Glabella tap sign. Is it due to a lack of R2-habituation?

In 30 patients with Parkinson's disease, 55 patients with other neurological disorders and 25 normal subjects, both upper eyelid movements and orbicularis oculi reflexes to repetitive glabella taps were simultaneously recorded using a newly devised apparatus for the measurement of eyelid movement. Upper lid movement during the blink reflex has been thought to correspond to the late component of the two components of the orbicularis oculi reflex, and failure of habituation of the late component to repetitive stimuli has been considered to be responsible for the glabella tap sign. However, the present study showed that the eyelid lowered after the early component (R1), and habituation of the late component (R2) was recognized in 31% of subjects with the glabella tap sign. This shows that there is no direct causal relationship between the glabella tap sign and lack of the habituation of the late component.