Hi Paul,
There is a long history in neuroscience regarding the layers of cells in the cortex. It includes the types of cells in each layer, the connections between the layers, and the connections between regions. The central and widely held belief is that this connectivity architecture is nearly identical for every cortical region regardless of what the region does. A theory of neocortex should explain the function of each layer of cells and why they are connected together the way they are.
The diagrams I sometimes use (that you show) are much simpler than the real anatomy. It is common in the neuroscience literature to show more or less of the detail depending on the context. These diagrams aren’t wrong but leave out many details that are more subtle and often speculative. For example we are currently working at a much more granular level of detail that includes L2, L3a, L3b, L4, L5a, L5b, L6a, and L6b. These have all been distinguished in the literature and we are working at understanding the function of each. Our theory has progressed a lot recently where we now find we need these distinctions.
If you read journal papers about these connections you will find that “what connects to what”, heavily depends on how the connections were determined. It takes a lot of practice to interpret these papers. A good place to start if you want to delve into this is Alex Thompson’s 2007 “Functional maps of neocortical local circuity”.
To say Layer 4 projects to Layer 3 generally means all or most of the excitatory neurons in L4 send an axon into Layer 3 and form synapses with neurons in Layer 3. The axons typically split and go elsewhere as well, so it isn’t an exclusive connection In HTM theory it makes a big difference where on the neuron the synapse is formed. Generally, neurons in the cortex have three integration zones, proximal, basal, and apical, and the affect on the post-synaptic neuron varies depending on the zone. HTM sequence memory is the only theory I know of that explains the role of the different integration zones in the context of a large network of neurons. Unfortunately, most of the connectivity data about the cortex does not include this critical piece of data about which zones are the termination of axons.
This spring we had a major leap in understanding what region does and one consequence was I abandoned some ideas that I held for a long time. I still believe a region builds a predictive model and a stable representation, but the changes are not passed up the hierarchy. It is too much for me to explain here, if you are interested check out some of the material we posted earlier this year on sensory-motor inference.
Our current belief is the following. Layer 4 works nearly identically to the Layer 3 model we use in the HTM temporal memory. It has mini-columns. The big difference is that distal dendrites receive most of their input from L6a, not other L4 cells. L4 forms a unique SDR for different locations on an object whereas L3 forms a unique SDR for each element in a sequence. In our current model L3 and L4 share minicolumns, together they try to predict the next input either by high order sequences or by knowing the location on object determined by movement of the sensory.
Before the end of the year I hope to be able to give a much richer explanation of what is going on in the different layers of cells and what mechanisms are being used in the different layers. We are still sorting through the details. Hopefully this comment will help you. I recommend developing a strong understanding of the HTM sequence memory algorithm as it will likely be reused in other layers, and after that I recommend learning about the new sensory-motor concept we posted earlier this year.
Jeff