The human eye suffers various optical aberrations that degrade the retinal image. These aberrations include defocus and astigmatism, as well as the higher order aberrations that also play an important role in our vision. The optics of the eye are not static, but are continuously fluctuating. The work reported in this thesis has studied the nature of the microfluctuations of the wavefront aberrations of the eye and has investigated factors that influence the microfluctuations. The fluctuations in the ocular surface of the eye were investigated using high speed videokeratoscopy which measures the dynamics of the ocular surface topography. Ocular surface height difference maps were computed to illustrate the changes in the tear film in the inter-blink interval. The videokeratoscopy data was used to derive the ocular surface wavefront aberrations up to the 4th radial order of the Zernike polynomial expension. We examined the ocular surface dynamics and temporal changes in the ocular surface wavefront aberrations in the inter-blink interval. During the first 0.5 sec following a blink, the tear thickness at the upper edge of the topography map appeared to thicken by about 2 microns. The influence of pulse and instantaneous pulse rate on the microfluctuations in the corneal wavefront aberrations was also investigated. The fluctuations in ocular surface wavefront aberrations were found to be uncorrelated with the pulse and instantaneous heart rates. In the clinical measurement of the ocular surface topography using videokeratoscopy, capturing images 2 to 3 seconds after a blink will result in more consistent results. To investigate fluctuations in the wavefront aberrations of the eye and their relation to pulse and respiration frequencies we used a wavefront sensor to measure the dynamics of the aberrations up to the Zernike polynomial 4th radial order. Simultaneously, the subject's pulse rate was measured, from which the instantaneous heart rate was derived. An auto-regressive process was used to derive the power spectra of the Zernike aberration signals, as well as pulse and instantaneous heart rate signals. Linear regression analysis was performed between the frequency components of Zernike aberrations and the pulse and instantaneous heart rate frequencies. Cross spectrum density and coherence analyses were also applied to investigate the relation between fluctuations of wavefront aberrations and pulse and instantaneous heart rate. The correlations between fluctuations of individual Zernike aberrations were also determined. A frequency component of all Zernike aberrations up to the 4th radial order was found to be significantly correlated with the pulse frequency (all > 2R0.51, p<0.02), and a frequency component of 9 out of 12 Zernike aberrations was also significantly correlated with instantaneous heart rate frequency (all>2R0.46, p<0.05). The major correlations among Zernike aberrations occurred between second order and fourth order aberrations with the same angular frequencies. Higher order aberrations appear to be related to the cardiopulmonary system in a similar way to that reported for the accommodation signal and pupil fluctuations. A wavefront sensor and high speed videokeratoscopy were used to investigate the contribution of the ocular surface, the effect of stimulus vergence, and refractive error on the microfluctuations of the wavefront aberrations of the eye. The fluctuations of the Zernike wavefront aberrations were quantified by their variations around the mean and using power spectrum analysis. Integrated power was determined in two regions: 0.1 Hz ─ 0.7 Hz (low frequencies) and 0.8 Hz ─ 1.8 Hz (high frequencies). Changes in the ocular surface topography were measured using high speed videokeratoscopy and variations in the ocular wavefront aberrations were calculated. The microfluctuations of wavefront aberrations in the ocular surface were found to be small compared with the microfluctuations of the wavefront aberrations in the total eye. The variations in defocus while viewing a closer target at 2 D and 4 D stimulus vergence were found to be significantly greater than variations in defocus when viewing a far target. This increase in defocus fluctuations occurred in both the low and high frequency regions (all p<0.001) of the power spectra. The microfluctuations in astigmatism and most of the 3rd order and 4th order Zernike wavefront aberrations of the total eye were found to significantly increase with the magnitude of myopia. The experiments reported in this thesis have demonstrated the characteristics of the microfluctuations of the wavefront aberrations of the eye and have shown some of the factors that can influence the fluctuations. Major fluctuation frequencies of the eye's wavefront aberrations were shown to be significantly correlated with the pulse and instantaneous heart rate frequencies. Fluctuations in the ocular surface wavefront aberrations made a small contribution to those of the total eye. Changing stimulus vergence primarily affected the fluctuations of defocus in both low and high frequency components. Variations in astigmatism and most 3rd and 4th order aberrations were associated with refractive error magnitude. These findings will aid our fundamental understanding of the complex visual optics of the human eye and may allow the opportunity for better dynamic correction of the aberrations with adaptive optics.

Accommodative microfluctuations (AMFs) are temporal variations in the accommodative response. AMFs can be divided into two categories, the low frequency component (LFC) measuring below 0.6 Hz and the high frequency component (HFC) measuring between 1.0 and 2.3 Hz. This thesis examined occupational factors that may affect AMFs comprising of cognitive demand, visual fatigue and the type of digital display used. These effects were measured by assessing changes in the LFC, mid frequency component (MFC), HFC, and RMS accommodation. Analysis of chaos and the overall accommodative response (AR) was also used. Furthermore, an investigation into inter- and intra-participant variability was completed. The HFC was shown to increase in line with increasing cognitive demand. A significant effect for task duration was found at higher accommodative demands for the overall AR, LFC and MFC. There was a significant effect on the LFC and AR depending on the type of display used during near work. The HFC and MFC were found to be less repeatable in the same participant relative to LFC, RMS accommodation and AR measurements. AMFs appear to have an effect on occupational factors, however more research is required to assess how much of an effect they may have relative to other ocular factors that affect occupational settings.

Yes / Background: Soft contact lenses have been developed and licensed for reducing myopia progression. These lenses have different designs, such as extended depth of focus (EDOF) and dual focus (DF). In this prospective, doublemasked, cross-over study, different lens designs were investigated to see whether these had impact on accommodative microfluctuations and eye movements during reading. Methods: Participants were fitted with three lenses in a randomised order; a single vision (SV) design (Omafilcon A2; Proclear), a DF design (Omafilcon A2; MiSight), and an EDOF lens design (Etafilcon A; NaturalVue),. Accommodative microfluctuations were measured at 25 cm for at least 60s in each lens, using a Shin-Nippon SRW-5000 autorefractor adapted to continuously record accommodation at 22Hz. Eye movement data was collected with the Thomson Clinical Eye Tracker incorporating a Tobii Eye bar. Eye movements include fixations per row, fixations per minute, mean regressions per row, total number of regressions, and total rightward saccades. Accommodation data was analysed using power spectrum analysis. Differences between the lenses were compared using a related sample two-way Friedman test. Results: Twenty-three participants (18–29 years) were recruited to take part. The average mean spherical error was − 2.65D ± 1.42DS, with an average age of 23.4 ± 3.5 years. No significant difference for accommodative microfluctuations was found. Significant differences were found for fixations per row (P = 0.03), fixations per minute (P = 0.008), mean regressions per row (P = 0.002), and total number of regressions (P = 0.002), but not total rightward saccades (P = 0.10). Post-hoc analysis indicated the EDOF lens results were significantly different from the other lenses, with more regressive eye movements observed. Conclusions: Regressive saccades appear to increase when wearing EDOF lens designs, which may impact visual comfort. Further studies in children, over a longer period of adaptation are necessary to assess the potential impact of this finding on daily reading activities in children. / This project was supported by the British Contact Lens Association Summer Studentship Award (recipient Muskaan Hussain).