Those Who Want to See Must Close Their Eyes

Medical Facts

Anatomy of the eyelid

The eyelid, or palpebra (lat.), is a thin, muscular skin-fold that covers the eye. Humans have two eyelids, palpebra superior ('upper eyelid') and palpebra inferior ('lower eyelid') 4. The lid serves many purposes - the protection of the eyeball, its humidification and the coverage of the pupil. It is possible to perceive light through the eyelids, as they partially let light pass into the eye.

Some species (for instance camels, polar bears and crocodiles) have an additional nictating membrane, a third, transparent eyelid for further protection. The plica semilunaris (a small fold of skin in the corner of the eye) is suggested to be the evolutionary legacy of this third eyelid in human anatomy.

Eyelid movement

Movement of the upper eyelid is achieved through contraction and relaxation of levator palpebrae, Mueller's muscle and orbicularis oculi, the three muscles that are connected to the lid. Additionally, lid-closure is caused through a passive force, resulting from the construction of the apparatus. It has also been shown that the basic eyelid movements, blinks, and saccadic lid movements, can uniquely be determined solely from their movement patterns 5.

Three different types of blink can be differentiated: as a reflex, as a voluntary movement or spontaneous, as for the moisturizing of the eyeball. Evinger, Manning and Silbony 5 found differences in the movement patterns of each of these.

	Source	Down Phase	Up Phase
Spontaneous Blink	VanderWerf et al. 6	92 ± 17	242 ± 55
	Guitton et al. 7	85	200
	Sun et al. 8	85	198
Voluntary Blink	VanderWerf et al. 6	88 ± 13	187 ± 34
	Sun et al. 8	92	221
Reflex Blink	VanderWerf et al. 6	53 ± 6	152 ± 6

The blink reflex (or corneal reflex) is a widely used measure in the assessment of learning capability, more specifically in experiments that use Pavlovian conditioning 9 10. In these experiments, a puff of air is blown into the eye of the participant; who immediately blinks. In the test group, a sound is played to the participants a certain, constant time in advance; in the control group, the sound is played without timely relation to the air puff. It has been demonstrated that participants of the test groups learned that the sound preceded the air puff, and later the sound alone induced the blink reflex 9.

The movements of eyeball and eyelid are compound: an upward movement of the eyeball causes the eyelid to lift, and a downward glance involves lowering of the eyelid. This lowering results from a passive force, opposed to blinking, which is always performed actively (by muscle contraction). During a blink, the eyeball slightly rotates, leaving and returning to its original angle afterwards 5 11.

Squinting the eyes

The expression 'to squint' in the eye-context is ambiguous. Either it can mean looking 'cross-eyed' or looking with the eyes partly closed 12. In the following (and the remainder of this thesis), the latter meaning is intended whenever 'squinting' is mentioned.

When we squint at something, we perceive only a smaller field of vision, and as less light enters the eye, the perception is darker. However, squinting is a state in which things can be seen sharper, as the hole through which light enters the eye is made even smaller than the pupil: in principle, a smaller hole in a 'camera obscura' causes a sharper image. This is useful, for example, when we want to guide a yarn string through the eye of a needle or see something far away.

Ergonomics

As the average blink rate lowers during screen work, it is often recommended to take small breaks at the workplace 13 14.

In a similar vein to left- or right-handedness, every human has a dominant eye 15. It can be determined by looking at a stretched out finger and closing one eye after the other: the dominant eye will show an image more similar to the binocular image. If this eye is too dominant, the remaining eye will no longer be included in visual perception - this is regarded as the 'lazy eye syndrome'. This prevalent illness causes 'more visual loss in the under 40 group than all the injuries and diseases combined in this age group [does]'. 16

Lastly, it has been demonstrated 17 that closing the eyes induces alpha waves. Our brain potentials resemble this pattern especially when we are awake and relaxed, as during the onset of sleep 18 19.