FND's Flawed Analogy: When 'Software' Problems can Hide Undetected Hardware Damage

12 Mar 2026 7 comments

One of the most common analogies that you would come across within Functional Neurological Disorder (FND) literature is the software/hardware analogy. Neurosymptoms.org – Patient Information on Functional Neurological Disorder

Basically, your CNS (that is, your brain, spinal cord, and signals carried via the PNS, the nerves) is described as hardware, while the processing of those signals that flow from that hardware is described as the software.

In this analogy, the CNS and PNS are considered structurally intact, while the processing of those signals (the software) is considered flaky or glitchy. They are believed to not be processing those signals properly and correctly.

The rule-in criteria (that is, the various patterns perceived by the clinician) are basically used to distinguish between hardware and software issues, comparable to a software developer looking at a stack trace or a computer technician looking at system event logs.

At this stage, when we start to elaborate or extend on the analogy, proponents tend to get a bit fidgety and annoyed, since the analogy is supposed to be partial and bound to a specific context and not really part of a broader, deeper, and more complex discussion, for example, "it's just a helpful metaphor, not literal" or "don't overcomplicate it".

Let's ignore the fluff. From here on, I will extend the analogy in my own way to illustrate the point.

Now admittedly, the CNS/PNS system is vastly more complex than any of our current computer systems.

However, I believe that reducing this complexity (that is, applying the Feynman technique here, reducing the CNS/PNS to a computer system) will prove useful as to why we cannot invoke a partial analogy here.

Okay, so many decades ago when I had just finished high school, I worked at a little computer repair shop as a technician. As a client brought in a computer, we would initially assess if we were dealing with hardware failure or simply software that went corrupt or awry in some other way.

Most of the time it was quite easy to assess the difference. Hardware problems tend to be quite easy to spot most of the time, since they tend to be quite dramatic at times, for example, a flickering screen from a damaged connector, rolling displays from a damaged graphics card, or even the smell of burnt circuits or capacitors.

And generally, when software issues arose, we could do a simple software update and in extreme cases we could simply back up data and just reformat the system and easily restore it back to its former glory.

But sometimes things were not this clean and easy.

Sometimes we would reinstall software only to discover later that the real problem was subtle hard-drive damage. Other times systems passed every stress test in our lab but failed immediately when the client used them in a specific way. In one case the problem turned out not to be the computer at all, but poor ventilation in the client's server room.

These are designed systems for which we have full specifications, and even then subtleties can be very tricky to find.

If diagnosing computers with full specifications can still miss subtle hardware faults, it should not surprise us that diagnosing the vastly more complex nervous system can also miss them.

As for humans themselves (that is, systems that evolved from other systems over billions of years), we do not have full specifications, only the ones we have discovered and are still discovering as we investigate medical issues. For example, as recently as 2020 Dutch researchers described the tubarial salivary glands. The tubarial salivary glands: a potential new organ at risk for radiotherapy (Radiotherapy and Oncology, 2020)

If we take this back to the initial FND discussion, how many of these rule-in patterns are truly "software" issues and not rather a misunderstanding, misattribution, or an overreach?

Rule-in signs (like Hoover’s) function like basic diagnostics A practical review of functional neurological disorder (FND) for the general physician (Clin Med Lond, 2021) This review explains that Hoover’s sign, hip abductor sign and inconsistency between voluntary and automatic movements are reliable positive signs used to make a rule‑in diagnosis of FND. . They spot inconsistencies, but they do not rule out subtle hardware contributions any more than initial tests rule out bad sectors or ventilation problems. For instance, fluctuating neurological deficits caused by structural issues (such as dynamic compression, apraxia, ataxia etc) could potentially mimic patterns interpreted as inconsistency.

For example, the facial asymmetry sometimes attributed to FND. In my wife’s case we found actual reproducible mechanical actions that are directly responsible for what is seen.

The issue is not whether functional symptoms exist. The issue is whether the label is sometimes applied before the limits of structural investigation have truly been reached.

If a symptom is believed to be functional rather than structural, surely we cannot assume we already know what triggered it.

Surely we need to find triggers and monitor the situation instead of slapping a diagnostic label on it.

Why take the presentation of symptoms, give it a name, and push patients into therapy?

For example, if you go to a dentist with a sore tooth and the dentist cannot identify the source of the pain, in practice they would investigate triggers (for example, bruxism, diet, or posture), monitor the problem, and refer to specialists such as orofacial pain experts rather than declaring the pain “functional”.

FND risks becoming a diagnosis that closes investigation rather than one that manages uncertainty. Systematic review of misdiagnosis of conversion symptoms and “hysteria” (BMJ, 2005)

It is great to come up with hypotheses and dream up new ideas, but at the end of the day we need to acknowledge uncertainty and limitations. We cannot afford to treat unknowns as knowns; we need to keep diagnostic doors open. End of the day, you are not there to win some doctor an award. This is your life, this is your health.

References

Neurosymptoms.org – Patient Information on Functional Neurological Disorder
The tubarial salivary glands: a potential new organ at risk for radiotherapy (Radiotherapy and Oncology, 2020)
A practical review of functional neurological disorder (FND) for the general physician (Clin Med Lond, 2021) This review explains that Hoover’s sign, hip abductor sign and inconsistency between voluntary and automatic movements are reliable positive signs used to make a rule‑in diagnosis of FND.
Systematic review of misdiagnosis of conversion symptoms and “hysteria” (BMJ, 2005)

Comments

7 months ago by Anonymous Neurologist Neurologist

Replied to FND's Flawed Analogy: When 'Software' Problems can Hide Undetected Hardware Damage

It’s just an analogy. Trying to delve deeper into it doesn’t offer any benefit because the nervous system isn’t actually like a computer. The analogy is meant to help patients understand that it’s different from conditions like Parkinson’s, stroke, MS, etc. “For instance, fluctuating neurological deficits caused by structural issues (such as dynamic compression) could potentially mimic patterns interpreted as inconsistency.” - Please explain how. While sitting or lying down, the patient is asked to extend their right hip (press down into the clinician’s hand placed under the right heel). If there’s no pressure, the clinician places their hand on top of the left leg and asks the patient to lift the left leg against the clinician’s resistance. The clinician feels pressure pushing down on their hand from the patient’s right heel. This is a positive test. How can that possibly be explained by a structural issue, especially “dynamic compression”? “For example, the facial asymmetry sometimes attributed to FND. In my wife’s case, we found actual reproducible mechanical actions that are directly responsible for what’s seen.” - This doesn’t mean anything. It’s like someone in a clinical trial experiencing remission after receiving only a placebo. Beliefs and expectations (conscious and unconscious) from past experiences shape our current perceptions of the world, movement generation, and more. Clearly, you and your wife reject the FND diagnosis (despite having substantial evidence supporting it). Consequently, both of you will seek out evidence and modify your behavior to support your belief. However, since you lack education in anatomy, physiology, and clinical care, you don’t realize that something mechanical cannot directly cause the characteristic facial motor symptoms seen in FND. This is common. For instance, a person might press on their palm, and their leg starts jerking. They believe there’s a direct mechanical causation, but that’s not possible. We know enough to confidently conclude that. Perhaps you don’t, but people with the proper education and training do. “Why take the presentation of symptoms, give it a name, and push patients into therapy?” - Um, that’s the whole point of medicine. Diagnostic labels are essential for organizing complex symptoms into recognizable patterns, enabling tailored treatment plans, predicting prognosis, and facilitating communication between providers. It’s quite absurd to argue that FND is at risk of being a label that stifles investigations. If someone is diagnosed with Parkinson’s Disease (which can only be confirmed through an autopsy), they won’t undergo endless and extensive testing that’s not relevant to their presentation. We have so much more to learn about Parkinson’s disease (and most other conditions, including FND) but we know enough to reliably diagnosis and treat it with evidence-based treatments. FND is not different. It’s easy for someone like you, lacking formal clinical education, training, and experience, to claim that we don’t know everything. Of course, there’s always more to learn, but you can’t possibly know the things healthcare professionals do know. You don’t, but others do. There’s something quite narcissistic about acting like you understand medicine and neurology more than neurologists.

3 months ago by BlueTarg4 advocate

Replied to FND's Flawed Analogy: When 'Software' Problems can Hide Undetected Hardware Damage · Replying to Anonymous Neurologist

The problem is you are assuming you understand everything there is to know about the 'mechanical' body and how it operates. Setting aside the bizarreness of FND claiming to get rid of the dichotomy between mind and body and in the next breath talking about hardware v software, lol, I see a far more complex picture. (For clarity I'm not arguing there is no such thing as a psychosomatic symptom, just that FND is a meaningless construct and is wildly overdiagnosed and inappropriately applied to large numbers of patients.) For example, ME/CFS is a proven biological disease - nobody with any credibility in biomedical research is even asking that question any more. There's mountains of published research, billions going into furthering the research - which may not have all the answers but is no longer at an early stage - and we know the disease impacts the whole body, and multiple systems. Researchers have so far identified in lab research116 differences in blood tests between those with ME and healthy controls (having matched for sedentary lifestyle). Some can correctly distinguish in lab tests between those with ME of different types and those who do have and do not have fibromyalgia. There are identified genes emerging as culprits with the second stage of the research under way to confirm the specific addresses, they are all associated with neuro and immune systems. The research is increasingly converging between teams and AI's applying NICE's GRADE analysis evaluate much of it as medium and some as potentially high and very significant - streets ahead of it's analysis of the scientific underpinnings of FND which are rated poor and very poor. Allocating ME/CFS to 'FND' is completely inappropriate. Similarly assuming epilepsy is FND until an (increasingly rare and brief) EEG captures a seizure to prove it's epilepsy is cruel, harmful and has done many patients devastating damage. We have real concerns. You can take a patient and with the same patient examined, same history, same symptoms, same test results and they will be diagnosed with ME/CFS by an ME specialist and as having FND by a neurologist - this isn't acceptable. It's undermining all the science behind ME. You stand a physio who specialises in neurology at the end of the ward and let them watch a patient walk normally from bed to bathroom, and then on the way back start to walk uncertainly - I've overheard the physio then saying look, he can walk normally if he's distracted, it's clearly FND. No it's not, it's the muscles running out of energy in ME. You can show them the patient walking uncertainly down a corridor, but then the phone by the bed rings and they walk normally to the phone to pick it up. The neuro is saying look the patient is distracted, they have FND. The ME specialist is saying look, the patient is taking time and care not to fall because they know their legs can give way, but when the phone goes it's more important to catch the call so they take the risk of walking faster (which they can do for a brief time before the energy in the muscles runs out). This isn't FND it's normal and rational behaviour in a patient who knows they have frequent falls. It's common sense, not a 'clinical sign'. An ME specialist shows his team training videos of doctors training doctors to diagnose FND and they are laughing at the patients on the video for being fooled by a name that makes it sound as though there's something wrong, when, ha ha ha, we all know there's nothing wrong, it's just in their imagination. It's all out there. The name has been invented to fool patients. You don't actually have a clue what's happening or what's causing it to the vast majority of these patients, but rather than admit it, you have invented something that makes life easier for you. It's the classic situation - the doctor looks seriously and kindly (because research has found you can manipulate people more easily if you are kind to them) and says 'I know your symptoms are real'. The patient hears 'your symptoms have a physical explanation'. The doctor means 'your symptoms are really psychosomatic'. Patients are being manipulated into taking a route into therapy which in the vast majority of patients has absolutely dire results.

7 months ago by Christoff Trüter Spousal caregiver

Replied to FND's Flawed Analogy: When 'Software' Problems can Hide Undetected Hardware Damage

Thank you for your feedback. May I ask about your background? From what neurosurgeons have explained to me, dynamic spinal compression, for example lumbar spinal stenosis, can cause nerve signals to vary depending on posture or strain. As a result, a person may struggle to push a leg down voluntarily, but when the opposite leg is lifted, the nervous system automatically generates extension, and some force appears. This difference can resemble a positive Hoover’s sign, even when a structural problem exists. Of course, Hoover’s sign is normally interpreted within the broader clinical context rather than in isolation. The key point is that apparent inconsistencies in motor output do not automatically exclude structural causes. Regarding my wife’s facial asymmetry, this is not a theory. It was verified by a neurosurgeon through examination of her biokinetics. The facial nerve branches, particularly the marginal mandibular branch, control muscles that pull the jaw and lower lip downward, and this movement can be easily demonstrated and reproduced on demand. Therefore, your assessment here is a serious overreach, since you lack the context of our situation. As for labelling me as narcissistic, you have completely missed the point of this post. As stated in other communications, I am under no delusion about who or what I am. I have never claimed to know more about medicine than any doctor. I am simply offering my perspective. Regardless of whether you, or anyone sceptical, respect me, it is important to keep dialogue open and have meaningful discussions rather than whatever this approach is that you are attempting here.

7 months ago by FND Nope Moderator

Replied to FND's Flawed Analogy: When 'Software' Problems can Hide Undetected Hardware Damage

I think everyone understands that we are dealing with analogies here, and the post itself even pre-empts the “it is just an analogy” criticism. However, the discussion here involves hypotheticals, and it does not take much imagination to see that the analogy can also be extended from a sceptical perspective. It is well known and openly discussed by neurologists such as Jon Stone that organic conditions can sometimes mimic functional presentations. Because of that, considerable care is required when applying a diagnosis like this. My greater concern, however, is the suggestion from Anonymous that only neurologists should be allowed to discuss issues like this. If dialogue is discouraged in that way, the topic risks becoming a sacred cow rather than something that can be openly examined. This also avoids the elephant in the room. Yes, Functional Neurological Disorder is accepted within mainstream medicine and used as a clinical diagnosis. But the field itself does not hide the fact that the underlying explanatory framework is still evolving and remains partly hypothetical. In that sense, we are inevitably dealing with models and abstractions rather than a fully settled mechanism.

Functional neurological disorder (FND) provides a significant diagnostic challenge because of its phenotypic heterogeneity https://t.co/SnIFREidii pic.twitter.com/zccm1f8e7J
— FndNope (@FndNope) March 15, 2026

7 months ago by Anonymous Neurologist Neurologist

Replied to FND's Flawed Analogy: When 'Software' Problems can Hide Undetected Hardware Damage

I’m a neurologist with over 15 years of experience, and I have no personal or professional stake in the FND field. However, I do encounter these patients frequently in my clinic. Skepticism is healthy, but it shouldn’t be driven by a complete lack of understanding of what we already know about medicine, the nervous system, and FND. You can’t argue what we don’t know when you don’t know what we do know. “As a result, a person may struggle to push a leg down voluntarily, but when the opposite leg is lifted, the nervous system automatically generates extension, and some force appears. This difference can resemble a positive Hoover’s sign, even when a structural problem exists.” - No, what you described is the Hoover’s sign. That’s the point. To lift a leg against resistance, the other leg must automatically extend. This demonstrates impaired voluntary movement with intact automatic movement, which is consistent with FND annd supports a diagnosis of FND when used in conjunction with the clinical history and the rest of the examination. Impairments caused by structural problems will persist with both voluntary and automatic movement. You can sit there all day and claim that perhaps we haven’t discovered the mechanism for that yet because we still have a lot to learn, but you can do that for everything in medicine. That ignores the fact that these yet-to-be-discovered mysterious mechanisms would be incompatible with what we already know, making them unlikely to exist. We already have a sufficient explanation for it. “Regarding my wife’s facial asymmetry, this is not a theory. It was verified by a neurosurgeon through examination of her biokinetics. The facial nerve branches, particularly the marginal mandibular branch, control muscles that pull the jaw and lower lip downward, and this movement can be easily demonstrated and reproduced on demand. Therefore, your assessment here is a serious overreach, since you lack the context of our situation.” - What do you mean by “examination of her biokinetics” in this context? Are you saying your wife has unilateral lip pulling downward? A structural lesion to the marginal mandibular branch causes paralysis or paresis of the innervated muscle, leading to an inability to pull the lip downward. In the context of FND, a downward lip pull is commonly mistaken for facial droop due to a stroke. I see you’re claiming your wife was diagnosed with MS, but then clarify it as CIS MS. CIS is not MS but has the risk of developing into MS. You state your wife has had several relapses with progression over the years, starting back in 2019, without any lesions in the brain or spinal cord. That’s not consistent with CIS or MS, so I’m not sure who told you she has CIS or why you’ve stated she was diagnosed with MS when you meant CIS. However, the extensive evidence you’ve provided about her condition demonstrates that she doesn’t have CIS or MS. I suspect one neurologist said CIS, the other said FND, and you decided the CIS must be correct despite all evidence suggesting otherwise. Regardless of what you say, you’re clearly not interested in honest constructive critique and skepticism to improve care for all patients, including those who do have FND.

7 months ago by Christoff Trüter Spousal caregiver

Replied to FND's Flawed Analogy: When 'Software' Problems can Hide Undetected Hardware Damage

Do you have any self-awareness at all? I've consistently been open to dialogue, but that's not what I'm receiving from you. From the outset, you engaged in personal attacks, demonstrated no willingness to listen, and consistently overstated your points while making fundamental errors (see). I believe you are misrepresenting yourself as a neurologist. If this is considered acceptable practice in neurology, it only underscores that the evidence is not being properly examined. I am well aware of what CIS "presenting" as MS is, will probably not help to mention that it can take decades (as per the literature) for lesions to present, since I am apparently not qualified to read nor quote research papers. The reasons we reject the FND diagnosis is simply because positive rule in signs were never found, the clinician literally based his evidence on a MRI, our current neurologist is leaning towards MS due to optic neuritis being present. I would welcome corrections from qualified neurologists, but I dont feel being bullied is fair here.

7 months ago by FND Nope Moderator

Replied to FND's Flawed Analogy: When 'Software' Problems can Hide Undetected Hardware Damage

Just a reminder for the clinician in the comment section: FND requires positive rule-in signs. Some context about Christoff's wife: even though Hoover’s sign is mentioned here, functional rule-in signs were never found, and the FND diagnosis was made purely because a diagnostic impasse was reached. (the dynamic spinal cord compression part is purely hypothetical, not sure if you make use of dynamic MRI's in your practice at all?) In the South African context, the term “biokinetical” as per biokineticist reports, refers to genuine mechanical movements that consistently and repeatedly produce facial asymmetry. These movements have been evident (in her case) for the past 30 years, long before the onset of any neurological symptoms.

This post explores a hypothesis. It should not be read as an established fact or settled scientific conclusion.

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