State-dependent auditory evoked hemodynamic responses recorded optically with indwelling photodiodes.

Implantable optical technologies provide measurements of cerebral hemodynamic activity from freely behaving animals without movement constraint or anesthesia. In order to study state-dependent neural evoked responses and the consequential hemodynamic response, we simultaneously measured EEG and scattered light changes in chronically implanted rats. Recordings took place under freely behaving conditions, allowing us to compare the evoked responses across wake, sleep, and anesthetized states. The largest evoked electrical and optical responses occurred during quiet sleep compared to wake and REM sleep, while isoflurane anesthesia showed a large, late burst of electrical activity synchronized to the stimulus but an earlier optical response.