Bombay Hospital Journal Issue SpecialContentsHomeArchivesSearchBooksFeedback

PROSPECTS FOR PERFECT VISION

Anand A Shroff
Hon. Ophthalmic Surgeon, Bombay Hospital.

The theoretical concept that perfect vision may be within our grasp is a 21st century quantum leap forward from the foundations of ophthalmic practice. The assumption that the cause of poor vision is the poor optical quality of the retinal image forms the basis for refractive procedures aimed at achieving perfect vision. The elimination of almost all optical errors from the optical system of the eye through wavefront analysis has been found to enhance the visual acuity to beyond conventional ‘normal vision’. Will it be possible to enhance the visual acuity of patients already enjoying 6/6 vision for the purpose of occupational needs?

Uncorrected refractive errors cause the retinal image to become blurred, reducing visual performance by decreasing contrast in the retinal image. The elimination of a sphero-cylindrical blur (lower order optical errors) by using spectacles, contact lenses or refractive surgery restores what is traditionally called ‘normal vision’. The goal, however, is to eliminate all traces of optical blur due to higher order optical errors (aberrations) of the eye which clinicians referred to as ‘irregular astigmatism’. The possible optical limitation to achieving an optically perfect eye will be the unavoidable effects of diffraction (according to the wave theory of light, limitation of the aperture by the pupil causes a spread of light even in a fully focused system), which can be limited.

Various issues to be considered are:

• Is a diffraction-limited ‘perfect’ image quality really possible?

• What would be the potential benefits of perfect retinal images if they can be obtained?

• Is there a down side to having perfect retinal images?

Limits imposed by the retina

Unlike the film plane of a camera, the ‘grain’ of the human retina is not uniform. The central part of the retina, the foveola, has the highest density of cones. This provides the normal fixating eye with its highest spatial resolving ability. As a consequence, the optical quality of the retinal image needs to be optimized over the small area of the foveola.

Visual acuity is limited by photoreceptor diameter, packing and biological variation to between 6/3 and 6/2.4. Will a person used to a best corrected vision of 6/6 see 6/2.4 if the retinal image is optimized later in life? Refractive amblyopia (reduced visual development in an uncorrected refractive state - in this case higher order optical errors of the optical system) generally improves by one or more lines of visual acuity and can improve more over time. However, visual performance in the real world is further complicated by factors that include past visual experience, cognitive ability, expectation and information content. These factors increase visual acuity and may help decipher standard Snellen letters beyond 6/2.4.

Limits imposed by the optics of the eye

Improving the optics of the eye by removing aberrations increases the contrast and spatial detail of the retinal image. These effects are pupil-dependent. The larger the pupil in a diffraction-limited system, the higher the contrast and the crisper the edges of the retinal image. As the viewing distance changes so does the ideal compensating optic. No single compensating optic can optimally correct the eye for all viewing distances. Additionally, chromatic aberrations can reduce the eye’s resolution limits compared to a monochromatic system (Chromatic aberrations of the eye prevent the simultaneous focusing of all visible wavelengths. This may result in poor contrast).

The most significant question is - can an ideal compensating optic based on wavefront measurements provide better vision than sphero-cylindrical corrections alone?

If a patient is best corrected with sphero-cylindrical lenses to 6/4-6/3, reducing the higher order aberrations will most likely not improve high contrast acuity but will increase perceived contrast. If on the other hand, a patient’s best-correction with sphero-cylindrical lenses is 6/6-6/9 due to optical aberrations in an otherwise normal eye, reducing the higher order aberrations will improve visual performance (visual acuity and other measurements of spatial vision). In all cases, eliminating the sphero-cylindrical error of the eye and reducing the higher order aberrations will increase image contrast. Objects in the visual world have higher contrast and crisper borders. Most patients would prefer the gain in contrast and sharper borders but some prefer the softer view of the world offered by lower contrast and blurred edges.

Potential benefits of perfect retinal images

Vision generally fluctuates for any visual task dependent on the amount of contrast in the retinal image and by the distortions of spatial phase induced by optical aberrations. Effects such as defocus occur on the spatial phase of retinal images. Defocus introduces an effect called "spurious resolution" - which is a phase-reversal of images so that dark regions become light and light regions become dark. [1] Correcting ocular aberrations significantly minimizes such effects and improves spatial vision. Therefore correcting aberrations yields improvements in visual performance greater than that on the basis of contrast increases alone.

Potential penalties of perfect retinal images

Aberration correction by wavefront guided treatments may succeed in increasing the optical bandwidth beyond the optical resolution limit of the fovea. Such vision may become unreliable because neural undersampling misrepresents fine spatial details as coarse details, a phenomenon known as ‘aliasing’. [2] Aliasing produces a kind of misperception in which objects appear to have a different spatial scale, orientation, form or direction of motion compared to the physical stimulus.

Many industries depend upon the eye’s imperfections to make their products look good. For example, television and computer monitors use rasters, which produce pixel patterns that are about equal to the resolution limit of the eye for viewing at arm’s length. Similarly, the printing industry uses half-tone images which may become objectionable if the individual dots of the image are clearly visible to a person with ‘supernormal’ vision.

Possible public health and safety issues associated with perfect retinal images may need consideration. Current safety standards for exposure to light hazards and lasers are based on the assumption that eyes are aberrated. Therefore light from a bright point source is spread out across the retina, which helps dissipate the damaging heat. If we improve image quality significantly, we may be putting the retina at risk for accidental exposure to bright point sources of light.

We should also consider costs and benefits to the national and global communities of refractive surgery aimed at reducing the eye’s higher-order aberrations. Would striving for marginal improvement in the vision of normal individuals who already have good vision be more important than devoting our energies and resources to treating eye diseases and other visual disabilities, especially in the developing world?

CONCLUSION

A better understanding of individual biomechanical responses is needed to minimize unpredictable adverse physiological responses. Wavefront-guided refractive treatments have revolutionized the way we see the world around us. Researchers are further refining the procedure and addressing the various issues arising out of perfect vision.

REFERENCES

1. Campbell FW, Green DG. Optical and retinal factors affecting visual resolution. J Physiol 1965; 181 : 576-93.

2. Williams DR. Aliasing in human foveal vision. Vision Res 1985; 25 : 195-205.

3. Adler’s Physiology of the Eye. Ninth ed. Mosby.


To Section TOC
Sponsor-Dr.Reddy's Lab