Single Dose Double-blind, Placebo-controlled Cross-over (SDDBPCCO) Shiftability Study, Will be Followed by a 10-week Open-label Study With Arbaclofen (4 Weeks of Titration and Then 6 Weeks of Active/Stable Treatment). The Effects of Arbaclofen on Target EEG and ERG Metrics Will be Associated With th

Inclusion Criteria: * Signed Written Informed Consent a.Participants or their legal representative must have signed and dated an IRB/IEC approved written informed consent form * Diagnosis of an Autism Spectrum Disorder according to the DSM-5 criteria * Participation in the AIMS-2 CT1 (ages at recruitment 5 to 17). * Current pharmacological treatment regimen affecting behaviour has been stable for at least 6 weeks prior to screening and is expected to be stable during the duration of the study * Current psychotherapeutic/psychosocial interventions affecting behaviour stable for 3 months prior to screening and expected to be stable during the duration of the study * Participants with a history of seizure disorder must currently be receiving stable treatment with anticonvulsant medication and must have been seizure free for 6 months prior to screening or must be seizure free for 3 years prior to screening if not currently on a stable (\>3 months) dose of antiepileptics * Male or female participants 7 to 23 years of age at the time of providing consent, inclusive. * Reside or regular contact (at least twice a week) with the parent/carer who is interviewed for the study. * Negative pregnancy test for females of childbearing potential (participant has experienced onset of menses) * Females of childbearing potential who are sexually active must agree to use a highly effective form of contraception (i.e., existing surgical sterilization, complete or abstinence or a combination of tw…