Display on Cortex (Time Frequency Map)

I had a question regarding the interpolation techniques used for displaying TF maps on the cortical surface. How does brainstorm interpolate the TF maps to be displayed on the cortical surface? Are there any relevant literature that can help me understand it better?

This will largely depend on which source imaging method you used, and whether you used constrained vs unconstrained methods. This tutorial on source estimation is helpful. For example, if you have 3 orientations unconstrained, it will perform the vectorial sum operation.

I forgot to mention that I am working on intracranial EEG and TF plots are directly estimated from the intracranial EEG. I suspect it to some sort of interpolation technique.

When EEG/SEEG/ECOG data are displayed on a surface (head, gray matter, white matter, skull...), the values at the surface vertices are computed from the recordings using a 3D nearest-neighbor interpolation using Shepard's weighting.
This is done in the same way for temporal (EEG recordings), spectral (PSD) and time-frequency data.

The relevant functions: