Functional Neurologic Disorder - also known as Conversion Disorder, hysteria or psychosomatic illness - is a a common neuro-behavioral / neuro-psychiatric condition where patients experience limb weakness, paralysis, blackouts, movement disorders, vision disorders, speech disorders and other varied symptoms - but without any obvious physical cause.

It was originally thought to be closely related to traumatic incidents in the patient's past, but recent studies suggest that only around 30% report any obvious stressful incidents.

There is currently no consensus as to the cause(s)

Converging evidence from several studies using different techniques and paradigms has now demonstrated distinctive brain activation patterns associated with functional deficits, unlike those seen in actors simulating similar deficits. New research has uncovered pathways in the brain's white matter that may be altered in patients with functional neurological disorder. The new findings advance current understanding of the mechanisms involved in this disease, and offer the possibility of identifying markers of the condition and patients’ prognosis.

Source : Wikipedia

There are indications that it may be related to changes in the brain's anterior cingulate cortex (ACC) and middle cingulate cortex (MCC). What these changes are, however, and why they arise, remain unknown. See : Handbook of Clinical Neurology, 166: 267–279.