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.

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Depth perception requires finding matching features between the two eye’s images to estimate binocular disparity. This process has been successfully modelled using local cross-correlation. The model is based on the known physiology of primary visual cortex (V1) and has explained many aspects of stereo vision including why spatial stereoresolution is low compared to the resolution for luminance patterns, suggesting that the limit on spatial stereoresolution is set in V1. We predicted that this model would perform better at detecting square-wave disparity gratings, consisting of regions of locally constant disparity, than sine-waves which are slanted almost everywhere. We confirmed this through computational modelling and performed psychophysical experiments to test whether human performance followed the predictions of the model. We found that humans perform equally well with both waveforms. This contradicted the model’s predictions raising the question of whether spatial stereoresolution may not be limited in V1 after all or whether changing the model to include more of the known physiology may make it consistent with human performance. We incorporated the known size-disparity correlation into the model, giving disparity detectors with larger preferred disparities larger correlation windows, and found that this modified model explained the new human results. This provides further evidence that spatial stereoresolution is limited in V1. Based on previous evidence that MT neurons respond well to transparent motion in different depth planes we predicted that the spatial resolution of joint motion/disparity perception would be limited by the significantly larger MT receptive field sizes and therefore be much lower than the resolution for pure disparity. We tested this using a new joint motion/disparity grating, designed to require the detection of conjunctions between motion and disparity. We found little difference between the resolutions for disparity and joint gratings, contradicting our predictions and suggesting that a different area than MT was used.

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The work presented in this thesis has investigated, both computationally and psychophysically, the chromatic conditions for the phenomenon of perceptual transparency. A series of physical measurements and numerical simulations have been carried out to show that the cone-excitation ratios between two opaque surfaces and the cone-excitation ratios between the same surfaces covered by a transparent filter are almost statistically invariant (see also Westland & Ripamonti, 2000) for many physically transparent systems. The relationship between physical transparency and perceptual transparency has been explored and strong psychophysical evidence has been presented to show that, when the invariance holds, the filter is perceived as transparent. Furthermore, the degree of invariance seems to be a good measure of the degree of perceived transparency. The more invariant the cone-excitation ratios are, the more the filter is perceived to be transparent. This shows that perceptual transparency is not a categorical perception but rather a continuously graded perception: the more the cone-excitation ratios approximate invariance the more the filtered area is perceived to be transparent. A set of further psychophysical experiments has been carried out to ascertain whether the invariance is required for all three classes of cones or whether some cone classes are more important than others. The data seem to support the notion that the S-cone class responses contribute relatively little to transparency perception. The role of image complexity on transparency perception was also explored. Specifically it has been shown that the strength of the transparency percept increases with the number of patches in a Mondrian display partially covered by a transparent filter. Finally, it is acknowledged that the chromatic conditions for transparency perception that are defined by the invariance model may well be alternatively expressed by other models or transparency or at other levels of visual processing. However, the invariance model makes explicit an intriguing link between transparency perception and colour constancy.

Recording eye movement behaviour can help us understand visual and cognitive processes, but eye movements also have a communicative role in social and learning interactions by attracting and directing another's attention. This thesis examined whether viewing another person's eye movement patterns could influence the perceptions of observers and facilitate performance during complex tasks. In addition, given that experts demonstrate enhanced visual processing of task-specific items, this thesis also investigated whether it would be more beneficial for observers to view the eye movements of experts rather than novices, and the extent to which observer expertise influences the effectiveness of these gaze cues. Chapter 1 provides an integrative review of eye movement behaviour, expertise, and gaze following, and discusses how eye movements can cue attention. Chapter 2 presents an experiment that explored whether another's eye movement behaviour could increase the likelihood that observers would solve Duncker's radiation problem, whereas Chapters 3-5 reports experiments that examined whether radiographers would benefit from viewing where another person looked for pulmonary nodules during chest x-ray inspection. Chapter 3 found that novice and experienced radiographers performed better when shown the search behaviour of either a novice radiographer or an expert "radiologist. However, Chapter 4 established that only novices consistently improved when shown an expert's search behaviour, and that these benefits only arose when the eye movements shown were related to the search for nodules. Chapter 5 re-examined the contribution of model expertise and found that even a naive observer's search behaviour could help scaffold the decisions made by novices. Chapter 6 discusses these findings and highlights some of the theoretical and methodological issues associated with guiding observers' attention via another person's eye movement patterns.