Conflict? What conflict?

The dominant theory in the literature regarding the cause of the complaints people make about symptoms of ‘eyestrain’ or headache following the viewing of 3D stereoscopic images is that the so-called ‘accommodation convergence conflict’ is responsible. This explanation is not universally accepted, however, and the empirical evidence is not compelling. In Shibata et al. (2011), for example, statistically-significant changes are seen in the control conditions, and the size of any effect reported is small. An analysis of the stimulus leads one to question the basis of the theory, inasmuch as there is a difference in the stimulus provided by a 3D stereoscopic display and that provided by the real world, but it is realistic to suppose that symptoms will only arise as a consequence of the visual system’s response to a stimulus, and not because of the stimulus itself. The Optometric literature, stretching back decades, provides clear evidence that the visual system has a certain amount of ‘slack’, ‘slop’, or ‘play’ in it – for example, the amount of accommodation produced is usually only enough to provide a ‘good-enough’ retinal image, rather than the exact amount required to meet the optical demand. The presence of this depth of focus indicates that the response of the accommodation system can differ from the response of the vergence system when the stimulus to the two systems is the same, and it follows that a difference in the stimulus to the two systems will not necessarily cause problems. Although alternative explanations for the syptoms reported have been suggested (E.G. VIMS – see the abstract entitled “Visually Induced Motion Sickness”) we cannot dismiss out of hand the idea that the stimulus provided by a 3D stereoscopic display will produce symptoms – particularly if large disparities are present. However, an alternative explanation for the genesis of these symptoms is found by considering that both the accommodation and vergence systems have a number of inputs. To take the former as an example, when viewing a near target proximal accommodation and vergence accommodation will increase the optical power of the eye, and the ‘fine tuning’ will be provided by the blur accommodation system. When viewing a 3D stereoscopic stimulus, the amount of accommodation provided by the cross-links with the vergence system will change, and so if the total amount of accommodation remains fixed the proportion of this value provided by the vergence-accommodation system and by the blur-accommodation systems will alter. The same argument applies to the vergence system – the proportion of the vergence elicited which is provided by accommodative-convergence and by disparity-vergence will be different when viewing a 3D stereoscopic display than when viewing the real world, and these differences provide an alternative explanation to the ‘accommodation-convergence conflict’ theory for the genesis of the discomfort