The avian cortex had been hiding in plain sight all along. Humans were just too birdbrained to see it
- By Bret Stetka on September 24, 2020
With enough training, pigeons can distinguish between the works of Picasso and Monet. Ravens can identify themselves in a mirror. And on a university campus in Japan, crows are known to intentionally leave walnuts in a crosswalk and let passing traffic do their nut cracking. Many bird species are incredibly smart. Yet among intelligent animals, the “bird brain” often doesn’t get much respect.
Two papers published today in Science find birds actually have a brain that is much more similar to our complex primate organ than previously thought. For years it was assumed that the avian brain was limited in function because it lacked a neocortex. In mammals, the neocortex is the hulking, evolutionarily modern outer layer of the brain that allows for complex cognition and creativity and that makes up most of what, in vertebrates as a whole, is called the pallium. The new findings show that birds’ do, in fact, have a brain structure that is comparable to the neocortex despite taking a different shape. It turns out that at a cellular level, the brain region is laid out much like the mammal cortex, explaining why many birds exhibit advanced behaviors and abilities that have long befuddled scientists. The new work even suggests that certain birds demonstrate some degree of consciousness.
The mammalian cortex is organized into six layers containing vertical columns of neurons that communicate with one another both horizontally and vertically. The avian brain, on the other hand, was thought to be arranged into discrete collections of neurons called nuclei, including a region called the dorsal ventricular ridge, or DVR, and a single nucleus named the wulst.https://e80c6291491128d9c5bbe61cb3987e4e.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.htmlADVERTISEMENT
In one of the new papers, senior author Onur Güntürkün, a neuroscientist at Ruhr University Bochum in Germany, and his colleagues analyzed regions of the DVR and wulst involved in sound and vision processing. To do so, they used a technology called three-dimensional polarized light imaging, or 3D-PLI—a light-based microscopy technique that can be employed to visualize nerve fibers in brain samples. The researchers found that in both pigeons and barn owls, these brain regions are constructed much like our neocortex, with both layerlike and columnar organization—and with both horizontal and vertical circuitry. They confirmed the 3D-PLI findings using biocytin tracing, a technique for staining nerve cells.
“We can now claim that this layered, corticallike organization is indeed a feature of the whole sensory forebrain in most, if not all, birds,” says Martin Stacho, co-lead author of the study and Güntürkün’s colleague at Ruhr University Bochum.
“It’s not that the DVR is the neocortex,” says Vanderbilt University neuroscientist Suzana Herculano-Houzel, who wrote a commentary accompanying the two new papers and was not involved in either of them, “but rather that the whole of the pallium in mammals and in birds has similar developmental origins and connectivity, and therefore [the pallia of both classes] should be considered equivalent structures. Stacho shows that settling for what the naked eye sees can be misleading.”
The idea that the DVR was somehow related to the neocortex was proposed in the 1960s by neuroscientist Harvey Karten. Yet it didn’t stick. Others subsequently claimed the DVR actually corresponded with other mammalian brain regions, including the amygdala, which, among other tasks, carries out the processing of emotion. “The theory about a DVR [correlation] has been possibly one of the biggest disputes in the field of comparative neurobiology,” Stacho says. But his new work lends credibility to Karten’s original hypothesis.https://e80c6291491128d9c5bbe61cb3987e4e.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.htmlADVERTISEMENT
Stacho and his colleagues think the findings also represent a glimpse into ancient animal brain evolution. The last common ancestor of birds and mammals was a reptile that roamed the earth around 320 million years ago. And its brain, the team believes, was probably a precursor to that of the two lineages that diverged through evolution. “Nobody knows how exactly the brain of the last common ancestor looked like,” Stacho says. “Most likely, it wasn’t like the neocortex or the DVR. It was probably something in between that, in mammals, developed to a six-layered neocortex and, in birds, to the wulst and DVR.”
The other new paper, by a group at the University of Tübingen in Germany, lends still more insight into the avian brain, suggesting that birds have some ability for sensory consciousness—subjective experiences in which they recall sensory experiences. Consciousness has long been thought to be localized in the cerebral cortex of smart primates—namely, chimps, bonobos and us humans. Yet crows appear to have at least a rudimentary form of sensory consciousness.
In the Tübingen group’s experiment, two carrion crows were trained to recall a previous experience to guide their behavior. When their training was completed, they went through a testing phase in which a gray square might appear followed by either a red or blue square 2.5 seconds later. In this exercise, the crows were trained to move their head if they saw a gray square and then a red one. And they learned to keep their head still if they saw a gray square and then a blue one. When the birds saw no stimulus followed by the appearance of a colored square, the sequence was reversed: blue signaled them to move their head, and red told them not to. So to correctly respond to the colored squares, the crows had to recall whether or not they had seen a gray one first—equating to a past subjective experience.
It was crucial to the experiment to present the gray square in six different intensities, including at the threshold of the birds’ perception. This way, lead author and neurobiologist Andreas Nieder and his colleagues could confirm that the crows were not simply carrying out conditioned responses to stimuli but instead drawing on a subjective experience.
Further, by implanting electrodes in an avian brain region called the nidopallium caudolaterale (NCL), the researchers were able to monitor activity of individual neurons in response to the stimuli. When the crows viewed a dim gray square at their perceptual threshold, NCL neurons became active in the period between that stimulus and the presentation of a colored square—but only if the crows reported seeing the gray one. If they could not detect that square, the neurons remained silent. This result suggests a unique subjective experience was being manifested through neuronal activity.https://e80c6291491128d9c5bbe61cb3987e4e.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.htmlADVERTISEMENT
Nieder does not claim crows have the self-conscious existence and self-awareness of apes but simply that the birds can partake in a unique, multipart sensory experience in response to a stimulus. “I am generally not a big fan of ascribing complex humanlike cognitive states to animals and prefer to maintain a conservative attitude,” he says. “Humans easily start to project their own mental states to other living (or even nonliving) beings. But in terms of sensory consciousness in other species, it is probably fair to assume that advanced vertebrates, such as mammals and birds, possess it.”
Nieder’s team’s findings suggest that the neural underpinnings of sensory consciousness either were in place before mammals evolved or developed independently in both lineages—with the avian line showing that being conscious does not necessarily depend on a bulky cerebral cortex.
Work by Herculano-Houzel demonstrates that the brains of corvids—members of a family of so-called “smart birds” such as crows, ravens and magpies—are very densely populated with interconnected neurons. Her studies jibe with the new Science papers. “With Güntürkün’s findings that pallium connectivity is indeed very similar between birds and mammals…, it all comes together very nicely,” she says, pointing out that the corvid pallium holds about as many neurons as you’d find in primates with a much larger brain.
This latest research also undercuts primate exceptionalism. “I hope that more people will be tempted to drop the notion that there is something very unique and exclusive about the human brain,” Herculano-Houzel says.