Neural phase transitions that made us mammals

I review two studies that both deal with radical changes in neuronal circuitry, presumed to have occurred at the transition from early reptilians to mammals: the lamination of sensory cortex and the differentiation into sub-fields of the mammalian hippocampus. In neither case the qualitative structural change seems to be accompanied by an equally dramatic functional change in the operation of those circuits. Both studies discuss the evolution of cortical networks in terms of their computations, quantified by simulating simplified formal models. The models can be conceived as variants of a basic autoassociative neural network model, whose storage capacity plays an important role in the results. Both studies dwell on the interrelationship between qualitative and quantitative change, and both studies include, as a necessary ingredient of the relevant computational mechanism, a simple feature of pyramidal cell biophysics: firing rate adaptation