Only mammals have a neocortex. But we know from behaviour that birds and reptiles think a lot like mammals. They must have structures and processes similarly to a neocortex.
ScienceDaily has a report (here) on research by Dugas-Ford and others in a PNAS paper, Cell-type homologies and the origins of the neocortex. The group looked for neurons similar to those in the neocortex in birds. This supports a 50 year old hypothesis.
Both the mammalian neocortex and a structure in the bird brain called the dorsal ventricular ridge (DVR) originate from an embryonic region called the telencephalon. But the two regions mature into very different shapes, with the neocortex made up of six distinct cortical layers while the DVR contains large clusters of neurons called nuclei.
in the 1960s, neuroscientist Harvey Karten studied the neural inputs and outputs of the DVR, finding that they were remarkably similar to the pathways traveling to and from the neocortex in mammals. As a result, he proposed that the DVR performs a similar function to the neocortex despite its dramatically different anatomy.
Dugas-Ford, Ragsdale and co-author Joanna Rowell decided to test Karten’s hypothesis by using recently discovered sets of molecular markers that can identify specific layers of mammalian cortex: the layer 4 “input” neurons or layer 5 “output” neurons. The researchers then looked for whether these marker genes were expressed in the DVR nuclei.
They found the level 4 and 5 markers in chicken and zebra finch. But instead for being in layers as in the neocortex, in the DVR the marked cells are in distinct nuclei. They looked at turtles as well and found the level 4 and 5 markers but this time in a single layer on the dorsal cortex. There may be advantages and disadvantages to the different structures using these neurons.
The complex language and tool-use of some bird species suggests that the nuclear organization of this pathway is also capable of supporting advanced functions — and even may offer advantages over the mammalian brain.
“If you wanted to have a special nuclear processing center in Broca’s area to carry out language processing, you can’t do that in a mammal,” Ragsdale said. “But in a bird they have these special nuclei that are involved in vocalization. It’s as if you have additional flexibility: You can have shorter circuits, longer circuits, you can have specialized processing centers.”
We can look forward to this putative homology being productive in studying the embryonic development of the brain and in understanding the mechanisms of neocortex activity.