Hi Rich,
I wouldn’t equate a neuron with a hierarchical level. That is maybe the case for deep neural networks but when I talk about levels in the hierarchy I am talking about V1, V2, V4,…
Each cortical column contains several thousands of neurons and encodes information through a population code. A cortical column itself is made up of several layers (classically described as 6 layers but if you look close you can divide some of them into more sublayers) with intricate wiring between them. When I talk about levels in a hierarchy I also didn’t refer to the layers within a cortical column but the different regions in the brain. A hierarchical relationship in neuroscience is classically described as a lower-level column sending output from layer 3 to layer 4 in the higher level column. The higher level column connects back to the lower level column (top-down connection) from it’s layer 6 to the lower column’s layer 6 and 1. This kind of connectivity and classical definition of hierarchy in the neocortex is for instance described here https://academic.oup.com/cercor/article-abstract/1/1/1/408896?redirectedFrom=fulltext&login=false (and the paper I mentioned in my previous response)
Following this definition, there are certainly more than two hierarchical levels of processing in the primatehuman neocortex (probably around 4-5, i.e. V1, V2, V4, posterior inferior temporal cortex (TEO) and the anterior inferior temporal cortex (TE)), but less than 25. When I discuss two levels of hierarchy being used at a given time, I’m referring to attention being brought to bear on any two of these ~4-5 levels of hierarchy.
Another important thing to point out again is that information doesn’t always have to flow through the entire hierarchy of cortical processing before we can begin to generate an action output. Even V1 has projections to subcortical motor regions and can, for instance, directly control saccades of the eyes.
- Viviane