Spatial and temporal aspects of visual performance in relation to light level and normal aging

The research contained in this thesis describes three studies designed to investigate the ability of the observer to detect stimuli defined by changes in luminance in space and/or time in mesopic conditions, including contrast sensitivity, temporal flicker sensitivity and visual acuity. The first two studies determined the effect of the aging of the retina on spatial and temporal contrast sensitivity at photopic and mesopic light levels. The literature states that older people experience losses of retinal neurons including rods, cones and ganglion cells. Furthermore, older people tend to have particular difficulties with vision at low light levels which can be attributed to greater loss of rods than cones, particularly at parafoveal eccentricities. Spatial and temporal contrast sensitivity was measured separately in two groups of participants, aged 20-73 (n=74) and 20-74 (n=80) years of age, respectively. Measures were taken to ensure that thresholds largely reflected age-related changes to the retina rather than the optics of the eye. Spectral content of the stimuli was restricted to the middle and long wavelength regions of the visual spectrum and the pupil was measured continuously so as to obtain participant-specific retinal illuminances for each condition. The HRindex was derived and calculated for each participant as a single number which summarized performance from photopic to mesopic light levels. As age increased both spatial and temporal contrast vision worsened and older participants showed particularly elevated thresholds at lower light levels when compared to younger participants. Spatial contrast thresholds show a steady linear decline with age, whereas temporal modulation thresholds were relatively stable up to 50 years of age and then demonstrated a rapid decline. These different trends of changes in performance with increasing age suggests that contrast and temporal HRindex may be measuring the aging of different retinal mechanisms. The normal limits of HRindex values were calculated which could be used in the future to detect abnormal performance. A secondary aim of the first two studies was to determine if binocular summation of spatial and temporal contrast thresholds declined with age, while accounting for differences in retinal illuminance between monocular and binocular conditions. For spatial contrast vision, binocular summation declined significantly with age and 18% showed binocular inhibition. However, the binocular summation of flicker signals did not change significantly with age and only 1% of participants showed binocular inhibition. Interocular differences cannot explain our results. The third study determined whether altering the scotopic/photopic luminous efficiency ratio could improve spatial acuity at mesopic light levels. This was achieved by altering the spectral power distribution of illuminating lights to increase the contribution of rods to vision at constant levels of photopic illumination. It was found that visual acuity at the fovea was improved by low levels of increased scotopic luminance, but peripheral acuity was improved by larger increases of scotopic luminance. The three studies demonstrate that the detection of luminance defined stimuli can be compromised in a number of external conditions such as low light levels, as well as due to internal changes caused by aging to the optics of the eye, retina and/or the central visual system.