Analysis of new methodologies for the measurement of optometric parameters in natural vision conditions

Resumen

Summary Objectives The main goal of this thesis is the development and testing of new tests for the measurement of optometric parameters. These tests will be based in existing paradigms and procedures but introducing new approaches in the methodology combining objective and subjective procedures. Natural vision refers to binocular viewing without any artificial restriction or asymmetry. As a second goal, we have decided to limit the cost of the developed tests and the necessary equipment to make it suitable for the clinical context. To achieve these goals four experiments with their own methodologies and results are described in chapters 2 to 5. In the first experiment the fixation stability under binocular viewing is analysed. The second experiment is centred in the implementation of a low-cost technique to increase the luminance resolution of common displays. The third experiment extents the classical paradigm for measuring the contrast sensitivity to the peripheral vision under binocular viewing. The last experiment focuses on the pupil size fluctuations and how changes of illumination, the position of the stimulus and the binocularity affect the pupil. Experiment 1. Measurement of the fixation disparity under natural viewing conditions The fixational eye movements prevent the eye from remaining still, this implies that the visual axes do not cross on the point of interest. The stereopsis is allowed by the existence of the Panum’s areas. The angular difference between the actual angle formed by the visual axes and the required angle to fixate the point of interest is known as fixation disparity (FD). The clinical tests for measuring the FD are based in subjective methods and do not measure the FD in its original definition. Methods Using the Web Cam Eye-Tracker (Annex II) the position of the visual axes of both eyes were determined in each frame of a video sequence. Two stimuli were designed, a black dot over a white background and a white dot over a black background. The task for the subject was to stare the fixation point for 45 seconds. Fifteen subjects participated. Results For the black dot the median FD was -4.68 minutes of arc, for the white dot the median FD was -7.22 minutes of arc. Regards the evolution of the FD over time different trends were found: positive, negative trends of all the measurement and convergent, divergent trends of the maxima and minima values. The differences in the results depending on the polarity of the stimulus were not statistical significant. Experiment 2. Measurement of the suprathreshold sensitivity with common displays The contrast sensitivity function measures the capabilities of the system at the threshold level, but in the real world we are dealing with suprathreshold contrasts most of the time. This task can be assessed by means of the contrast discrimination function. Methods A two alternative forced choice (2AFC) test was developed implementing a bit-stealing technique (Annex I) in order to increase the luminance resolution of the display. Three pedestal contrasts were tested: 0.3, 0.5 and 0.7 for the following spatial frequencies: 2, 4, 8 and 16 cycles per degree. A modified staircase method was implemented, the test finishes after five reversals and the sensitivity is calculated by averaging the four last reversals. 52 subjects were recruited. Results The incremental contrast discrimination thresholds increase between the pedestal contrasts 0.3 and 0.5 but decrease between 0.5 and 0.7. The threshold also increases for increasing spatial frequencies. When adjusting the results to the power law model defined by Legge the values obtained are in agreement with those reported in the literature when excluding the thresholds measured for the pedestal contrast of 0.7. Experiment 3. Peripheral contrast sensitivity under binocular natural viewing conditions The CSF has been deeply studied for the central vision. But central vision is not enough to achieve a high degree of satisfaction with the vision or for common tasks such as walking and avoiding obstacles. Previous studies about the peripheral sensitivity were not done under binocular natural conditions. Methods To test three different areas of the retina (foveal, perifoveal and the near periphery) three different stimuli were designed, a Gabbor patch for the fovea, and two rings for the other regions. The area of the three stimuli was exactly the same. Two versions of the stimuli were generated, with and without a fixation target. The task for the subject, 4 in total, was to press a key when the stimulus was detected. Results The sensitivity measured when the fixation cross was present was lower for the low frequencies in comparison with the stimuli without the cross. The sensitivity diminishes when increasing eccentricity, changing the shape of the curve from a band-pass shape in central vision to a low-pass shape. Experiment 4. Objective measurement of the pupil size variation over time under different viewing conditions The pupil regulates the amount of light reaching the retina. The muscles that modify its diameter are never still. Many different factors can affect the size of the pupil. In this experiment we want to test the influence of the illumination, the stimulus proximity and the binocular conditions. Methods Eight combinations of illumination, stimulus proximity and binocularity were defined. The pupil size of the right eye was measured during 45 seconds at a framerate of 250 Hz when staring at a fixation target. Seven young adults participated in this study. Results All the measurements did not follow a normal distribution. When comparing with the Kruskal Wallis test all the conditions were statistical different except from the photopic monocular far and near conditions of subject 5. The mean effect of the change in illumination was 2.50 ± 0.20 mm, the proximity of the stimulus generated a mean change of 0.34 ± 0.15 mm and the variation in the binocularity produces a change of 0.71 ± 0.28 mm. When adapting a disposable contact lens to match the vergence of the near stimulus the mean change due to the position of the stimulus was -0.07 ± 0.12 mm. When reducing the sampling rate from 250 to 25 and 5 Hz no statistical significant differences were found. When dividing the 45 second measurement into 2 second measurements the differences were statistically significant. General conclusions - The objective measurement of the FD by means of the Web Cam Eye-Tracker is reliable. - The results provided by our methodology cannot be compared directly with other studies because we have not found any data about the temporal evolution of the FD or the influence of the polarity of the stimulus. - The use of bit-stealing for increasing the luminance resolution of common displays provides reliable results when measuring suprathreshold contrast discrimination and for the contrast sensitivity measurement for different retinal areas. - The power law for contrast discrimination formulated by Legge may be not accurate for pedestal contrasts higher than 0.5 and for high spatial frequencies. - When placing a fixation target in a contrast test the sensitivity for low frequencies diminishes. - The use of Gabbor patches for measuring the sensitivity for central vision is reliable for natural viewing conditions, the equivalent for peripheral areas are the ring patches. - Pupil size cannot be considered constant under constant conditions of illumination, stimulus proximity and binocularity - The variations of the pupil size do not follow a normal distribution - When the pupil size is measured under not natural conditions the values will differ from those obtained under natural vision - The best strategy for determining the pupil size is to measure it under natural viewing conditions and taking measurements for a reasonable period of time even at a low capture speed