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.