During a migraine, the brain goes through a cascade of electrical, chemical, and blood flow changes that light up on imaging scans in ways distinctly different from a normal, pain-free brain. What researchers see isn’t a single event but a sequence: areas deep in the brain activate hours before the headache starts, a slow wave of electrical disruption can sweep across the surface, blood vessels dilate and constrict in shifting patterns, and the brain’s pain-filtering systems lose their grip. Here’s what each phase actually looks like.
The Hypothalamus Fires First
The earliest visible change in the brain happens hours before migraine pain begins, during what neurologists call the prodrome. This is when you might feel unusually tired, irritable, or hungry without knowing a migraine is coming. On PET and functional MRI scans, the hypothalamus, a small structure deep in the brain that regulates sleep, hunger, and hormonal cycles, shows a clear spike in activity and increased blood flow. The brainstem’s pain-regulation center (the periaqueductal gray) and areas tied to emotion and memory, including the amygdala and hippocampus, also ramp up.
This isn’t a single structure misfiring. Imaging shows the hypothalamus acting as a hub, strengthening its connections to pain-sensing neurons in the brainstem and to the limbic system that processes emotion and stress. That coordinated pattern of increased connectivity is one reason researchers now view the hypothalamus as the likely trigger point for the entire migraine sequence, not just a bystander.
A Slow Electrical Wave Crosses the Cortex
About one in three people with migraine experiences aura: visual distortions like shimmering zigzag lines, blind spots, or tingling on one side of the body. What causes this is a phenomenon called cortical spreading depression, and it’s one of the most dramatic things imaging has captured in a migraine brain.
Cortical spreading depression is a wave of intense nerve cell firing followed immediately by a period of electrical silence. It moves across the brain’s surface at roughly 2 to 5 millimeters per minute, about the speed of a snail. Using high-field functional MRI, researchers have watched this wave travel across the visual cortex in real time while subjects reported seeing aura. The blood oxygen signal in the brain first increases by about 5% as the wave arrives (corresponding to the bright, scintillating visuals people see), then drops by a similar amount as the nerve cells go quiet (corresponding to the dark blind spots that often follow the sparkles). The initial surge lasts about three to four minutes in any given spot, while the suppression that follows can linger for one to two hours.
The wave starts in area V3A, a region of the visual cortex involved in processing motion and spatial information, then spreads contiguously across the occipital lobe. Its path maps precisely onto the visual disturbances patients report: as the wave moves to areas representing different parts of the visual field, the aura shifts accordingly. This tight match between the wave’s location and the person’s perceptual experience was one of the strongest confirmations that cortical spreading depression is the physical basis of migraine aura.
How the Pain Signal Gets Generated
The brain itself has no pain receptors. So how does a migraine hurt? The answer involves a network of nerve fibers called the trigeminovascular system, which wraps around blood vessels in the membranes (meninges) surrounding the brain. These fibers belong to the trigeminal nerve, the same nerve responsible for sensation in your face and head.
When cortical spreading depression sweeps through, the intense burst of neural activity triggers signaling cascades that reach the meninges and activate these trigeminal nerve fibers. Once activated, the fibers release powerful vasodilating chemicals, the most important being a peptide called CGRP. CGRP is one of the most potent dilators of blood vessels inside the skull and is now the primary target of newer migraine medications. During an active migraine, CGRP levels in blood drawn from the head’s circulation rise significantly, roughly doubling compared to healthy controls in early landmark studies. Importantly, those elevated levels drop within one to two hours in people who respond to treatment, while they stay high in people whose medication isn’t working.
The released chemicals cause blood vessels to swell and surrounding tissue to become inflamed, which further stimulates the pain-sensing nerve fibers. Those fibers relay signals through the trigeminal ganglion to the brainstem, and from there up to higher brain regions. The brain, in effect, is both generating the signals that initiate the headache and processing the pain those signals create.
Blood Flow Shifts During an Attack
One of the earliest ways researchers studied migraines was by measuring cerebral blood flow, and the patterns are distinct. During common migraine (without aura), blood flow increases broadly across the brain, particularly in the frontal and temporal regions, by an average of about 37% during the acute headache phase. This elevated flow decreases gradually over the following days.
In migraines with aura, the picture is more complex. The cortical spreading depression wave initially causes a brief surge of blood flow (hyperemia) as nerve cells fire intensely, followed by a mild but prolonged reduction in blood flow (hypoperfusion) that can persist for one to two hours after the wave passes. This is why aura symptoms like blind spots don’t resolve instantly. The brain tissue in the wave’s wake is functionally suppressed, receiving less blood and responding poorly to normal stimulation.
The Thalamus Reshapes Sensory Processing
During the headache phase, the thalamus, the brain’s central relay station for sensory information, shows abnormal connectivity patterns on imaging. Its connections to the cortex on the side of the headache weaken, particularly to areas that process touch and body sensation. At the same time, connections to the opposite side’s emotional and decision-making regions strengthen. This rewiring helps explain why, during a migraine, normal sensory input like light, sound, and touch becomes painful or unbearable. The thalamus is essentially misrouting and amplifying signals that would normally be filtered as harmless.
White Matter Changes in Frequent Migraines
Beyond what happens during a single attack, MRI scans reveal that people with recurring migraines often develop small bright spots in the brain’s white matter, the tissue that connects different brain regions. These white matter hyperintensities show up in roughly 41% of people with migraine without aura and 44% of those with migraine with aura. They’re small areas where the insulating material around nerve fibers has been damaged, likely from repeated episodes of blood flow disruption.
These spots are typically tiny and don’t cause noticeable symptoms on their own. They’re similar to what doctors see in aging brains or in people with high blood pressure, but they appear earlier and more frequently in people with migraine. Their clinical significance is still being worked out, but their presence on a scan is one visible marker of migraine’s cumulative effect on brain tissue.
How Brain Networks Rewire Between Attacks
Even between migraines, the brains of people who get them frequently look different from those who don’t. Resting-state functional MRI, which measures brain activity when a person is lying quietly doing nothing, shows that a major network called the default mode network has altered connectivity in migraine patients. This network is active during daydreaming, self-reflection, and internal thought, and it plays a role in how the brain manages stress responses.
Compared to healthy controls, people with migraine show increased connectivity in visual processing areas and regions tied to memory, but decreased connectivity in frontal lobe areas involved in planning and impulse control. The sensory cortex, which processes touch, shows particularly disrupted connections, and the degree of disruption correlates with how long a person has been living with migraines. These aren’t changes you’d notice in daily life, but they suggest migraine isn’t just a series of isolated attacks. It gradually reshapes how the brain’s networks communicate, particularly the systems that process sensory input and regulate the body’s response to stress.