Traditionally, people with Parkinson’s disease (PD) are advised to avoid performing dual-tasks while walking. Rehabilitation programs also emphasise the need to train walking under single-task conditions to improve gait and reduce risk for falls. There are findings that people with PD can walk faster and with longer strides while performing a secondary motor or cognitive task, when cued by a metronome or visual floor stripes. There are also findings that people with PD can walk faster and with longer strides while performing a secondary motor task simply by prioritising their attention to take big steps. Using attention is a convenient strategy favoured by people with PD to manage their gait difficulties. / This thesis examined the immediate and training effects of two attention-priority strategies on dual-cognitive task walking in people with mild to moderate PD. Two groups of participants received 30 minutes of training to prioritise attention to take big steps while performing serial three subtractions (gait-priority strategy) or to divide their attention between taking big steps and the cognitive task (equal-priority strategy). Control groups received no training. Measures of gait hypo-bradykinesia (stride length and gait velocity), stride variability (Coefficient of variation [CV] of stride length and CV of stride time) and cognitive task performance (accurate enumeration rate) were assessed at baseline, during training, immediately after training and 30 minutes after training. Both attention-priority strategies improved stride length and gait velocity during training. The improvement was retained for at least 30 minutes after training. Both strategies have no effect on CVs of stride length, stride time and accurate enumeration rate. / Many daily routines require our ability to overcome single-, dual- and multi-task demands while walking. Rehabilitation strategies should encompass real life demands in order to minimise functional impairments, activity limitations and participation restrictions, as recommended by the World Health Organisation. Putting together the findings of this thesis and the evidence provided by previous studies, it is concluded that traditional recommendations need qualification. Avoiding dual-tasks during walking or gait retraining in people with mild to moderate PD may not be necessary. Gait-priority and equal-priority strategies can be used as compensatory strategies to improve gait during dual-tasks. The two strategies can also be used in training programs for walking rehabilitation.

In Canada, as baby boomers age, there will be an increase in the percentage of seniors within the general population (Statistics Canada, 2006). Seniors often have difficulty in performing certain everyday tasks and have greater risk of having health issues. As such, it becomes increasingly important to understand factors that pose difficulty for this group of people. As people get older, many visual functions such as visual acuity, contrast sensitivity, and glare are known to deteriorate (Haegerstrom-Portnoy, 2005). However, when considering activities that aid mobility such as driving and walking, vision related variables are not the only ones that create difficulty for older individuals. A sensory variable such as attention, in conjunction with vision, has been shown in previous studies to be a good predictor of difficulties encountered by the elderly (McGwin, Owsley, & Ball, 1998; Owsley, McGwin, Sloane, Stalvey, & Wells, 2001). Moreover, inattention and distraction seem to be common causes of automobile accidents as well as falls. The work load imposed on the working memory can impact distractibility and inattention. In mobility related activities such as driving and walking, individuals perceive objects that are increasing in size. Experiments were designed to investigate the factors that affect the perception of targets that are enlarging in size. Size matching of expanding targets to a previously presented static target, was investigated in a group of younger participants with normal vision using central or peripheral vision. The results show that size estimates differ depending on whether the target appears in the central visual field or in the periphery. The participants respond faster to targets that appear in the periphery compared to those in the centre/midline. In the subsequent set of experiments we compared the performance of younger and older participants using a dual task paradigm where individuals had to perform two tasks concurrently, one of which was to match the size of an enlarging target. Attention was modulated in the dual tasks by varying the difficulty of the secondary task. It has been found that older individuals have difficulty processing multiple visual tasks or performing multiple tasks in general (Pashler, 1994a, 1994b, 1998; Verhaeghen et al., 2003). Compared to younger individuals, older individuals were found to have greater performance difficulty in the highly demanding dual tasks. These results are compared to those observed in studies of psychological refractory period effects. The differences between the young and older individuals are discussed with respect to limited capacity and bottle neck models of attention. Furthermore, eye movement measures in the dual tasks seem to provide evidence of difficulty in task switching for the older observers. The thesis also investigated the functional field of view of younger and older individuals. By assessing the functional field of view (FFOV) using a method employed earlier by Coeckelbergh et al., (2004a), significant overall age related differences were found. Multiple characteristics of what might affect the FFOV as measured by the attended field of view (AFOV) were also investigated (e.g., impact of a pop out distracter and divided attention). It was found that differences between the two age groups occurred in all conditions. The presence of irrelevant distracters had a greater impact on the older individuals compared to the younger group, whereas divided attention or the presence of the pop out distracter did not affect either age group. Attention processing seemed to be similar for both the younger and older individuals and, therefore, the differences between the age groups appear to be at a quantitative level rather than a qualitative level.

In Canada, as baby boomers age, there will be an increase in the percentage of seniors within the general population (Statistics Canada, 2006). Seniors often have difficulty in performing certain everyday tasks and have greater risk of having health issues. As such, it becomes increasingly important to understand factors that pose difficulty for this group of people. As people get older, many visual functions such as visual acuity, contrast sensitivity, and glare are known to deteriorate (Haegerstrom-Portnoy, 2005). However, when considering activities that aid mobility such as driving and walking, vision related variables are not the only ones that create difficulty for older individuals. A sensory variable such as attention, in conjunction with vision, has been shown in previous studies to be a good predictor of difficulties encountered by the elderly (McGwin, Owsley, & Ball, 1998; Owsley, McGwin, Sloane, Stalvey, & Wells, 2001). Moreover, inattention and distraction seem to be common causes of automobile accidents as well as falls. The work load imposed on the working memory can impact distractibility and inattention. In mobility related activities such as driving and walking, individuals perceive objects that are increasing in size. Experiments were designed to investigate the factors that affect the perception of targets that are enlarging in size. Size matching of expanding targets to a previously presented static target, was investigated in a group of younger participants with normal vision using central or peripheral vision. The results show that size estimates differ depending on whether the target appears in the central visual field or in the periphery. The participants respond faster to targets that appear in the periphery compared to those in the centre/midline. In the subsequent set of experiments we compared the performance of younger and older participants using a dual task paradigm where individuals had to perform two tasks concurrently, one of which was to match the size of an enlarging target. Attention was modulated in the dual tasks by varying the difficulty of the secondary task. It has been found that older individuals have difficulty processing multiple visual tasks or performing multiple tasks in general (Pashler, 1994a, 1994b, 1998; Verhaeghen et al., 2003). Compared to younger individuals, older individuals were found to have greater performance difficulty in the highly demanding dual tasks. These results are compared to those observed in studies of psychological refractory period effects. The differences between the young and older individuals are discussed with respect to limited capacity and bottle neck models of attention. Furthermore, eye movement measures in the dual tasks seem to provide evidence of difficulty in task switching for the older observers. The thesis also investigated the functional field of view of younger and older individuals. By assessing the functional field of view (FFOV) using a method employed earlier by Coeckelbergh et al., (2004a), significant overall age related differences were found. Multiple characteristics of what might affect the FFOV as measured by the attended field of view (AFOV) were also investigated (e.g., impact of a pop out distracter and divided attention). It was found that differences between the two age groups occurred in all conditions. The presence of irrelevant distracters had a greater impact on the older individuals compared to the younger group, whereas divided attention or the presence of the pop out distracter did not affect either age group. Attention processing seemed to be similar for both the younger and older individuals and, therefore, the differences between the age groups appear to be at a quantitative level rather than a qualitative level.

Motion capture technology and Magnetic Resonance Imaging with Diffusion Tensor Imaging (MRI-DTI) were used in this work to detect subtle abnormalities in patients with mild traumatic brain injury (MTBI). A new concept, termed dynamic variability, is introduced in this work to quantify and localize gait variability. Three chronic MTBI patients were recruited from the Veterans Affair Medical Center in Iowa City, IA, and three healthy controls with height, weight, and gender matched to the patients were recruited from the Reserve Officers' Training Corps in Iowa City, IA. Kinematic and kinetic data of the subjects were collected during the performance of three gait testing scenarios. The first test involved single-task walking and was used as a baseline. The second and third tests were dual tasks that involved walking while performing a cognitive or motor task and were designed to magnify gait abnormalities. The results showed that MTBI patients had reduced gait velocity, shortened stride length, and larger step width; findings that are consistent with those published in the literature. The new dynamic variability factor found that, as compared to controls, MTBI patients had more variability in their hip and ankle joint moments. MRI-DTI has been used to detect dysfunction of the major white matter tracts in chronic MTBI patients; although, the sample size of this study was too small to detect a difference between the MTBI and control subjects. The imaging and gait abnormalities are suggestive of frontal lobe-white matter tracts dysfunction.

Dual task studies have found that Task 2 response information is activated during Task 1 response selection, and can have a priming effect on Task 1. This is called the backward compatibility effect (BCE). Giammarco et al. (2016) found that single-task practice of Task 2 in the context of a random, filler task (Practice-T2 condition) extinguished BCE development in a subsequent dual-task. On the other hand, practicing Task 2 in the context of Task 1 (Practice-Both condition) promoted BCE development in subsequent dual-tasks. Experiment 1a sought to replicate this context-specific disruption of BCE development by presenting participants with a single-task practice phase where they practiced Task 2 along with a filler task, and then observed BCE development in a subsequent dual-task phase. Experiment 1b addressed a counterbalancing issue in Experiment 1a. Experiment 2 was an exact replication of the Practice-T2 condition used in Giammarco et al. (2016). Overall, we conceptually replicated the context-specific disruption of BCE in Experiment 2, but not in Experiments 1a and 1b. Further study is warranted to determine the effect of specific response features on the learning context of Task 2. / Thesis / Master of Science (MSc) / Backward compatibility effects (BCEs) have been consistently observed in dual task paradigms. BCEs occur when Task 1 and Task 2 response information are congruent: participants respond faster to Task 1 when the two tasks require congruent responses than when they require incongruent ones. This suggests that there is some parallel processing of Task 2 while performing Task 1. The purpose of this study was to explore the episodic account of BCE development. Since episodic memories are context-specific, BCEs should also be context-specific, according to the episodic account. By manipulating the context of Task 2 learning, we tested whether this affected subsequent BCE development. Our findings suggest that context-specific disruption of BCE development is possible, but depends on other factors as well.

Zahlreiche Studien belegen, dass Menschen Schwierigkeiten bei der simultanen Ausführung von 2 Aufgaben haben. Diese Schwierigkeiten sind durch zusätzliche Leistungskosten in Doppelaufgabensituationen im Vergleich zu Einzelaufgabensituationen gekennzeichnet (d.h. Doppelaufgabenkosten). Allerdings konnten jüngere Studien eine deutliche Reduktion der Doppelaufgabenkosten am Ende von Übung zeigen. Der Befund von Doppelaufgabenkosten am Beginn und die deutliche Reduktion davon am Ende der Übung indiziert, dass diese Reduktion durch spezifische Lernmechanismen geleistet wird. Obwohl sich frühere Studien bereits mit diesen Mechanismen befassen, bleiben die genauen Mechanismen der Reduktion der Doppelaufgabenkosten durch Übung unbekannt. Das Ziel von vier Studien der vorliegenden Arbeit ist die Spezifizierung dieser Mechanismen durch die Anwendung einer Doppelaufgabensituation mit einer visuellen und einer auditiven Teilaufgabe (Schumacher et al., 2001). Untersuchungen zu Lernmechanismen innerhalb der Teilaufgaben zielten auf die genaue Lokalisation von Verkürzungen der Verarbeitungszeit in diesen Aufgaben während der Übung. Erstmalig konnte ich zeigen, dass die Verkürzung der Aufgabenverarbeitung in Doppelaufgabensituationen vor allem durch eine verkürzte Antwortauswahlstufe erfolgt. Demgegenüber haben Untersuchungen zu Lernmechanismen zwischen den Teilaufgaben auf den Erwerb von Fertigkeiten der Aufgabenkoordination gezielt. Hier habe ich Nachweise erbracht, dass diese Fertigkeiten während der Übung von Doppelaufgaben erwerbbar sind, aber dass kein Erwerb während Einzelaufgabenübung erfolgt. Weiterhin habe ich gezeigt, dass diese Fertigkeiten in alternative Doppelaufgabensituationen transferierbar sind. Allerdings gibt es keinen Nachweis für den Fertigkeitstransfer in Aufgabenwechsel- und Attentional-Blink-Situationen. Um das Ergebnis von Lernmechanismen weiter zu spezifizieren, habe ich gezeigt, dass die Doppelaufgabenleistung in der visuellen aber nicht in der auditiven Teilaufgabe stabil ist nachdem die Übung beendet wurde. Die vorliegenden Befunde zu Lernmechanismen werden in ein Modell von geübter Doppelaufgabenleistung, das Latent Bottleneck Model, integriert und neue Annahmen im Rahmen dieses Modells diskutiert. / Numerous studies showed that people have difficulty performing two tasks at the same time. This difficulty is indicated by additional performance costs in dual-task situations when compared to single-task situations, i.e. dual-task costs. However, recent evidence has shown a substantial reduction of dual-task costs through practice. The finding of dual-task costs at the beginning and the reduction thereof at the end of practice indicates that this reduction must be accomplished via specific learning mechanisms. Although such mechanisms have been addressed in previous studies, the specific mechanisms contributing to practice related dual-task cost reduction remained unknown. The aim of four studies in the present work is to specify these mechanisms by applying a dual-task situation including a visual and an auditory component task (Schumacher et al., 2001). Investigations on learning mechanisms within the component tasks aimed to identify loci of stage shortening in these tasks through practice. For the first time I showed that component task processing in dual-task situations is mainly shortened through a shortening at the response selection stage. In contrast, investigations on learning mechanisms between the component tasks focussed on the acquisition of task coordination skills. Here I provided evidence that these skills are acquired during dual-task practice and there is no acquisition of these skills during single-task practice. Additionally, I demonstrated that these skills are transferable to alternative dual-task situations. There is, however, no evidence for transfer of these skills to task switching and attentional blink paradigms. In order to further specify the result of the learning mechanisms, I showed that dual-task performance in the visual but not in the auditory task is stable after practice has finished. The present findings on learning mechanisms are integrated into a model of practiced dual-task performance, the latent bottleneck model, and new assumptions in the framework of this model are discussed.

Werden zwei Aufgaben in einem Doppelaufgaben-Paradigma gleichzeitig bearbeitet, dann treten oft sogenannte Doppelaufgabenkosten auf (längere Reaktionszeiten und/oder höhere Fehlerzahlen). Diese Doppelaufgabenkosten werden durch einen zentralen “Flaschenhals” erklärt, der die gleichzeitige Verarbeitung der beiden Aufgaben an der zentralen Verarbeitungsstufe der Reaktionsauswahl verhindert. Das Ziel der vorliegenden Arbeit war es, einige mögliche Faktoren der Verarbeitungsreihenfolge an diesem Flaschenhals zu untersuchen. Die Studie zeigt, dass die Ankunftszeit am Flaschenhals ein wichtiger Faktor bei der Festlegung der Verarbeitungsreihenfolge ist. Zusätzlich ist der Einfluss der Ankunftszeit auf die Verarbeitungsreihenfolge unabhängig davon, welche der beiden Aufgaben manipuliert wurde um den Einfluss dieser Aufgabe zu untersuchen (visuelle oder auditorische Aufgabe). Ein zweiter Faktor der manipuliert wurde, ist die Instruktion an die Probanden. Die Ergebnisse zeigen, dass unter bestimmten Instruktionsbedingungen kognitive Kontrollprozesse aktiviert zu werden scheinen. Als dritten Faktor wurden Aufgabenanforderungen untersucht, indem eine Aufgabe mit zeitlicher Reihenfolge-Entscheidung mit einer Doppelaufgabe mit zufälliger Aufgabenreihenfolge, d.h.: Bestimmung der zeitlichen Reihenfolge mit der zusätzlichen Anforderung einer Reaktionswahl-Aufgabe, verglichen wurde. Die Ergebnisse dieser Experimente deuten darauf hin, dass die Entscheidung über die zeitliche Reihenfolge der beiden Aufgaben zwischen der Wahrnehmungsstufe und der Reaktionswahlstufe getroffen wird. / The simultaneous performance of two tasks in a dual-task paradigm is often accompanied by dual-task costs (longer reaction times and/or higher error rates). These dual-task costs have been explained by the existence of a central bottleneck which prohibits the simultaneous processing of the two tasks at the central response-selection stage of information processing. The aim of the present work was to investigate several of the possible factors which determine the task processing order at this central bottleneck. The study shows that the arrival time of the two tasks at the bottleneck plays an important role in the determination of task order. Additionally, the influence of the arrival time on processing order is independent of the component task which is manipulated to test the influence of that task (i.e., visual & auditory task). A second factor that was manipulated is the instruction given to the participants. The results show that cognitive control processes are activated under certain instruction conditions. As a third factor, task requirements were investigated by comparing a temporal order judgement task with a dual task with random task order (i.e., temporal order judgement with the additional requirement to do a choice-RT task). The results suggest that the decision about the temporal order of the two tasks is located between the perception stage and the response-selection stage of processing.

Zahlreiche Untersuchungen belegen fundamentale Grenzen in der menschlichen Fähigkeit, mehrere Dinge gleichzeitig zu tun. Aktuelle Theorien zur Verarbeitung von Doppelaufgaben gehen davon aus, dass kognitive Kontrollprozesse den Verarbeitungsstrom mehrerer Aufgaben koordinieren. Funktionell-neuroanatomisch wird insbesondere der laterale Präfrontalcortex (lPFC) mit der Kontrolle zielgerichteten Verhaltens in Verbindung gebracht. Mittels funktioneller Magnetresonanztomographie (fMRT) wurde bereits eine Beteiligung des lPFC an der Verarbeitung von Doppelaufgaben nachgewiesen. Die neuronalen Mechanismen der Doppelaufgabenkoordination sind jedoch weitgehend ungeklärt. Die drei fMRT Studien der vorliegenden Dissertation spezifizieren die funktionelle Rolle des lPFC bei der Interferenzverarbeitung in Doppelaufgaben. Die Ergebnisse zeigen (1) die Allgemeinheit der lPFC-Beteiligung über verschiedenen Doppelaufgabensituationen hinweg, (2) die funktionell-neuroanatomische Dissoziierbarkeit verschiedener doppelaufgabenrelevanter Kontrollfunktionen im lPFC , (3) die Bedeutung der Interaktion des lPFC mit posterioren aufgabenrelevanten Regionen für die Kontrolle von Doppelaufgabenverarbeitung. / Numerous studies indicate fundamental limitations in the human ability to do multiple things at the same time. Recent theories on dual-task processing postulate the involvement of cognitive control processes in the coordination of the processing stream of multiple tasks. The most prominent neuroanatomical structure associated with the control of goal-directed human behavior is the lateral prefrontal cortex (lPFC). It has been show with functional Magnetic Resonance Imaging (fMRI) that the lPFC is also involved in the processing of dual tasks. However, the precise role of the lPFC for the control of dual-task processing and the neural mechanisms of dual-task coordination are still widely unknown. The three fMRI studies presented in this dissertation specify the functional role of the lPFC in interference processing in dual tasks.The results show (1) the generality of lPFC involvement across different types of dual-task situations, (2) the functional neuroanatomical dissociability of different dual-task relevant control process in the lPFC, (3) the role of the interaction of the lPFC with posterior task-relevant brain regions for the control of dual-task processing