Reviewer #1's Review

"This is an excellent study and should be published in PLoS ONE. However, I will still raise some tough questions, which I hope would improve the study as well as PLoS ONE standards.

In this manuscript, Han and Boyden investigated optical inactivation of neurons expressed Halo, a light-driven chloride pump, by millisecond yellow light. They found that the photocurrents have rapid time courses and are reliable with high fidelity. Next, they showed the light-induced hyperpolarization could inactivate neuronal firing at single-spike temporal resolution. Finally, they co-expressed ChR2 with Halo, which allows bi-directional optical control of neuronal excitability. Fast neuronal inactivation by Halo is interesting and will open a precise way to manipulate neural circuits. Therefore, the study will help to understand the role of neurons and brain regions in physiological behaviors and brain diseases.

In general, experiments appear well executed and most results from each category of analyses are agreeable and convincing. The paper is well-written. Also, there is clear logic in experimental designs. I have some concerns need to be addressed, which I hope would improve the study."

n.b. These are the general comments made by the referee when reviewing this paper. Specific points addressed during revision of the paper are not shown.