***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Much research on visual function in migraine has examined early aspects of visual processing , often using detection or discrimination measures and stimuli reported to trigger an attack , e.g. striped patterns or flickering lights . Differences between people with and without migraine have been attributed to abnormal cortical processing in migraine , variously described by interictal hyperexcitability , heightened responsiveness , a lack of habituation and /or a lack of intra-cortical inhibition . Here , two experiments are presented that explore a uniquely cortical phenomenon , pattern or contrast adaptation , one using the motion after-effect , one the tilt after-effect . Pattern adaptation reflects specific interactions between groups of neurones and is therefore ideally suited to address proposed models of cortical function in migraine . These models lead to specific predictions in an adaptation study : there should be smaller effects in people with migraine than in people without . The results from both adaptation experiments , however , revealed larger effects in migraine sufferers than in headache-free control subjects . There were no differences between migraine subgroups classified according to the presence or absence of aura . These results are discussed in terms of models of cortical function in migraine Between attacks , migraine patients are characterized by potentiation instead of habituation of stimulation-evoked cortical responses . It is debated whether this is due to increased or decreased cortical excitability . We have studied the changes in visual cortex excitability by recording pattern-reversal visual evoked potentials ( PR-VEP ) after low- and high-frequency repetitive transcranial magnetic stimulation ( rTMS ) , known respectively for their inhibitory and excitatory effect on the cortex . In 30 patients ( 20 migraine without , 10 with aura ) and 24 healthy volunteers , rTMS of the occipital cortex was performed with a focal figure-of-eight magnetic coil ( Magstim ) . Nine hundred pulses were delivered r and omly at 1 or 10 Hz in two separate sessions . Stimulus intensity was set to the phosphene threshold or to 110 % of the motor threshold if no phosphenes were elicited . Before and after rTMS , PR-VEP were averaged sequentially in six blocks of 100zztieresponses during uninterrupted 3.1 Hz stimulation . In healthy volunteers , PR-VEP amplitude was significantly decreased in the first block after 1 Hz rTMS and the habituation normally found in successive blocks after sustained stimulation was significantly attenuated . In migraine patients , 10 Hz rTMS was followed by a significant increase of first block PR-VEP amplitude and by a reversal to normal habituation of the potentiation ( or dishabituation ) characteristic of the disorder . This effect was similar in both forms of migraine and lasted for at least 9 min . There were no significant changes of PR-VEP amplitudes after 1 Hz rTMS in migraineurs and after 10 Hz rTMS in healthy volunteers , nor after sham stimulation . The recovery of a normal PR-VEP habituation pattern after high-frequency rTMS is probably due to activation of the visual cortex and the dishabituation in healthy volunteers to cortical inhibition . We conclude , therefore , that the deficient interictal PR-VEP habituation in migraine is due to a reduced , and not to an increased , pre-activation excitability level of the visual cortex 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 st and ard 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 OBJECTIVE To determine the excitability of the visual cortex by  phosphene thresholds ( PT ) in patients with migraine using transcranial magnetic stimulation ( TMS ) with single- and paired-pulses . METHODS Nineteen patients with migraine with aura ( MWA ) , 19 patients with migraine without aura ( MWoA ) , and 22 control subjects were included . Patients were free from preventive anti-migraine treatment and were investigated within 3 days before or after an acute migraine attack . In each subject , PT were assessed by single-pulse and paired-pulse TMS with an interstimulus interval of 50 ms . RESULTS The main effect of diagnosis indicated that mean PT were significantly lower in migraine patients than in control subjects ( P = .001 ) . Using single-pulse TMS , mean PT tended to be lower in MWoA- patients ( 57.7 + /- 11.8 % ) compared with control subjects ( 64.4 + /- 10.5 % ) ( P = .064 ) . In MWA- patients , mean PT ( 53.1 + /- 5.7 % ) were significantly lower compared with controls ( P < .001 ) . Using TMS with paired pulses , mean PT were significantly reduced in MWoA- patients ( 40.3 + /- 4.9 % , P = .017 ) as well as in MWA- patients ( 39.6 + /- 4.2 % , P = .005 ) compared with controls ( 44.6 + /- 6.0 % ) . The main effect of stimulation type indicated that mean PT were lower determined with paired-pulse stimulation than with single pulses ( P < .001 ) . CONCLUSIONS PT are reduced in patients with migraine in the interictal state suggesting an increased excitability of visual cortical areas . Compared with single-pulse TMS , paired-pulse magnetic stimulation is more efficient to elicit phosphenes . This technique provides the opportunity to evaluate visual cortex excitability with lower stimulus intensities and less discomfort The present study used recordings of visual potentials evoked by pattern reversal ( VEPs ) to investigate the central effects of three drugs used in migraine prophylaxis : the calcium channel blocker nifedipine , the beta‐1‐selective blocker metoprolol , and the nonselective beta adrenoreceptor blocker propranolol . The study involved 58 patients with common or classical migraine who were treated in a double‐blind r and omized study over a period of 7 months , while the effectiveness of prophylactic treatment was recorded in headache diaries that were subjected to time series analysis . VEPs were recorded at the beginning of a 2‐month baseline period without treatment , after 4 months of treatment , and at the end of a 3‐month washout period . At baseline , migraine patients had significantly higher VEP amplitudes and longer latencies than did a group of 87 healthy control subjects . Patients were separated by statistical analysis into responders and nonresponders to each prophylactic treatment . Nifedipine had no effects on the frequency , intensity , and duration of migraine attacks , nor on amplitude and latency of the VEPs . In contrast , the use of beta blockers result ed in a significant decrease in VEP amplitude , both in responders and nonresponders , whereas VEP latency remained unchanged . VEP amplitudes returned to the initial values at follow‐up in the nonresponders , but stayed at lower levels in responders . Beta blockers thus appear to have a significant effect on the increased excitability of the visual system in patients with migraine , although their action is not directly related to their reduction of migraine frequency We performed transcranial magnetic stimulations of the motor and visual cortices in healthy controls ( n = 27 ) and in patients suffering from migraine without ( n = 33 ) or with ( n = 25 ) aura between attacks . By using a 13‐cm circular coil placed over the vertex and recordings of the first dorsal interosseus muscle , we measured thresholds ( at rest and during contraction ) , amplitudes of motor evoked potentials and cortical silent periods .  Paired stimulations with short ( 1–20 msec ) interstimulus intervals were performed to assess intracortical inhibition . The visual cortex was stimulated with the same coil placed over the occipital scalp ( 7 cm above the inion ) and the prevalence and threshold of phosphene production was determined . In patients with migraine with aura  , motor thresholds during isometric contraction were significantly higher , whereas the prevalence of stimulation‐induced phosphene production was lower compared with healthy controls . These changes were not correlated with attack frequency or disease duration . No differences were found between subject groups in thresholds at rest , motor evoked potential amplitudes , cortical silent periods , or response curves after paired stimuli . These results are in favor of cortical hypoexcitability rather than hyperexcitability in patients with migraine with aura between attacks Magnetic  coil ( MC ) stimulation percutaneously of human occipital cortex was tested on perception of 3 briefly presented , r and omly generated alphabetical characters . When the visual stimulus-MC pulse interval was less than 40 - 60 msec , or more than 120 - 140 msec , letters were correctly reported ; at test intervals of 80 - 100 msec , a blur or nothing was seen . Shifting the MC location in the transverse and rostro-caudal axes had effects consistent with the topographical representation in visual cortex , but incompatible with an effect on attention or suppression from an eyeblink . The MC pulse probably acts by eliciting IPSPs in visual cortex . The neural activity subserving letter recognition is probably transmitted from visual cortex within 140 msec of the visual stimulus 
    Output:
    
Overall considered , these results support the hypothesis of a primary visual cortex hyper-excitability in MA , providing not enough evidence for MwA. A significant statistical heterogeneity reflects clinical and method ological differences across studies , and higher temporal variabilities among PT measurements over time , related to unstable excitability levels .