Cortical Layers
The cortical layers are the six horizontal bands of the neocortex, numbered I to VI from the surface down, each with its own cell types and connections; a penetrating probe crosses them in order along its shank.
Their anatomy and function are standard textbook material; what an electrode adds is that the layers turn depth along a shank into a coordinate that can be read from the recording.
The layers set two probe numbers. The cortical thickness sets the span the sites must cover to record the whole column — about 1 mm in mouse, about 2 mm in rat, 2 to 3 mm in primate — and the thickness of a single layer, one to several hundred micrometers, sets the site pitch needed to place more than one site in each. A 32-site linear layout at 50 µm pitch spans 1.55 mm and puts several sites in every layer of a rodent cortex; a 100 µm pitch covers a thicker cortex with the same site count at coarser resolution. Layouts and spans are under Probe Geometry.
The layers are also read back from the signal. Layer IV receives thalamic input and layer V holds the large output cells, so the pattern of spikes and the current sources in the local field potential across sites identify where in the column each site sits, often more reliably than the depth reading on the manipulator. The hippocampus is under its own entry, Hippocampus; coordinates are set from bregma.
Typical values
| Number of layers | six (I to VI) |
|---|---|
| Cortical thickness, mouse | about 1 mm |
| Cortical thickness, rat | about 2 mm |
| Cortical thickness, primate | 2 to 3 mm |
| Span of a 32-site linear layout at 50 µm pitch | 1.55 mm |