This clinical study will extend our current understanding of EFV neurotoxicity by further defining the nature of these biological changes. Further elucidation of the neurobiological underpinnings of EFV-induced CNS toxicity will have clinical relevance in improving the quality of life and drug adherence of HIV-infected patients on ART, especially among older patients or those with baseline neuropsychiatric disorders, whom at baseline are more vulnerable to neurocognitive decline from long-term HIV infection.