Abstract: Neural activity in the brain arising from a stimulus is monitored. A stimulus is applied to a target structure of the brain and a neural measurement is obtained from at least one electrode implanted in contact with the target structure. The neural measurement is configured to capture a measure of any late response arising in the target structure, typically being a neural response arising after conclusion of an ECAP, such as in the period 1.5-10 ms after stimulus onset. The late response(s) can be a useful biomarker such as of therapeutic ranges of deep brain stimulation, disease progression, medication efficacy, and intra-operative changes.