This thesis describes results of experimental studies designed to investigate some functional aspects of visual performance in the mesopic range. The principal part of the study involved extensive measurements of contrast-acu- ity thresholds in a large number of subjects, age range: 17 to 61 years. The tests were carried out at high mesopic levels of light adaptation and provided the basis for establishing the statistical limits for normal subjects. Further studies were carried out to attempt to explain the differences between pho- topic and mesopic conditions and the increased intersubject variability mea- sured at mesopic light levels. These studies examined the extent to whic higher-order optical aberrations at larger pupil sizes, macular pigment opti- cal densities and susceptibility to increased crowding in the mesopic range are potential sources of the variability. Other aspects of visual performance have also been examined, such as sensitivity to red-green and blue-yellow colour differences measured at mesopic light levels of retinal adaptation and the effects that selective absorption by the lens and macular pigment may have on chromatic sensitivity. Results revealed much increased variability in contrast acuity thresholds-at mesopic light levels when compared to the results measured in the photopic range. Around 50% of the subjects achieved mesopic contrast acuity thresh- olds comparable to those measured for the whole population in the photopic range. 11.2% and 12.6% of the variance in contrast thresholds at the fovea and parafovea were due to increased higher-order aberrations. Age made a sig- nificant contribution accounting for 18% of the measured variability. Neither peak macular pigment optical density nor average macular pigment optical density (over the centre ±2.8°) were found to have a significant difference on contrast acuity thresholds when comparing the extremes of the distribution. Susceptibility to crowding was also not significant suggesting that the effects of visual crowding do not increase significantly when contrast acuity is mea- sured in the mesopic range. Colour thresholds showed no significant corre- lation with either lens absorption or selective absorption of short wavelength light by the macular pigment.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:554384 |
| Date | January 2011 |
| Creators | Walker, Grace |
| Publisher | City University London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
Page generated in 0.015 seconds