Are functional maps in the cortex used by the brain to carry out computations or are they just a byproduct of wiring minimization?
A key element to answer this question is to know if, when neurons from a cortical map project their axons to the dendrite of a downstream neuron, they retain any spatial order proportional to their location in the map.
For example, in the cartoon below, the four neurons from a cortical map (in black) project their axons to a downstream neuron's dendrite (in green). The relative spatial position of the synapses (black circles) is proportional to the relative position of the neuron in the map.
I would be very grateful if you could point me to any relevant paper addressing this question, in particular in the cortex of the primate (e.g. axon tracing experiments).
Thanks!
Ju et al. 2020 characterized the spatial organization of synaptic input in superficial V1 neurons' dendrites based on their orientation preference (2p imaging of glutamate receptors in the behaving macaque).
They found spatial clustering of synaptic inputs encoding similar orientation preferences (even if the RF positions of these input were different). That may actually reflect a topographic pooling of the underlying V1 orientation map (i.e. superficial layers are the output layers of the cortical circuit).
Unfortunately, since this approach was very challenging, they were able to record only a very small number of neurons, only from the superficial cortical layers. Therefore, even if this is a very interesting study (actually addressing other questions) it does not answer the question in this post.
I though it was worth it to share it here anyway. I believe this is the kind of approach that can answer the question in this post.
Giac
--- Reference ----
Ju N, Li Y, Liu F, Jiang H, Macknik SL, Martinez-Conde S, Tang S. 2020. Spatiotemporal functional organization of excitatory synaptic inputs onto macaque V1 neurons. Nat Commun 11:697. doi:10.1038/s41467-020-14501-y
Poloarized distribution of HCN channel on pyramidal cell prevent spatial bias of synaptics input on dendrites, but inputs do different function between dendrite and soma.
Article Polarized and compartment-dependent distribution of HCN1 in ...
Article Towards deep learning with segregated dendrites
But other cell types have special receptor distribution may have some computational consequence.
Article Presynaptic localization of an AMPA-type glutamate receptor ...
Thank you for your answer Yan-Bo Jiang !
Could you please explain why polarized distribution of channels should prevent spatial organization of synaptic input?
Check this!
Article Magee, J.C. Dendritic integration of excitatory synaptic inp...
Check this lab if you want in person detail discussion. Daniel is a giant in this field. Jeff was his postdoc.
https://www.researchgate.net/lab/Daniel-Johnston-Lab
Output layers usually are deep layers in cortex, Layer5 and 6. Pooling as my understanding should happen in Layer 4 and LGN neurons which have no apical dendrite.
Thank you very much Yan-Bo Jiang ! The review you suggested seems indeed very relevant to the question I have asked. I need to read it more carefully but here is an important point I was not aware about (I copy from the paper online)
- "Although theoretical analyses predict a marked location dependence for the integration of spatially segregated synaptic input, evidence indicates that the dendrites of some cell types can counteract the influence of filtering on the three main elements of synaptic integration — unitary EPSP amplitude, and temporal and spatial summation.
- As a result of countering the filtering properties of dendrites, synaptic integration is essentially independent of input location. Three broad categories of cellular properties are involved in the normalization of synaptic integration — neuron morphology, active properties of the membrane and synaptic mechanisms."
However, most of these results seems to come from the mice hippocampus, that as far as I know does not receive direct input from a clear functional map and is more concerned with response patterns timing (as György Buzsáki put it "the Rhythms of the brain").
I wonder if similar mechanisms "erase" topographic information from the dendrites of cortical neurons pooling information from well defined functional maps (as the orientation map in V1 cortex).. it seems to me that these neurons would actually benefit of retaining that information..
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