval center toward lower frequencies. The second aim of the present study was to evaluate the aftereffects of prism adaptation on an expanded auditory spectrum. The results showed aftereffects of adaptation to a leftward optical deviation for high frequency intervals in musicians and nonmusicians. Adaptation to a leftward optical deviation affects the auditory perception on an extended auditory spectrum, by shifting the subjective interval center toward high frequencies. The present study provides innovative data about representation of auditory perception and its modulation by prism adaptation.

Prism Adaptation Modulates Connectivity of the Intraparietal Sulcus with Multiple Brain Networks

Prism adaptation (PA) alters spatial cognition according to the direction of visual displacement by temporarily modifying sensorimotor mapping. Right-shifting prisms (right PA) improve neglect of left visual field in patients, possibly by decreasing activity in the left hemisphere and increasing it in the right. Left PA shifts attention rightward in healthy individuals by an opposite mechanism. However, functional imaging studies of PA are inconsistent, perhaps because of differing activation tasks. We measured resting-state functional connectivity (RSFC) in healthy individuals before and after PA. When contrasted, right versus left PA decreased RSFC in the spatial navigation network defined by the right posterior parietal cortex (PPC), hippocampus, and cerebellum. Within-PA-direction comparisons showed that right PA increased RSFC in subregions of the PPCs and between the PPCs and the right middle frontal gyrus and left PA decreased RSFC between these regions. Both right and left PA decreased RSFC between the PPCs and bilateral temporal areas. In summary, right PA increases connectivity in the right frontoparietal network and left PA produces essentially opposite effects. Furthermore, right, compared with left, PA modulates RSFC in the right hemisphere navigation network.

Using Dual-Site Transcranial Magnetic Stimulation to Probe Connectivity between the Dorsolateral Prefrontal Cortex and Ipsilateral Primary Motor Cortex in Humans

Dual-site transcranial magnetic stimulation to the primary motor cortex (M1) and dorsolateral prefrontal cortex (DLPFC) can be used to probe functional connectivity between these regions. The purpose of this study was to characterize the effect of DLPFC stimulation on ipsilateral M1 excitability while participants were at rest and contracting the left- and right-hand first dorsal interosseous muscle. Twelve participants were tested in two separate sessions at varying inter-stimulus intervals (ISI: 4, 5, 6, 7, 8, 9, 10, 11, 12, 15, and 20 ms) at two different conditioning stimulus intensities (80% and 120% of resting motor threshold). No significant effect on ipsilateral M1 excitability was found when applying a conditioning stimulus over DLPFC at any specific inter-stimulus interval or intensity in either the left or right hemisphere. Our findings suggest neither causal inhibitory nor faciliatory influences of DLPFC on ipsilateral M1 activity while participants were at rest or when performing an isometric contraction in the target hand muscle.

Prismatic Adaptation Modulates Oscillatory EEG Correlates of Motor Preparation but Not Visual Attention in Healthy Participants

Prismatic adaption (PA) has been proposed as a tool to induce neural plasticity and is used to help neglect rehabilitation. It leads to a recalibration of visuomotor coordination during pointing as well as to aftereffects on a number of sensorimotor and attention tasks, but whether these effects originate at a motor or attentional level remains a matter of debate. Our aim was to further characterize PA aftereffects by using an approach that allows distinguishing between effects on attentional and motor processes. We recorded EEG in healthy human participants (9 females and 7 males) while performing a new double step, anticipatory attention/motor preparation paradigm before and after adaptation to rightward-shifting prisms, with neutral lenses as a control. We then examined PA aftereffects through changes in known oscillatory EEG signatures of spatial attention orienting and motor preparation in the alpha and beta frequency bands. Our results were twofold. First, we found PA to rightward-shifting prisms to selectively affect EEG signatures of motor but not attentional processes. More specifically, PA modulated preparatory motor EEG activity over central electrodes in the right hemisphere, contralateral to the PA-induced, compensatory leftward shift in pointing movements. No effects were found on EEG signatures of spatial attention orienting over occipitoparietal sites. Second, we found the PA effect on preparatory motor EEG activity to dominate in the beta frequency band. We conclude that changes to intentional visuomotor, rather than attentional visuospatial, processes underlie the PA aftereffect of rightward-deviating prisms in healthy participants.

SIGNIFICANCE STATEMENT Prismatic adaptation (PA) has been proposed as a tool to induce neural plasticity in both healthy participants and patients, due to its aftereffect impacting on a number of visuospatial and visuomotor functions. However, the neural mechanisms underlying PA aftereffects are poorly understood as only little neuroimaging evidence is available. Here, we examined, for the first time, the origin of PA aftereffects studying oscillatory brain activity. Our results show a selective modulation of preparatory motor activity following PA in healthy participants but no effect on attention-related activity. This provides novel insight into the PA aftereffect in the healthy brain and may help to inform interventions in neglect patients.

A Brief Exposure to Leftward Prismatic Adaptation Enhances the Representation of the Ipsilateral, Right Visual Field in the Right Inferior Parietal Lobule

A brief exposure to rightward prismatic adaptation (PA) was shown to shift visual field representation within the inferior parietal lobule (IPL) from the right to the left hemisphere. This change in hemispheric dominance could be interpreted as (1) a general effect of discrepancy in visuomotor alignment caused by PA or (2) a direction-specific effect of rightward PA. To test these hypotheses, we compared the effects of rightward and leftward PA on visual representation in normal human subjects. Three groups of normal subjects underwent an fMRI evaluation using a simple visual detection task before and after brief PA exposure using leftward- or rightward-deviating prisms or no prisms (L-PA, R-PA, neutral groups). A two-way ANOVA group × session revealed a significant interaction suggesting that PA-induced modulation is direction specific. Post hoc analysis showed that L-PA enhanced the representation of the right visual field within the right IPL. Thus, a brief exposure to L-PA enhanced right hemispheric dominance within the ventral attentional system, which is the opposite effect of the previously described shift in hemispheric dominance following R-PA. The direction-specific effects suggest that the underlying neural mechanisms involve the fine-tuning of specific visuomotor networks. The enhancement of right hemispheric dominance following L-PA offers a parsimonious explanation for neglect-like symptoms described previously in normal subjects.

Combining tDCS with prismatic adaptation for non-invasive neuromodulation of the motor cortex

BACKGROUND

Prismatic adaptation (PA) shifts visual field laterally and induces lateralized deviations of spatial attention. Recently, it has been suggested that prismatic goggles are also able to modulate brain excitability, with cognitive after-effects documented even in tasks not necessarily spatial in nature.

OBJECTIVE

The aim of the present study was to test whether neuromodulatory effects obtained from tDCS and prismatic goggles could interact and induce homeostatic changes in corticospinal excitability.

METHODS

Thirty-four subjects were submitted to single-pulse transcranial magnetic stimulation (TMS) over the right primary motor cortex to measure Input-Output (IO) curve as a measure of corticospinal excitability. Assessment was made in three experimental conditions: before and after rightward PA and anodal tDCS of the right motor cortex; before and after rightward PA; before and after anodal tDCS of the right motor cortex.

RESULTS

A significant decrease of MEPs amplitude and of IO curve slope steepness was found after the combination of rightward PA and anodal tDCS; on the other hand, an increase of MEPs amplitude and of the steepness of IO curve slope on the right motor cortex was found following either rightward PA or anodal tDCS.

CONCLUSION

These findings suggest that priming of motor cortex excitability using PA could be an additional tool to modulate cortical metaplasticity.