An apparatus that performs transcranial stimulation, wherein the apparatus comprises circuitry that is configured to: generate a stimulation signal having a set of first oscillation parameters; apply the stimulation signal transcranially to a patient; monitor the stimulation signal as applied to the patient; receive a brain activity signal from the patient; generate a feedback signal based on the monitored stimulation signal as applied to the patient; generate a modified activity signal by subtracting the feedback signal from the brain activity signal; determine one or more second oscillation parameters of the modified activity signal; adjust the set of first oscillation parameters of the stimulation signal based on the one or more second oscillation parameters of the modified activity signal; characterised in that the apparatus comprises circuitry configured to: receive an external input to control how the one or more second oscillation parameters are determined and/or how the stimulation signal is adjusted based on the one or more second oscillation parameters, control how the one or more second oscillation parameters are determined by controlling which one or more second oscillation parameters are determined and/or when the one or more second oscillation parameters are determined, wherein the external input is based on at least one of a task provided to a patient, and an external observation of the patient's behaviour or environment. generate a stimulation signal having a set of first oscillation parameters; apply the stimulation signal transcranially to a patient; monitor the stimulation signal as applied to the patient; receive a brain activity signal from the patient; generate a feedback signal based on the monitored stimulation signal as applied to the patient; generate a modified activity signal by subtracting the feedback signal from the brain activity signal; determine one or more second oscillation parameters of the modified activity signal; adjust the set of first oscillation parameters of the stimulation signal based on the one or more second oscillation parameters of the modified activity signal; characterised in that the apparatus comprises circuitry configured to: receive an external input to control how the one or more second oscillation parameters are determined and/or how the stimulation signal is adjusted based on the one or more second oscillation parameters, control how the one or more second oscillation parameters are determined by controlling which one or more second oscillation parameters are determined and/or when the one or more second oscillation parameters are determined, wherein the external input is based on at least one of a task provided to a patient, and an external observation of the patient's behaviour or environment. receive an external input to control how the one or more second oscillation parameters are determined and/or how the stimulation signal is adjusted based on the one or more second oscillation parameters, control how the one or more second oscillation parameters are determined by controlling which one or more second oscillation parameters are determined and/or when the one or more second oscillation parameters are determined, wherein the external input is based on at least one of a task provided to a patient, and an external observation of the patient's behaviour or environment.