A system comprising: processing circuitry configured to: control a medical device to deliver a first electrical stimulation at a first frequency to a brain of a patient via a first electrode combination: receive information representative of a first electrical signal sensed by a second electrode combination of a medical lead from the brain after delivery of the first electrical stimulatior; determine, from the information representative of the first electrical signal, a peak in a spectral power of the first electrical signal at a second frequency lower than the first frequency of the first electrical stimulation; characterised in that said processing circuitry is further configured to control the medical device to deliver a second electrical stimulation at the first frequency to the brain of the patient via the first electrode combination; receive information representative of a second electrical signal sensed by the second electrode combination from the brain after delivery of the second electrical stimulation; determine, from the information representative of the second electrical signal, a change in a magnitude of the peak in a spectral power of the second electrical signal at the second frequency; and responsive to determining the change in the magnitude of the peak in the spectral power of the second electrical signal at the second frequency, determine, based on the change, that the medical lead has moved with respect to the brain of the patient. processing circuitry configured to: control a medical device to deliver a first electrical stimulation at a first frequency to a brain of a patient via a first electrode combination: receive information representative of a first electrical signal sensed by a second electrode combination of a medical lead from the brain after delivery of the first electrical stimulatior; determine, from the information representative of the first electrical signal, a peak in a spectral power of the first electrical signal at a second frequency lower than the first frequency of the first electrical stimulation; control a medical device to deliver a first electrical stimulation at a first frequency to a brain of a patient via a first electrode combination: receive information representative of a first electrical signal sensed by a second electrode combination of a medical lead from the brain after delivery of the first electrical stimulatior; determine, from the information representative of the first electrical signal, a peak in a spectral power of the first electrical signal at a second frequency lower than the first frequency of the first electrical stimulation; characterised in that said processing circuitry is further configured to control the medical device to deliver a second electrical stimulation at the first frequency to the brain of the patient via the first electrode combination; receive information representative of a second electrical signal sensed by the second electrode combination from the brain after delivery of the second electrical stimulation; determine, from the information representative of the second electrical signal, a change in a magnitude of the peak in a spectral power of the second electrical signal at the second frequency; and responsive to determining the change in the magnitude of the peak in the spectral power of the second electrical signal at the second frequency, determine, based on the change, that the medical lead has moved with respect to the brain of the patient. control the medical device to deliver a second electrical stimulation at the first frequency to the brain of the patient via the first electrode combination; receive information representative of a second electrical signal sensed by the second electrode combination from the brain after delivery of the second electrical stimulation; determine, from the information representative of the second electrical signal, a change in a magnitude of the peak in a spectral power of the second electrical signal at the second frequency; and responsive to determining the change in the magnitude of the peak in the spectral power of the second electrical signal at the second frequency, determine, based on the change, that the medical lead has moved with respect to the brain of the patient.