How many layers of neurons are in the cortex of the human brain?

Officially there are 6 layers in the cortex, but this layering is somewhat arbitrary, and layer “1” contains almost no neurons.

Here is an idealized cross-section of the cortex showing the layering:

 In the visual cortex, layer 4 is subdivided into 4A, 4B, and 4C; and layer 4C is further subdivided into 4Cα and 4Cβ.

In truth, there are 20 or more distinct neuron types organized into a complex multi-layer circuit that repeats across the surface of the cortex. It is the horizontally repeating pattern of the circuit in all directions that creates the layered appearance under a microscope. But what is important is the circuit structure and not the layers per se.

Unfortunately the circuit has not yet been fully reverse-engineered or decoded. Hopefully that will happen in our lifetime.

 Reference: Paul King