All-optical interrogation

Simultaneously reading neural activity with a genetically-encoded indicator (imaging) and writing activity with an opsin (photostimulation) in the same preparation — a fully optical closed loop. It demands spectral separation between reporter and actuator so that the imaging light does not spuriously drive the opsin, and the stimulation light does not swamp the imaging channel. The canonical pairing is a green calcium (or voltage) indicator with a red-shifted, soma-targeted opsin.

All-optical interrogation is the optical counterpart of a high-channel electrical interface but with genetic cell-type specificity and true single-cell write-in that electrodes cannot match. It has enabled genuinely causal experiments — activating a chosen ensemble and reading the network's response — and closed-loop schemes where stimulation is updated in real time from the imaged state. Demonstrations are in rodents and, increasingly, non-human primates; it remains a laboratory method rather than a fielded device.