Frequently during major vascular procedures the patient is conscious for a portion of the operation to monitor how the brain is functioning.
Once the incision is made, bone is removed and the brain is exposed, the patient's sedation is lightened in order for the patient to communicate with the surgeon or speech therapist.
The patient is needed to be awake in surgeries that require working on parts of the brain that control speech and motor function. Therefore, the patient is usually awake as the surgeon stimulates and tests those areas before working on them. If stimulation of those areas results in speech arrest or paralysis (where patient is unable to talk during the surgery or move a limb), the surgeon would avoid those areas