Investigating the Neuropsychological Effects of 5-HT2a Antagonism

Inclusion Criteria: * Willing and able to give informed consent for participation in the research * Aged 18-45 years * Good vision and hearing * Body mass index (BMI) within the range of 18-35kg/m2 (This is to ensure an appropriate pharmacokinetic profile for pimavanserin is achieved by all participants) * Sufficiently fluent in English to understand tasks * Willing to avoid drinking any alcohol the day before the research visit Exclusion Criteria: * Currently receiving or seeking treatment for any mental health condition * Any past or current history of severe and/or serious psychiatric disorder, including but not limited to schizophrenia, psychosis, bipolar affective disorder, severe major depressive disorder, obsessive compulsive disorder (covered in SCID-5 assessment in screening procedures) * ADHD requiring treatment with stimulant or other centrally-acting drugs * Regular alcohol consumption of more than 21 units per week * A head injury causing concussion or unconsciousness in the past 6 months * Pregnancy / intention to become pregnant during the study or breastfeeding * Any use of recreational drugs in the last three months * Participation in any other drug study in the last three months * Participation in any other study with the same tasks in the last year * History of cardiac disease or cardiac arrhythmias * Prolonged QTc interval on baseline ECG * Current usage of other drugs known to prolong QT interval including Class 1A or 3 antiarrhythmics, e.g. certain antibiotics (getifloxacin, moxifloxacin) * Current use of drugs that inhibit CYP3A4 (eg Clarithromycin. Diltiazem. Erythromycin. Fluconazole). Participants will be asked to avoid grapefruit juice in the week before the study. * Current use of psychoactive medication that in the opinion of the Chief Investigator may interfere with the study measures * History of, or current medical condition(s) which, in the opinion of the Investigator may interfere with the safety of the participant or the scientific integrity of the study, including epilepsy/seizures, brain injury, severe hepatic or renal disease, severe gastro-intestinal problems, Central Nervous System (CNS) tumours, severe neurological problems (e.g. Parkinson's disease; blackouts requiring hospitalisation); * Any physical (including visual and auditory), cognitive or language impairment that would make complying with the study protocol challenging * Excessive caffeine consumption, i.e., consumption higher than 8 cups of standard caffeinated drinks (tea, instant coffee) or higher than 6 cups of stronger coffee or other drinks containing methylxanthines such as coca cola or Red Bull per day; * Smoking \>10 cigarettes per day; or equivalent nicotine consumption * Participant who is unlikely to comply with the clinical study protocol or is unsuitable for any other reason, in the opinion of the chief investigator. * Inability to ingest up to 95mg of lactose Additional Exclusion Criteria for Participants in the Sleep Study Cohort: * Unable to undergo cardiac monitoring * Unable to wear the sleep patch device for full monitoring period * Implanted neurostimulator