Phosphene and motor transcranial magnetic stimulation thresholds are correlated: A meta-analytic investigation

Transcranial magnetic stimulation (TMS) is commonly delivered at an intensity defined by the resting motor threshold (rMT), which is thought to represent cortical excitability, even if the TMS target area falls outside of the motor cortex. This approach rests on the assumption that cortical excitability, as measured through the motor cortex, represents a 'global' measure of excitability. Another common approach to measure cortical excitability relies on the phosphene threshold (PT), measured through the visual cortex of the brain. However, it remains unclear whether either estimate can serve as a singular measure to infer cortical excitability across different brain regions. If PT and rMT can indeed be used to infer cortical excitability across brain regions, they should be correlated. To test this, we systematically identified previous studies that measured PT and rMT to calculate an overall correlation between the two estimates. Our results, based on 16 effect sizes from eight studies, indicated that PT and rMT are correlated (ρ = 0.4), and thus one measure could potentially serve as a measure to infer cortical excitability across brain regions. Three exploratory meta-analyses revealed that the strength of the correlation is affected by different methodologies, and that PT intensities are higher than rMT. Evidence for a PT-rMT correlation remained robust across all analyses. Further research is necessary for an in-depth understanding of how cortical excitability is reflected through TMS.

TMS-evoked EEG potentials demonstrate altered cortical excitability in migraine with aura

Migraine is associated with altered sensory processing, that may be evident as changes in cortical responsivity due to altered excitability, especially in migraine with aura. Cortical excitability can be directly assessed by combining transcranial magnetic stimulation with electroencephalography (TMS-EEG). We measured TMS evoked potential (TEP) amplitude and response consistency as these measures have been linked to cortical excitability but were not yet reported in migraine.We recorded 64-channel EEG during single-pulse TMS on the vertex interictally in 10 people with migraine with aura and 10 healthy controls matched for age, sex and resting motor threshold. On average 160 pulses around resting motor threshold were delivered through a circular coil in clockwise and counterclockwise direction. Trial-averaged TEP responses, frequency spectra and phase clustering (over the entire scalp as well as in frontal, central and occipital midline electrode clusters) were compared between groups, including comparison to sham-stimulation evoked responses.Migraine and control groups had a similar distribution of TEP waveforms over the scalp. In migraine with aura, TEP responses showed reduced amplitude around the frontal and occipital N100 peaks. For the migraine and control groups, responses over the scalp were affected by current direction for the primary motor cortex, somatosensory cortex and sensory association areas, but not for frontal, central or occipital midline clusters.This study provides evidence of altered TEP responses in-between attacks in migraine with aura. Decreased TEP responses around the N100 peak may be indicative of reduced cortical GABA-mediated inhibition and expand observations on enhanced cortical excitability from earlier migraine studies using more indirect measurements.

An entorhinal-visual cortical circuit regulates depression-like behaviors

Major depressive disorder is viewed as a 'circuitopathy'. The hippocampal-entorhinal network plays a pivotal role in regulation of depression, and its main sensory output, the visual cortex, is a promising target for stimulation therapy of depression. However, whether the entorhinal-visual cortical pathway mediates depression and the potential mechanism remains unknown. Here we report a cortical circuit linking entorhinal cortex layer Va neurons to the medial portion of secondary visual cortex (Ent→V2M) that bidirectionally regulates depression-like behaviors in mice. Analyses of brain-wide projections of Ent Va neurons and two-color retrograde tracing indicated that Ent Va→V2M projection neurons represented a unique population of neurons in Ent Va. Immunostaining of c-Fos revealed that activity in Ent Va neurons was decreased in mice under chronic social defeat stress (CSDS). Both chemogenetic inactivation of Ent→V2M projection neurons and optogenetic inactivation of the projection terminals induced social deficiency, anxiety- and despair-related behaviors in healthy mice. Chemogenetic inactivation of Ent→V2M projection neurons also aggravated these depression-like behaviors in CSDS-resilient mice. Optogenetic activation of Ent→V2M projection terminals rapidly ameliorated depression-like phenotypes. Optical recording using fiber photometry indicated that elevated neural activity in Ent→V2M projection terminals promoted antidepressant-like behaviors. Thus, the Ent→V2M circuit plays a crucial role in regulation of depression-like behaviors, and can function as a potential target for treating major depressive disorder.

Transcranial magnetic stimulation of the brain: What is stimulated? - A consensus and critical position paper

Transcranial (electro)magnetic stimulation (TMS) is currently the method of choice to non-invasively induce neural activity in the human brain. A single transcranial stimulus induces a time-varying electric field in the brain that may evoke action potentials in cortical neurons. The spatial relationship between the locally induced electric field and the stimulated neurons determines axonal depolarization. The induced electric field is influenced by the conductive properties of the tissue compartments and is strongest in the superficial parts of the targeted cortical gyri and underlying white matter. TMS likely targets axons of both excitatory and inhibitory neurons. The propensity of individual axons to fire an action potential in response to TMS depends on their geometry, myelination and spatial relation to the imposed electric field and the physiological state of the neuron. The latter is determined by its transsynaptic dendritic and somatic inputs, intrinsic membrane potential and firing rate. Modeling work suggests that the primary target of TMS is axonal terminals in the crown top and lip regions of cortical gyri. The induced electric field may additionally excite bends of myelinated axons in the juxtacortical white matter below the gyral crown. Neuronal excitation spreads ortho- and antidromically along the stimulated axons and causes secondary excitation of connected neuronal populations within local intracortical microcircuits in the target area. Axonal and transsynaptic spread of excitation also occurs along cortico-cortical and cortico-subcortical connections, impacting on neuronal activity in the targeted network. Both local and remote neural excitation depend critically on the functional state of the stimulated target area and network. TMS also causes substantial direct co-stimulation of the peripheral nervous system. Peripheral co-excitation propagates centrally in auditory and somatosensory networks, but also produces brain responses in other networks subserving multisensory integration, orienting or arousal. The complexity of the response to TMS warrants cautious interpretation of its physiological and behavioural consequences, and a deeper understanding of the mechanistic underpinnings of TMS will be critical for advancing it as a scientific and therapeutic tool.

Transcranial magnetic stimulation entrains alpha oscillatory activity in occipital cortex

Parieto-occipital alpha rhythms (8-12 Hz) underlie cortical excitability and influence visual performance. Whether the synchrony of intrinsic alpha rhythms in the occipital cortex can be entrained by transcranial magnetic stimulation (TMS) is an open question. We applied 4-pulse, 10-Hz rhythmic TMS to entrain intrinsic alpha oscillators targeting right V1/V2, and tested four predictions with concurrent electroencephalogram (EEG): (1) progressive enhancement of entrainment across time windows, (2) output frequency specificity, (3) dependence on the intrinsic oscillation phase, and (4) input frequency specificity to individual alpha frequency (IAF) in the neural signatures. Two control conditions with an equal number of pulses and duration were arrhythmic-active and rhythmic-sham stimulation. The results confirmed the first three predictions. Rhythmic TMS bursts significantly entrained local neural activity. Near the stimulation site, evoked oscillation amplitude and inter-trial phase coherence (ITPC) were increased for 2 and 3 cycles, respectively, after the last TMS pulse. Critically, ITPC following entrainment positively correlated with IAF rather than with the degree of similarity between IAF and the input frequency (10 Hz). Thus, we entrained alpha-band activity in occipital cortex for ~ 3 cycles (~ 300 ms), and IAF predicts the strength of entrained occipital alpha phase synchrony indexed by ITPC.

Precise oculocentric mapping of transcranial magnetic stimulation-evoked phosphenes

OBJECTIVE

Transcranial magnetic stimulation (TMS)-evoked phosphenes are oculocentric; their perceived location depends upon eye position. We investigated the accuracy and precision of TMS-evoked phosphene oculocentric mapping.

METHODS

We evoked central phosphenes by stimulating early visual cortical areas with TMS, systematically examining the effect of eye position by asking participants to report the location of the evoked phosphene. We tested whether any systematic differences in the precision or accuracy of responses occurred as a function of eye position.

RESULTS

Perceived phosphene locations map veridically to eye position, although there are considerable individual differences in the reliability of this mapping.

CONCLUSIONS

Our results emphasize the need to carefully control eye movements when carrying out phosphene localization studies and suggest that individual differences in the reliability of the reported position of individual phosphenes must be considered.

Inducing lateralized phosphenes over the occipital lobe using transcranial magnetic stimulation to navigate a virtual environment

Electrical stimulation involving visual areas of the brain produces artificial light percepts called phosphenes. These visual percepts have been extensively investigated in previous studies involving intracortical microsimulation (ICMS) and serve as the basis for developing a visual prosthesis for the blind. Although advances have been achieved, many challenges still remain with implementing a functional ICMS for visual rehabilitation purposes. Transcranial magnetic stimulation (TMS) over the primary occipital lobe offers an alternative method to produce phosphenes non-invasively. A main challenge facing blind individuals involves navigation. Within the scientific community, methods to evaluate the ability of a visual prosthesis to facilitate in navigation has been neglected. In this study, we investigate the effectiveness of evoking lateralized phosphenes to navigate a computer simulated virtual environment. More importantly, we demonstrate how virtual environments along with the development of a visual prosthesis share a mutual relationship benefiting both patients and researchers. Using two TMS devices, a pair of 40mm figure-of-eight coils were placed over each occipital hemisphere resulting in lateralized phosphene perception. Participants were tasked with making a series of left and right turns using peripheral devices depending on the visual hemifield in which a phosphene is present. If a participant was able to accurately perceive all ten phosphenes, the simulated target is able to advance and fully exit the virtual environment. Our findings demonstrate that participants can interpret lateralized phosphenes while highlighting the integration of computer based virtual environments to evaluate the capability of a visual prosthesis during navigation.

Retinal and Cortical Contributions to Phosphenes During Transcranial Electrical Current Stimulation

It is generally believed that the phosphenes induced by transcranial electric current stimulation (tECS) are a product of retinal activation, even when electrode placement is directly over the primary visual cortex. However, the origins of these tECS-induced phosphenes have not yet been conclusively determined. In this study, phosphene detection thresholds using an FPz-Oz montage were compared with those from (i) an Oz-Cz montage to determine whether prefrontal regions, such as the retina, contribute to phosphenes and (ii) an FPz-Cz montage to determine whether the visual cortex in the occipital lobe contributes to phosphenes. Twenty-two participants received transcranial current stimulation with each of these montages (as well as a T3-T4 montage included for exploratory purposes) at 6, 10, 16, 20, 24, 28, and 32 Hz. To estimate differences in current density at the retina and occipital lobe across montages, modeling of current density at phosphene thresholds was measured across 20 head models. Consistent with the proposal that tECS-induced phosphenes are generated in the retina, increasing current density near the retina (FPz-Oz relative to Oz-Cz montage) reduced phosphene thresholds. However, increasing current density near the occipital cortex (FPz-Oz relative to FPz-Cz montage) also reduced phosphene thresholds while also requiring less current density at the retina according to the modeling estimates. This suggests that tECS of this occipital cortex also contributed to phosphene perception. © 2020 Bioelectromagnetics Society.

Task-related functional magnetic resonance imaging-based neuronavigation for the treatment of depression by individualized repetitive transcranial magnetic stimulation of the visual cortex

To determine whether repetitive transcranial magnetic stimulation (rTMS) of the visual cortex (VC) provides effective and well-tolerated treatment and whether magnetic resonance imaging (MRI) measures functional change of the VC as a biomarker of therapeutic effect in major depressive disorder (MDD), we performed a sham-controlled, double-blind, randomized, three-arm VC rTMS treatment study in 74 MDD patients. Neuronavigated rTMS (10 Hz, 90% of resting motor threshold, 1,600 pulses over 20 min twice per day) was performed over the VC for five days. Clinical outcome was measured by Hamilton Depression Rating Scale (HAMD-24) at days 0, 1, 3, 5 and after terminating rTMS, with follow-up at four weeks. MRI was measured at days 0 and 5. The individualized group exhibited the greatest change in HAMD-24 scores after VC rTMS for 5 days (F=5.53, P=0.005), which were maintained during follow-up period (F=4.22, P=0.016). All patients reported good tolerance. Changes in VC task-related functional MRI correlated with symptomatic reduction in the individualized group. Treatment reduced the initially abnormal increase in resting state functional connectivity from the VC to the pre/subgenual anterior cingulate cortex at day 5, especially in the individualized group. We demonstrated therapeutic potential and good tolerance of VC rTMS in MDD patients, indicated by biomarkers of fMRI measurement.

Separation effect of early visual cortex V1 under different crowding conditions: a transcranial magnetic stimulation study

The visual crowding makes it difficult to identify the patterns in peripheral vision, but the neural mechanism for this phenomenon is still unclear because of different opinions. To study the separation effect of early visual cortex V1 under different crowding conditions, single-pulse transcranial magnetic stimulation (TMS) is applied within the right V1. The experimental design includes two factors: TMS intensity (10%, 65%, and 90% of the phosphene threshold) and crowding conditions (high and low). The accuracy results show that there is a strong interaction between crowding and TMS conditions. When the TMS intensity is 65% or 90% of the phosphene threshold, more crowding will be perceived under the high crowding condition, and less crowding under the low crowding condition. We concluded that the high and the low crowding conditions can be separated by TMS. These results support the assumption that the crowding is related to V1 and occurs in the visual coding phase.

Cerebellar-Stimulation Evoked Prefrontal Electrical Synchrony Is Modulated by GABA

Cerebellar-prefrontal connectivity has been recognized as important for behaviors ranging from motor coordination to cognition. Many of these behaviors are known to involve excitatory or inhibitory modulations from the prefrontal cortex. We used cerebellar transcranial magnetic stimulation (TMS) with simultaneous electroencephalography (EEG) to probe cerebellar-evoked electrical activity in prefrontal cortical areas and used magnetic resonance spectroscopy (MRS) measures of prefrontal GABA and glutamate levels to determine if they are correlated with those potentials. Cerebellar-evoked bilateral prefrontal synchrony in the theta to gamma frequency range showed patterns that reflect strong GABAergic inhibitory function (r = - 0.66, p = 0.002). Stimulation of prefrontal areas evoked bilateral prefrontal synchrony in the theta to low beta frequency range that reflected, conversely, glutamatergic excitatory function (r = 0.66, p = 0.002) and GABAergic inhibitory function (r = - 0.65, p = 0.002). Cerebellar-evoked prefrontal synchronization had opposite associations with cognition and motor coordination: it was positively associated with working memory performance (r = 0.57, p = 0.008) but negatively associated with coordinated motor function as measured by rapid finger tapping (r = - 0.59, p = 0.006). The results suggest a relationship between regional GABA levels and interregional effects on synchrony. Stronger cerebellar-evoked prefrontal synchrony was associated with better working memory but surprisingly worse motor coordination, which suggests competing effects for motor activity and cognition. The data supports the use of a TMS-EEG-MRS approach to study the neurochemical basis of large-scale oscillations modulated by the cerebellar-prefrontal connectivity.

Current understanding of cortical structure and function in migraine

Objective

To review and discuss the literature on the role of cortical structure and function in migraine.

Discussion

Structural and functional findings suggest that changes in cortical morphology and function contribute to migraine susceptibility by modulating dynamic interactions across cortical and subcortical networks. The involvement of the cortex in migraine is well established for the aura phase with the underlying phenomenon of cortical spreading depolarization, while increasing evidence suggests an important role for the cortex in perception of head pain and associated sensations. As part of trigeminovascular pain and sensory processing networks, cortical dysfunction is likely to also affect initiation of attacks.

Conclusion

Morphological and functional changes identified across cortical regions are likely to contribute to initiation, cyclic recurrence and chronification of migraine. Future studies are needed to address underlying mechanisms, including interactions between cortical and subcortical regions and effects of internal (e.g. genetics, gender) and external (e.g. sensory inputs, stress) modifying factors, as well as possible clinical and therapeutic implications.

The mixed-polarity benefit of stereopsis arises in early visual cortex

Depth perception is better when observers view stimuli containing a mixture of bright and dark visual features. It is currently unclear where in the visual system sensory processing benefits from the availability of different contrast polarity. To address this question, we applied transcranial magnetic stimulation to the visual cortex to modulate normal neural activity during processing of single- or mixed-polarity random-dot stereograms. In line with previous work, participants gave significantly better depth judgments for mixed-polarity stimuli. Stimulation of early visual cortex (V1/V2) significantly increased this benefit for mixed-polarity stimuli, and it did not affect performance for single-polarity stimuli. Stimulation of disparity responsive areas V3a and LO had no effect on perception. Our findings show that disparity processing in early visual cortex gives rise to the mixed-polarity benefit. This is consistent with computational models of stereopsis at the level of V1 that produce a mixed polarity benefit.

Phosphene perception is due to the ultra-weak photon emission produced in various parts of the visual system: glutamate in the focus

Phosphenes are experienced sensations of light, when there is no light causing them. The physiological processes underlying this phenomenon are still not well understood. Previously, we proposed a novel biopsychophysical approach concerning the cause of phosphenes based on the assumption that cellular endogenous ultra-weak photon emission (UPE) is the biophysical cause leading to the sensation of phosphenes. Briefly summarized, the visual sensation of light (phosphenes) is likely to be due to the inherent perception of UPE of cells in the visual system. If the intensity of spontaneous or induced photon emission of cells in the visual system exceeds a distinct threshold, it is hypothesized that it can become a conscious light sensation. Discussing several new and previous experiments, we point out that the UPE theory of phosphenes should be really considered as a scientifically appropriate and provable mechanism to explain the physiological basis of phosphenes. In the present paper, we also present our idea that some experiments may support that the cortical phosphene lights are due to the glutamate-related excess UPE in the occipital cortex.

Serotonergic Hallucinogen-Induced Visual Perceptual Alterations

Serotonergic hallucinogens, such as lysergic acid diethylamide (LSD), psilocybin, and N,N-dimethyltryptamine (DMT), are famous for their capacity to temporally and profoundly alter an individual's visual experiences. These visual alterations show consistent attributes despite large inter- and intra-individual variances. Many reports document a common perception of colors as more saturated, with increased brightness and contrast in the environment ("Visual Intensifications"). Environmental objects might be altered in size ("Visual illusions") or take on a modified and special meaning for the subject ("Altered self-reference"). Subjects may perceive light flashes or geometrical figures containing recurrent patterns ("Elementary imagery and hallucinations") influenced by auditory stimuli ("Audiovisual synesthesia"), or they may envision images of people, animals, or landscapes ("Complex imagery and hallucinations") without any physical stimuli supporting their percepts. This wide assortment of visual phenomena suggests that one single neuropsychopharmacological mechanism is unlikely to explain such vast phenomenological diversity. Starting with mechanisms that act at the cellular level, the key role of 5-HT2A receptor activation and the subsequent increased cortical excitation will be considered. Next, it will be shown that area specific anatomical and dynamical features link increased excitation to the specific visual contents of hallucinations. The decrease of alpha oscillations by hallucinogens will then be introduced as a systemic mechanism for amplifying internal-driven excitation that overwhelms stimulus-induced excitations. Finally, the hallucinogen-induced parallel decrease of the N170 visual evoked potential and increased medial P1 potential will be discussed as key mechanisms for inducing a dysbalance between global integration and early visual gain that may explain several hallucinogen-induced visual experiences, including visual hallucinations, illusions, and intensifications.

Possible role of biochemiluminescent photons for lysergic acid diethylamide (LSD)-induced phosphenes and visual hallucinations

Today, there is an increased interest in research on lysergic acid diethylamide (LSD) because it may offer new opportunities in psychotherapy under controlled settings. The more we know about how a drug works in the brain, the more opportunities there will be to exploit it in medicine. Here, based on our previously published papers and investigations, we suggest that LSD-induced visual hallucinations/phosphenes may be due to the transient enhancement of bioluminescent photons in the early retinotopic visual system in blind as well as healthy people.

Mapping the visual brain areas susceptible to phosphene induction through brain stimulation

Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation technique whose effects on neural activity can be uncertain. Within the visual cortex, phosphenes are a useful marker of TMS: They indicate the induction of neural activation that propagates and creates a conscious percept. However, we currently do not know how susceptible different areas of the visual cortex are to TMS-induced phosphenes. In this study, we systematically map out locations in the visual cortex where stimulation triggered phosphenes. We relate this to the retinotopic organization and the location of object- and motion-selective areas, identified by functional magnetic resonance imaging (fMRI) measurements. Our results show that TMS can reliably induce phosphenes in early (V1, V2d, and V2v) and dorsal (V3d and V3a) visual areas close to the interhemispheric cleft. However, phosphenes are less likely in more lateral locations (hMT+/V5 and LOC). This suggests that early and dorsal visual areas are particularly amenable to TMS and that TMS can be used to probe the functional role of these areas.

Cerebellar contributions to motor control and language comprehension: searching for common computational principles

The past 25 years have seen the functional domain of the cerebellum extend beyond the realm of motor control, with considerable discussion of how this subcortical structure contributes to cognitive domains including attention, memory, and language. Drawing on evidence from neuroanatomy, physiology, neuropsychology, and computational work, sophisticated models have been developed to describe cerebellar function in sensorimotor control and learning. In contrast, mechanistic accounts of how the cerebellum contributes to cognition have remained elusive. Inspired by the homogeneous cerebellar microanatomy and a desire for parsimony, many researchers have sought to extend mechanistic ideas from motor control to cognition. One influential hypothesis centers on the idea that the cerebellum implements internal models, representations of the context-specific dynamics of an agent's interactions with the environment, enabling predictive control. We briefly review cerebellar anatomy and physiology, to review the internal model hypothesis as applied in the motor domain, before turning to extensions of these ideas in the linguistic domain, focusing on speech perception and semantic processing. While recent findings are consistent with this computational generalization, they also raise challenging questions regarding the nature of cerebellar learning, and may thus inspire revisions of our views on the role of the cerebellum in sensorimotor control.

Phosphene Perception Relates to Visual Cortex Glutamate Levels and Covaries with Atypical Visuospatial Awareness

Phosphenes are illusory visual percepts produced by the application of transcranial magnetic stimulation to occipital cortex. Phosphene thresholds, the minimum stimulation intensity required to reliably produce phosphenes, are widely used as an index of cortical excitability. However, the neural basis of phosphene thresholds and their relationship to individual differences in visual cognition are poorly understood. Here, we investigated the neurochemical basis of phosphene perception by measuring basal GABA and glutamate levels in primary visual cortex using magnetic resonance spectroscopy. We further examined whether phosphene thresholds would relate to the visuospatial phenomenology of grapheme-color synesthesia, a condition characterized by atypical binding and involuntary color photisms. Phosphene thresholds negatively correlated with glutamate concentrations in visual cortex, with lower thresholds associated with elevated glutamate. This relationship was robust, present in both controls and synesthetes, and exhibited neurochemical, topographic, and threshold specificity. Projector synesthetes, who experience color photisms as spatially colocalized with inducing graphemes, displayed lower phosphene thresholds than associator synesthetes, who experience photisms as internal images, with both exhibiting lower thresholds than controls. These results suggest that phosphene perception is driven by interindividual variation in glutamatergic activity in primary visual cortex and relates to cortical processes underlying individual differences in visuospatial awareness.