There are many advantages to the light and opsin-based approach as compared to the electrical stimulation one, including:
1) More (dynamic) control over the size of the area in the CNS affected.
2) The ability to target specific cell types with different gene implantation techniques.
3) The light stimulation would not cause electrical interference, thus eliminating recording artifacts and allowing for continuous feedback.
4) The stimulation can be precisely controlled by varying the light intensity.
The chief drawback to the approach, and it is a big one, is that the technique would rely upon the precise genetic manipulation of human cells.
Despite being in clinical trials for 21 years, according to the U.S. Department of Energy Genome Programs, a gene therapy treatment has yet to be approved by the FDA.
Neurons do have some particular advantages for gene therapy, however, because they are terminally differentiated (i.e., they no longer divide). Plus, optogenetics has already been used successfully in at least two monkeys.
The potential of optogenetics to help treat brain disorders provides an often overlooked reason to avidly support research into gene therapy.
Gilja V, et al. 2011 Challenges and Opportunities for Next-Generation Intracortically Based Neural Prostheses. IEEE TBE. doi: 10.1109/TBME.2011.2107553.