The mind arises from a collection of many maps, all working coherently to provide a model of the self in the environment. But it is the maps, no one is looking.

What is a homunculus anyway?

The term homunculus literally means “little man” in Latin. Medieval alchemists thought there was a little, fully formed human in sperm that would implant and grow in the womb. The mother was just an incubator for a preformed human.

The term homunculus to describe the somatotopic map of the body laid out across the motor and sensory cortex of the brain was popularized by Wilder Penfield, one of the pioneering neurosurgeons of the 20th century. Using electrical stimulation during awake brain surgery, Penfield identified how specific regions of the precentral and postcentral gyri corresponded to distinct parts of the body. Penfield created these maps by directly stimulating the cortex during awake neurosurgery for epilepsy. As he stimulated, the patient either reported sensation in some part of the body or involuntary movement in the form of evoked twitches or jerks —creating a distorted but systematic representation that came to be visualized as a “little man” stretched across the cortical surface.

Cortical maps

The idea that the cortex was organized as a series of maps was, of course, not new. Of course, the ideas of cortical maps were not new. At the dawn of neurology and neuroscience in the 1870s, people like David Ferrier stimulated and lesioned monkey cortex and established the mapping of the “motor centers”. Hulings Jackson noticed that motor seizures progressively spread across body parts in a clear somatotopic pattern— leading him to infer an organized map in the cortex as Ferrier had shown.

At the same time, those studying sensory systems also realized the brain mapped the sensory environment. Evidence accumulated that the retina’s visual fields were mapped onto the visual cortex. It was inferred first from lesion studies but with the development of electrophysiological recording, the spatial organization of the visual cortex became clear. And we all know it was Hubel and Wiesel in the 1950s that not only showed that the map of visual field was distorted like the body maps. The fovea with its high density of color-sensitive photoreceptors was given more area than the visual periphery. Hubel and Wiesel also showed that there were parallel maps overlayed in V1 for processing of binocular disparity, providing the basis for depth perception.

In the 1970s, it became apparent that there were often duplicated adjacent maps. In the visual system,there were secondary maps that preserved the retinotopic map, but laid out in stripes for color, motion, binocular disparity, form, or orientation. So in V2, we get sensitivity to figure-ground separation, border ownership, and contours. And there’s more! V3 has maps that infer changing shape over time. V4 appears to primarily process color and shade. And MT (V5 sometimes), which is highly specialized for motion perception.

But no theater of the mind

Continue reading “There’s nothing wrong with your homunculus— there’s just no homunculus looking at your homunculus”

We only experience a single, stable perception at a time. How  bistable viusal figures and Karl Friston’s big idea explain how we build a coherent prediction of ourselves in the environment and keep our selves sane in an uncertain world.

By James Vornov, MD PhD
Neurologist, drug developer and philosopher exploring the neuroscience of decision-making and personal identity.

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It’s not just an accident of biology that our brains work as single-threaded engines, engaging only one input stream at a time, one view of the world at a time. I’m going to argue here that a real-time control system operating under conditions of uncertainty needs to minimize error and settle on its best guess prediction of the state of the world. Maybe developers of autonomous systems like self-driving cars could learn something from the nature of human consciousness.

One stable perception at time

Give it a bit of thought and you’ll see that we have a stable awareness of just one thing at a time. Reading comes one chunk of meaning at a time. We can’t listen to the radio while we’re reading. Pausing, we may turn our attention to the background music, but the sound was excluded from awareness while engaged with the text.

The brain is processing ambient sound all the while; we are just not attending to it, so it is not presented in awareness. If among the sound is a doorbell ringing, brain systems outside of awareness signal the system controlling the flow of sensory information to switch from reading to listening to the environment. We become aware, a bit after the fact, that the doorbell rang. If it rings again, it’s less of an echo of a recent event and more present in awareness.

Continue reading “The Unity of Experience: How the free energy principle builds reality”

When the recent season of the Apple TV show Severance concluded, I sent a pitch to a few mainstream publications to write about how close to reality the series is from the point of view of a neurologist. I never got a reply, so I’m going to move the discussion here as a post.

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It turns out that the premise of the show, splitting human identity, is pretty close to some real, albeit pathological conditions that I’ve seen clinically as a neurologist. And there’s an important point to be made about how the brain constructs its model of self-identity as a continuous experience of the world.

If you haven’t seen the show, maybe you’ve at least heard of the premise. No real spoilers here, as I can base this discussion on just the opening episode of the series.

It goes like this. You open your eyes and don’t know who you are.

A woman’s voice is asking you questions. You’re in a conference room, wearing a suit. You don’t remember arriving, don’t recognize your own name on the folder in front of you. “Who are you?” she asks again. Behind this is a brain implant that splits your life in two. One is your normal, daily continuous existence outside of your work at this mysterious company, Lumen. That’s your Outie. Then there’s a self that exists only at work, the Innie. When the Outie descends to work in the elevator, the device is triggered, and the Innie emerges. The two selves share some base personality, but no narrative knowledge is carried over from one to the other.

A real case of identity loss

Let me tell you a story that happened to a colleague and friend. He found himself in an emergency room and had no idea how he had gotten there. But worse, when the nurse asked his name, he was unable to remember it. He couldn’t provide an address, telephone number, or employer. He found his wallet, which helped the ER staff contact family. Other than this confusion, he felt normal. The staff would have seen him as someone who was cooperative and alert but adrift.

This is called Transient Global Amnesia or TGA. For my friend, it lasted for about 90 minutes in the ER. As things came back, he felt embarrassed and silly but was reassured that this was a relatively rare occurrence and was unlikely to repeat itself or be indicative of a more serious condition. He stayed in the hospital overnight with scans and, as I recall, a cerebral angiogram, since this was in the days before we had the software and scanners to do angiograms with CTs and MRIs as we do now. In my days as an academic neurologist, I saw a few cases, one in the ER while it was happening. It looked like an odd, dreamy, disconnected state for an older woman. It’s like floating outside of identity. Dissociated from self.

In general, TGA comes on suddenly: the person remains alert but unable to recall recent—or even distant—events. They may ask the same questions repeatedly: “Where am I? What happened?” In some cases but not all, they can’t recall personal details, like their address or even their name. Then it fades away as quickly as it started.

What’s remarkable is that this happens without permanent brain injury. Most people still speak fluidly, understand language, and retain general knowledge of the world—they know what a hospital is, even if they’re not sure why they’re in one.

On MRI, there’s a consistent but very odd finding reported in some cases. As you might have guessed from the effect on global core memory, the findings have been in the hippocampus, a seahorse-shaped structure buried deep in the temporal lobe that’s part of the ancient “inner cortex”. We don’t think that memory is actually stored there, but it seems to be how cortical activity gets linked together and time-stamped to form episodic memory, the story of past experience. Like other brain systems, it seems to act as a gating system to store and recall previous activity stored diffusely in the cortex itself.

An abnormal signal has been detected in the hippocampus after TGA, generally after a day or two. These bright spots show up late and disappear, leaving no sign of permanent damage. The kind of MRI sequence that shows the change is called Diffusion Weighted Imaging (DWI), which is sensitive to the movement, the diffusion, of water molecules. When there’s injury, stress, or inflammation, water movement becomes restricted, showing up as bright spots. So something’s going on in the hippocampus, but the nature of the change that causes the hippocampus to go offline temporarily is a matter of speculation. Early in my career in studying mechanisms of stroke damage, this was an area of interest for me since the hippocampus is uniquely sensitive to metabolic stress. So we’ll guess for now this is some kind of localized metabolic disturbance that gets corrected after a while, some vascular or metabolic glitch that “turns off” episodic memory.

So in the world of Severance, we have an implant that switches off the Outie’s hippocampal function, leaving the Innie with TGA, at least at first. I can’t speculate how one could then turn it back on in a segregated, separate new identity. For fun, I’ll speculate that it’s using lateralization to achieve this. There’s a hippocampus in both the right and left temporal lobes. Like many of these global control systems, losing one side has no observable effect; the other side can carry on the function perfectly well. It’s only bilateral lesions that cause devastating loss of memory function, as I discuss in a moment. So we’ll say that the Severance implant works by creating something like a “split brain” condition where the outie continues to use the left hippocampus, but the innie uses the right for new storage and retrieval.

Time stamped memories

Now, is this really believable? Are memories time-stamped so that you could actually turn off some based on when the events occurred? Actually, there’s another neurological condition where this time stamp becomes very clear: the person who wakes up every day believing it’s the morning of their accident.

The first such report was in the 1950s—a man injured in an oven explosion who, for decades, believed it was still the day before the accident. Every morning, Henry Molaison, or HM as he was known in the published reports, awoke in the same world. He could speak, reason, and recognize family, but he couldn’t lay down new memories. The brain’s machinery for encoding experience into lasting memory had been irreversibly disrupted. We call this anterograde amnesia— loss of memory going forward with previously recorded memory preserved.

But what gets really strange is there have been other cases reported where memory isn’t simply turned off, but gets frozen in the past. In some cases, it’s been hours or days before the injury, but in one case, there was a near 20-year gap!

WS had a case of severe brain inflammation, encephalitis, that damaged his hippocampus and temporal lobes, just like Henry Molaison. But when WS recovered in 1997, his memory was frozen in 1979. In extensive interviews and testing with neuropsychologists, it appears he believed Jimmy Carter was president, that he was still in his early 30s, and his children were still young even though they were now adults. His recall of pertinent details perfectly lined up with the facts of 1979 with evidence of leakage from the ensuing years.

So you might wonder what happened when he looked in the mirror and saw a man in his 50s? When he saw a newspaper or documentary about the events between 1979 and 1997? Well, he’d become a bit disoriented and distressed about these facts that contradicted his beliefs. Sometimes he would make excuses for the discrepancy, like it was a joke or bad lighting. We call this confabulation, and it’s a very common human coping mechanism when memory systems fail. But since he had lost the ability to form new memories, there was no lasting effect. He would reset back to 1979 with no recall of what he had been shown minutes before. He had the usual inability to update the timeline, but for some reason, his contextual time stamp had been moved years back from his injury.

So if the Severance implant could keep the timestamp context intact, it’s possible that memory could be stopped and started reversibly. Personal identity is preserved in anterograde amnesia. The person knows who they are, what they value, their life history—up until the moment of trauma. After that, memory becomes patchy, and the ability to carry events forward is lost.

Choosing to split the self

But in truth, the real-world condition that Severance most closely resembles isn’t from brain injury or malfunction at all. It’s the condition we now call dissociative identity disorder (DID), part of the spectrum that included classic multiple personalities.

In the emergency room, you’ll see a patient walk in completely coherent, well-spoken, and oriented—except they insist they’re 20 years old filling out forms when they appear much older. They’re shocked when they see themselves in a mirror. They can recount vivid details from earlier in life but draw a blank on anything that happened afterward. There’s no injury. No drugs. Just a complete, time-bound void in memory.

What makes these episodes so remarkable is their precision. It’s not that the memory is fuzzy—it’s surgically absent. The patient remembers everything up to a specific point—a breakup, a trauma, the loss of a child—and nothing after. The person sitting before you seems to have stepped out of the past. It’s as if the timestamped memory has been tampered with to turn off access to everything beyond a certain date.

It appears that time-delimited memory shutdown is a real capability, engaged without injury. In the DID we recognize as multiple personalities, personal identity is fractured—different states or “alters” emerge, often with different postures, voices, preferences, and worldviews. Memory between them is sometimes leaky, sometimes sealed.

The two identities in Severance seem to most resemble dissociation controlled by the implant, rather than intrinsic brain processes as we see in the clinic. It’s as if the implant induces dissociation, but with a blank identity to be filled in by the Lumen workplace. In DID, the switch is often involuntary, triggered by stress or context. In Severance, the transition is mechanical—an elevator controls the shift. But what’s imagined in fiction already happens in function: the brain can selectively partition identity, assigning one set of memories to one context and hiding the rest.

Personal identity is a brain construct

The neurological conditions I’ve discussed, TGA and anterograde amnesia with frozen memory, reveal something about the mechanism of personal identity and timestamping of episodic memory. Cases of DID show the ability of these systems to be controlled by intrinsic brain mechanisms.

I thought, for example, that some patients develop DID as a way to survive trauma by disconnecting experience from awareness. If the brain can do that on its own, maybe a future where it’s done on command really might be possible.

So Severance is taking the real events of identity fragmentation not as disorder, but as a plot device. It doesn’t invoke trauma, repression, or psychological conflict. It posits a world where the brain’s natural capacity for compartmentalization is switched on and off like software.

But take the lesson that this is how all of our brains work all the time. We live as if the self is fixed: I am who I am. But neurologically, that’s never been true. The brain maintains identity through dynamic integration of memory. Personal identity is a story we present to ourselves and the world. But the self can be dissociated under the right conditions. Whether through trauma, inflammation, or an elevator at Lumen, what we see is this: the mind feels whole, but the brain is capable of dividing or at least putting on convincing mass that temporarily allows us to change identity based on context to achieve our aims.

Reposted from my Substack as an experiment

Our friend, C. elegans

“What do you consider the largest map that would be really useful?”

‘About six inches to the mile.’

‘Only six inches!’ exclaimed Mein Herr.

‘We very soon got to six yards to the mile. Then we tried a hundred yards to the mile. And then came the grandest idea of all! We actually made a map of the country, on the scale of a mile to the mile!’

‘Have you used it much?’ I inquired.

‘It has never been spread out, yet,’ said Mein Herr: ‘the farmers objected: they said it would cover the whole country, and shut out the sunlight!’

—Lewis Carroll from Sylvie and Bruno Concluded, 1893

If you’ve been following my posts over the last few weeks on the failure of uploading a mind and not simply emulating what would appear like a person, you might object that this is all very sci-fi and not grounded in reality. Maybe the technology now just can’t be imagined, but someday a high-resolution scan or some very precise mapping function will provide a model so precise that the emulation will be for all intents and purposes an uploaded individual mind. Who knows, it might be self-aware with preserved self-identity.

I don’t think so. I think this is, as I’ve said, a confusion between copying a thing and building a model of a thing. An uploaded toaster can’t make toast, and a model of a hurricane won’t make you wet. The idea of uploading a brain is seductive — but it confuses a structural map with the thing itself.

Uploading the world’s simplest brain

Lets put aside the human brain for now. We can see this confusion of map and territory clearly in the failures and successes in uploading the brain of the worm, Caenorhabditis elegans into a computer. We’ll see that the bottom-up approach of mapping the C. elegans connectome didn’t work, but a top-down use of the model as explanatory has been increasingly useful as a way of understanding how a simple nervous system generates complex, state dependent behaviors. Models can be powerful tools to understand and predict behavior, but aren’t a replication of the thing itself, expected to just be an uploaded copy in the computer.

Continue reading “You want to upload a human mind, but you can’t even upload a worm”