Experiments which have investigated differences of function of the right and left cerebral hemispheres by studying ear asymmetry are reviewed, and literature relating to the two click threshold, the task used in this series of experiments, is summarised. Two reaction time studies establish a perceptual factor in the ear asymmetry effect and subsequent experiments explore the nature of this perceptual asymmetry_ A signal detection method is developed which, unlike the psychophysical threshold method, reveals greater sensitivity in detection of two clicks presented to the left ear than to the right ear when clicks are presented with a contralateral burst of white noise, or with preceding shock, but little difference between ears without white noise or shock. Caffeine citrate also increases the right/left ear difference. The effects of temporal intervals between white noise onset or shock and click presentation, and of laterality of white noise or shock are investigated. Interaction effects are again obtained, and periodicity effects are noted which suggest the role of attention mechanisms in the ear asymmetry effect. It is suggested that two major factors are involved in the task: detection of two clicks and pitch discrimination. The former is considered to be an index of cortical arousal. The possibility of hemispheric differences in arousal is discussed and a method for investigating individual differences in arousal is suggested. Thelatter is considered to reveal differentiation of function of the cerebral hemispheres in an auditory discrbnination task mainly involving pitch discrimination. The phenomenon is interpreted as being primarily perceptual, but attention mechanisms are also probably involved. Although the results obtained are reliable, the interpretation is tentative and much further investigation is suggested

This thesis explored the role of reference memory in human scalar timing. Chapters 3 and 4 examined if there is a distinction between reference memory and working memory. Using a combination of normal temporal generalization and pseudoepisodic temporal generalization conditions, the same physical durations (comparisons and standards) were asked to be compared by participants and judge for their equality (Chapter 3). Even though conditions consisted of the same comparison and standard durations participants showed difference in behaviour between the two tasks: that is it appeared that in normal temporal generalization they appeared to be more conservative in responding YES to whether a comparison was the same as a standard. Computer modelling supported these findings (Chapter 4). Chapter 5 examined a simple possible interference task. Participants were asked to provide judgements in a test block after being exposed to interfered blocks towards different standard durations (short, long or same interference) with the use of shorter or longer standards and comparisons. An interference effect was discovered and supported by computer modelling that raised logical questions towards the stability of reference memory, not only through interference from different standards but from comparison durations as well. The instability of reference memory due to bias manipulation was examined in Chapters 6 and 7. PartiCipants were biased by the use of shorter or longer comparisons (bias sequences) in order to misjudge the standard as being shorter or longer in a test sequence after they have completed a short or long bias sequence. Chapter 6 also explored the biasing effect with the use of post-response feedback. The effect was also supported by computer modelling in Chapter 7. The thesis overall suggested a clear distinction between reference and working memory and provided evidence supporting the instability of reference memory through interference (standard manipulation) and bias (comparison manipulation).

The present work presents a series of studies investigating neuropsychological aspects of visual hallucinations (VHs) in Parkinson's Disease (PD) and high proneness to VHs in the normal population. The aim of the thesis is to investigate whether the same risk factors are implicated in both hallucinating PD patients and in high-prone individuals from the normal population, i.e. the continuum hypothesis of VHs. To this end, new instruments were designed to assess the nature of VHs in PD and to differentiate among , high and low hallucination-prone individuals. PD patients with and without VHs, agematched normal controls, and high and low-prone normal young individuals are assessed on visual memory and executive tests from the CANTAB test battery, alongside tests examining personality factors, sleep patterns, and demographic factors. The findings suggest that VHs in PD and hallucination-proneness in the normal population are both associated with a combination of different factors, particularly aspects of visual processing and sleep patterns. Results from the five studies are interpreted with the multifactorial models of VHs, suggesting that both VHs in PD and hallucinationproneness in the normal population stem from concurrent neuropsychological dysfunctions of several processing systems. However, a specific personality profile is predictive of high hallucination-proneness in the normal population, but not in PD patients. Therefore, two different models are proposed, arguing for similar, but not identical set of risk factors in hallucinating PD patients and in high-prone normal individuals.

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