Visual performance in the mesopic range
The aims of this work were to assess the effects of different stimulus parameters on chromatic sensitivity, with particular emphasis on the effect of retinal illumination, and to investigate aspects of suprathreshold visual performance under mesopic conditions. All investigations were performed using visual psychophysical techniques. Chromatic thresholds were obtained using dynamic luminance contrast noise to isolate responses to colour signals. The effects of stimulus size and spatial distribution were examined in normal trichromats, dichromats and subjects with acquired colour vision deficiency. The chromatic sensitivity of normal trichromats was investigated with reduction in light level. Measurements were also performed to assess the possible involvement of rods in chromatic processing at threshold. Suprathreshold performance in the mesopic range was assessed in terms of the relative contributions of colour and luminance contrast to a measure of stimulus conspicuity and to visual search time. The conspicuity of a stimulus defined by colour and luminance contrast was defined as the value of achromatic contrast of a similar stimulus with an equal perceived conspicuity. An empirical model was developed from an extensive data set of conspicuity matches, to enable prediction of conspicuity for a wide range of coloured stimuli. Visual search performance for both achromatic and coloured stimuli was investigated under mesopic conditions, and results for the coloured stimuli were compared to the measure of stimulus conspiculty combined with an achromatic search time calibration. The results revealed that chromatic sensitivity is dependent on stimulus size, spatial distribution, eccentricity of presentation and level of illumination. These factors are suggested to reflect changes in cone performance and the relative cone contributions to the postreceptoral chromatic channels. Chromatic sensitivity was found to be independent of rod activity in the mesopic range, suggesting separate processing of rod signals and threshold colour signals under mesopic conditions. Measurements of stimulus conspicuity under mesopic conditions revealed individual variations in response to both luminance contrast and chromatic signals indicative of individual differences in gain control of postreceptoral mechanisms. Conspicuity was successfully modelled as a function of photopic contrast, scotopic contrast, chromatic difference to the background and the level of illumination. The nonlinear relationship between search time and luminance contrast was found to change with reduction in light level, reflecting increased contrast thresholds and diminishing effectiveness of unit physical contrast. Mesopic visual search was also found to depend on the photopic contrast, scotopic contrast and chromatic content of the stimulus, but with an apparent greater emphasis on scotopic contrast and reduced emphasis on colour compared to the measure of stimulus conspicuity. Conspicuity was successfully used to predict visual search times, and was found to be an improved indicator of search performance than either photopic or scotopic contrast.