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Signals for accommodative responses in humans

Final Report Summary - SACCO (Signals for accommodative responses in humans)

The present project aimed to identify the optical cues that drive and control the accommodative system of the human eye. Considering the human accommodation as a kind of short-term defocus adaptation, the signals involved in this process could be also influencing the long-term defocus adaptation, known as emmetropization process, that occurs during the first years of life. Then, the purpose of the project was to evaluate whether human accommodation responds to the wavefront characteristics of light under different circumstances without the feedback from the change of several optical aberrations. To determine whether accommodation responds really to wavefront aberration per se, or simply to the shape or the skewing of blur, aberrations were selectively removed while simulation of defocus targets were imaged on the retina, with and without the effects of lower- and higher-order aberrations on the blur. The results, confirmed by four different major experiments, positively indicate that, under monochromatic light and monocular conditions, the eye must have a retinal mechanism that identifies the sign of defocus. This retinal mechanism, used for short-term defocus adaptation, could perhaps be also used during long-term defocus adaptation (emmetropization process). Thus, as accommodation no longer responds to blur from defocus, astigmatism, and higher-order aberrations when optical vergence is removed, the results obtained in this project allow to hypothesize that, under the tested conditions, there should be such a retinal mechanism able to provide a directional cue for accommodation from the information given by the optical vergence. Therefore, optical vergence seems to be an important optical cue that could be used by the accommodative system of the eye, allowing for faster, accurate and reliable responses than trial and error mechanisms. Arguing that the eye under the defined conditions uses the same mechanism during eye axial growth and emmetropization, the findings obtained in this project could be potentially useful for the design and development of new optical solutions able to influence and control the ametropia development.

Another important achievement attained in the project was the design and development of “an artificial eye”, which has the property to change its accommodation dynamically, miming thus the human eye behavior. Based on an opto-mechanical design, it includes two new features: a nearly punctual light source in the center of the artificial retina and a variable optical power lens to collimate the light beam or change to focus it at different vergences from the eye exit pupil. The artificial eye built inside the project can be used to reproduce any refractive or accommodative state as it does the human eye, including also simple dynamic accommodative responses, with, e.g. smooth continuous changes or fast steep transitions between two well-defined accommodative states. Other potential optical features can be implemented with this type of optical configuration. For instance, the wavelength of the light source used in the artificial eye can be changed in order to obtain the optical dispersion data of an optical element to be tested. Besides, the ability to change electronically and very fast the power of the lens can be used to study and analyze the effect of potential multifocal solutions for ametropia correction. In addition, as the artificial eye increases its optical power, the fourth-order spherical aberration decreases coincidentally in a similar fashion as it does for the human eye during accommodation. The reason of this concordance may be due to the fact that the artificial eye is using a similar procedure to increase its power to the one present in the human eye, and in both cases the surfaces of the variable lens had a negative asphericity. However, the rate of decrease of this aberration per diopter of refractive change in the artificial eye was lower than the one found in the human eye. To our knowledge, the developed artificial eye is the first accommodative design that mimics the natural behavior of the eye from the accommodative point of view.