Here’s one of the most disturbing projects to emerge in Silicon Valley recently: human clones grown without conscious brains. At least one biotech start-up reportedly has quixotic ambitions to produce spare, unsentient meat bags to pave the ethical way for procedures called “body transplants” – and, hypothetically, immortality. The idea seems to be that if these surrogate bodies are completely unconscious, without even the slightest awareness of the world or themselves, then there is no harm.
It’s unclear how much or how much brain these clones would have, but they would certainly lack a cerebral cortex, the wrinkled outer layer that is responsible for sophisticated cognitive functions such as language, self-reflection and abstract thought. Most theorists We have long assumed that the cortex is where consciousness, or our subjective experience of the world, arises.. If they are right, an organism without this system would have no thoughts, sensations, emotions, or inner life at all.
But what happens if they are wrong? A growing number of consciousness researchers are seriously considering the possibility that consciousness may originate deep within the brain’s most evolutionarily ancient domain: the belowcortex. They argue that, just as astronomy once operated according to a false geocentric model, consciousness research is captivated by the misconception that cortical processing is central to all experience – the corticocentric model. The idea is “as old as any attempt to establish a link between the brain and the mind” in neuroscience, says Mark Solms, a neuropsychologist at the University of Cape Town in South Africa. “It’s a fundamental theory of where the mind is.”
On supporting science journalism
If you enjoy this article, please consider supporting our award-winning journalism by subscribe. By purchasing a subscription, you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
Yet over the past several decades, Solms and others have marshaled contrary evidence in hopes of bringing about a Copernican upheaval in their field. The subcortical revolution, if it occurred, would have massive implications for how we define and measure consciousness – and which creatures we deem worthy of moral consideration.
The brain in two parts
The cortex is the latest innovation in neuroanatomy and it has performed well. Its size varies between species, but in humans and many other mammals, the cortex now swells to epic proportions, approximately 75 percentage of brain massin our case – and envelops the older structures underneath. The deepest region, the subcortex, assumes more fundamental responsibilities than the region upstairs: maintaining alertness, processing emotions, regulating the body and relaying sensory information.
Amanda Montánez
The cortex and subcortex are closely interconnected. When most sensory information enters the brain, it travels through relay points deep in the subcortex back up to the cortex, which then responds with feedback signals in a continuous communication loop. Virtually all neuroscientists agree that, in a healthy human brain, consciousness depends on this ongoing dialogue between the cortex and subcortex; it has been clear for almost a century that if parts of the subcortical brainstem are damaged“the lights are going out,” as Solms puts it. The question is whether the subcortex is simply a power supply keeping the cortex’s consciousness onlineas the corticalists maintain, or if it can maintain fundamental consciousness by itself.
Unconscious zombies
The most intuitive evidence that the subcortex is more powerful than we thought is that many organisms without cortex nonetheless seem aware. We don’t need to wait for Silicon Valley clones: children with a rare developmental disorder called hydranencephaly are already born without a cortex and, on this basis, are often classified as being in an unconscious vegetative state.
But in 2004, at what proved to be a pivotal moment in how researchers think about the subcortex, Swedish neuroscientist Bjorn Merker joined five families including children with hydranencephaly at Disney World.
He spent a week observing the children’s behavior. They laughed, played with toys, and generally showed “reactivity to their environment in the form of emotional or orienting reactions to environmental events.” as he later wrote. They seemed completely normal to Merker, even though they were developmentally delayed. Even though they couldn’t speak and therefore couldn’t account for their internal state, he simply couldn’t believe he was in the presence of them. philosophical zombies– hypothetical beings who act like normal humans but have no felt experiences.
Solms, following Merker’s lead, also spent time with children with hydranencephaly. “The proof that they are not ‘zombies’ is exactly the same proof that your dog and your cat are not zombies,” he says. “They declare by their behavior that they feel things.”
Of course, the appearance consciousness and consciousness itself are not the same thing. Strictly speaking, we cannot determine whether an organism is conscious unless it can somehow narrate its experience, which leaves us to speculate on the matter. babies, cerebral organoids And non-human animals. (What this means for major language modelswhich can recounting their “experience”, is another question.) So we seem to be at an impasse: how can non-verbal life forms prove that they are not mindless automatons?
When language is not an option, most researchers use other information to infer consciousness. Matthias Michel, a philosopher at the Massachusetts Institute of Technology, is willing to attribute consciousness to other mammals, which have a cortex, and to birds, which have a functional equivalent in the pallium, but not to fish or birds. insectswhich is not the case.
This belief is based in part on studies showing how visual information enters consciousness consciousness – investigations that constitute the bulk of consciousness research. Visual information travels from the eye to subcortical structures, then to the primary visual cortex located at the back of the brain. From there, neuronal activity quickly cascades to higher cortical areas. This initial passage seems to take place unconsciously, below the threshold of consciousness; only later, when feedback loops begin to reverberate through the cortex, do people report being aware of what they are seeing.
Michel argues that animals with similar brain organization to humans probably enjoy a similar type of consciousness, and animals without it probably do not. After all, we don’t need to be aware of the information for it to affect our body’s reaction. “Consciousness is just the tip of the iceberg,” says Michel. “Much of your behavior is driven by unconscious processes.” In the phenomenon known as blind visionFor example, people whose visual cortex has been damaged appear to respond to visual stimuli, even if they insist that they see nothing. (There are also auditory and olfactory versions, called hearing-deaf and smell-blind, respectively).
The result is that you don’t have to be aware of something to respond to it. This may be what happens in children with hydranencephaly and in some nonhuman animals: subcortical activity results in reactive but not subjective behavior. experience which is associated with it.
Feelings (and who feels them and why)
Most research on consciousness has focused on vision, but it is clear that visual perception can continue. Andconsciously. What about other modes of experience? Solms argues that researchers should investigate feelings, which he considers to be intrinsically conscious. “If [we] “If we had started with feeling rather than seeing,” he says, “there would have been no mystery to begin with. »
There is a case to be made that feelings are the most fundamental form of consciousness. From an evolutionary perspective, Solms says, they appear once organisms become complex enough to have many competing needs: eating, sleeping, finding mates, escaping predators, and so on. Such organizations must constantly reprioritize these evolving needs; feelings guide their actions. “We feel,” Solms says, “so that we can transcend instinct.”
This balance of needs is widespread throughout the animal kingdom, unlike the complex brain processing that most neuroscientists consider essential to experience. For subcorticalists, this ubiquity suggests that the ability to feel – even if only incoherently – may be just as widespread. “Consciousness is not, as many people think, a sophisticated cognitive function but something much more basic,” says Merker.
Even some experts without strong subcortical tendencies find this an attractive evolutionary explanation. “You have to take all of this in and act in real time,” says Tim Bayne, a philosopher at Monash University in Australia. “Maybe that’s what consciousness is, and that’s what fairly simple animals have.” This fair story suggests that consciousness might be widely distributed across the tree of life.
Unlike the mammalian cortex, which began coalescing around 300 million years ago, the subcortical architecture to which Solms and others attribute the experience evolved during the Cambrian Explosion – the dawn of vertebrate life – more than 500 million years ago. Drawing the line here would admit reptiles, amphibians and fish into the club of consciousness; Some researchers believe that analogous brain structures may also support consciousness in insects and cephalopods, including octopuses.
In humans and other mammals, complex cortical processing increase experience. Even the most convinced subcorticalists admit it. As Merker says: “Everything that makes our world rich is a gift of the cortex. And in the same way, without the cortex, such wealth does not exist.” It’s ju ste that he is convinced that cortical processing leads to consciousness not in situ but deep in the brain.
According to subcorticalists, this is the most parsimonious way of accounting for the unified nature of consciousness. The human brain receives a torrent of information about the world, but we do not perceive the whole scene in detail. “Our experiment is so simple in comparison,” says Daniel Freeman, a neurophysiologist at MIT. One way or another the thought is this: cortical information must be distilled in a single stream of consciousness. Subcorticalists argue that this happens when the cortex channels its diffuse electrical signals downward, toward a neuronal bottleneck in lower regions of the brain. There, subcorticalists argue, chaos is transformed into a finished product worthy of consciousness.
In children with hydranencephaly and in primitive animals, Solms says, the subcortex alone could give rise to “only the most rudimentary form of consciousness.” With this in mind, research has shown that deep brain stimulation can invoke basic (but powerful) emotions such as severe depression and fear. But corticalists deny even this possibility, arguing that such feelings could result from downstream activity higher up in the brain. “Even in dim light, the most boring experience,” insists Michel, “would require something like the cortex.”
Stimulate the deep brain
So, does consciousness originate from the cortex or the subcortex? “I don’t think we have enough evidence to fully support either version,” says Anil Seth, a neuroscientist at New York University. Sussex in England. He agrees with Michel that “there is a wealth of evidence” linking cortical processing to consciousness, but the precise brain activity that corresponds to experience – the so-called neural correlates of consciousness – has remained elusive. From the myriad theories of consciousnessnone inspires a consensus. Perhaps this is because we understand so little about what is happening in the subcortex during experience.
But a new technique, called transcranial focused ultrasound (tFUS), could allow researchers to directly probe subcortical structures, the terra incognita hidden beneath relatively well-mapped cortex. Although Freeman and Michel have opposing views on the place of consciousness, they recently co-authored an article suggesting that tFUS offers “an exciting opportunity for breakthroughs in consciousness research.” Some results could persuade Michel to rethink his cortical commitments. If, for example, a person who had lost pain sensation due to damage to the cortex regained it during subcortical stimulation, he says, “I would be blown away.”
Corticalism currently dominates the field, but a reevaluation of the subcortex appears to be underway. When Bayne and Seth wrote an influential article review of theories of consciousness in 2022, they relegated subcortical considerations to a single sentence in a box, reflecting the absence of these ideas in the broader discourse. Today, Bayne says: “I think the debate has opened up a bit. » A sign of this change is the New York Declaration on Animal Consciousnesspublished in 2024 and signed by nearly 600 scientists. After noting that other mammals and birds are almost certainly conscious, he asserts that “empirical evidence indicates at least a realistic possibility of conscious experience in all vertebrates…and many invertebrates” and that we must therefore consider their well-being.
Alongside the signatures of Solms and other confirmed subcorticalists, Seth placed his own. Although he is still cortical, he too has opened his mind. “I wouldn’t be terribly surprised if we looked at the book of truth,” he says, “and the answer is that some fundamental forms of consciousness are supported entirely by the subcortex.”
