Gheorghe Marinescu and Anghel Radovici in their seminal paper hypothesize that both the afferent (receptive) and efferent (motor) arms of the reflex are on the same side (ipsilateral) to the hand stimulated; however this hypothesis remains unsubstantiated.
The PMR has been found to be present more frequently in various neurological conditions both localized and diffuse. These include congenital conditions such as Down's syndrome where it is unclear whether the reflex persists throughout life or disappears and then re-appears in association with the onset of Alzheimer's disease pathology. The reflex is common in the elderly population and should not be taken as indicative of a dementing process.
The thenar eminence is stroked briskly with a thin stick, from proximal (edge of wrist) to distal (base of thumb) using moderate pressure. A positive response is considered if there is a single visible twitch of the ipsilateral mentalis muscle (chin muscle on the same side as the hand tested).
A study set in a neurosurgical in-patient population showed there is no significant association between the side of the reflex and the side of the hemispheric lesion in patients with unilateral (one-sided) reflexes and unilateral (one-sided) lesions.
Primitive reflexes such as the PMR were classically viewed as signs of disorders that affect the frontal lobes, however this traditional view is questionable as the reflex has been noted for example in cases of one-sided temporal lobe pathology.