In the holographic brain, what wave like phenomenon is the brain using to create a internal hologram?

The term holographic means that the Information is distributed over the whole object (brain or hologram). It does not mean that the mechanisms are the same, or even similar, for brain and hologram.

The mechanics of information storage in optical holograms is extremely very well understood in terms of 2D interference pattern of light waves from 3D objects. Note that the 2D pattern is stored on a continuous medium

However the distributed nature of information storage in the brain is not well understood, but there is no reason to assume any sort of wave is involved. The mechanism(s) will depend on the distribution of biochemical and bioelectrical states off different discrete neutrons and their interconnections in the neural network. This is not comparable to the way optical holograms store information as far as I know.

So, I believe there is no wavelike mechanism involved. However, you might want to ask in the Biology section for a better informed opinion.