ME/CFS and Long COVID Show Different Brain Connectivity Patterns During Cognitive Fatigue

Cognitive fatigue — the progressive failure of mental effort under sustained demand — is one of the most disabling and least understood features of both ME/CFS and long COVID. Standard clinical evaluation dismisses it as subjective. Neuropsychological testing fails to capture it because it measures performance, not the cost of producing that performance. A 2026 study by Inderyas and colleagues in the Journal of Translational Medicine used 7 Tesla fMRI to look at what is actually happening in the brain during cognitive fatigue in both conditions simultaneously. What they found settles one question definitively: cognitive fatigue in ME/CFS and long COVID is not the same neurologically. The two conditions produce distinct, non-overlapping brain connectivity signatures. And the differences are measurable with the right tools — tools that clinical assessment does not currently use.

The Study: 7 Tesla fMRI During a Cognitive Fatigue Protocol

Inderyas, Thapaliya, Marshall-Gradisnik, and Barnden recruited 78 participants across three groups: 32 with ME/CFS, 19 with long COVID, and 27 healthy controls. Each participant underwent task-based functional MRI at 7 Tesla field strength — a higher field strength than most clinical scanners, providing greater spatial resolution and sensitivity for detecting subcortical connectivity differences that lower-field imaging often misses.

The cognitive task was a Stroop colour-word paradigm, which places sustained demands on executive function and working memory while progressively depleting cognitive resources. The protocol was divided into two sequential phases: a Pre phase designed to build cognitive fatigue by working through the task, and a Post phase captured once fatigue had set in. This design allowed the researchers to compare brain connectivity patterns at two points in the fatigue trajectory rather than collapsing across it. The protocol acquired 450 sagittal volumes per session, and functional connectivity between brain regions was mapped using the CONN toolbox.

The Findings: Two Distinct Connectivity Signatures

The two patient groups showed distinct and non-overlapping patterns of connectivity disruption. They were not the same condition presenting with the same neurological picture. They were different conditions producing different patterns of brain network dysfunction during cognitive fatigue.

In long COVID participants during the Pre phase — before full fatigue had developed — functional connectivity between the nucleus accumbens and cerebellar vermis 3 was significantly reduced (p=0.02). The nucleus accumbens is a core node of the reward and motivation network, and its connectivity to the cerebellum plays a role in the prediction and timing of motivated motor and cognitive actions. A reduction in this connectivity before fatigue is fully established suggests that the motivational drive system in long COVID is already compromised at the beginning of cognitive effort, not only as a result of sustained effort. Once fatigue had set in during the Post phase, long COVID participants showed increased connectivity between the prefrontal cortex and hippocampus (p=0.02) — a pattern suggesting compensatory memory recruitment as the cognitive system attempts to support declining executive function.

ME/CFS participants showed a different pattern: increased connectivity between the left cuneiform nucleus and the right medulla (p=0.03). The cuneiform nucleus is a brainstem structure involved in pain modulation, locomotor control, and arousal regulation. The medulla is the site where autonomic cardiovascular and respiratory regulation is coordinated. Disrupted connectivity between these structures points to impaired integration of central autonomic control at the brainstem level — a finding that connects directly to the autonomic dysregulation documented throughout ME/CFS research, but localized to a specific neural circuit that standard neuroimaging at lower field strength typically does not resolve.

What the Accumbens and Brainstem Findings Mean

The accumbens-hippocampal-prefrontal circuit disruptions in long COVID describe a motivation and memory system that is not coordinating normally during cognitive effort. The nucleus accumbens governs the subjective experience of cognitive effort as worthwhile — the signal that this task is worth sustaining attention on. When accumbens connectivity is reduced, this signal is attenuated. The patient is not choosing not to try. The neural signal that drives sustained trying is not being generated at normal strength. The subsequent prefrontal-hippocampal recruitment in the Post phase suggests the brain is shifting toward a memory-based strategy as executive resources deplete — a compensatory reorganization that consumes additional resources in an already fatigued system.

The cuneiform-medullary finding in ME/CFS describes something different at a different level of the nervous system. The brainstem is not primarily a cognitive structure. The cuneiform nucleus coordinates between arousal systems and motor output, and the medulla coordinates autonomic cardiovascular and respiratory regulation. When connectivity between these structures is disrupted during cognitive effort, the implication is that cognitive fatigue in ME/CFS is not simply a cortical resource depletion phenomenon. It is happening in the context of, or being driven by, disrupted coordination between arousal and autonomic regulation at the brainstem level. The cognitive effort is imposing demands on a system that is already managing impaired autonomic control — and the neural evidence of that impairment is visible in brainstem connectivity during the fatigue task.

Why Standard Clinical Assessment Cannot Detect This

Standard clinical assessment for ME/CFS and long COVID does not include neuroimaging. Cognitive fatigue is typically evaluated through self-report questionnaires, neuropsychological test batteries, or structured clinical interviews. None of these methods can detect the subcortical and brainstem connectivity differences this study identified.

Neuropsychological testing measures cognitive output — how many items a patient completes, how fast, with how many errors. It does not measure what it costs the neural system to produce that output. A patient can perform within normal limits on a standardized cognitive test while the neural architecture supporting that performance is visibly dysregulated at 7 Tesla. The test says "normal performance." The fMRI says "abnormal connectivity under the conditions required for that performance." These are measuring different things. Clinical practice currently uses only the former.

This matters for formal assessments — particularly disability and capacity evaluations — in which cognitive symptoms must be documented with "objective" evidence. Standard neuropsychological testing often fails to show deficits in ME/CFS and long COVID patients whose functional cognitive capacity is genuinely impaired, because the testing captures peak output rather than the cost structure and fatigue trajectory of that output. The Inderyas findings establish that the objective abnormalities are present — they simply require imaging tools that clinical assessment does not currently deploy.

ME/CFS and Long COVID Are Not the Same Condition

One important implication of the distinct connectivity signatures is that ME/CFS and long COVID, despite sharing symptom profiles including cognitive fatigue and post-exertional worsening, are not the same neurological condition. Their brain network dysfunction during cognitive fatigue involves different circuits at different levels of the nervous system, with different temporal patterns relative to fatigue onset.

This matters for research design: studies that pool ME/CFS and long COVID patients as a single group to improve statistical power are mixing populations with distinct underlying neurology. Treatment trials that produce no effect in a pooled population may be masking specific effects in one subgroup that are canceled out by the absence of effect in the other. Mechanistic research that attributes findings in one condition to the other is drawing conclusions the data do not support.

For patients, the finding matters because it establishes that the cognitive symptoms you experience — the inability to sustain mental effort, the worsening that follows cognitive exertion, the failure to think clearly under conditions that once felt effortless — have measurable neural correlates. They are not a perception shaped by mood or illness belief. They are visible as specific patterns of disrupted brain connectivity in high-resolution imaging. The tools capable of detecting them are not part of standard clinical assessment. That is a limitation of the clinical system, not evidence that the findings are absent.