When you compare three-layer and six-layer cortex, it appears that there are two types of cortical microcircuits. What's interesting is that hippocampus and olfactory (piriform) cortex have two spatially seperated types of circuits, while neocortex has the two circuits stacked on top of each other.
The first type of circuit is what you see in CA3, anterior piriform cortex and the superficial layers of neocortex. The pyramidal neurons in these areas are characterized by massive recurrent connections. They receive the primary afferent inputs. It is widely believed that these recurrent circuits are "pattern completion" circuits. They learn associations and can recover the association given only partial input - like hopfield nets.
The second type of circuit is what you see in CA1, posterior piriform cortex, and the deep layers of neocortex. Here the input is mainly from the pattern completion circuit. It is thought that these circuits are then "pattern separation" - finding new features that distinguish the current state from other states. This allow for new associations to be learned.
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