Visual contrast gain control in migraine: measures of visual cortical excitability and inhibition

No Evidence of Cross-Orientation Suppression Differences in Migraine with Aura Compared to Healthy Controls

It has been suggested that there may be an imbalance of excitation and inhibitory processes in the visual areas of the brain in people with migraine aura (MA). One idea is thalamocortical dysrhythmia, characterized by disordered oscillations, and thus disordered communication between the lateral geniculate nucleus and the cortex. Cross-orientation suppression is a visual task thought to rely on inhibitory processing, possibly originating in the lateral geniculate nucleus. We measured both resting-state oscillations and cross-orientation suppression using EEG over occipital areas in people with MA and healthy volunteers. We found evidence of cross-orientation suppression in the SSVEP responses, but no evidence of any group difference. Therefore, inhibitory processes related to cross-orientation suppression do not appear to be impaired in MA.

Relating the cortical visual contrast gain response to spectroscopy-measured excitatory and inhibitory metabolites in people who experience migraine

Objective

This study aimed to determine whether the visual response to flickering checkerboard patterns measured using electroencephalography (EEG) relate to excitatory or inhibitory metabolite levels measured using ultra-high (7Tesla/7T) magnetic resonance spectroscopy (MRS).

Background

Electrophysiological studies have shown altered visual cortical response amplitudes and contrast gain responses to high contrast flickering patterns in people with migraine. These contrast response anomalies have been argued to represent an imbalance between cortical inhibition and excitation, however the specific mechanism has not been elucidated.

Methods

MRS-measured metabolite levels were obtained from the occipital cortex of 18 participants with migraine and 18 non-headache controls. EEG contrast gain response functions were collected on separate days from a subset of 10 participants with migraine and 12 non-headache controls. Case-control outcome measures were statistically compared between groups both before and after checkboard exposure.

Results

No significant difference in GABA and glutamate levels were found between groups nor checkerboard timepoint. Glucose levels were significantly reduced after checkerboard exposure in both participant groups. There was no metabolic signature in visual cortex in response to high-contrast flickering checkboards that distinguished those with migraine and without. There was also no correlation between MRS and EEG measurements in response to the flickering checkerboard.

Conclusion

Our findings suggest that the mechanisms driving contrast-flickering stimulus aversion are not simplistically reflected by gross changes in metabolic activity in the primary visual cortex.

Chromatic Induction in Migraine

The human visual system is not a colorimeter. The perceived colour of a region does not only depend on its colour spectrum, but also on the colour spectra and geometric arrangement of neighbouring regions, a phenomenon called chromatic induction. Chromatic induction is thought to be driven by lateral interactions: the activity of a central neuron is modified by stimuli outside its classical receptive field through excitatory-inhibitory mechanisms. As there is growing evidence of an excitation/inhibition imbalance in migraine, we compared chromatic induction in migraine and control groups. As hypothesised, we found a difference in the strength of induction between the two groups, with stronger induction effects in migraine. On the other hand, given the increased prevalence of visual phenomena in migraine with aura, we also hypothesised that the difference between migraine and control would be more important in migraine with aura than in migraine without aura. Our experiments did not support this hypothesis. Taken together, our results suggest a link between excitation/inhibition imbalance and increased induction effects.

No Evidence of Reduced Contrast Sensitivity in Migraine-with-Aura for Large, Narrowband, Centrally Presented Noise-Masked Stimuli

Individuals with migraine aura show differences in visual perception compared to control groups. Measures of contrast sensitivity have suggested that people with migraine aura are less able to exclude external visual noise, and that this relates to higher variability in neural processing. The current study compared contrast sensitivity in migraine with aura and control groups for narrow-band grating stimuli at 2 and 8 cycles/degree, masked by Gaussian white noise. We predicted that contrast sensitivity would be lower in the migraine with aura group at high noise levels. Contrast sensitivity was higher for the low spatial frequency stimuli, and decreased with the strength of the masking noise. We did not, however, find any evidence of reduced contrast sensitivity associated with migraine with aura. We propose alternative methods as a more targeted assessment of the role of neural noise and excitability as contributing factors to migraine aura.

Illusory Motion Perception Is Associated with Contrast Discrimination but Not Motion Sensitivity, Self-Reported Visual Discomfort, or Migraine Status

Purpose

Altered visual processing of motion and contrast has been previously reported in people with migraine. One possible manifestation of this altered visual processing is increased self-reported susceptibility to visual illusions of contrast and motion. Here, we use the Fraser–Wilcox illusion to explore individual differences in motion illusion strength in people with and without migraine. The motion-inducing mechanisms of the Fraser–Wilcox illusion are purported to be contrast dependent. To better understand the mechanisms of the illusion, as well as visual processing anomalies in migraine, we explored whether migraine status, susceptibility to visual discomfort, contrast discrimination, or motion sensitivity are related to quantified motion illusion strength.

Methods

Thirty-six (16 with aura, 20 without aura) people with migraine and 20 headache-free controls participated. Outcome measures were motion illusion strength (the physical motion speed that counterbalanced the illusory motion), motion sensitivity, and contrast discrimination thresholds (measured for each contrast pair that formed part of the illusory motion stimulus). Typical daily visual discomfort was self-reported via questionnaire.

Results

Motion illusion strength was negatively correlated with contrast discrimination threshold (r = –0.271, P = 0.04) but was not associated with motion sensitivity or migraine status. People with migraine with aura reported experiencing visual discomfort more frequently than the control group (P = 0.001). Self-reported visual discomfort did not relate to quantified perceptual motion illusion strength.

Conclusions

Individuals with better contrast discrimination tend to perceive faster illusory motion regardless of migraine status.

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.

Typical Lateral Interactions, but Increased Contrast Sensitivity, in Migraine-With-Aura

Individuals with migraine show differences in visual perception compared to control groups. It has been suggested that differences in lateral interactions between neurons might account for some of these differences. This study seeks to further establish the strength and spatial extent of excitatory and inhibitory interactions in migraine-with-aura using a classic lateral masking task. Observers indicated which of two intervals contained a centrally presented, vertical Gabor target of varying contrast. In separate blocks of trials, the target was presented alone or was flanked by two additional collinear, high contrast Gabors. Flanker distances varied between 1 and 12 wavelengths of the Gabor stimuli. Overall, contrast thresholds for the migraine group were lower than those in the control group. There was no difference in the degree of lateral interaction in the migraine group. These results are consistent with the previous work showing enhanced contrast sensitivity in migraine-with-aura for small, rapidly presented targets, and they suggest that impaired performance in global perceptual tasks in migraine may be attributed to difficulties in segmenting relevant from irrelevant features, rather than altered local mechanisms.

Visual processing in migraine

Background

Migraine is a common neurological condition that often involves differences in visual processing. These sensory processing differences provide important information about the underlying causes of the condition, and for the development of treatments.

Review of psychophysical literature

Psychophysical experiments have shown consistent impairments in contrast sensitivity, orientation acuity, and the perception of global form and motion. They have also established that the addition of task-irrelevant visual noise has a greater effect, and that surround suppression, masking and adaptation are all stronger in migraine.

Theoretical signal processing model

We propose utilising an established model of visual processing, based on signal processing theory, to account for the behavioural differences seen in migraine. This has the advantage of precision and clarity, and generating clear, falsifiable predictions.

Conclusion

Increased effects of noise and differences in excitation and inhibition can account for the differences in migraine visual perception. Consolidating existing research and creating a unified, defined theoretical account is needed to better understand the disorder.

Motion Perception in Migraineurs: Abnormalities are Not Related to Attention

Migraine groups have impaired ability to identify global motion direction in noisy random dot stimuli, an observation that has been used as evidence for cortical hyperexcitability. Several studies have also suggested abnormalities in cognitive processing, particularly in the domains of attention, visuo-spatial processing and memory. This study aimed to determine whether poor performance by migraineurs in motion coherence tasks could be explained by non-visual cognitive factors such as attention. Twenty-nine migraineurs and 27 non-headache controls participated. Global motion coherence thresholds were measured along with measures of neuropsychological function, using the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). The migraine group had significantly higher motion coherence thresholds than controls. No significant difference in attention or any other RBANS index score was found between groups. Index scores did not correlate with motion perception thresholds. This study does not support inattention or other cognitive abnormality as an explanation for motion perception anomalies in migraine.

Motion Processing Deficits in Migraine

This study was designed to determine whether cortical motion processing abnormalities are present in individuals with migraine. Performance was measured using a visual motion coherence task (motion coherence perimetry, MCP) thought to depend on the operation of cortical area V5. Motion coherence thresholds were measured using stimuli composed of moving dots at 17 locations in the central ± 20° of visual field. Pre-cortical visual function was also measured using frequency doubling perimetry (FDP) at the same 17 locations. Several migraine subjects demonstrated significant pre-cortical visual functional abnormalities, however, most subjects had normal visual fields measured with FDP. Abnormal MCP performance was measured in 15 of 19 migraine-with-aura subjects, and 11 of 17 migraine-without-aura subjects. A decreased ability to detect coherent motion may possibly be explained by an increase in baseline neuronal noise, such as would be consistent with the concept of cortical hyperexcitability in migraine.

An Analysis of the Factors Associated with Visual Field Deficits Measured with Flickering Stimuli in-between Migraine

We have previously demonstrated that perimetric performance measured with flickering stimuli is not normal in some individuals who experience migraine with aura in the period between their attacks. In this study, flicker perimetric performance is measured in a broad group of migraineurs to determine whether the existence of such visual field deficits is dependent on the presence of visual aura, is correlated with the duration of migraine history, or frequency of attacks. Twenty-eight migraine with aura, 25 migraine without aura, and 24 non-headache control subjects participated. The performance of the migraine groups was not significantly different from each other. The migraine groups showed significantly lower general sensitivity across the visual field and higher incidence of localized visual field deficits relative to controls. Both length of migraine history and frequency of migraine occurrence over the past 12 months were significantly correlated with lower general sensitivity to flickering visual stimuli.

Melatonin Secretion is Supersensitive to Light in Migraine

The present study examined the sensitivity to light of melatonin (MLT) secretion in familial migraine during a headache-free interval. Twelve female patients and 12 healthy controls were included in the trial. All subjects were studied twice. In each session, light exposure (300 lx) or placebo was randomly administered for 30 min between 00.30 and 01.00 h. Blood was sampled hourly between 20.00 and 24.00 h, and 02.00 and 04.00 h and every 15 min between 00.30 and 01.30 h. Plasma MLT levels were determined by radioimmunoassay. MLT suppression was more marked in the migraine group than in the control group [difference of area under curve (δAUC) = −53.8 ± 16.2 vs. 18.5 ± 12.7 pg/h/ml, P < 0.005; maximum of MLT suppression (δ) = −35.7 ± 10.2 vs.- 6.7 ± 5.8 pg/ml, P < 0.05]. These findings show a clear hypersensitivity to light in young female migraineurs during the headache-free period.

Acquired equivalence and related memory processes in migraine without aura

Introduction

Interictal deficits of elementary visuo-cognitive functions are well documented in patients with migraine and are mostly explained in terms of neocortical hyperexcitability. It has been suggested that the basal ganglia and the hippocampi might also be affected in migraine. If so, a deterioration of learning and memory processes related to these structures is expected.

Methods

A visual learning paradigm thought to be capable of dissociating learning/memory processes mediated by the basal ganglia from processes mediated by the hippocampus (the Rutgers Acquired Equivalence Test) was applied to a group of patients with migraine without aura and to age- and sex-matched controls.

Results

Patients with migraine showed a significantly poorer performance in both main phases of the test and the deficit in the phase considered to be dependent on the hippocampi was especially marked.

Conclusions

These results can be interpreted as behavioural support for findings that have suggested the involvement of the basal ganglia and the hippocampi in migraine, but further research is needed to clarify these findings.

Clinical impact of migraine for the management of glaucoma patients

Migraine is a common and debilitating primary headache disorder that affects 10-15% of the general population, particularly people of working age. Migraine is relevant to providers of clinical eye-care because migraine attacks are associated with a range of visual sensory symptoms, and because of growing evidence that the results of standard tests of visual function necessary for the diagnosis and monitoring of glaucoma (visual fields, electrophysiology, ocular imaging) can be abnormal due to migraine. These abnormalities are measureable in-between migraine events (the interictal period), despite patients being asymptomatic and otherwise healthy. This picture is further complicated by epidemiological data that suggests an increased prevalence of migraine in patients with glaucoma, particularly in patients with normal tension glaucoma. We discuss how migraine, as a co-morbidity, can confound the results and interpretation of clinical tests that form part of contemporary glaucoma evaluation, and provide practical evidence-based recommendations for the clinical testing and management of patients with migraine who attend eye-care settings.

Cortical hyperexcitability in migraine and aversion to patterns

BACKGROUND

Patients with migraine are averse to certain visual stimuli, such as flicker and striped patterns that evoke paroxysmal EEG activity in patients with photosensitive epilepsy. Migraineurs demonstrate a hyper-responsiveness to such stimuli, and there is debate as to whether the aversion and hyper-responsiveness are due to a hyperexcitability of the cortex similar to that in patients with photosensitive epilepsy. In these patients grating patterns with certain spatial characteristics can be epileptogenic, depending critically on their movement. If the contours of the grating drift continually, the grating is not epileptogenic, but if the contours are static or if their direction is repeatedly and rapidly reversed so as to vibrate, the grating then becomes highly epileptogenic.

METHODS

We compared aversion to vibrating, drifting and static gratings in migraineurs and controls. The contrast of each grating was gradually increased, but only until the participant felt discomfort, so as to obtain a contrast threshold for aversion with minimal exposure.

RESULTS

Migraineurs had lower thresholds than the control group, indicating greater aversion. For both groups the threshold was higher (aversion was lower) for static than for both types of moving gratings. The drifting gratings were more aversive than the vibrating gratings when both groups were combined.

CONCLUSION

The findings suggest that the aversion shown by migraineurs is not attributable to a cortical hyperexcitability similar to that in photosensitive epilepsy.

Increased internal noise cannot account for motion coherence processing deficits in migraine

Aim: This study aimed to revisit previous findings of superior processing of motion direction in migraineurs with a more stringent direction discrimination task and to investigate whether increased internal noise can account for motion processing deficits in migraineurs.

Methods: Groups of 13 migraineurs (4 with aura, 9 without aura) and 15 headache-free controls completed three psychophysical tasks: one detecting coherence in a motion stimulus, one discriminating the spiral angle in a glass pattern and another discriminating the spiral angle in a global-motion task. Internal noise estimates were obtained for all tasks using an N-pass method.

Results: Consistent with previous research, migraineurs had higher motion coherence thresholds than controls. However, there were no significant performance differences on the spiral global-motion and global-form tasks. There was no significant group difference in internal noise estimates associated with any of the tasks.

Conclusions: The results from this study suggest that variation in internal noise levels is not the mechanism driving motion coherence threshold differences in migraine. Rather, we argue that motion processing deficits may result from cortical changes leading to less efficient extraction of global-motion signals from noise.

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.

Avaliação da sensibilidade ao contraste em pacientes com migrânea

O objetivo foi medir a Função de Sensibilidade ao Contraste (FSC) de pacientes com migrânea e de voluntários saudáveis sem a patologia. Participaram dos testes 12 voluntários do sexo feminino, seis com migrânea e seis sem migrânea na faixa etária de 20 a 37 anos. As medidas de FSC foram realizadas utilizando estímulos visuais estáticos de grades senoidais angulares com frequências espaciais de 2, 3, 4, 24 e 64 ciclos/360º Foi utilizado o método psicofísico da escolha forçada entre duas alternativas temporais, condições de luminância fotópica (luminância média da tela de 41 cd/m²) e visão binocular com pupila natural. Os resultados demonstram que a percepção visual de contraste dos voluntários com migrânea foi menor nas frequências de 2, 3, 4 e 64 ciclos/360º. Esses achados preliminares sugerem alterações na FSC relacionadas a essa patologia.

Visual metacontrast masking in migraine

Background: In visual metacontrast masking, the visibility of a brief target stimulus can be reduced substantially if it is preceded (forward masking) or followed (backward masking) by a non-overlapping mask. These effects have been attributed to inhibitory processes within the visual system. Two previous studies have used metacontrast masking to assess inhibitory function in migraine and control groups, however, each used different types of masking and obtained different results.

Subjects and Methods: Forward, backward and combined forward and backward masking were compared in migraine (15 with visual aura, 15 without) and control ( n = 15) groups. Baseline trials were also included (target only).

Results: For all types of masking, both migraine groups were more accurate than the control group. When performance for the masking trials was expressed relative to baseline, however, there were no significant group differences. Performance in certain conditions nevertheless correlated significantly with migraine frequency and with the recency of the last attack.

Conclusions: The inhibitory processes involved in the masking tasks employed in this study do not appear to be impaired in migraine. Their better overall performance may reflect a sensitivity difference, perhaps as a consequence of a heightened neuronal response, which varies with the migraine cycle.

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.

Low spatial frequency contrast sensitivity deficits in migraine are not visual pathway selective

Some people who experience migraine demonstrate reduced visual contrast sensitivity that is measurable between migraines. Contrast sensitivity loss to low spatial frequency gratings has been previously attributed to possible impairment of magnocellular pathway function. This study measured contrast sensitivity using low spatial frequency targets (0.25-4 c/deg) where the adaptation aspects of the stimuli were designed to preferentially assess either magnocellular or parvocellular pathway function (steady and pulsed pedestal technique). Twelve people with migraine with measured visual field abnormalities and 17 controls participated. Subjects were tested foveally and at 10 degrees eccentricity. Foveally, there was no significant difference in group mean contrast sensitivity. At 10 degrees , the migraine group demonstrated reduced contrast sensitivity for both the stimuli designed to assess magnocellular and parvocellular function (P < 0.05). The functional deficits measured in this study infer that abnormalities of the low spatial frequency sensitive channels of both pathways contribute to contrast sensitivity deficits in people with migraine.

Binocular rivalry in migraine

Cortical hyperexcitability in migraine could arise from abnormally weak inhibition or from strengthened intracortical excitatory mechanisms. The present study employed binocular rivalry to differentiate between these possibilities. Rivalry between static oriented grating patterns was examined in migraine with aura (MA), migraine without aura (MoA) and headache-free control participants. A non-significant trend toward elevated mean dominance intervals (monocular percepts, in seconds) was seen in both migraine groups at all contrasts. Second, significant interocular differences in rivalry dominance durations were seen in the MoA group compared with controls; this difference also approached significance in the MA group. Finally, both MA and MoA exhibited significantly greater visual discomfort than the control group in the presence of both static stripes and flickering visual stimuli. The rivalry results provide no support for weakened intracortical inhibition in migraine. Optical or neural precortical differences in the eyes' input strengths paired with enhanced recurrent cortical excitation can explain these findings.

Abnormal visual processing in migraine with aura: a study of steady-state visual evoked potentials

BACKGROUND

Although a number of studies reported different interictal findings between migraine with aura (MA) and migraine without aura (MO), the pathophysiology of the visual aura in migraine remains unclear.

OBJECTIVE

To investigate the visual processing in patients who experience MA between attacks using steady-state visual evoked potentials (SSVEPs).

METHODS

SSVEPs to high (98%) and low (29%) contrast black and white checkerboard gratings with two spatial frequencies (0.5 and 2.0 cpd) at 5 and 10 Hz (10 and 20 reversal/s) were recorded binocularly from 10 patients with MA, 10 patients with MO between attacks and 20 healthy controls (HC). The SSVEPs were Fourier analyzed to obtain the amplitude and phase of the second (2F) and fourth (4F) harmonic response.

RESULTS

In the amplitude of 2F, at 0.5 cpd, there was significant increased amplitude in both MA and MO in comparison to HC at 5 Hz in high and low contrast. However, no significant differences were detected at 2.0 cpd in both 5 and 10 Hz in high and low contrast. In the amplitude of 4F, at 2.0 cpd, there was significant increased amplitude in MA in comparison to MO and HC at 10 Hz in high contrast. However, there were no significant differences at 0.5 cpd at both 5 and 10 Hz in high and low contrast. There were no significant phase differences between MA, MO, and HC.

CONCLUSION

The high amplitude of the SSVEPs suggests that interictally migraine patients have abnormal excitability in the primary visual cortex, and this change in excitability may exist, at least partially, in the visual association cortex in MA.

[Cortical dysbalance in the brain in migraineurs--hyperexcitability as the result of sensitisation?]

A cortical dysbalance has a pivotal role in the pathophysiology of migraine. Numerous electrophysiological and transcranial magnetic stimulation (TMS) studies have investigated the interictal excitability level in migraineurs and have shown a consistent lack of habituation during repetitive stimulation. There is some controversy in the current literature over whether this deficit is based on a lowered or an elevated preactivation level. However, the current discussion may be misguided. It seems that multiple external and intrinsic factors influence the level of cortical excitability and the frequency and intensity of attacks: Habituation is specific neither to migraine nor even to pain; the same phenomenon is found in tinnitus patients, for example. Cortical hyperexcitability is presumably the result of chronicity and the concomitant central sensitisation process.

Subclinical Dysfunction of Cochlea and Cochlear Efferents in Migraine: An Otoacoustic Emission Study

Otoacoustic emission (OAE) testing enables us to identify the cochlear component of a hearing disorder and to monitor objectively minute changes in cochlear status undetectable by other audiological methods. Contralateral sound-induced suppression is mediated by medial superior olivary complex efferents which induce hyperpolarization counteracting the amplifying effects of outer hair cell (OHC) activity. The aim of this study was to assess functions of cochlea and its efferents in migraine using OAE testing and contralateral suppression of transiently evoked OAEs (TEOAE). Fifty-three migraineurs (106 ears) and 41 healthy subjects (82 ears) were included and pure tone audiometry (PTA), speech discrimination scores (SDS), distortion product OAE (DPOAE), TEOAE and contralateral suppression of TEOAEs were tested. PTA and SDS of migraineurs and controls were not different ( P > 0.05). DPOAEs were tested between 1 and 6 kHz and a significant difference was detected only at 5 kHz frequency, where DPOAE amplitudes in migraine with aura (MA) were lower than in controls ( P < 0.03). The mean amplitudes of TEOAEs were statistically insignificant between controls and migraine groups. Contralateral sound stimulus induced significant decrease in amplitudes of TEOAE ( P = 0.005) in controls. In patients with migraine without aura and MA, mean amplitudes of TEOAEs were not suppressed by contralateral sound stimulus ( P > 0.05). As PTA, SDS and DPOAE tests demonstrate normal functioning of inner ear between 1 and 4 kHz, absence of suppression of the TEOAEs by contralateral sound stimulation indicates the presence of dysfunction either in the medial olivocochlear complex in the brainstem or at the synaptic transmission between olivocochlear efferents and OHCs in the cochlea. Disruption in the contralateral suppression may be one of the mechanisms predisposing to the phonophobia symptom associated with migraine headache.

The brain is hyperexcitable in migraine

Migraine is a very common disorder occurring in 20% of women and 6% of men. Central neuronal hyperexcitability is proposed to be the putative basis for the physiological disturbances in migraine. Since there are no consistent structural disturbances in migraine, physiological and psychophysical studies have provided insight into the underlying mechanisms. This is a review of the neurophysiological studies which have provided an insight to migraine pathogenesis supporting the theory of hyperexcitability.

Higher variability of phosphene thresholds in migraineurs than in controls: a consecutive transcranial magnetic stimulation study

The aim of this study was to characterize the temporal course of phosphene thresholds (PT) using transcranial magnetic stimulation (TMS) in control subjects and in subjects with migraine and to observe whether changes in PT over time can predict a subsequent migraine attack. PTs were measured in 16 migraineurs [nine with aura (MA) and seven without aura (MoA)] and nine controls five times over an approximately 10-week period. Mean PTs were not significantly different between migraineurs and controls; however, there was a trend in MA showing lower thresholds. The minimum threshold values were also smaller in MA subjects than in MoA or control subjects. Generally, PTs had higher variance in migraineurs than in controls, revealing a significant increase in standard deviation of PTs in MA subjects. There was no significant difference of thresholds from the first to the last stimulation in controls and in MoA subjects, but the 3rd, 4th and 5th measurements of MA subjects were significantly lower than their first measurements. Four migraineurs experienced headache within 1 day after one of the measurements. They had either very low or very high PTs compared with the PT values which were not followed by a migraine attack. Our results imply that migraineurs show a higher variability among PT measurements over time than controls, revealing unstable excitability levels in these patients. Additionally, both particularly high and low PTs might predict a subsequent headache in some individuals.

Do migraineurs have difficulty judging direction of simulated heading?

Some migraineurs have increased thresholds for the detection of global dot motion. We investigated whether migraineurs show consequential abnormalities in the determination of direction of self-motion (heading) from simulated optic flow. The ability to determine heading from optic flow is likely to be necessary for optimal determination of self-motion through the environment. Twenty-five migraineurs and 25 controls participated. Global dot motion coherence thresholds were assessed, in addition to performance on two simulated heading tasks: one with a symmetrical flow field, and the second with differing velocity of optic flow on the left and right sides of the participant. While some migraineurs demonstrated abnormal global motion coherence thresholds, there was no difference in performance on the heading tasks at either simulated walking (5 km/h) or driving (50 km/h) speeds. Increased global motion coherence thresholds in migraineurs do not result in abnormal judgements of heading from 100% coherent optic flow.

Visual cortical inhibitory function in migraine is not generally impaired: evidence from a combined psychophysical test with an fMRI study

A robust, visual masking test that was developed to be feasible with functional magnetic resonance imaging (fMRI) was used to examine the visual cortical inhibitory function in migraine patients with visual aura at both psychophysical and cortical levels. The study showed that the decreased visibility of a visual target was associated with a reduction in cortical activation in the primary visual cortex. The suppression of the transient on-response and after-discharge of neurons to the target was most likely to be responsible for reducing cortical activation, rendering the target less visible or invisible. The migraine patients were equally susceptible to visual masking and showed no difference in cortical activation when compared with age- and sex-matched non-headache controls, demonstrating that visual cortical inhibitory function was not impaired under the experimental conditions. Although these results are not in conflict with the general cortical hyperexcitability theory in migraine, they provide evidence to show the limitation to the theory.

Assessment of spatial–contrast function and short-wavelength sensitivity deficits in patients with migraine

AIMS

To study spatial-contrast function and short-wavelength sensitivity deficits in a migraine population with a disease duration of 30 years or less.

MATERIALS AND METHODS

In this prospective, cross-sectional study, we evaluated 28 subjects with migraine headache and 15 nonheadache healthy controls. Visual fields were evaluated using the Humphrey Field Analyzer 750i and the 30-2, blue and yellow threshold programme. Contrast sensitivity (CS) was measured at 1.5, 3, 6, 12, and 18 cpd spatial frequencies, using the Functional Acuity Contrast Test (F.A.C.T.). The results of the visual field parameters (mean defect (MD) and pattern standard defect (PSD)) and CS were compared with 15 age-equivalent normal subjects.

RESULTS

Short-wavelength amplitude perimetry (SWAP) parameters and CS scores at all spatial frequencies were significantly altered in the migraine patients when compared with the control subjects. Visual field parameters correlated significantly with contrast sensitivity scores: positively for MD (r=0.39, P=0.01; r=0.43, P=0.005; r=0.56, P=0.0001; r=0.45, P=0.003; r=0.48, P=0.0001) and negatively for PSD (r=-0.45, P=0.003; r=-0.45, P=0.003; r=-0.51, P=0.001; r=-0.53, P=0.0001; and r=-0.67, P=0.0001) at all (1.5, 3, 6,12, and 18 cpd) spatial frequencies, respectively. Migraine duration correlated negatively with MD (r=-0.42, P=0.04) and positively with PSD (r=0.42, P=0.03).

CONCLUSION

Migraineurs had significantly altered visual field and contrast function at all spatial frequencies to the normal population. These defects share some features with early stages of glaucoma and may relate a possibility for a common vascular disease pathogenesis in these two conditions.

Altered motion perception in migraineurs: evidence for interictal cortical hyperexcitability

Much research on visual functions in migraine has pinpointed the existence of abnormal visual processing between attacks. However, it is not clear if this is due to cortical hyper- or hypoexcitability. We aimed to clarify this issue by comparing motion perception thresholds of subjects with migraine with (MA) or without aura (MoA) and control subjects. Two types of dot kinetograms were used: in the first experiment coherently moving dots were presented in an incoherent environment, while in the second only coherent motion was seen. Subjects with migraine displayed significantly impaired motion perception compared with controls when they had to detect the direction of the coherently moving dots in an incoherent environment, while they were slightly better in a direction discrimination task, where only coherent motion was presented. This pattern of results is comparable to those achieved by an external excitability enhancement of V5 induced in healthy human subjects in a former study of our group. According to this, a cortical excitability enhancement can result in an impaired focusing on a given signal against a noisy background, but improves perception of non-ambiguous stimuli. Thus we conclude that migraineurs display enhanced visual cortical excitability between attacks in V5.

Processing of global form and motion in migraineurs

Previous studies have identified anomalies of cortical visual processing in migraineurs that appear to extend beyond V1. Migraineurs respond differently than controls to transcranial magnetic stimulation of V5, and can demonstrate impairments of global motion processing. This study was designed to assess the integrity of intermediate stages of both motion and form processing in people with migraine. We measured the ability to integrate local orientation information into a global form percept, and to integrate local motion information into a global motion percept. Control subjects performed significantly better than migraineurs on both tasks, suggesting a diffuse visual cortical processing anomaly in migraine.

Brain Hyperexcitability: The Basis for Antiepileptic Drugs in Migraine Prevention

Abnormal brain excitability may provide the susceptibility for triggering migraine attacks. Antiepileptic drugs may diminish neuronal excitability and consequently reduce the frequency of migraine. Because migraine aura is predominantly visual, hyperexcitability of the occipital cortex has been the focus of investigations. Functional magnetic resonance imaging of the brain and magnetoencephalography provide the most consistent evidence for the role of brain hyperexcitability in migraine and confirm that triggering an abnormal electric and metabolic event consistent with the cortical spreading depression (CSD) of Leao is anatomically and functionally linked with migraine aura symptoms. Future drug discovery should focus on the interface between the excitable cell and the earliest events of CSD.

Are migraineurs hypersensitive? A test of the stimulus processing disorder hypothesis

The concept of hypersensitivity in migraineurs was advanced mainly on the basis of studies on information processing in which increased amplitudes and reduced habituation in cortical evoked and event related potentials were found in migraine sufferers. The present investigation examined whether migraineurs exhibit hypersensitivity within three different experimental paradigms and various non electrocortical response parameters. Samples of 24 migraine, 19 tension-type headache sufferers, and 24 normal controls were compared regarding their subjective estimation of intensity and discomfort due to visual and acoustical stimuli. Subjects also participated in an experiment using the eyeblink startle response paradigm. In a last experimental task the Stroop test was applied. The trait variables emotionality, arousal, and extraversion were also measured. None of the experimental tasks revealed the predicted hypersensitivity of migraineurs in relation to the control samples. The series of experiments was conducted a second time with half of the participants in order to replicate the findings. The conclusions remained the same. The results of earlier studies on cortical processing can not be interpreted as demonstrating general hypersensitivity in the sense of a dispositional trait in migraine afflicted individuals irrespective of the involved response system.

The electrophysiology of migraine

Migraine is currently regarded as a neurovascular disorder of trigeminal sensory processing, generated centrally, probably at the level of the brainstem. In the past, electrophysiological techniques have drawn no definite conclusions on either interictal or ictal changes in migraineurs compared with controls, largely because of methodological differences. Recently, two findings have been shown consistently: an interictal increasing lack of habituation of evoked potentials with a normalization at the start of the attack and strong intensity dependence of auditory evoked potentials. These findings substantiate migraine sufferers as having an abnormal trait interictally, with the attack characterized by a change in the state of central processing. Exploitation of these differences may be a useful tool to study the mechanism of action of drugs used for the treatment of migraine.

Assessment and psychological management of recurrent headache disorders

This article updates earlier reviews of recurrent headache disorders published in 1982 and 1992, selectively reviewing research published since 1990. Current issues in assessment (headache diagnosis, psychophysiology, comorbid psychopathology, quality-of-life assessment, and new assessment technologies) and psychological treatment (efficacy, therapeutic mechanisms, treatment delivery, and integration with drug therapy) are addressed. The author emphasizes the need to adapt psychological treatments to the severity of the headache disorder and to developments in drug therapy. Opportunities for the integration of biological, medical, and psychological science are highlighted.

Spatial contrast sensitivity of migraine patients without aura

Visual disturbances are frequent symptoms in migraine. Since there is a possibility of separate damage in the magno- or parvo-cellular visual pathway in migraine patients, we performed a study including the measurement of static and dynamic spatial contrast sensitivity on 15 patients suffering from migraine without aura under photopic and scotopic conditions. Fifteen healthy volunteers without primary headache served as controls. The results revealed a marked decrease in contrast sensitivity at low spatial frequencies in the migraine patients. Spatial contrast sensitivity demonstrated some lateralization, as the sensitivity to low spatial frequencies obtained through separate eyes showed significantly larger side-differences in migraine patients than in control subjects. These findings suggest that the mechanisms responsible for vision at low spatial frequencies are impaired in migraine patients. This might indicate impaired function of the magnocellular pathways in this condition.