Neurophysiological aspects of eye and eyelid movements during blinking in humans

The neural relationships between eyelid movements and eye movements during spontaneous, voluntary, and reflex blinking in a group of healthy subjects were examined. Electromyographic (EMG) recording of the orbicularis oculi ( OO) muscles was performed using surface electrodes. Concurrently, horizonta l and vertical eye positions were recorded by means of the double magnetic induction (DMI) ring method. In addition, movement of the upper eyelid was measured by a specially designed search coil, placed on the upper eyelid. T he reflex blink was elicited electrically by supraorbital nerve stimulation either on the right or the left side. It is found that disconjugate obliqu e eye movements accompany spontaneous, voluntary as well as reflex blinking . Depending on the gaze position before blinking, the amplitude of horizont al and vertical components of the eye movement during blinking varies in a systematic way. With adduction and downward gaze the amplitude is minimal. With abduction the horizontal amplitude increases, whereas with upward gaze the vertical amplitude increases. Unilateral electrical supraorbital nerve stimulation at low currents elicits eye movements with a bilateral late co mponent. At stimulus intensities approximately two to three times above the threshold, the early ipsilateral blink reflex response (R-1) in the OO mus cle can be observed together with an early ipsilateral eye movement compone nt at a latency of similar to 15 ms. In addition, during the electrical bli nk reflex, early ipsilateral and late bilateral components can also be iden tified in the upper eyelid movement. In contrast to the late bilateral comp onent of upper eyelid movement, the early ipsilateral component of upper ey elid movement appears to open the eye to a greater degree. This early ipsil ateral component of upper eyelid movement occurs more or less simultaneousl y with the early eye movement component. It is suggested that both early ip silateral movements following electrical stimulation do not have a central neural origin. Late components of the eye movements slightly precede the la te components of the eyelid movement. Synchrony between late components of eyelid movements and eye movements as well as similarity of oblique eye mov ement components in different types of blinking suggest the existence of a premotor neural structure acting as a generator that coordinates impulses t o different subnuclei of the oculomotor nucleus as well as the facial nerve nucleus during blinking independent from the ocular saccadic and/or vergen ce system. The profile and direction of the eye movement rotation during bl inking gives support to the idea that it may be secondary to eyeball retrac tion; an extra cocontraction of the inferior and superior rectus muscle wou ld be sufficient to explain both eye retraction and rotation in the horizon tal vertical and torsional planes.