Functional MRI does not take a picture of the brain. It takes several hundred pictures over several minutes and looks at what changed between them. What changes is blood oxygen. When a population of neurons becomes more active, blood flow to that tissue rises by more than local oxygen consumption does, shifting the ratio of oxygenated to deoxygenated hemoglobin — two forms that differ magnetically, so the scanner can detect the shift. That measurement is the BOLD signal, for blood-oxygen-level-dependent.
So fMRI does not record neurons firing. It records a vascular consequence of firing, seconds later, averaged across tissue holding hundreds of thousands of cells. A good proxy, still a proxy — worth remembering through this fourth entry in a series on the eight imaging types used for mild traumatic brain injury.
What Happens During the Scan?
**Task-based fMRI** asks you to do something — hold numbers in mind, respond to a cue, tap fingers in sequence — and looks at which regions activate above baseline, and how hard they work to deliver a given performance.
**Resting-state fMRI** asks you to do nothing. Even at rest, separated regions show slow BOLD fluctuations that rise and fall together; regions whose activity correlates over time are functionally connected, and the recurring sets of them are networks.
Resting state dominates concussion research because it does not require performing a task well — which matters when the complaint is that tasks have become hard.
Which Networks Matter After a Head Injury?
Four come up most often. The **default-mode network** runs when you are not directed outward — internal narration, memory, self-referential thought — and normally quiets when attention turns to the world. The **attention networks** orient and sustain focus. The **executive network** handles working memory, planning, and goal-holding under distraction. The **sensorimotor network** coordinates movement and its feedback.
Set that beside what people report after a concussion — [brain fog](condition-brain-fog.html), losing the thread mid-sentence, being unable to filter a noisy room, exhaustion after mental work that was once free. These are network-level complaints, and fMRI is the only widely available technique that looks at networks.
What Does the Research Actually Show?
The most current systematic review of resting-state fMRI after mTBI is essentially negative on consistency. Dogra and colleagues wrote: "Reported functional connectivity changes varied, even within the same region and network, at least partially reflecting differences in technical parameters, preprocessing software, and analysis methods as well as probable differences in individual injury" (Dogra et al., 2024).
Read that carefully. It is not saying nothing is found — connectivity changes are reported after mTBI, repeatedly. It says they do not point the same direction. The same network, sometimes the same region, shows increased connectivity in one study and decreased connectivity in the next. Some of that spread is technical; some is real, because two patients' injuries are not the same injury.
So when you meet a confident claim that concussion produces a characteristic default-mode signature, current evidence does not support it. Individual studies do examine the DMN — Nathan and colleagues explored variations in resting-state default-mode connectivity in mTBI and reported measurable differences from controls (Nathan et al., 2015). A real finding, and exactly the kind the review warns against turning into a rule.
One pattern has better footing, and it concerns location rather than direction. A review and meta-analysis of neuroimaging after mTBI found frontal regions particularly vulnerable, describing an anterior-to-posterior gradient of effects. Those authors were blunt: "much more work in this area is required... to achieve clinically-relevant capabilities for diagnosis" (Eierud et al., 2014). That was over a decade ago, and the 2024 review suggests the requirement is unmet.
Why Isn't fMRI a Standard Concussion Test?
Because the guideline-setting bodies have read this evidence and drawn the obvious conclusion. The American College of Radiology rates fMRI "usually not appropriate" for subacute or chronic head injury with cognitive deficits, and a consensus position holds that "there remains insufficient evidence... to conclude that these advanced techniques can be used for routine clinical use at the individual patient level" (Wortzel, 2022).
I will not soften that, and you should be suspicious of anyone who does.
The gap described is between group statistics and individual diagnosis. A study can find a reliable difference between forty concussed subjects and forty controls and still not say which group one person's scan belongs to. Advanced neuroimaging in mTBI clears the first bar far more often than the second.
Mild traumatic brain injury is diagnosed by its characteristic symptoms and neurological effects. It is not diagnosed by a scan, and fMRI does not change that.
Could the Neck Be Driving What the Cortex Shows?
What follows is my clinical reasoning rather than a cited finding, and the line between the two should be unmistakable.
The lower brainstem and upper spinal cord pass through the foramen magnum at the base of the skull. Ascending pathways from there — arousal, autonomic regulation, vestibular and proprioceptive input from the upper neck — project widely upward and shape cortical behavior. Mechanical problems at that junction, the kind produced when [whiplash and head impact arrive together](post-why-whiplash-and-concussion-produce-the-same-symptoms.html), sit on that traffic.
My working position is that when a patient shows both demonstrable craniocervical dysfunction and a network-level functional picture, the two are related and the neck is the more plausible driver. No study in the reference list below shows that upper cervical dysfunction produces a specific cortical pattern, and I know of none that does. This is a hypothesis about what I expect to see, held because it fits the anatomy and because these patients frequently improve when the mechanical problem is addressed.
A reason to be interested; not a reason to bill a research scan as diagnostic.
What Is fMRI Good For, Then?
Structural MRI tells you whether tissue is damaged, diffusion imaging speaks to white-matter tracts, flow studies to circulation and drainage. None tells you how the brain behaves as a system while it works — which is what the patient is complaining about. fMRI is the only one that takes up [the mismatch between normal structure and abnormal function](post-why-coordination-fails-while-a-brain-scan-looks-normal.html) from the function side.
A research-grade tool can still earn a place in an integrated workup, because it contributes to a pattern rather than delivering a verdict. When functional imaging, [symptom picture](condition-post-concussion-syndrome.html), and examination of the craniocervical junction point the same direction, the convergence carries weight none carries alone.
It must not stand alone. A single fMRI offered as proof of injury asks for the one thing the field says it cannot yet give.
Does This Help Separate a Brain Problem From a Neck Problem?
It contributes to that separation, and I want to be honest that it does so imperfectly.
Most of my clinical work concerns the craniocervical junction, and it would be fair to ask whether I find the neck wherever I look. A protocol that could only ever return one answer would not be worth ordering, and functional imaging is one of the studies that can argue against me.
Network activity is cortical. Whatever a functional study shows is happening in the brain, not in a ligament. Where it becomes interesting is that a cortical finding does not, by itself, establish a cortical cause — a network can be underperforming because the tissue is damaged, or because the environment it is operating in has degraded, or both.
That is the genuine ambiguity, and it is why functional imaging is read against the rest of the protocol rather than alone. Paired with a clean craniocervical examination, an abnormal functional study points inward. Paired with a measurably displaced junction, restricted fluid flow, and position-dependent neurological findings, the same result reads differently.
Sometimes the problem is in the brain, and when it is I would rather know and say so. Sometimes it is the neck. Most often, in the people who reach me, it is both — and the reason to run more than one study is that you cannot tell which from any single one.
Has Your Workup Only Looked at Structure?
If every scan you have had asked "is anything broken" and every answer was no, that is good news that does not explain your symptoms.
Looking at how your brain is functioning, and at the craniocervical structure underneath, is the work we do at Cerebral. If you'd like a real evaluation, we're here.
References
- Dogra S, Arabshahi S, Wei J, et al. Functional connectivity changes on resting-state fMRI after mild traumatic brain injury: a systematic review. *American Journal of Neuroradiology*. 2024;45(6):795–801. https://pubmed.ncbi.nlm.nih.gov/38637022/
- Nathan DE, Oakes TR, Yeh PH, et al. Exploring variations in functional connectivity of the resting state default mode network in mild traumatic brain injury. *Brain Connectivity*. 2015;5(2):102–114. https://pubmed.ncbi.nlm.nih.gov/25222050/
- Eierud C, Craddock RC, Fletcher S, et al. Neuroimaging after mild traumatic brain injury: review and meta-analysis. *NeuroImage: Clinical*. 2014;4:283–294. https://pubmed.ncbi.nlm.nih.gov/25061565/
- Wortzel HS. Advanced neuroimaging and mild traumatic brain injury litigation, revisited. *Journal of the American Academy of Psychiatry and the Law*. 2022;50(3):336–341. https://jaapl.org/content/50/3/336