In the present study, we demonstrate a reduced NVC of the visual network in patients with MwA, (explored during the interictal period) when compared to patients with MwoA and HC.

In the last decades, advanced neuroimaging and neurophysiological investigations have consistently demonstrated functional abnormalities within the extrastriate cortex, both during ictal and interictal periods, a strategic hub within the visual network, widely considered to be involved in the genesis of the “CSD”, the neurophysiological process underpinning migraine aura [3,4,5,6,7,8,9,10, 35,36,37]. While increased resting functional connectivity and responsiveness could make visual areas more prone to be overwhelmed by the CSD propagation [10], the mechanisms underpinning the proneness to CSD ignition are still matter of debate [9]. Therefore, it could be arguable that also an increased rCBF at rest could characterize visual areas of patients with MwA [13]. Indeed, according to the physiological concept of “functional hyperemia”, increased neural activity is accompanied by the local dilation of arterioles and micro-vessels to supply local blood flow and volume, and oxygenation [38]. More in depth, “functional hyperemia” is part of a more complex biological process – the so-called NVC - aimed to govern the mutual relationship between neuronal activity demand and perfusion supply [39]. The neuronal energy demands at rest are physiologically oversupplied so that further increase of rCBF is not always required to support an increased neuronal activity.

The rCBF physiological oversupply is the reason why, despite the observed NVC reduction in patients with MwA, it remains sufficient to support the hyperresponsive and hyperconnected visual cortices in these patients [40]. However, we speculate that when patients with MwA face with experience associated with an increased energy demand (as putatively happens when they come across trigger factors, such as bright or flickering lights, sleep deprivation, or physical exercise) the physiological rCBF oversupply may become inadequate [12].

In other words, the reduced NVC characterizing the visual network in patients with MwA might represent the link between the ignition of the aura phenomenon and the exposure to the trigger factors. It is important to note that while regional NVC is estimated by correlating ReHo and CBF maps within the same region, the regional CBF is obtained from the average of CBF across all voxels of the region. In other words, when calculating the NVC, both ReHo and CBF maps are converted to regional z-scores, thereby any possible bias of the regional CBF (as well as the regional ReHo) is removed as NVC should only quantify whether (and to what extent) the spatial change in two measures is more (or less) coupled within the region.

This is in line also with recent findings from provocative studies showing that in about 40% of patients with migraine with aura intravenous infusion of CGRP is able to induce migraine aura although, due to its size, CGRP is unable to pass through the blood-brain barrier [41]. Indeed, it has been suggested that CGRP-induced vasodilation may mechanically stimulate perivascular primary afferents, leading to the transmission of nociceptive information to the brain cortex through second-order and third-order neurons [42] able to provide sufficient excitatory stimuli to brain areas such as the visual cortex, lowering, in turn, the threshold for CSD initiation in “predisposed” patients [8, 43]. Our present arterial spin labeling findings suggest that a reduced NVC may properly represent the evoked MwA “predisposition”.

The present findings are in agreement also with observations showing that hypoxia, a trigger factor capable of ignite aura phenomenon in patients with MwA, is able to induce, despite the hypoxia-related increase in rCBF, an abnormal increase in the lactate concentration of the visual cortex, suggesting a metabolic uncoupling in terms of an increment of blood flow not adequate to support the escalating energy demand in these patients [44].

Intriguingly, although further observations are warranted, also the patent foramen ovale, frequently found as a MwA comorbidity, could affect NVC, likely due to microembolism-related occipital cortex hyperexcitability [45].

Moreover, in support to the suggested pathophysiological model, interventions capable of suppressing primary visual cortex hyperresponsiveness to specific visual stimuli (i.e. shielding light lenses) [37] as well as preventive pharmacological therapies such as lamotrigine and topiramate (by inhibiting the occipital cortical hyperexcitability to visual stimuli probably enhancing GABA and reducing glutamate levels) [6, 46, 47] result in a reduced frequency of migraine attacks. More in depth, it could be argued that the effects of such antiepileptic drugs in the prevention of MwA attacks can be achieved by restoring the demand-supply mismatch between brain energy demands and rCBF (that is nothing other than “NVC”) within the visual network [48].

It is noteworthy that logistic regression analysis has shown that the full model, considering rCBF in VIS-4 and VIS-5 and NVC in the VIS-4 parcel from the Schaefer atlas, can discriminate MwA patients from MwoA (AUC = 0.87). Moreover, as demonstrated by the analysis of the coefficient, the VIS-5 rCBF and, above all, the VIS-4 NVC were associated with a significantly increased likelihood of experiencing an aura phenomenon.

It is important to remark that higher CBF values in visual cortical areas were here found only in MwA compared to MwoA patients, whereas the same differences were not identically found between MwA patients and controls. Therefore, future ASL MRI studies are needed to accurately reproduce more general CBF findings concerning MwA and MwoA patients in comparison to HC. Nonetheless, although the aim of the study was to investigate putative NVC abnormalities in the visual network in patients with MwA, rCBF abnormalities were also detected in patients with MwoA across two different large-scale brain networks, in comparison to HC. Particularly, the increased rCBF within the salience and executive networks in patients with MwoA compared with HC, might indicate the involvement of these large-scale networks in the detection and integration of emotional and sensory stimuli, as well as in the control of externally directed attention [49]. Interictal dysfunction of these networks may contribute to the dysregulation of emotional and cognitive functions sometimes observed in migraine as well as to the different susceptibility to migraine attacks along with the transition from interictal to ictal states [50].

The present study is not exempt from limitations. First, although calculating the local spatial correlation between rCBF and ReHo maps represents a straightforward approach to capture the relationship between local neural activity and blood flow [51,52,53], further exploration is mandatory as happens with all current MRI methods estimating NVC, to date. Second, in the present study we used ReHo for the NVC estimate, which primarily index the local input and processing of neuronal information within functional parcels. Thereby, assuming that higher or lower similarities of one voxel with the surrounding voxels are also affected by the amount of neurovascular coupling around that voxel itself, the resulting spatial correlation between ReHo and CBF distributions should provide a suitable metric for the region-specific neurovascular coupling.

Third, both signal-to-noise and spatio-temporal resolution are higher for fMRI, compared to ASL images and this is the reason why we did not derive voxel-based estimates for NVC (e.g., as a CBF/ReHo ratio). However, the registration of the images to a common brain space at a higher (isotropic) spatial resolution (2 mm), together with the partial volume correction and the region-specific estimation based on a functionally stable parcellation, provides a reasonable technical solution to address the issue, mitigating the impact of the spatial mismatch between two modalities.

Fourth, our interpretations assume that neurovascular “decoupling”, underlying aura phenomenon ignition, is triggered by experience able to increase brain energy demand, such as intense light stimulation, physical activity, or alterations in the sleep-wake rhythm. Although these trigger factors are extensively reported by a significant percentage of patients [12] they are not consistently confirmed in provocative studies [11], suggesting either a putative recall bias in patients’ reports or, more likely, the difference between experimental settings and ecological systems being the “brain predisposition to migraine/aura” ignition due to maladaptive changes of brain function closely related to incremental allostatic load [54] where the relative contribution of each ‘‘effector’’ is unknown but seems to be additive or cumulative over time [55].

On the other hand, homogeneity of the patients’ sample still represents a strength of the present study, also considering the rarity of patients experiencing exclusively migraine with visual aura.