I have read in some papers about the layers L1-L5 of the brain (e.g. in this paper). I could not find a definition of these layers. I have found information about the layers V1-V5 in the visual cortex.
Where exactly are L1-L5? And do these definitions vary across mammals?
V1-V5 are different (sub) regions of the visual cortex itself.
The layers L1-L5 (some regions have a layer 6) refer to the different cellular strata in the depth dimension of the cortical mass. This stratification occurs to some extent or another in various areas. Different sections of the cortex, e.g., the primary motor cortex have a predominant level 5 (for the pyramidal tract). Something like the visual cortex has more cells in layers 4 and 6.
From http://www.unige.ch/cyberdocuments/theses2003/RivaraC-B/images/fig.2.jpg
Layers 2 and 3 tend to carry interconnections between different areas of the cortex, and the topmost layer is generally less dense.
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