Some people experience auditory sensations when seeing visual flashes or movements. This prevalent synaesthesia-like visually evoked auditory response could result either from overexuberant cross-activation between brain areas and/or reduced inhibition of normally occurring cross-activation. The authors of this paper have used transcranial alternating current stimulation to test these theories. Their results suggest that “hearing” visual stimuli may depend on disinhibition of normally occurring sensory cross-activation, and that this may be expressed more strongly in some individuals. Furthermore, endogenous alpha- and gamma-frequency oscillations in the brain may respectively inhibit or promote this cross-activation.

Fassnidge C, Ball D, Kazaz Z, Knudsen S, Spicer A, Tipple A, Freeman E: Hearing through Your Eyes: Neural Basis of Audiovisual Cross-activation, Revealed by Transcranial Alternating Current Stimulation. J. Cognitive Neuroscience 31(6):922-935 (2019).

https://www.ncbi.nlm.nih.gov/pubmed/30883286