Dichotic listening of auditory stimuli is used to assess brain lateralisation by simultaneously presenting different stimuli to the left and right ears to determine which syllable was perceived as being the clearest. There is a limited, albeit dated number of studies that have examined dichotic listening performance in adults who stutter (AWS) and the results remain inconclusive. The aim of this research was to investigate whether AWS show a difference in the magnitude of the right ear advantage (REA) in both undirected and directed attentional tasks when compared with adults who do not stutter (AWNS). There were 14 right-handed participants, consisting of seven AWS and seven age and sex matched AWNS controls. All participants were screened for normal hearing. They completed a dichotic listening task, which included undirected and directed attentional listening tasks. Participants were to select the consonant-vowel (CV) pair they heard the clearest. The interaural intensity difference (IID) was modulated randomly during the undirected attention task. The results for the undirected task revealed: (1) a significant REA for AWS for the IID conditions of 0 to +21 dB and significant left ear advantages (LEA) for IIDs of -15 to -21 dB; (2) a significant REA for AWNS for the IID conditions of -9 to +21 dB and significant LEAs for IIDs of -18 to -21 dB; (3) laterality index scores with a significant IID effect but no significant group or group-by-ear interaction effects using parametric statistics. Further analysis of laterality using non-parametric statistics found significant differences between the fluency groups. In general, the findings in this study were revealing of differences between AWS and AWNS when performing dichotic listening tasks with speech stimuli. The primary difference observed between groups was in regards to the IID point at which a previous REA became a LEA. This “crossing-over” point occurred later for AWNS, indicating a strong left hemisphere advantage for the processing of speech. The earlier “crossing-over” for AWS would indicate that the right hemisphere was activated sooner for the processing of speech compared to AWNS. This activation of the right hemisphere is assumed to reflect more diffuse cerebral lateralisation for speech processing for the AWS and confirms past brain imaging studies. In the directed attention task, there was no significant difference between AWS and AWNS indicating that instances of stuttering may occur due to more automatic (bottom-up) speech processing. These findings have implications for theories of laterality and hemispheric asymmetry for phonological processing for AWS, which has been suggested to reflect a subgroup of AWS for whom cerebral dominance is related to their disfluency.

Neuromodulation of the immune system has been described to be influenced by hemispheric lateralisation (HL), the stable tendency to relatively utilise one hemisphere or its functions over another. To date there has not been a systematic review of research in this phenomenon conducted, and only one study has examined the effects of HL on the progression of a disease – Human Immunodeficiency Virus (HIV). That research was conducted on a small sample with little control for confounders. The present work sought to compile a systematic review of literature concerning HL and immunity in humans, using effect size analysis. Further, the present work also describes an empirical advancement of this earlier HIV study with stricter control over confounds in a larger sample. The findings corroborated the theory of asymmetrical immune influence by HL via the systematic review showing clear, relatively consistent and strong relationships between left-HL and immunopotentiation. The empirical prospective study extended current knowledge of this relationship in HIV to identify a moderator – HAART treatment. Specifically, left-HL predicted better immunity in HIV-1 patients independent of confounders, with further findings of the same pattern in untreated patients, but not in HAART-treated patients. Further observations were made between HL and HIV-relevant behaviours, again adding to current knowledge. The finding of left-HL being associated with fewer sexual partners in Europeans presents new information of relevance to public health. The combined findings of the present work suggest that left-HL has predictive value in illness (HIV-1) and in general immunity. The present work adds to the existing knowledge new information concerning a moderating factor of the HLimmunity relationship in HIV, and behavioural implications of HL which impact upon HIV disease. Potential explanations for moderation, proposals for neurobiological mechanisms and direction towards future, more rigourous study in the field, both in HIV and immunity, are discussed.

Purpose: The aim of this research was to examine the relationship between stuttering and bilingualism to hemispheric asymmetry for the processing and production of language. Methods: A total of 80 native speakers of German were recruited for the study, ranging in age from 15 to 58 years. Out of those 80 participants, 40 participants were also proficient speakers of English as a second language (L2). The participants were organised into four speaker groups (20 per group) according to language ability and speech status, consisting of monolinguals who stutter (MWS), monolinguals who do not stutter (MWNS), bilinguals who stutter (BWS), and bilinguals who do not stutter (BWNS). Each of the four groups comprised 12 males and 8 females. All participants completed a battery of behavioural assessments measuring functional cerebral hemispheric asymmetry during language processing and production. The behavioural tests included (1) a dichotic listening paradigm, (2) a visual hemifield paradigm, and (3) a dual-task paradigm. Results: Overall, the results showed no significant differences in language lateralisation between participant groups on the three behavioural tests. However group differences were identified in regard to executive functions on the visual hemifield and dual-task paradigms. Both bilingual groups showed significantly faster reaction times and fewer errors than the two monolingual groups on the visual hemifield paradigm. The bilingual groups also performed similarly on the dual-task paradigm, while the MWS group tended to show greater task disruption. No meaningful relationship was found between stuttering severity and the majority of results obtained for the test conditions. However, all four language modalities were found to correlate significantly with results obtained for the visual hemifield and dual-task paradigms, suggesting that performance on these tests increased with higher L2 proficiency. Conclusion: Although no differences in language lateralisation were found, it appears that bilingualism had a greater influence on functional cerebral hemispheric processing than stuttering. A prevailing finding was that bilingualism seems to be able to offset deficits in executive functioning associated with stuttering. Brain reserve and cognitive reserve are thought to have a close interrelationship with the executive control system. Cognitive reserve may have been reflected in the present study, resulting in a bilingual cognitive advantage. Hence, the results of the present study lend support to previous findings implicating the benefits of bilingualism.

Introduction: Schizotypy is a personality trait that shares some of the characteristics of clinical disorders such as schizophrenia. Similarities are found in expression of psychotic-like experiences and presence of attenuated negative signs. Furthermore, schizotypal samples are associated with impairments in cognitive tasks, albeit in a less comprised form. For these reasons and others, schizotypy is considered a part of the extended-phenotype of schizophrenia and as such can be utilised as an analogue sample without some of theconfounds associated with illness. Objective: The aim of the PhD is to examine the relationship of schizotypal features and brain function and structure in a sample of adolescents and young adults (age 16-25 years). This will attempt to provide further evidence for the placement of schizotypy on the continuum, along with insights into pathophysiological mechanisms involved in schizophrenia and related disorders. Methods: The study involved three main phases: recruitment via an online survey, further neuropsychological testing and brain imaging on selected high schizotypes and controls. The thesis comprises 5 papers/experiments. Paper 1 utilises confirmatory factor analysis (CFA) to examine the factorial structure of the schizotypal personality questionnaire (SPQ) in a community sample aged 16-25 years. It also examined the effects of demographics on schizotypal levels. Paper 2 examined the association between schizotypy and measures of sustained attention and spatial working memory both in a total sample, and in samples split by age and by sex. Paper 3 further examined the association between schizotypy and cognition laboratory tests of attention, executive function and verbal learning/memory. Paper 4 tested the same participants on measures of functional brain asymmetry. Paper 5 used diffusion tensor imaging (DTI) to examine white matter structures in a sample of high schizotypes and controls. Results: Paper 1 confirmed that the SPQ is most appropriately modelled by a four-factor structure in an adolescent and young adult sample. Demographic effects on SPQ subscales scores mirrored those seen in clinical samples. Paper 2 found that where small associations between schizotypy and sustained attention/spatial working memory function occurred, these were in relation to either age of sex. Paper 3 demonstrated an association between increased schizotypal features and a slight reduction in performance on verbal learning/memory, but no association with tasks of executive function or attention. In Paper 4, schizotypy was associated with a left-hemifield bias on a computerised line bisection task. Paper 5 found that a group of high schizotypes had an increase in tract coherence in the uncinate fasciculus compared to controls. Furthermore, increasing subclinical hallucinatory experiences were associated with increased tract coherence in the right hemisphere arcuate fasciculus. Conclusions: Schizotypy was associated with changes in brain function and structure similar to that demonstrated in more serious mental illness, although to a lesser degree. The current studies suggested that schizotypy is associated with relatively intact prefrontal function, but slight performance bias on measures of medial temporal lobe function. There was also evidence for structural brain changes in schizotypes, with these being indicative of either a protective factor, or a marker of a pathological process. Correlations between hallucinatory experiences and white matter tracts between language regions support theories implicating hyperconnectivity and presentation of symptoms in clinical groups. The functional and structural data collected from this study suggests that the ‘schizotypal’ brain may represent an ‘early’ stage of pathology, but which is likely to be compensated enough such that transition to serious mental illness is unlikely. Further studies could examine similarities and differences between the schizotypal profile and clinical conditions, which would provide further insights into aetiological mechanisms in schizophrenia/psychosis.

Dichotic listening of auditory stimuli is a method of assessing brain lateralisation. Different stimuli are presented simultaneously to the left and right ears, with the listener reporting which stimulus is perceived most clearly. To date, several studies that have examined the effects of aging on dichotic listening performance have indicated a pronounced right ear advantage (REA) with increasing age, but few studies have considered the effects of sex, and findings to date have been inconclusive. The aim of this research was to investigate whether the effects of age and sex resulted in a difference in the magnitude of the REA in both undirected and directed attentional tasks. Forty sex-matched, right-handed subjects with normal hearing or symmetrical bilateral sensorineural hearing loss participated in a series of directed and undirected dichotic listening tasks using consonant-vowel (CV) stimuli. The interaural intensity difference (IID) was modulated randomly during the undirected attention task. Results indicated that all groups (age & sex) showed a REA for both undirected and directed dichotic listening tasks. No age or sex-related differences were found. The findings were suggestive of a task-linked effect for dichotic listening performance. The use of CV stimuli, in combination with detailed testing via manipulation of the IID, appeared to minimize any possible age or sex-related differences. These findings have implications for theories on laterality and hemispheric asymmetry for older adults.

Many complex decisions require integrating and assessing multiple streams of dynamic information whilst determining how to act. This dynamic information often contains rhythmic structures which our sensory systems can adapt to and use to anticipate future events. Despite the close relationship between rhythmic temporal expectations and complex decision making being self evident, no experiments explicitly attempt to understand this interdependence. If the theories that have emerged from both domains are to generalise to complex interactive behaviour, the effects of dynamic context on complex decisions must be considered. I argue that timing research must move beyond simple decisions and develop a new experimental framework for addressing the problem. This includes increasing the complexity of experimental tasks, testing the effects of timing on perceptual averaging and subjective value decisions, incorporating timing as an inherent dimension of targets, testing degrees of aperiodicity and exploring the effects that prior knowledge about the temporal structure of a stimulus has on choice. Seven behavioural experiments are reported that implement the new experimental framework. Five use a complex auditory-spatial averaging task to examine effects of periodicity, expectation, prior knowledge and related parameters such as IOI variance. One tests the effects of rhythmic variability and stimulus duration on auditory detection to determine specificity to complex decision making, and one investigates the effects of timing on audio-visual subjective value decisions. The results show that existing theories of temporal expectation do not necessarily generalise to complex decision making. Periodicity reduces the amount of information that is needed to form complex decisions. However, the effects of periodicity (or degree of aperiodicity) on choice are dependent on a number of factors associated with prior knowledge, stimulus rate, variance, decision type and task complexity. Using these findings I develop an explanatory framework called "dynamic inhibition and boosting" that better accounts for behavioural data in the literature compared with existing theories. This explanation is supported by the novel proposal that temporal expectations influence confidence and perceived risk.

Une des hypothèses concernant la fonction du sommeil suggère que ce dernier permettrait la plasticité neuronale et l'organisation (ou la réorganisation) synaptique, phénomènes sous-tendant des fonctions cognitives. Des perturbations spécifiques du sommeil faisant suite à un apprentissage ont en effet montré une diminution significative des performances aux niveaux des gènes et du comportement. Notre travail à visé à étudier les conséquences d'une privation de sommeil faisant suite à un apprentissage d'une tâche mnésique au niveau de l'abondance de protéines dans l'hippocampe de rat. Pour ce faire, une première étude protéomique de l'hippocampe de rat en l'absence d'apprentissage spatial spécifique a d'abord été réalisée ; elle montre l'absence de différence quantitative d'abondance protéique entre les hippocampes gauche et droit. Ensuite, une seconde étude protéomique montre qu'une privation de courte durée affecte différents réseaux de protéines, principalement liés au métabolisme cellulaire, aux voies biochimiques de l'énergie, des transports, du trafic vésiculaire, du cytosquelette et du traitement des protéines dans l'hippocampe de rat. Finalement, une troisième étude protéomique montre les effets d'un apprentissage d'une tâche spatiale en début de période d'activité diurne sur le protéome d'hippocampe de rat. Les principales protéines affectées ont ici des fonctions liées au métabolisme cellulaire et au cytosquelette.

The performance of most tasks requires some interaction between the cerebral hemispheres. Despite this fact, research has focused on demonstrating that each hemisphere is specialised for certain processes and has largely neglected this interaction. ¶ Recent research has recognised the need for a better understanding of how resources are shared between the cerebral hemispheres. While these studies have shed light on factors external to the participants being tested, such as the type of task and stimuli used, presentation times, and different measurement methods, they have neglected variables that differ between individuals. The studies reported here focused on factors internal to the participants. They include sex, age, handedness, functional lateralisation, practice, attention, and hemispheric activation, which vary between individuals or within individuals across time, and have been shown to influence the structure and morphology of the corpus callosum which is the main pathway for hemispheric interactions. ¶ This thesis examines the relationship of these variables to the efficiency of hemispheric interactions. ¶ A literature review of the factors affecting hemispheric interactions and interhemispheric transfer is presented in Chapter 1, and methodological issues relating to the measurement of these variables in Chapter 2. Based upon this research, two tasks, the Poffenberger paradigm and a letter-matching task, were selected to assess interhemispheric transfer time and hemispheric interactions, respectively, and to investigate the relationship between these two variables. ¶ Chapters 3 and 4 present the findings of the principal study, using a large sample of participants and regression analysis, which demonstrate that both faster interhemispheric transfer and more extreme left-handedness are associated with greater efficiency of hemispheric interaction. Surprisingly, other factors which were expected to influence hemispheric interactions (age, sex, functional lateralisation, and attention) did not have a significant effect on this variable. ¶ A strong practice effect found in the task used in Chapters 3 and 4 is analysed in Chapter 5. Contrary to previous findings, this practice effect seems not to be due to a shift from sequential, rule-based processing to memory-retrieval, but rather, is a more general practice effect consistent with progressively more efficient use of neural resources. ¶ Chapter 6 shows that individuals with dyslexia not only demonstrate an abnormally fast interhemispheric transfer, but also attentional deficits, due probably to decreased efficiency in hemispheric interactions. Because some clinical populations, such as individuals with dyslexia, have been shown to have hemispheric interaction deficits, the study of such clinical samples can provide valuable information about the relationship between hemispheric interactions and other individual variables. ¶ In Chapter 7 it is demonstrated that both latent and induced patterns of lateralised hemispheric activation affect hemispheric interactions. This suggests that assessment of hemispheric activation is important not only in this field, but probably also more generally in neuropsychological research. These findings highlight the need for a simple, inexpensive measure of hemispheric activation that can be applied routinely in cognitive experiments. ¶ Chapter 8 presents a new technique to measure lateralised brain activation in typical psychological experiments using functional tympanic membrane thermometry (fTMT). This measure relies on the measurement of ear membrane temperature as an index of hemispheric activation. The technique is simple and inexpensive, and is shown to be suitable for the assessment of hemispheric activation patterns during typical experiments. ¶ In conclusion, individual characteristics such as the efficiency of interhemispheric transfer, handedness, functional lateralisation, attention, and hemispheric activation are important factors to consider when researching hemispheric interactions in both normal and clinical populations. Furthermore, future research will benefit from this newly developed measure, fTMT, by allowing the systematic study of the effects of hemispheric activation in brain processes.

Previous studies have suggested that mechanisms for neural compensation involve a reorganisation to right hemisphere processing in people with post-traumatic stress disorder (PTSD), and are associated with functional alterations in the capacity for behavioural flexibility. However, research has not established a direct relationship between the complex physiological and psychological processes of the heterogeneous disorder and right hemisphere cortical activity. The present study examined cognitive information processing in people with PTSD, reaction patterns associated with perceived traumatic stressors, and quantitative electroencephalographic (qEEG) indices of hemispheric asymmetry. Individuals with PTSD (N=34) and age and sex-matched normal controls (N=136) completed standardised web-based self-report questionnaires assessing traumatic stressor events and reaction patterns to those events. Neuropsychological indices of verbal, visuospatial, sensori-motor performance, and electrophysiological recordings, were examined for right hemisphere coding. The relationships among traumatic characteristic reaction patterns of numbing and avoidance, cognitive performance, and frontal and posterior EEG alpha asymmetry were also investigated. Structural and functional alterations were shown in those with PTSD, using indices of working memory for the retrieval of verbal and psychomotor information, indicating a reduced speed of processing and alterations to background cortical arousal in left hemisphere frontal regions. The study supported and extended previous findings of verbal working memory abnormality, alterations to left frontal cortical rhythmic oscillations, and low EEG alpha amplitudes in those diagnosed with PTSD. Results indicated a pattern of compensatory mechanisms associated with reduced speed of information processing and right-sided activation patterns in PTSD participants and control participants who experienced strong reactions to perceived traumatic events. Findings support the impact of traumatic events on psychobiological health in high-risk populations, implicating an association with specific patterns of neural and cognitive functioning in characteristic numbing and avoidance behaviours. / Thesis (Ph.D.) -- University of Adelaide, School of Population Health and Clinical Practice, 2008

The main aim of this dissertation was to investigate the difference in neural language patternsrelated to language ability in healthy adults. The focus lies on unraveling the contributions of theright‐hemispheric homologues to Broca’s area in the inferior frontal gyrus (IFG) and Wernicke’s areain the posterior temporal and inferior parietal lobes. The functions of these regions are far from fullyunderstood at present. Two study populations consisting of healthy adults and a small group ofpeople with generalized epilepsy were investigated. Individual performance scores in tests oflanguage ability were correlated with brain activation obtained with functional magnetic resonanceimaging during semantic and word fluency tasks. Performance‐dependent differences were expectedin the left‐hemispheric Broca’s and Wernicke’s area and in their right‐hemispheric counterparts. PAPER I revealed a shift in laterality towards right‐hemispheric IFG and posterior temporal lobeactivation, related to high semantic performance. The whole‐brain analysis results of PAPER IIrevealed numerous candidate regions for language ability modulation. PAPER II also confirmed thefinding of PAPER I, by showing several performance‐dependent regions in the right‐hemispheric IFGand the posterior temporal lobe. In PAPER III, a new study population of healthy adults was tested.Again, the right posterior temporal lobe was related to high semantic performance. A decrease in lefthemisphericIFG activation could be linked to high word fluency ability. In addition, task difficultywas modulated. Increased task complexity showed to correlate positively with bilateral IFGactivation. Lastly, PAPER IV investigated anti‐correlated regions. These regions are commonly knownas the default mode network (DMN) and are normally suppressed during cognitive tasks. It wasfound that people with generalized epilepsy had an inadequate suppression of regions in the DMN,and showed poorer performance in a complex language test. The results point to neural adaptabilityin the IFG and temporal lobe. Decreased left‐lateralization of the IFG and increased rightlateralizationof the posterior temporal lobe are proposed as characteristics of individuals with highlanguage ability.