What is consciousness?

Much of the time, consciousness is something of a “you know it when you see it” phenomenon. We’re conscious right now; the phone we’re reading this on is not. A raven is conscious; a writing desk, not so much. Rock star mid-set: yes. Rock, mid-settle: no. Simple.

But science is rarely so cut-and-dry – so, when the lines get blurry, what separates the nearly-conscious from the barely-conscious?

It’s a question whose answer is yet to be pinned down. “Each of us might have a slightly different definition” of consciousness, neuroscientist Lorina Naci of Trinity College Dublin told Science this week. For her, it’s the capacity to have an experience or a subjective point of view, she explained; for others, like psychologists Josef Perner and Zoltán Dienes writing in 2002, it’s the ability to recognize and comprehend your own mental states. Some experts go even further, arguing that understanding the concepts of “appearing” or “seeming” is necessary for consciousness – that unless you realize the world may look one way to you and another to somebody else, you don’t fully qualify.

None of these viewpoints are necessarily without merit – but as the bar for consciousness gets pushed higher, the consequences become ever more counter-intuitive. If a fully developed theory of mind is the benchmark, for instance, then “consciousness” is potentially only available to humans above the age of five or so. To those with a beloved pet dog or a precocious toddler, such a stance likely feels wrong on a gut level – and it’s ethically thorny too, allowing people to argue that those with conditions such as autism or schizophrenia are, in some way, not fully conscious.

Recently, however, a new approach has been gaining popularity. Rather than set a strict definition and exclude everything that falls short, researchers are moving towards a more “cluster-based” model of consciousness, basing the state on a collection of criteria being met rather than one all-consuming yes-or-no question.

“This [earlier] approach to the ascription of consciousness might be appropriate if consciousness were […] akin to (say) dirt,” explains one 2010 paper by philosophers Nicholas Shea and Tim Bayne. “There is no underlying nature to dirt, and it makes little sense to suppose that something might have the appearance of dirt without really being dirt.” 

But suppose that’s not the case, they argue – perhaps, they suggest, “being conscious is more like having hepatitis C than being dirty.”

Differences in desirability aside, it’s an interesting comparison. Hepatitis C was being treated well before it was formally known to exist, and certainly before there was any test for it: “Tests of liver inflammation, together with the clinical history and exclusion of other causes, gave a good indication, but were not determinative,” point out Shea and Bayne.

Facing a laundry list of symptoms and with no known culprit, physicians in the 1980s treated their patients anyway. “In medicine, clinicians gather signs of a disease and then examine whether they are ‘syndromic’ – whether they seem to be found together better than chance,” the pair explain. “Now, sequencing for viral RNA directly gives a highly accurate diagnosis [of hepatitis C]. However, having strong evidence for the existence of an underlying natural property does not depend on there ever being a surefire gold standard test.”

In other words: just because we can’t test definitively for consciousness, that doesn’t mean we can’t reasonably infer its presence, given enough evidence. But here’s the question: if “consciousness” is merely the culmination of a collection of properties occurring at once… then who makes the grade?

When does consciousness start?

Babies are many things: cute; messy; often milk-drunk off their asses – but deep thinkers, they are not. They can’t talk; they aren’t aware of circadian rhythms or social cues; they can’t even control their own bodies, pooping and peeing all over the place as their little limbs flail reflexively about. 

But are they conscious? Some new parents would probably say yes – babies might not be able to hold a conversation about the virtues of nihilism versus stoicism, but they can sure let you know when they’re unhappy. Clinicians and researchers, though, have traditionally not been so sure. It wasn’t so long ago that doctors would routinely operate on infants without anesthetic, assuming that they couldn’t really experience pain like adults do; that practice is now (thankfully) obsolete, but debate over when consciousness truly begins in children still rages on. 

“It is really hard to establish when babies become conscious,” explained Andrew Bremner, Professor of Developmental Psychology at the University of Birmingham, in a statement last year. “This is mostly because infants can’t report their experiences and, as most parents will know, can be rather uncooperative particularly when it comes to experimental tasks.”

So, if asking is out of the question, what can we do? Well, here’s where the cluster-based approach comes in: “the best approach is to try to identify a broad range of markers of consciousness, which appear in early development and late development, and then group them together,” Bremner said. “This could help us identify when consciousness emerges.” 

Of course, now we have a different problem – namely, which markers should be counted? Unsurprisingly, researchers have yet to settle on a definitive list, but one headline-grabbing paper from last year pared it down to four key areas. 

Testing babies

First, they required activity in the default mode network, a collection of areas in the brain that take over when we’re daydreaming or ruminating. It’s not easy to measure, not least because babies aren’t great at staying still in an fMRI machine – but Naci and colleagues managed it, and in 2022 concluded that “the reciprocal relationship between the default mode and dorsal attention network was present at full-term birth or term-equivalent age.” 

“Our results suggest that, at full-term birth or by term-equivalent age, infants possess key features of the neural circuitry that enables integration of information across diverse sensory and high-order functional modules, giving rise to conscious awareness,” they wrote in their paper.

So, that’s one criterion for consciousness fulfilled; next on the list is attention. Do babies direct their attention to stimuli, or are they just passively experiencing the world? The answer, once again, seems to point toward consciousness: “the attentional blink […] is widely taken to involve conscious processing,” write Joel Frohlich and Tim Bayne, the two philosophers and authors of last year’s paper. “When two stimuli are presented in succession, the second is often prevented from entering conscious awareness due to the fact that the first monopolizes attention.” 

“The attentional blink can be seen in infants from 5 months,” they note, “but it is much longer in infants than it is in adults.”

Third is the marker of “multisensory integration” – the ability to take sensory input from different organs and combine them to create one overall experience. There’s a neat way to test for this: just watch this video and tell us what noise is being said.

It’s called the McGurk effect, and it’s the audio-visual equivalent of an optical illusion. You’re actually hearing a “ba” sound – but you’re seeing a “fa”. Because you have the capacity for multisensory integration, your brain tries to combine these two conflicting pieces of information, eventually settling on “va” as a compromise. And as far as we can tell, that’s what babies hear as well: “Consistent McGurk-type effects can be found consistently from 5 months of age,” point out Frohlich and Bayne, “and less consistently from 4 months.”

Finally, the pair lists the “local-global effect” as a requirement for consciousness. This is, they explain “an auditory oddball paradigm” – basically, it comes down to whether you get surprised when a pattern is broken. And this is where the question of consciousness starts getting weird. 

If earlier researchers set the bar too high, excluding everyone pre-grade school from possessing consciousness, then this, some might argue, is the pendulum swinging too far in the opposite direction. Because it turns out that yes, babies do show evidence of the local-global effect – and, actually, so do fetuses.

“We don’t rule out the possibility that consciousness might already start some weeks before birth,” Frohlich wrote in an article for Psychology Today last year. “My coauthor Dr. Julia Moser, currently at the University of Minnesota, has led work showing that third-trimester fetuses appear to be capable of learning sequences of auditory beeps. When an auditory tone deviates from a pattern established earlier in the experiment, the fetus shows this surprise response in its magnetic brain activity. The neural activity shows a field deflection as if the fetus is saying, ‘Huh?’”

So, are fetuses conscious? 

Certainly, it seems that consciousness arises earlier than we previously thought. But can it really be the case that it’s present before birth? That’s a tricky question – and one with thorny moral implications, whatever the result.

“It is widely believed that consciousness requires a thalamocortical structure, and this system develops at around 26 weeks of pregnancy, so it is unlikely that consciousness is present before that time,” said Claudia Passos-Ferreira, assistant professor of bioethics at the NYU School of Global Public Health, in January this year. 

“But even after fetuses develop the brain structures needed for consciousness – meaning they might have the capacity for consciousness – they might not be deploying this capacity because of chemicals in the womb that keep them sedated,” she added.

One idea favored by a few researchers is that birth itself is the watershed – that being forced from a warm, muted pool, through a narrow channel and into a harsh, cold, bright delivery room necessitates the development of stronger mental faculties. 

Still others remain unconvinced of the early-onset consciousness idea entirely. The four markers proposed by Frohlich and Baynes are too simplistic, argue Taylor and Bremner; yes, they may point to consciousness emerging between the third trimester of pregnancy and around five months of age, but other markers, equally important in Taylor and Bremner’s eyes but ignored by Frohlich and Baynes, place its development much later. 

“One of the complicated issues is that it does not look like all the markers point to the same age for the emergence of consciousness,” Taylor pointed out. “Because there are so many different markers of consciousness which appear in early and late development it is extremely hard to come to a conclusion.”

So, the question of when consciousness arises? It’s still open. We’ve more data than ever before, and yet a definitive answer remains tantalizingly out of reach.

“We may, in the fullness of time, develop a kind of crucial test that can act like a thermometer for consciousness,” Bayne told New Scientist in February this year. “But we’re a million miles away from that right now.”

“I think the best way to go is to look for lots of different tests, and if they all point in roughly the same direction, then you know you’re onto something,” he said.