4
Short- and Long-term Consequences

4.1. Short-term effects

4.1.1. Decreasing ease of accommodation

Ease of accommodation is the speed at which the curve of our crystalline lens changes in moving from a nearby object to a distant object or vice versa (Figure 4.1). The more tired the visual system, the more time it will take to accommodate to a point [LER 09]. This is normally imperceptible, but it can have serious consequences. Imagine that we are leaving a film and we go directly to our car. Our eyes will shift back and forth between the nearby dashboard and the distant road. If our visual system does not respond quickly enough, it will take us longer to notice cars, which might be very dangerous.

4.1.2. Decrease in stereoscopic acuity

Stereoscopic acuity is the accuracy with which we can perceive the difference in depth between two objects. It is expressed as an angle (Figure 4.2).

This acuity decreases sharply when the visual system is subject to the constraints which we have mentioned [LER 09]. This implies that when we leave a 3D cinema, our depth perception is lessened.

This reduction in perception will be much more pronounced for those whose stereoscopic acuity was already weak to begin with. In fact, such people put greater demands on their visual system in viewing a film, which causes more fatigue. In contrast, those with very good depth perception will become less tired and will maintain better stereoscopic acuity after the film.

Figure 4.1. Ease of accommodation. For a color version of this figure, see www.iste.co.uk/leroy/stereoscopy.zip

Figure 4.2. Stereoscopic acuity is the minimum amount of depth perceived by a person, measured as an angle

4.1.3. Effects on the punctum proximum

The punctum proximum is the shortest distance between our nose and our book, which still allows for clear vision. To measure this, we can gradually bring the book closer to our nose. We can see that past a certain distance, the words will become blurred. This distance is our punctum proximum, the lower limit of the amplitude of accommodation (Figure 4.3). The younger we are, the shorter this distance is. If we have problems with presbyopia, this distance increases greatly and we will probably wear glasses adapted to reduce it to a more convenient distance for everyday life.

Figure 4.3. Punctum proximum

This distance increases after immersion in artificial stereoscopic vision [LER 09]. Simply put, this means that we will have more difficulty reading text close-up after seeing a 3D film.

4.1.4. More subjective effects

Many more subjective effects have been reported. At the level of the eyes, there may be feelings of heaviness, tingling, burning, watering, redness of the eyeballs or of the eyelids. Eyelid tremors or temporary myopia may occur. Disordered, blurred or double vision can also occur.

More generally, there may be dizziness, headache and neckache. Generally, these symptoms disappear after a good nights’ sleep. However, it may sometimes occur that visual fatigue accumulates over several days [ALB 02].

4.2. Long-term consequences

At present, we do not yet have enough hindsight to know what would be the long-term impact of such constraints on the visual system. Some studies have shown that among healthy adults, the visual system eventually gets used to it. People confronted with very tiring stereoscopic images find them progressively less tiring [NEV 12]. However, we do not yet know if the fact that the visual system gets used to these images means that it gets less used to normal images. In other words, through watching 3D films, it might be conceivable that we perceive the real world less well. All this is still being studied. It is possible that our brain eventually creates two “stable states”, so that the brain may become able to switch from an “artificial” to a “normal” configuration. But this is still only a hypothesis, and it would be ethically very tricky to immerse people into a stereoscopic world in the hope of being able to study the adaptation of their visual system over the long term.

The ocular muscles are linked to the muscles of the spinal column. It is thus very possible that, in forcing our visual system to see images in artificial depth, we are also putting strain on our spinal muscles and this may produce back pain [TIG 14].

4.2.1. Long-term effects on children

The case of children is even more tricky. In effect, the visual system builds its depth perception between 6 and 12 years, and if we disturb the control loops of the visual system at the moment of their formation, this can have unforeseen consequences. From a strictly preventative point of view, it is probably a good thing that 3D televisions are not more widely sold. Indeed, if television channels had developed 3D cartoons to be watched by children for 3 hours a day and 1 meter from the screen, this could have been the source of a good deal of unsuspected damage.