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

Participants with any condition that might interfere with the conduct of the study, confound interpretation of the study results, or endanger their own well-being.

Participants who are currently receiving treatment with racemic baclofen, vigabatrin, tiagabine, or riluzole or other GABA-related medications (e.g. gabapentin or pregabalin) other than arbaclofen in the context of AIMS-2 CT1

Participants who are currently receiving pharmacologic treatment affecting behaviour (see concomitant medication section) need to have a stable dose during the 6 weeks prior to the screening visit and for the duration of the study.

Participating in programs including non-pharmacologic educational, behavioural, and/or dietary interventions affecting behaviour, participation in these programs must have been continuous during the 3 months prior to screening and participants or their parent/caregiver/LAR may not electively initiate new or modify ongoing interventions for the duration of the study. Typical school vacations are not considered modifications of stable programming