readings> bird brain
Do birds seem a little bit alien to you? It’s those beady
expressionless eyes, the sudden darting movements, a stop/start sort of
mind. No question that a parrot or raven is as smart as a monkey or
dog, but surely their brains are wired up differently somehow?
This is the lay impression. And the avian experts agree. Yet exactly how the brains of birds differ is causing great ructions at the moment. Avian neuroscientists have finally put one century old view about its architecture to rest. But still they can’t quite decide the story that ought to replace it!
The layout of the lower brain of birds and mammals is of course much the same -
brainstem, cerebellum, midbrain and thalamus. However the cortical lobes look very different. There is no six-layer sheet of cortex wrapped around a mass of white matter connections. Instead the avian telencephalon seems a dense mass of nuclei. This led early neuroanatomists – who believed that birds ranked lower on the evolutionary scale and were thus largely instinctual in nature – to conclude that their cortical lobes were merely elaborated basal ganglia. And so all the higher bits of a bird’s brain got labelled as striatal this or striatal that. Even a strip of undoubted cortex on the dorsal surface was named hyperstriatum as if it wasn’t quite up to its mammalian counterpart.
Within ornithological circles, it was soon realised birds weren’t basal ganglia-brained. Only the most ventral nuclei were actual striatal structures. But by then the terminology had stuck, generating vast and continuing confusion. Even in 1998, a Journal of Neuroscience paper mistakenly compared the neostriatal control of grooming “syntax” in rats to the neostriatum song memory area in birds - bird neostriatum being more properly equivalent to temporal cortex.
Last year researchers finally agreed to a complete overhaul of the neuroanatomical nomenclature. Largely this was done by replacing each striatal reference with a pallidial one. So now, for example, the neostriatum is the nidopallium. But the experts remain divided on the deeper question of how to view the actual organisation of the bird brain.
One camp take the startling view that birds have a six-layer cortex like mammals after all – it’s just that the layers are split up into processing blobs! So layer four, the cortical “input” layer, is rolled up as a central nuclei known as the entopallium (formerly the ectostriatum). This then feeds into an adjacent lump, our friend the neostriatum or rather nidopallium, which serves the processing functions of cortical layers two and three. The nidopallium then feeds into what used to be thought of as the bird’s equivalent of the amygdala, the arcopalluium. This handles the chores of mammalian cortex layers five and six.
This way of looking at the connectivity of the avian brain suggests that birds and mammals have taken different anatomical routes but arrived at a remarkably similar processing architectures. However recent evidence based on homeobox genes questions the cortical layer hypothesis. Instead it seems the bird’s higher brain is the result of a massive expansion of that mysterious region, the claustrum.
The accepted story on mammals is that they branched from the reptile line about 300 million years ago as small nocturnal grubbers. So the part of the brain that ballooned was the dorsal cortex, an associative area linking the olfactory bulb to the hippocampus – the right sort of brain architecture for “filling in” the what and where of smells. Visual and auditory input then got diverted from midbrain to cortical areas to allow the same associative processing of the confusing noises and degraded images of the night-time jungle.
Birds on the other hand didn’t diverge from the dinosaurs until 100 million years later, so plenty of time to follow a different neurodevelopmental track. And birds were daylight fliers who just needed sharp eyes and snappy reflexes. Hence birds stuck mainly to a “collicular” style of sensory processing, expanding their midbrain optic tectum. The cortical expansion that did take place happened along an embryonic amygdala-claustrum axis.
Now the idea that birds may have a more “encephalised” amygdala is a bit of a poser for those who say birds aren’t emotional. But perhaps it does fit with the idea they are more instinctual – snap decision makers rather than associative ponderers. However a claustral origin for their cortical regions has left neuro-ornithologists floundering. What does it even do in humans except perhaps some kind of cross-modal sensory integration? Oh well, still more questions than answers then. But it does show that there must be surprising number of different ways for evolutionary tinkering to construct a brain.