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The human eye is a remarkable optical device. In less than a second, a young human eye can accommodate from infinity to closer than 10 cm. Accommodation occurs with minimal effort and can be rapidly repeated with no apparent evidence of fatigue. Unfortunately, maximum accommodation decreases throughout life and by the fifth decade leads to presbyopia, the inability to read at a normal working distance. Interestingly, the mechanism by which the human eye is able to adjust focus has been debated for over 300 years. No previous theory has been put forth that can account for all the physical chang.
This conference was instigated by a combination of factors: The nature of the problem, the wide spread occupational epidemiology reported on eye symptoms and eye fatigue in the workplace, and the organizers' awareness of the complexity of the scientific and clinical bases of knowledge that might be usefully applied. The introduction of new methods into system neurobiology provides new insights into how we receive and process information from the external world, and act upon it. New, non-invasive methods have opened the way to direct observation of the human brain in action. Due particularly to the interaction between the visual and oculomotor requirements involved, several clinical and scientific fields intersect when these issues are considered. To provide clear vision the accommodative and pupillary mechanisms are used. To maintain binocularity, the ver gence oculomotor system, sensitive to fatigue, must attain congruence with accommodative levels. This accommodation-vergence linkage was a focus of our symposium.
Examining established and emerging treatments for the correction of hyperopia and presbyopia, this reference offers guidance on technologies such as thermal or conductive keratoplasty, corneal implants, laser scleral relaxation, scleral expansion rings, intraocular lenses, and LASIK modifications.
Covering all major components of the ocular system, this state-of-the-art text is essential for vision scientists, biomedical engineers, and advanced clinicians with an interest in the role of mechanics in ocular function, disease, therapeutics, and surgery. With every chapter, leading experts strengthen the arguments that biomechanics is an indispensable and rapidly evolving tool for understanding and managing ocular disease.
Computational models of human accommodation hold the promise of an improved under- standing of the mechanism and of the development of presbyopia. A detailed and reliable model could greatly assist the design of treatments to restore accommodation to presbyopic eyes. However, a large quantity of data is required for such an endeavour. Currently, the details of the age-related increase in the stiffness of the lens is a major source of uncertainty as the published data differ markedly depending on the form of testing employed. A new version of the spinning lens test is presented, based on the method originated by Fisher, R. F. (1971) 'The elastic constants of the human lens', Journal of Physiology, 212(1):147-180. This test assesses the stiffness of the lens substance by photographically measuring the deformations induced by rotation of the lens about its axis of symmetry. The principal changes introduced in the present version are the removal of the capsule from the lens prior to testing, the synchronization of the photography with the orientation of the lens, and the use of a hyperelastic finite-element model of the test coupled with a numerical op- timization procedure to quantify the heterogeneous stiffness of the lens. These alterations, together with further improvements, provide a substantially more accurate means of measur- ing the stiffness of the lens 'substance'. Measurements made with the new test on a series of human lenses are reported. Good- quality tests were obtained for 29 lenses aged from 12 to 58 years. The older lenses were found to be much stiffer than younger lenses. In younger lenses the cortex of the lens is found to be stiffer than the nucleus, but the nucleus stiffens more rapidly, surpassing the cortex by about 44 years. These results differ substantially from those of the original spinning test. The stiffness values calculated for the lens substance are used in a series of hyperelastic finite-element models of the accommodation mechanism. Models corresponding to subjects aged 29 and 45 years follow clinical measurements of the decline in accommodation am- plitude between these ages. Adjusting the material parameters values indicates that it is the increase in stiffness which is largely responsible for the modelled fall in accommodation am- plitude. The 45-year model is adapted to represent the effect of laser lentotomy, a proposed presbyopia treatment. Among the lentotomy options trialled, the best result is a modest O.4D increase in the modelled accommodation amplitude.
Some of the best vision scientists in the world in their respective fields have contributed to chapters in this book. They have expertise in a wide variety of fields, including bioengineering, basic and clinical visual science, medicine, neurophysiology, optometry, and psychology. Their combined efforts have resulted in a high quality book that covers modeling and quantitative analysis of optical, neurosensory, oculomotor, perceptual and clinical systems. It includes only those techniques and models that have such fundamentally strong physiological, control system, and perceptual bases that they will serve as foundations for models and analysis techniques in the future. The book is aimed first towards seniors and beginning graduate students in biomedical engineering, neurophysiology, optometry, and psychology, who will gain a broad understanding of quantitative analysis of the visual system. In addition, it has sufficient depth in each area to be useful as an updated reference and tutorial for graduate and post-doctoral students, as well as general vision scientists.