Francis Edward Mitchell. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1995. / Includes bibliographical references (leaves 80-81 (2nd gp.)). / Acknowledgments --- p.iii / Summary --- p.iv / Introduction --- p.1 / Sharpness and contrast --- p.2 / Causes of unsharpness --- p.3 / Chapter i) --- Photographic unsharpness --- p.3 / Chapter ii) --- Movement unsharpness --- p.6 / Chapter iii) --- Geometric unsharpness --- p.8 / Chapter iv) --- Exposure unsharpness --- p.12 / Contrast factors --- p.15 / Chapter i) --- Subject/object contrast --- p.15 / Chapter ii) --- Radiation --- p.16 / Chapter iii) --- Film and screen --- p.18 / Chapter iv) --- Subjective --- p.20 / "How do we see ""Sharpness & Contrast"" ？" --- p.21 / Factors in perception --- p.26 / Chapter i) --- The individuals eyesight (and age) --- p.26 / Chapter ii) --- Light intensity --- p.26 / Chapter iii) --- Colour --- p.27 / Chapter iv) --- Pupil diameter --- p.27 / Chapter v) --- Size and shape of the object --- p.28 / Chapter vi) --- Eccentricity --- p.28 / Chapter vii) --- Edge enhancement --- p.29 / Chapter viii) --- Background luminance --- p.30 / Chapter ix) --- Maximising information retrieval from an image --- p.30 / Experiment I - Production of an image with controlled sharpness and contrast --- p.31 / Chapter i) --- Choice of film & cassette --- p.31 / Chapter ii) --- Prevention of movement unsharpness --- p.34 / Chapter iii) --- Prevention of parallax --- p.34 / Chapter iv) --- Control of penumbra --- p.35 / Chapter v) --- Verification of image unsharpness --- p.40 / Control of contrast --- p.46 / Radiography of test objects --- p.48 / Experiment II - Perception of sharpness at different contrast levels --- p.53 / Chapter i) --- Experiment --- p.53 / Chapter ii) --- "Viewing the data in terms of ""sharpness""" --- p.56 / Chapter iii) --- Viewing the data in terms of contrast levels --- p.61 / Analysis of data from an expanded group size (N=55) --- p.66 / Experiment III - Effect of room lighting conditions on the perception of sharpness --- p.69 / Overall conclusions --- p.78 / Bibliography --- p.79 / References --- p.80 / Appendix A (VBV calculation) --- p.82 / Appendix B (Line-pair test-tool) --- p.83 / Appendix C (Scattered radiation) --- p.85 / Photo-electric --- p.85 / Compton --- p.85 / Pair-production --- p.85 / Net result --- p.86 / Appendix D (Metal discs) --- p.87 / "Appendix E (OFD, magnification and penumbra)" --- p.88 / Appendix F (Processor developer temperature) --- p.90 / Appendix G (Viewing contrast-sharpness data) --- p.91 / Appendix H (Viewing conditions) --- p.92 / Appendix I (Comparison of data - light and dark viewing conditions) --- p.94 / Appendix J (Curix film & screens) --- p.97

A cardinal role of selective visual attention is to serve our action by selecting all the relevant information. One task that has been applied extensively to explore attention is visual search for a target among distractors. Given the extensive practice of human observers with visual search, and its ecological relevance, one could expect a high level of efficiency when performing visual search tasks. Indeed, a prominent in the literature Ideal Observer model (Najemnik & Geisler, 2005, 2008) suggests, that human visual system is extremely efficient, in that every eye movement during visual search is executed to the locations that could be expected to yield maximum information; similarly to the Ideal Observer, humans require the minimum number of eye moments possible to find the target (Najemnik & Geisler, 2005, 2008). The present programme of research tests the prediction of the Ideal Observer model (Najemnik & Geisler, 2005, 2008) against a simpler, but similarly effective stochastic selection model (Clarke, Green, Chantler, & Hunt, 2016), by examining human visual search strategies under normal viewing conditions, and conditions that simulate visual field deficit. Across nine experiments, I observed strikingly inefficient search behaviour that speaks against the assumptions of the Ideal Observer model. Although on the surface these cumulative results appear to be in line with the random process of fixation selection (Clarke et al., 2016), such conclusion appears to be valid only for the observed group results. The individual observer's data that was carefully documented in each of the experiments did not allow such a conclusion. The individual observers' data showed full spectrum of search strategies, with some observers being extremely efficient, some being average searchers, and others applying a very inefficient strategy. The large individual differences between participants suggest that the fixation selection process is neither optimal nor random, but rather idiosyncratic.

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There is a growing body of evidence that the main psychoactive ingredient of the recreational drug “ecstasy” (methylendioxymethamphetamine; MDMA) causes lasting changes to the serotonin system in both animals and humans, including the hippocampus (involved in memory) and the occipital lobe (involved in visual perception). Previous studies have often found memory deficits in ecstasy users. However, the results have been far from consistent across studies. None of the methods used to date have adequately isolated the hippocampal component of memory from the contribution of other brain regions. Three memory studies were conducted in this thesis to clarify which components and processes of memory are in deficit in ecstasy users.¶ In the first memory study, ecstasy users (n=32) did not differ from non-drug using controls (n=29) on implicit memory (automatic non-conscious retrieval, as revealed by a stem-completion task), or explicit memory (conscious recollection, as revealed by stem-cued recall). In the second memory study, no significant differences were found between ecstasy users (n=30) and non-drug using controls (n=34) on tests designed to clarify the findings on explicit memory, or on two standard neuropsychological tests of long-term memory (prose recall and Auditory Verbal Learning Test) that allowed greater use of elaborative processing at study. In the third memory study, a number of tests were applied that differed in their elaborative processing demands, including the California Verbal Learning Test, Visual Paired Associates, and Verbal Paired Associates. Ecstasy users (n=32) had poorer recall, and made less strategic use of elaborative processing compared to both cannabis-using controls (n=33) and non-drug using controls (n=33). Also, on a novel test of elaborative processing (“Verbal Triplet Associates”), both cannabis users and ecstasy users had memory deficits on the first trial, but only ecstasy users had a significant learning deficit over successive trials. On the basis of the localisation of the components and processes of memory in literature, it was concluded that long-term memory deficits in ecstasy users may reflect changes in elaborative processes localised in the frontal lobes, or global deficits, rather than just changes to the memory functions of the hippocampus.¶ With regard to visual perception, no studies have been published to date that have examined MDMA-related changes to the behavioural functioning of the occipital lobe in humans. In the current thesis, this was investigated using the tilt aftereffect illusion. In accordance with expectations, ecstasy users had a larger tilt aftereffect compared to non-drug using controls (n=34). Unexpectedly, this result was only obtained for a subset of 12 ecstasy users (out of n=30) who had not used amphetamines in the recent past. It was concluded that the results for ecstasy users who had not recently used amphetamines were consistent with the proposal that ecstasy-related serotonergic changes in the occipital lobe broaden the tuning bandwidth of orientation sensitive neurons, and that the recent use of amphetamines appears to counteract that effect.

The goal of this thesis was to investigate the role of colour in visual selective attention. Previous experiments exploring this topic in tasks where location varied led to mixed results. Some studies only found evidence of colour as a guide to a specific location where selection then takes place (e.g., Nissen, 1985). Others reported an effect, but could not decide clearly if the benefit was due to direct selection of colour in perception (e.g., Humphreys, 1981). One major contributor to the inconsistencies of findings seems to be the confounding of colour and location in these tasks. For that reason the initial paradigm used here was a rapid serial visual presentation (RSVP) task. Previous studies using similar paradigms have found no evidence for direct selection by colour (Poder, 2001; Shih & Sperling, 1996), but in these studies advance colour information was of limited usefulness because it only reduced the set of candidate stimuli by half. To assess an effect of colour in selection similar to the one associated with location, in all experiments reported here valid colour information led to only one item, as is typical in location cuing tasks. The first RSVP experiment explored whether colour certainty improved performance over a colour uncertainty condition. Colour was the defining feature of the target participants had to discriminate. In one condition the target colour was certain; in the other it could be one of two colours. Performance was improved when participants could focus on one colour. Further experiments used colour not as a defining feature of the target but as additional information presented in the form of cues, similar to the typical use of location cues. The participants� task was to discriminate whether a target letter within the RSVP sequence appeared in its upper or lower case version, and an advance cue indicated the colour in which the target letter was most likely to occur. An accuracy benefit of valid colour information was found, supporting the hypothesis that colour cuing allows the direct selection of objects for further perceptual processing. In addition, an effect of invalid colour cues was also observed. Subsequent experiments investigated possible factors influencing the colour cuing effect. Together, task requirements and properties of the stimulus set were shown to have an influence on the effect size, whereas an increase in perceptual load had no impact. Furthermore, the colour cuing effect seems to be due partially to both automatic and strategic processes. In all these experimental variations, benefits of colour cuing remained, indicating that the effect is very robust. Colour cuing effects were also found in a design where location could vary, extending the previous findings from selection in the time domain to selection in space. The two last experiments investigated whether advance colour knowledge would also lead to a performance benefit in single item tasks. No effect of colour cuing was found, indicating that colour information is only helpful in multiple item displays when a selection of one target stimulus among distractor items is necessary.

Thesis (Ph. D. in Biomedical Engineering)--Vanderbilt University, Dec. 2007. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

Thesis (Ph. D.)--University of Oregon, 2006. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 166-174). Also available for download via the World Wide Web; free to University of Oregon users.

Four experiments investigated object persistence in conscious awareness as a function of the nature of the cues that permit the object to be segregated from the background, and identified. A number of factors were manipulated (cue type, [color, motion, color & motion] cue duration after object identification [1s vs 5s] and cue strength [strong vs weak]). Performance was fractionated into identification, maintenance and persistence components. The results show that (1) stronger cues yielded faster identification, and (2) persistence was independent of identification time, and (3) motion cues were associated with longer persistence than color cues. A distinction between dorsal and ventral visual pathways as used to segregate the object from the background provides one way to organize the data.

Four experiments investigated object persistence in conscious awareness as a function of the nature of the cues that permit the object to be segregated from the background, and identified. A number of factors were manipulated (cue type, [color, motion, color & motion] cue duration after object identification [1s vs 5s] and cue strength [strong vs weak]). Performance was fractionated into identification, maintenance and persistence components. The results show that (1) stronger cues yielded faster identification, and (2) persistence was independent of identification time, and (3) motion cues were associated with longer persistence than color cues. A distinction between dorsal and ventral visual pathways as used to segregate the object from the background provides one way to organize the data.