Suppression of perception in migraine: evidence for reduced inhibition in the visual cortex

Exploring sensory sensitivity, cortical excitability, and habituation in episodic migraine, as a function of age and disease severity, using the pattern-reversal task

BACKGROUND

Migraine is a cyclic, neurosensory disorder characterized by recurrent headaches and altered sensory processing. The latter is manifested in hypersensitivity to visual stimuli, measured with questionnaires and sensory thresholds, as well as in abnormal cortical excitability and a lack of habituation, assessed with visual evoked potentials elicited by pattern-reversal stimulation. Here, the goal was to determine whether factors such as age and/or disease severity may exert a modulatory influence on sensory sensitivity, cortical excitability, and habituation.

METHODS

Two similar experiments were carried out, the first comparing 24 young, episodic migraine patients and 28 healthy age- and gender-matched controls and the second 36 middle-aged, episodic migraine patients and 30 healthy age- and gender-matched controls. A neurologist confirmed the diagnoses. Migraine phases were obtained using eDiaries. Sensory sensitivity was assessed with the Sensory Perception Quotient and group comparisons were carried out. We obtained pattern-reversal visual evoked potentials and calculated the N1-P1 Peak-to-Peak amplitude. Two linear mixed-effects models were fitted to these data. The first model had Block (first block, last block) and Group (patients, controls) as fixed factors, whereas the second model had Trial (all trials) and Group as fixed factors. Participant was included as a random factor in both. N1-P1 first block amplitude was used to assess cortical excitability and habituation was defined as a decrease of N1-P1 amplitude across Blocks/Trials. Both experiments were performed interictally.

RESULTS

The final samples consisted of 18 patients with episodic migraine and 27 headache-free controls (first experiment) and 19 patients and 29 controls (second experiment). In both experiments, patients reported increased visual hypersensitivity on the Sensory Perception Quotient as compared to controls. Regarding N1-P1 peak-to-peak data, there was no main effect of Group, indicating no differences in cortical excitability between groups. Finally, significant main effects of both Block and Trial were found indicating habituation in both groups, regardless of age and headache frequency.

CONCLUSIONS

The results of this study yielded evidence for significant hypersensitivity in patients but no significant differences in either habituation or cortical excitability, as compared to headache-free controls. Although the alterations in patients may be less pronounced than originally anticipated they demonstrate the need for the definition and standardization of optimal methodological parameters.

Electrophysiological findings in migraine may reflect abnormal synaptic plasticity mechanisms: A narrative review

BACKGROUND

The cyclical brain disorder of sensory processing accompanying migraine phases lacks an explanatory unified theory.

METHODS

We searched Pubmed for non-invasive neurophysiological studies on migraine and related conditions using transcranial magnetic stimulation, electroencephalography, visual and somatosensory evoked potentials. We summarized the literature, reviewed methods, and proposed a unified theory for the pathophysiology of electrophysiological abnormalities underlying migraine recurrence.

RESULTS

All electrophysiological modalities have determined specific changes in brain dynamics across the different phases of the migraine cycle. Transcranial magnetic stimulation studies show unbalanced recruitment of inhibitory and excitatory circuits, more consistently in aura, which ultimately results in a substantially distorted response to neuromodulation protocols. Electroencephalography investigations highlight a steady pattern of reduced alpha and increased slow rhythms, largely located in posterior brain regions, which tends to normalize closer to the attacks. Finally, non-painful evoked potentials suggest dysfunctions in habituation mechanisms of sensory cortices that revert during ictal phases.

CONCLUSION

Electrophysiology shows dynamic and recurrent functional alterations within the brainstem-thalamus-cortex loop varies continuously and recurrently in migraineurs. Given the central role of these structures in the selection, elaboration, and learning of sensory information, these functional alterations suggest chronic, probably genetically determined dysfunctions of the synaptic short- and long-term learning mechanisms.

What has neurophysiology revealed about migraine and chronic migraine?

Since the first electroencephalographic recordings obtained by Golla and Winter in 1959, researchers have used a variety of neurophysiological techniques to determine the mechanisms underlying recurrent migraine attacks. Neurophysiological methods have shown that the brain during the interictal phase of an episodic migraine is characterized by a general hyperresponsiveness to sensory stimuli, a malfunction of the monoaminergic brainstem circuits, and by functional alterations of the thalamus and thalamocortical loop. All of these alterations vary plastically during the phases of the migraine cycle and interictally with the days following the attack. Both episodic migraineurs recorded during an attack and chronic migraineurs are characterized by a general increase in the cortical amplitude response to peripheral sensory stimuli; this is an electrophysiological hallmark of a central sensitization process that is further reinforced through medication overuse. Considering the large-scale functional involvement and the main roles played by the brainstem-thalamo-cortical network in selection, elaboration, and learning of relevant sensory information, future research should move from searching for one specific primary site of dysfunction at the macroscopic level, to the chronic, probably genetically determined, molecular dysfunctions at the synaptic level, responsible for short- and long-term learning mechanisms.

The not so hidden impact of interictal burden in migraine: A narrative review

Migraine is a highly prevalent neurological disease of varying attack frequency. Headache attacks that are accompanied by a combination of impact on daily activities, photophobia and/or nausea are most commonly migraine. The headache phase of a migraine attack has attracted more research, assessment tools and treatment goals than any other feature, characteristic, or phase of migraine. However, the migraine attack may encompass up to 4 phases: the prodrome, aura, headache phase and postdrome. There is growing recognition that the burden of migraine, including symptoms associated with the headache phase of the attack, may persist between migraine attacks, sometimes referred to as the "interictal phase." These include allodynia, hypersensitivity, photophobia, phonophobia, osmophobia, visual/vestibular disturbances and motion sickness. Subtle interictal clinical manifestations and a patient's trepidation to make plans or commitments due to the unpredictability of migraine attacks may contribute to poorer quality of life. However, there are only a few tools available to assess the interictal burden. Herein, we examine the recent advances in the recognition, description, and assessment of the interictal burden of migraine. We also highlight the value in patients feeling comfortable discussing the symptoms and overall burden of migraine when discussing migraine treatment needs with their provider.

Magnetic Suppression of Perceptual Accuracy Is Not Reduced in Visual Snow Syndrome

Objective: Patients with visual snow syndrome (VSS) suffer from continuous (“TV snow-like”) visual disturbance of unknown pathoetiology. In VSS, changes in cortical excitability in the primary visual cortex and the visual association cortex are discussed, with recent imaging studies tending to point to higher-order visual areas. Migraine, especially migraine with aura, is a common comorbidity. In chronic migraine and episodic migraine with aura but not in episodic migraine without aura, a reduced magnetic suppression of perceptual accuracy (MSPA) reflects a probably reduced inhibition of the primary visual cortex. Here we investigated the inhibition of the primary visual cortex using MSPA in patients with VSS, comparing that with MSPA in controls matched for episodic migraine.

Methods: Seventeen patients with VSS were compared to 17 age- and migraine-matched controls. Visual accuracy was assessed by letter recognition and modulated by transcranial magnetic stimulation delivered to the occipital cortex at different intervals with respect to the letter presentation (40, 100, and 190 ms).

Results: Suppression of visual accuracy at the 100-ms interval was present without significant differences between VSS patients and age- and migraine-matched controls (percentage of correctly recognized trigrams, control: 46.4 ± 34.3; VSS: 52.5 ± 25.4, p = 0.56).

Conclusions: In contrast to migraine with aura, occipital cortex inhibition, as assessed with MSPA, may not be affected in VSS.

Changes in face and face pareidolia processing in patients with migraine: an ERP study

Migraine is a multifactorial brain disorder characterized by recurrent disabling headache attacks. One of the possible mechanisms in the pathogenesis of migraine may be a decrease in inhibitory cortical stimuli in the primary visual cortex attributable to cortical hyperexcitability. The aim of this study was to investigate the neural correlates underlying face and face pareidolia processing in terms of the event-related potential (ERP) components, N170, vertex positive potential (VPP), and N250, in patients with migraine. In total, 40 patients with migraine without aura, 23 patients with migraine and aura, and 30 healthy controls were enrolled. We recorded ERPs during the presentation of face and face pareidolia images. N170, VPP, and N250 mean amplitudes and latencies were examined. N170 was significantly greater in patients with migraine with aura than in healthy controls. VPP amplitude was significantly greater in patients with migraine without aura than in healthy controls. The face stimuli evoked significantly earlier VPP responses to faces (168.7 ms, SE = 1.46) than pareidolias (173.4 ms, SE = 1.41) in patients with migraine with aura. We did not find a significant difference between N250 amplitude for face and face pareidolia processing. A significant difference was observed between the groups for pareidolia in terms of N170 [F(2,86) = 14,75, P < 0.001] and VPP [F(2,86) = 16.43, P < 0.001] amplitudes. Early ERPs are a valuable tool to study the neural processing of face processing in patients with migraine to demonstrate visual cortical hyperexcitability.NEW & NOTEWORTHY Event-related potentials (ERPs) are important for understanding face and face pareidolia processing in patients with migraine. N170, vertex positive potential (VPP), and N250 ERPs were investigated. N170 was revealed as a potential component of cortical excitability for face and face pareidolia processing in patients with migraine.

The Psychophysical Assessment of Hierarchical Magno-, Parvo- and Konio-Cellular Visual Stream Dysregulations in Migraineurs

Introduction

Although conscious, image-forming illusions have been noted in migraine, few studies have specifically sought to collectively evaluate the role of all three parallel visual processing streams in the retinogeniculostriate pathway involved with image-forming vision and their implications in the development of migraine symptoms.

Methods

We psychophysically assessed the functionality of the inferred magnocellular (MC), parvocellular (PC), and koniocellular (KC) streams at different hierarchical loci across three clinical groups: individuals who experience migraine with aura (MA; n=13), experience migraine without aura (MWO; n=14), and Controls (n=15). Participants completed four experiments: Experiment 1 designed to assess retinal short-wavelength-sensitive (S-) cone sensitivities; Experiment 2 intended to measure postretinal temporal and spatiochromatic contrast sensitivities; Experiment 3 intended to assess postretinal spatiotemporal achromatic contrast sensitivities; and Experiment 4 designed to measure thalamocortical color discriminations along the three cone-excitation axes.

Results

S-cone deficits were revealed with greater retinal areas being affected in MA compared to MWO participants. Findings across the four experiments suggest a prominent retinal locus of dysfunction in MA (lesser in MWO) with potential feedforward compensations occurring within the KC visual stream.

Conclusion

Complex, integrative network compensations need to be factored in when considering the dysregulating influences of migraine along the visual pathway.

Intracortical facilitation within the migraine motor cortex depends on the stimulation intensity. A paired-pulse TMS study

Introduction

Connectivity within the primary motor cortex can be measured using the paired-pulse transcranial magnetic stimulation (TMS) paradigm. This evaluates the effect of a first conditioning stimulus on the motor evoked potential (MEP) elicited by a second test stimulus when different interstimulus intervals are used. Aim of the present study was to provide, in patients suffering from migraine without aura (MwoA), additional information on intracortical facilitation (ICF), short intracortical inhibition (SICI), and long intracortical inhibition (LICI), using different intensities of the test stimulus (TS).

Methods

We enrolled 24 patients with episodic MwoA and 24 age- and sex-matched healthy volunteers. Both patients and controls were randomly assigned to two different experimental groups: the first group underwent evaluation of ICF, while in the second group we assessed SICI and LICI. All these measures were assessed by using three different suprathreshold intensities of the TS (110%, 130% and 150% of the resting motor threshold, RMT). Interstimulus intervals (ISIs) of 10 ms were used for testing ICF, while SICI and LICI were carried out by using 2 ms and 100 ms ISIs respectively. All migraine patients underwent the experimental protocol while in the interictal pain-free state.

Results

A main finding of the study was that an increased ICF could be seen in migraineurs as compared to the healthy subjects only by using a 110% intensity of the TS. Instead, no significant differences were observed between patients and controls as regards both measures of intracortical inhibition.

Conclusion

We show that hyperresponsivity of the glutamatergic intracortical circuits can be detected in the migraine motor cortex only by applying a low suprathreshold intensity of stimulation. Our results strengthen the notion that, to be reliable, the assessment of cortical excitability in migraine should always include evaluation of the cortical response to different stimulation intensities.

Chronic Migraine: An Update on Physiology, Imaging, and the Mechanism of Action of Two Available Pharmacologic Therapies

Several lines of research support the hypothesis that migraine is a spectrum of illness, with clinical symptoms that vary along a continuum from episodic migraine to chronic migraine. Physiologic changes may result in episodic migraine evolving into chronic migraine over months to years in susceptible individuals. With chronification, headache frequency increases, becoming more disabling and less responsive to therapy. Neurophysiologic and functional imaging research has reported that chronic migraine may be associated with severity‐specific metabolic, functional, and structural abnormalities in the brainstem. Without longitudinal studies, it is unclear whether these changes may represent a continuum of individual progression and/or are reversible. Furthermore, chronic migraine is associated with larger impairments in cortical processing of sensory stimuli when compared with episodic migraine, possibly caused by more pronounced cortical hyperexcitability.

Progressive changes in nociceptive thresholds and subsequent central sensitization due to recurrent migraine attacks in vulnerable individuals contribute to the chronic migraine state. This may result in changes to baseline neurologic function between headache attacks, evident in both electrophysiological and functional imaging research. Patients experiencing migraine chronification may report increased non‐headache pain, fatigue, psychiatric disorders (eg, depression, anxiety), gastrointestinal complaints, and other somatic conditions associated with their long‐term experience with migraine pain.

Recent research provides a foundation for differentiating episodic and chronic migraine based on neurophysiologic and neuroimaging tools. In this literature review, we consider these findings in the context of models designed to explain the physiology and progression of episodic migraine into chronic migraine, and consider treatment of chronic migraine in susceptible individuals. Advances in pharmacotherapy provide treatment options for chronic migraine. Of the currently available treatment options, only onabotulinumtoxinA and topiramate have received regulatory approval and have demonstrated efficacy in patients with chronic migraine, although the exact mechanisms of action are not fully elucidated.

Objective Assessment of Cortical Excitability in Migraine With and Without Aura

Recent progress in the genetics of migraine has refocused attention on cortical dysfunction as an important component of the pathophysiology of this disorder. In previous work, we have demonstrated functional changes in the visual cortex of migraine patients, using an objective transcranial magnetic stimulation technique, termed magnetic suppression of perceptual accuracy (MSPA). This study aimed to replicate previous findings in migraine with aura (MA) and to use the technique to examine migraine without aura (MoA). Eight MA patients, 14 MoA patients and 13 migraine-free controls participated. MSPA assessments were undertaken using a standardized protocol in which computer-presented letter targets were followed at a variable delay interval by a single magnetic pulse delivered over the occiput. MSPA performance is expressed as a profile of response accuracy across target-pulse delay intervals. The profiles of migraine-free controls exhibited a normal U-shape. MA patients had significantly shallower profiles, showing little or no suppression at intermediate delay intervals. MoA patients had profiles that were similar to controls. Recent animal evidence strongly indicates that the U-shape of the normal MSPA function is caused by preferential activation of inhibitory neurons. Shallower MPSA profiles in MA patients are therefore likely to indicate a functional hyperexcitability caused by impaired inhibition. The finding of normal MPSA profiles in MoA patients is novel and will require further investigation.

Cognitive Function is not Impaired in People with a Long History of Migraine: A Blinded Study

Little is known about the long-term consequences of migraine for cognitive functioning. This study compared older migraine patients with matched controls on four measures of cognitive ability, in a blinded design. Migraine patients and case-matched controls were recruited from the database records of a pre-existing study of ageing. Data were available from four tests of cognitive ability: verbal/arithmetic problem solving, spatial problem solving, processing speed, and vocabulary. There were no significant differences between the mean scores of migraine and control groups on any of the four cognitive tests. In addition, there were no significant differences between migraine and control groups in the effect of age on any of the four tests. A long history of migraine does not compromise scores on the four cognitive tests used in this study. These tests are predictive of memory and executive functioning in cognitive ageing, but it remains possible that lower-level cognitive processes, particularly as assessed by visual tasks, may be vulnerable to migraine.

Reduced threshold for inhibitory homeostatic responses in migraine motor cortex? A tDCS/TMS study

BACKGROUND AND OBJECTIVE

Neurophysiological studies in migraine have reported conflicting findings of either cortical hyper- or hypoexcitability. In migraine with aura (MwA) patients, we recently documented an inhibitory response to suprathreshold, high-frequency repetitive transcranial magnetic stimulation (hf-rTMS) trains applied to the primary motor cortex, which is in contrast with the facilitatory response observed in the healthy subjects. The aim of the present study was to support the hypothesis that in migraine, because of a condition of basal increased cortical responsivity, inhibitory homeostatic like mechanisms of cortical excitability could be induced by high magnitude stimulation. For this purpose, the hf-rTMS trains were preconditioned by transcranial direct current stimulation (tDCS), a noninvasive brain stimulation technique able to modulate the cortical excitability state.

METHODS

Twenty-two MwA patients and 20 patients with migraine without aura (MwoA) underwent trains of 5-Hz repetitive transcranial magnetic stimulation at an intensity of 130% of the resting motor threshold, both at baseline and after conditioning by 15 minutes of cathodal or anodal tDCS. Motor cortical responses to the hf-rTMS trains were compared with those of 14 healthy subjects.

RESULTS

We observed abnormal inhibitory responses to the hf-rTMS trains given at baseline in both MwA and MwoA patients as compared with the healthy subjects (P < .00001).The main result of the study was that cathodal tDCS, which reduces the cortical excitability level, but not anodal tDCS, which increases it, restored the normal facilitatory response to the hf-rTMS trains in both MwA and MwoA.

CONCLUSIONS

The present findings strengthen the notion that, in migraine with and without aura, the threshold for inducing inhibitory mechanisms of cortical excitability might be lower in the interictal period. This could represent a protective mechanism counteracting cortical hyperresponsivity. Our results could be helpful to explain some conflicting neurophysiological findings in migraine and to get insight into the mechanisms underlying recurrence of the migraine attacks.

Visual motion processing in migraine: Enhanced motion after-effects are related to display contrast, visual symptoms, visual triggers and attack frequency

Background Visual after-effects are illusions that occur after prolonged viewing of visual displays. The motion after-effect (MAE), for example, is an illusory impression of motion after viewing moving displays: subsequently, stationary displays appear to drift in the opposite direction. After-effects have been used extensively in basic vision research and in clinical settings, and are enhanced in migraine. Objective The objective of this article is to assess associations between ( 1 ) MAE duration and visual symptoms experienced during/between migraine/headache attacks, and ( 2 ) visual stimuli reported as migraine/headache triggers. Methods The MAE was elicited after viewing motion for 45 seconds. MAE duration was tested for three test contrast displays (high, medium, low). Participants also completed a headache questionnaire that included migraine/headache triggers. Results For each test contrast, the MAE was prolonged in migraine. MAE duration was associated with photophobia; visual triggers (flicker, striped patterns); and migraine or headache frequency. Conclusions Group differences on various visual tasks have been attributed to abnormal cortical processing in migraine, such as hyperexcitability, heightened responsiveness and/or a lack of intra-cortical inhibition. The results are not consistent with hyperexcitability simply from a general lack of inhibition. Alternative multi-stage models are discussed and suggestions for further research are recommended, including visual tests in clinical assessments/clinical trials.

Test-retest reliability of visual-evoked potential habituation

Objective

Habituation of visual-evoked potentials (VEPs) is typically described as deficient interictally in migraine patients, supposedly indicating altered cortical excitability. Use of this parameter for monitoring changes over time, e.g. under treatment, requires demonstration of test-retest reliability.

Methods

VEPs were recorded interictally in 41 episodic migraine patients and 40 controls. N75–P100 amplitudes were measured over six consecutive blocks of 75 VEPs each. Amplitude regression slopes and block ratios were used to quantify VEP habituation. Test-retest reliability was assessed over 15 minutes and two to three weeks.

Results

Controls showed significantly more negative VEP habituation slopes than migraine patients (−0.21 ± 0.40 vs. 0.04 ± 0.46 µV/block, p < 0.05). Results were similar for block ratios, though, in the migraine group, VEP habituation significantly increased from test to two- to three-week retest ( p < 0.05). In addition, VEP habituation test-retest correlations were mostly poor both in migraine patients and controls (intraclass correlation coefficients, 15 minutes: −0.13 to 0.30, two to three weeks: 0.07 to 0.59).

Conclusions

Deficient VEP habituation in migraine was confirmed. However, the test-retest reliability of VEP habituation was rather weak. Therefore, we suggest that VEP habituation should be used for evaluation of cortical excitability under treatment only at the group level and only when a control group with sham treatment is included.

Migraine and epilepsy: review of the literature

Migraine and epilepsy are disorders that are common, paroxysmal, and chronic. In many ways they are clearly different diseases, yet there are some pathophysiological overlaps, and overlaps in clinical symptomatology, particularly with regard to visual and other sensory disturbances, pain, and alterations of consciousness. Epidemiological studies have revealed that the two diseases are comorbid in a number of individuals. Both are now recognized as originating from electrical disturbances in the brain, although their wider manifestations involve the recruitment of multiple pathogenic mechanisms. An initial excess of neuronal activity in migraine leads to cortical spreading depression and aura, with the subsequent recruitment of the trigeminal nucleus leading to central sensitization and pain. In epilepsy, neuronal overactivity leads to the recruitment of larger populations of neurons firing in a rhythmic manner that constitutes an epileptic seizure. Migraine aura and headaches may act as a trigger for epileptic seizures. Epilepsy is not infrequently accompanied by preictal, ictal, and postictal headaches that often have migrainous features. Genetic links are also apparent between the two disorders, and are particularly evident in the familial hemiplegic migraine syndromes where different mutations can produce either migraine, epilepsy, or both. Also, various medications are found to be effective for both migraine and epilepsy, again pointing to a commonality and overlap between the two disorders.

Magnetic suppression of perceptual accuracy is not reduced in episodic migraine without aura

Background

Altered cortical excitability is thought to be part of migraine pathophysiology. Reduced magnetic suppression of perceptual accuracy (MSPA) has been found in episodic migraine with aura and in chronic migraine, and has been interpreted as reduced inhibition of the occipital cortex in these migraine subtypes. Results are less clear for episodic migraine without aura. In the present study we compared MSPA between 24 healthy controls and 22 interictally measured episodic migraine patients without aura. In addition, we investigated test-retest reliability in 33 subjects (24 controls, 9 migraine).

Findings

Visual accuracy was assessed by letter recognition and modulated by transcranial magnetic stimulation delivered to the occipital cortex at different intervals to the letter presentation (40, 100 and 190 ms). The results confirm suppression of visual accuracy at the 100 ms interval (p < 0.001), but there were no significant group differences (percentage of correctly recognized letters, control: 36.1 ± 36.2; migraine: 44.0 ± 32.3, p = 0.44). Controls and migraine patients were pooled for assessment of test-retest reliability (n = 33). Levels of suppression at 100 ms were similar at test (percentage of correctly recognized letters: 42.3 ± 32.6) and retest (41.9 ± 33.8, p = 0.90) and test-retest correlations were good (r = 0.82, p < 0.001).

Conclusions

The results demonstrate that occipital cortex inhibition as assessed with MSPA is not reduced in episodic migraine without aura. This suggests a larger role of occipital cortex excitability in episodic migraine with aura and in chronic migraine compared to episodic migraine without aura. Test-retest reliability of MSPA was good.

Motor and Occipital Cortex Excitability in Migraine Patients

OBJECTIVE

We evaluated motor and occipital cortex excitability in migraine patients using transcranial magnetic stimulation.

METHODS

In this study, we included 15 migraine patients with aura (MwA), 15 patients without aura (MwoA) between attacks, and 31 normal healthy controls. Motor thresholds at rest, amplitudes of motor evoked potentials, central motor conduction time and cortical silent period were measured by stimulation of the motor cortex by using 13.5 cm circular coil and recording from abductor digiti minimi muscle. Additionally, phosphene production and the threshold of phosphene production was determined by stimulation of the visual cortex with the same coil.

RESULTS

No significant differences were observed between the groups with respect to the motor thresholds, Motor evoked potential max/compound muscle action potential max (MEPmax/Mmax) amplitudes, central motor conduction times and duration of cortical silent period. Although not statistically significant, the proportion of the migraineurs with phosphene generation (90%) was found to be higher than that of normal controls (71%). Phosphene threshold levels in migraine patients, however, were significantly lower than those of the controls with MwA patients having the lowest levels.

CONCLUSION

Our findings indicate that the occipital cortex, but not the motor cortex, is hyperexcitable in migraine patients.

Exploring facial emotion perception in schizophrenia using transcranial magnetic stimulation and spatial filtering

Schizophrenia patients have difficulty extracting emotional information from facial expressions. Perception of facial emotion can be examined by systematically altering the spatial frequency of stimuli and suppressing visual processing with temporal precision using transcranial magnetic stimulation (TMS). In the present study, we compared 25 schizophrenia patients and 27 healthy controls using a facial emotion identification task. Spatial processing was examined by presenting facial photographs that contained either high (HSF), low (LSF), or broadband/unfiltered (BSF) spatial frequencies. Temporal processing was manipulated using a single-pulse TMS delivered to the visual cortex either before (forward masking) or after (backward masking) photograph presentation. Consistent with previous studies, schizophrenia patients performed significantly below controls across all three spatial frequencies. A spatial frequency by forward/backward masking interaction effect demonstrated reduced performance in the forward masking component in the BSF condition and a reversed performance pattern in the HSF condition, with no significant differences between forward and backward masking in the LSF condition. However, the group by spatial frequency interaction was not significant. These findings indicate that manipulating visual suppression of emotional information at the level of the primary visual cortex results in comparable effects on both groups. This suggests that patients' deficits in facial emotion identification are not explained by low-level processes in the retino-geniculo-striate projection, but may rather depend on deficits of affect perception occurring at later integrative processing stages.

Persistent aura with small occipital cortical infarction: implications for migraine pathophysiology

OBJECTIVE

The pathophysiology of migraine with aura is thought to be related to cortical spreading depression and cortical hypersensitivity, in which inhibitory interneurons may play a role. Persistent migraine aura (PMA) without infarction is defined as auras that last longer than 1 week in the absence of infarction. We describe a case of persistent aura with a small occipital cortical infarction and discuss implications of this case and PMA for pathophysiological concepts of migrainous auras.

METHODS

We present a case and discuss the implications for pathophysiological concepts.

RESULTS

The case presented cannot be diagnosed as PMA as the patient was found to have an occipital cortical infarction with hypoactivity on fluorodeoxyglucose-positron emission tomography. Nevertheless, the patient suffered from persistent aura (with infarction). We argue that the infarction may have been responsible for an increased imbalance in one of the primary visual cortex networks that was already hyperexcitable due to the migraine aura condition.

CONCLUSION

PMA with occipital infarction has not been reported previously. We believe the findings of the present case and PMA cases reported in the past may support the intracortical disinhibition hypothesis in migraine.

Elevated audiovisual temporal interaction in patients with migraine without aura

BACKGROUND

Photophobia and phonophobia are the most prominent symptoms in patients with migraine without aura. Hypersensitivity to visual stimuli can lead to greater hypersensitivity to auditory stimuli, which suggests that the interaction between visual and auditory stimuli may play an important role in the pathogenesis of migraine. However, audiovisual temporal interactions in migraine have not been well studied. Therefore, our aim was to examine auditory and visual interactions in migraine.

METHODS

In this study, visual, auditory, and audiovisual stimuli with different temporal intervals between the visual and auditory stimuli were randomly presented to the left or right hemispace. During this time, the participants were asked to respond promptly to target stimuli. We used cumulative distribution functions to analyze the response times as a measure of audiovisual integration.

RESULTS

Our results showed that audiovisual integration was significantly elevated in the migraineurs compared with the normal controls (p < 0.05); however, audiovisual suppression was weaker in the migraineurs compared with the normal controls (p < 0.05).

CONCLUSIONS

Our findings further objectively support the notion that migraineurs without aura are hypersensitive to external visual and auditory stimuli. Our study offers a new quantitative and objective method to evaluate hypersensitivity to audio-visual stimuli in patients with migraine.

Brain stimulation in migraine

Migraine is a very prevalent disease with great individual disability and socioeconomic burden. Despite intensive research effort in recent years, the etiopathogenesis of the disease remains to be elucidated. Recently, much importance has been given to mechanisms underlying the cortical excitability that has been suggested to be dysfunctional in migraine. In recent years, noninvasive brain stimulation techniques based on magnetic fields (transcranial magnetic stimulation, TMS) and on direct electrical currents (transcranial direct current stimulation, tDCS) have been shown to be safe and effective tools to explore the issue of cortical excitability, activation, and plasticity in migraine. Moreover, TMS, repetitive TMS (rTMS), and tDCS, thanks to their ability to interfere with and/or modulate cortical activity inducing plastic, persistent effects, have been also explored as potential therapeutic approaches, opening an interesting perspective for noninvasive neurostimulation for both symptomatic and preventive treatment of migraine and other types of headache. In this chapter we critically review evidence regarding the role of noninvasive brain stimulation in the pathophysiology and treatment of migraine, delineating the advantages and limits of these techniques together with potential development and future application.

Assessing temporal processing of facial emotion perception with transcranial magnetic stimulation

The ability to process facial expressions can be modified by altering the spatial frequency of the stimuli, an effect that has been attributed to differential properties of visual pathways that convey different types of information to distinct brain regions at different speeds. While this effect suggests a potential influence of spatial frequency on the processing speed of facial emotion, this hypothesis has not been examined directly. We addressed this question using a facial emotion identification task with photographs containing either high spatial frequency (HSF), low spatial frequency (LSF), or broadband spatial frequency (BSF). Temporal processing of emotion perception was manipulated by suppressing visual perception with a single-pulse transcranial magnetic stimulation (TMS), delivered to the visual cortex at six intervals prior to (forward masking) or following (backward masking) stimulus presentation. Participants performed best in the BSF, followed by LSF, and finally HSF condition. A spatial frequency by forward/backward masking interaction effect demonstrated reduced performance in the forward masking component in the BSF condition and a reversed performance pattern in the HSF condition, with no significant differences between forward and backward masking in the LSF condition. Results indicate that LSF information may play a greater role than HSF information in emotional processing, but may not be sufficient for fast conscious perception of emotion. As both LSF and HSF filtering reduced the speed of extracting emotional information from faces, it is possible that intact BSF faces have an inherent perceptual advantage and hence benefit from faster temporal processing.

Magnetic flimmers: 'light in the electromagnetic darkness'

Transcranial magnetic stimulation has become an important field for both research in neuroscience and for therapy since Barker in 1985 showed that it was possible to stimulate the human motor cortex with an electromagnet. Today for instance, transcranial magnetic stimulation can be used to measure nerve conduction velocities and to create virtual lesions in the brain. The latter option creates the possibility to inactivate parts of the brain temporarily without permanent damage. In 2008, the American Food and Drugs Administration approved repetitive transcranial magnetic stimulation as a therapy for major depression under strict conditions. Repetitive transcranial magnetic stimulation has not yet been cleared for treatment of other diseases, including schizophrenia, anxiety disorders, obesity and Parkinson's disease, but results seem promising. Transcranial magnetic stimulation, however, was not invented at the end of the 20th century. The discovery of electromagnetism, the enthusiasm for electricity and electrotherapy, and the interest in Beard's concept of neurasthenia already resulted in the first electromagnetic treatments in the late 19th and early 20th century. In this article, we provide a history of electromagnetic stimulation circa 1900. From the data, we conclude that Mesmer's late 18th century ideas of 'animal magnetism' and the 19th century absence of physiological proof had a negative influence on the acceptance of this therapy during the first decades of the 20th century. Electromagnetism disappeared from neurological textbooks in the early 20th century to recur at the end of that century.

Evidence for increased internal noise in migraineurs for contrast and shape processing

Aim: Increased contrast-level dependent internal noise has been reported in migraine. This study aimed to investigate whether a general increase in internal noise impacted on other tasks thought to assess functioning in cortical area V1 and was evident in global contour coding (V4).

Methods: Eleven migraineurs (six with aura) and 12 headache-free controls completed three psychophysical tasks: (i) contrast detection, (ii) discrimination of the angle of a spiral path and (iii) detection of deformation from circularity. Internal noise estimates were obtained using an N-pass method that compared responses to repeated presentations of identical stimuli. Internal noise results in inconsistent responses across different runs.

Results: Migraineurs had significantly higher contrast thresholds when there was high external luminance noise. There were no other significant group differences in thresholds. Increased multiplicative noise associated with contrast processing was replicated and increased additive noise, which is independent of the visual input, was found for the global form task.

Conclusions: This study provides further evidence for increased multiplicative internal noise associated with contrast processing in migraineurs. However there is no generalised increase in internal noise in V1 as noise estimates for angular discrimination were normal. Increased additive internal noise was associated with the global shape task, co-occurring with increased efficiency.

Significant differences in gene expression of GABA receptors in peripheral blood leukocytes of migraineurs

Migraine is a debilitating neurovascular disorder, with a substantial genetic component. The exact cause of a migraine attack is unknown; however cortical hyperexcitability is thought to play a role. As Gamma-aminobutyric Acid (GABA) is the major inhibitory neurotransmitter in the brain, malfunctioning of this system may be a cause of the hyperexcitability. To date, there has been limited research examining the gene expression or genetics of GABA receptors in relation to migraine. The aim of our study was to determine if GABA receptors play a role in migraine by investigating their gene expression using profile in migraine affected individuals and non-affected controls by Q-PCR. Gene expression of GABA(A) receptor subunit isoforms (GABRA3, GABRB3, GABRQ) and GABA(B) receptor 2 (GABBR2) was quantified in mRNA obtained from peripheral blood leukocytes from 28 migraine subjects and 22 healthy control subjects. Analysis of results showed that two of the tested genes, GABRA3 and GABBR2, were significantly down regulated in migraineurs (P=0.018; P=0.017), compared to controls. Results from the other tested genes did not show significant gene expression variation. The results indicate that there may be specific GABA receptor gene expression variation in migraine, particularly involving the GABRA3 and GABBR2 genes. This study also identifies GABRA3 and GABBR2 as potential biomarkers to select migraineurs that may be more responsive to GABA agonists with future investigations in this area warranted.

The biological basis of headache

This article covers the remarkable recent decades as clinicians and scientists have grappled with understanding headache. It is a challenge to understand how a 'normal' brain can become dysfunctional, incapacitating an individual, and then become 'normal' again. Does the answer lie in the anatomy, electrical pathways, the chemistry or a combination? How do the pieces fit together? The components are analyzed in this article. Animal models have provided potential answers. However, these processes have never been proven in man. The dynamic imaging of pain and headache is rapidly evolving and providing new insights and directions of research.

Transcranial magnetic stimulation reveals high test-retest reliability for phosphenes but not for suppression of visual perception

OBJECTIVE

To investigate test-retest reliability of visual cortical excitatory and inhibitory phenomena.

METHODS

Transcranial magnetic stimulation (TMS) was applied over occipital cortex twice in 22 healthy young adults with at least a one-month interval between both measurements. The test-retest reliability of the phosphenes and TMS-induced suppression of visual perception was assessed using correlation and calculation of the repeatability coefficient.

RESULTS

Both analyses revealed a high reliability for phosphenes but not for the suppression of visual perception.

CONCLUSIONS

It seems likely that the phosphenes may be better used than the TMS-induced suppression of visual perception in experiments which need repeated measurements (e.g., longitudinal studies or studies with pharmacological and non-pharmacological interventions).

SIGNIFICANCE

The study demonstrates a rather limited value of the TMS-induced suppression of visual perception for studies with repeated measurements.

Cortical inhibition at rest and under a focused attention challenge in adults with migraine with and without aura

Objectives: We aimed to further elucidate the functional and attentional regulation of cortical excitability in migraine patients.

Methods: We investigated the cortical silent period (CSP) after transcranial magnetic stimulation as a measure of cortical inhibition under three conditions: resting condition, cortical preactivation during reaction preparation, and during the post-processing of a motor response using a visual contingent negative variation paradigm in adults with migraine with aura, migraine without aura and healthy controls.

Results: CSP was reduced in individuals with migraine with aura and unaffected in those with migraine without aura under resting conditions. Along with the intensity of transcranial magnetic stimulation, CSP increased equally in all groups (equal slopes). Furthermore, the functional challenge by a contingent negative variation task requiring focused sustained attention led to a comparable reduction of CSP duration in all groups.

Discussion: Our data provide further hints towards the conclusion that a specific cortical inhibition deficit in migraine with aura but not migraine without aura is due to a tonic imbalance and not related to increased reactions to phasic stressors. Given that CSP at rest is related to GABA-ergic inhibition whereas the CSP reduction during late contingent negative variation is thought to be related to dopaminergic disinhibition in the basal ganglia, our results point towards reduced GABA-ergic cortical inhibition related to dysfunctional thalamo-cortical loops, especially in migraine with aura.

Mechanism of chronic migraine

Chronic migraine typically evolves from episodic migraine over months to years in susceptible individuals. Headaches increase in frequency over time, becoming less intense but more disabling and less responsive to treatment. Results of electrophysiologic and functional imaging studies indicate that chronic migraine is associated with abnormalities in the brainstem that may be progressive. Additionally, chronic migraine is associated with a greater degree of impairment in cortical processing of sensory stimuli than is episodic migraine, perhaps due to a more pervasive or persistent cortical hyperexcitability. These findings fit with the model of migraine as a spectrum disorder, in which the clinical and pathophysiological features of migraine may progress over time. This progression is postulated to result from changes in nociceptive thresholds and ensuing central sensitization caused by recurrent migraine in susceptible individuals, for whom a variety of risk factors have been described. This may lead to changes in baseline neurologic function between episodes of headache, evident not only in electrophysiologic and functional imaging studies, but also as an increase in depression, anxiety, nonhead pain, fatigue, gastrointestinal disorders, and other somatic complaints that may occur after years of episodic migraine. From the current research and migraine models, a conceptualization of chronic migraine, in which relatively permanent and pervasive central changes that warrant novel and tolerable treatments have occurred, is emerging. This model also implies that prevention of chronic migraine is an important goal in the management of episodic migraine, particularly in individuals who exhibit risk factors for chronic transformation.

Migraine increases centre-surround suppression for drifting visual stimuli

BACKGROUND

The pathophysiology of migraine is incompletely understood, but evidence points to hyper-responsivity of cortical neurons being a key feature. The basis of hyper-responsiveness is not clear, with an excitability imbalance potentially arising from either reduced inhibition or increased excitation. In this study, we measure centre-surround contrast suppression in people with migraine as a perceptual analogue of the interplay between inhibition and excitation in cortical areas responsible for vision. We predicted that reduced inhibitory function in migraine would reduce perceptual surround suppression. Recent models of neuronal surround suppression incorporate excitatory feedback that drives surround inhibition. Consequently, an increase in excitation predicts an increase in perceptual surround suppression.

METHODS AND FINDINGS

Twenty-six people with migraine and twenty approximately age- and gender-matched non-headache controls participated. The perceived contrast of a central sinusoidal grating patch (4 c/deg stationary grating, or 2 c/deg drifting at 2 deg/sec, 40% contrast) was measured in the presence and absence of a 95% contrast annular grating (same orientation, spatial frequency, and drift rate). For the static grating, similar surround suppression strength was present in control and migraine groups with the presence of the surround resulting in the central patch appearing to be 72% and 65% of its true contrast for control and migraine groups respectively (t(44) = 0.81, p = 0.42). For the drifting stimulus, the migraine group showed significantly increased surround suppression (t(44) = 2.86, p<0.01), with perceived contrast being on average 53% of actual contrast for the migraine group and 68% for non-headache controls.

CONCLUSIONS

In between migraines, when asymptomatic, visual surround suppression for drifting stimuli is greater in individuals with migraine than in controls. The data provides evidence for a behaviourally measurable imbalance in inhibitory and excitatory visual processes in migraine and is incompatible with a simple model of reduced cortical inhibitory function within the visual system.

Detection and discrimination of flicker contrast in migraine

AIMS

Flickering light is strongly aversive to many individuals with migraine. This study was designed to evaluate other abnormalities in the processing of temporally modulating visual stimulation.

METHODS

We measured psychophysical thresholds for detection of a flickering target and for the discrimination of suprathreshold flicker contrasts (increment thresholds) in 14 migraineurs and 14 healthy controls with and without prior adaptation to high-contrast flicker. Visual discomfort (aversion) thresholds were also assessed.

RESULTS

In the baseline (no adaptation) conditions, detection and discrimination thresholds did not differ significantly between groups. Following adaptation, flicker detection thresholds were elevated equivalently in both groups; however, discrimination thresholds were more strongly affected in migraineurs than in controls, showing greater elevation at moderate contrasts and greater threshold reduction (sensitisation) at high contrast (70%). Migraineurs also had significantly elevated discomfort scores, and these were significantly correlated with number of years with migraine.

DISCUSSION

We conclude that visual flicker not only causes discomfort but also exerts measurable effects on contrast processing in the visual pathways in migraine. The findings are discussed in the context of the existing literature on habituation, adaptation and contrast-gain control.

Peri-ictal changes of cortical excitability in children suffering from migraine without aura

In adult patients with migraine, transcranial magnetic stimulation (TMS) has been used to examine cortical excitability between attacks, but there have been discrepant results. No TMS study has examined cortical excitability in children or adolescents with migraine. Here, we employed TMS to study regional excitability of the occipital (phosphene threshold [PT] and suppression of visual perception) and motor (resting motor threshold and cortical silent period) cortex in ten children suffering from migraine without aura and ten healthy age-matched controls. Patients were studied 1-2 days before and after a migraine attack as well as during the inter-migraine interval. The motion aftereffect was also investigated at each time-point as an index of cortical reactivity to moving visual stimuli. Migraineurs had lower PTs compared to healthy participants at each time-point, indicating increased occipital excitability. This increase in occipital excitability was attenuated 1-2 days before a migraine attack as indicated by a relative increase in PTs. The increase in PTs before the next attack was associated with a stronger TMS-induced suppression of visual perception and a prolongation of the motion aftereffect. Motor cortex excitability was not altered in patients and did not change during the migraine cycle. These findings show that pediatric migraine without aura is associated with a systematic shift in occipital excitability preceding the migraine attack. Similar systematic fluctuations in cortical excitability might be present in adult migraineurs and may reflect either a protective mechanism or an abnormal decrease in cortical excitability that predisposes an individual to a migraine attack.

Topiramate modulates excitability of the occipital cortex when measured by transcranial magnetic stimulation

Summary

The aim of this study was to measure differences in occipital cortex excitability in migraineurs before and after administration of topiramate. We have previously demonstrated occipital cortex hyperexcitability in migraine using an objective technique of magnetic suppression of perceptual accuracy (MSPA). We hypothesized that a neuromodulator such as topiramate would demonstrate differences in MSPA in migraine compared with baseline. Ten migraine patients were recruited. To assess inhibitory function MSPA was measured using the following protocol. Timed transcranial magnetic stimulation were delivered at interstimulus intervals (ISI) varying from 40 to 190 ms (eight stimulations at each ISI) at 60% stimulus intensity. Subjects were asked to report letters projected at a fixed luminance on the screen. Visual suppression was calculated based on the number of errors the subjects made using automated analysis. This procedure was repeated at a minimum of two different dosages of topiramate when it was titrated for optimal migraine control. The interim dose was that at which an improvement in headache frequency was first observed, and the optimal dose was that at which the patient had a ≥ 50% reduction in headache frequency, or had reached a 100-mg dose. The mean [standard error (s.e.)] level of letters reported correct at baseline at 100-ms ISI was 91.6 (3.4) compared with 48.5 (6.0) ( P = 0.001) at an optimal dose of topiramate. Dose ranged from 50 to 100 mg; the average dose was 75 mg. The interim dose for most patients was 50 mg; the mean (s.e.) percentage of letters reported correct at interim was 75.9 (6.2) compared with baseline ( P = 0.01). Mean number of headaches at baseline was 27 per month, compared with eight headaches per month at interim dose and four headaches per month at optimal dose. There was no significant correlation between mean change in frequency of headache and mean change in inhibition from baseline to optimal dose (0.04, P = 0.89). Topiramate modulates occipital cortex excitability in chronic migraine possibly via mechanisms of cortical inhibition. Since there was not a strong correlation between the degree of inhibition and reduction of migraine frequency, it would appear that topiramate did have an independent effect on cortical excitability that was not dependent on reduction in migraine frequency.

Perceptive aspects of visual aura

Visual aura is the most common feature associated with migraine, though it can occur separately. In both cases it often represents a dramatic event, especially for patients who experience it for the first time. Besides, its subjective characteristics may illuminate on the functional architecture of the visual cortex. Repetitive events of migraine and visual aura have been suggested to affect the visual system in the long run, both on the cortical and precortical level. In effect, objective investigation of visual functions in patients support the idea that a selective damage does occur, so that more attention to visual examination seems to be justified. In this paper, subjective and psychophysical aspects of visual aura are examined, lastly highlighting and discussing the interesting correlations found between this condition and normal-tension glaucoma.

Is Chronic Migraine One End of a Spectrum of Migraine or a Separate Entity?

Chronic migraine is associated with abnormalities in the periaqueductal grey that may be progressive. The condition is also associated with a greater degree of impairment in cortical processing of sensory stimuli than episodic migraine, perhaps due to more pervasive or persistent cortical hyperexcitability These findings fit with the model of migraine as a spectrum disorder, in which the clinical and pathophysiological features may progress over time. This progression may result from changes in nociceptive thresholds and ensuing central sensitization caused by recurrent migraine in susceptible individuals. This may lead to changes in baseline neurological function between headaches, evident not only in electrophysiological and functional imaging studies, but also as psychological and somatic complaints that occur after years of episodic migraine. From current research and migraine models, a conceptualization of chronic migraine is emerging in which relatively permanent and pervasive central changes have occurred that warrant novel and tolerable treatments.