When folic acid, specifically its active form 5-methyltetrahydrofolate (5-MTHF), is reduced in the cerebrospinal fluid and its level in the blood is normal, cerebral folate deficiency develops.
Since folic acid and folate are actively involved in the reproductive system, embryonic and postnatal neuropsychiatric development, folic acid deficiency in the brain leads to developmental delays and various neuropsychiatric abnormalities.