https://www.nature.com/articles/s41583-020-0317-z.pdf
“The advent of optogenetics — using light to activate opsins expressed in selected neuronal subtypes — has revolutionized the field of neuroscience. It has certain limitations, however, in part owing to the poor penetration of light through brain tissue, which necessitates the surgical implantation of optical fibres in order to deliver light to the region
of interest at a sufficient strength
to activate opsins. The implantation procedure damages brain tissue, creating an inflammatory response, neuronal loss and altered spine turnover — thus creating a non-physiological neural environment. Now, Gong et al. demonstrate the use of a modified step-function opsin (SFO) with extreme light sensitivity in mice and macaques. They show that this SFO can be activated in deep brain structures (with minimal disturbance of
brain tissue) and can influence behaviour”.pdf