Visual stability refers to the apparent stability of the visual world given the displacement of retinal images induced by eye movements. Phenomenally visual stability involves both a stable representation of visual space and reduced sensitivities to perceptual changes at the temporal proximity of eye movements. While the psychophysics of the perisaccadic perceptual changes have been studied extensively, how visual stability is implemented in the human visual system remains to be explored. This dissertation examines the cortical mechanisms of perceptual stability in spatial vision with four series of experiments. Series 1 established a paradigm to induce saccadic suppression of displacement (SSD) and examined how the direction of saccades and displacements influence the strength of SSD. Series 2 examined the consequence of disrupting the posterior parietal cortex (PPC) with transcranial magnetic stimulation (TMS) in perceiving perisaccadic displacements. Series 3 examined psychophysical factors influencing perisaccadic mislocalization. Finally, series 4 explored how TMS on PPC impacts perisaccadic mislocalization. These experiments conjointly illustrate how the PPC contributes to a stable visuospatial perception during saccades.

This investigation is concerned with the control and organization of the "psychological present," or primary memory, and specifically with the implications of such control and organization for the suffix effect. The suffix effect arises when a nominally irrelevant speech item, or "suffix," is appended to a spoken sequence of items, and it consists of an impairment in the recall of the most recent items in the sequence, especially the last item (e.g., Crowder, 1967; Dallett, 1965). The dominant explanation of the suffix effect has been in terms of "bottom-up" masking in general (e.g., Nairne, 1990) and precategorical acoustic masking in particular (e.g., Crowder & Morton, 1969; Crowder, 1978, 1983; Greene & Crowder, 1984). The current version of this explanation is "two-component" theory, wherein the precategorical masking explanation is confined to the terminal component of the suffix effect (i.e., at the last position of the sequence), with the preterminal component being open to influences of top-down or conceptually-based interpretation and strategy (see Greene, 1992 for a review). Reported here are 12 experiments, each of which provides evidence inconsistent with two-component theory. Experiments 1--4 failed to replicate the principal findings proffered in support of the theory; Experiments 5--11 extended some of the findings of the first four experiments by showing additional evidence of postcategorical influences on the terminal suffix effect; and Experiment 12 demonstrated a suffix effect with static visual presentation. These findings, and indeed those in the suffix effect literature in general, are interpreted along the lines of the now largely ignored perceptual grouping account proposed by Kahneman (1973; Kahneman & Henik, 1981).

Researchers have tried to keep subjects from committing the conjunction fallacy since Tversky and Kahneman discovered it in 1983. Betting paradigms (Bar-Hillel, 1993) have been used to force subjects to use a mathematical interpretation of "probability", but past experiments have either not involved actual betting or have had subjects bet on fictitious situations. In the current experiments half of the subjects were asked to decide which of 2 statements (about future events) had a higher probability while the other half were asked which statement they would prefer to bet on (in view of an actual payoff). The hypothesis was that while subjects in the probability condition would commit the conjunction fallacy, those in the betting condition would not. This hypothesis was not supported---there was not a significant difference between the numbers of conjunction fallacies committed by subjects in the two conditions in either of two experiments.

Users of modern GUIs routinely engage in visual searches for control items such as buttons and icons. The current research is intended to deepen our understanding of how the spacing between icons affects search times. Two experiments based on previous icon sets (Fleetwood & Byrne, 2002) were conducted in which the spacing between icons was systematically manipulated, and for which there was a computational cognitive model that predicted performance. Although the model's prediction that larger spacing would lead to slower search times was supported, there was an unanticipated finding: users were substantially slower than in similar experiments that employed consistent smaller spacing. The results were better fit with a model that employed a fundamentally different, less efficient search strategy. Eye-tracking data from a third experiment confirmed the effect of spacing on users' visual search behavior, but the system could not provide adequate information to explain the change in search behavior.

The relation of short-term memory to sentence comprehension was investigated in 4 children who had sustained severe closed head injury. Two of the patients showed dissociation in performance on short term memory tasks. One of the patients, CS showed a pattern of performance on short term memory tasks consistent with a phonological short term memory deficit. Another patient, CB, showed a pattern of performance which suggests a deficit in semantic memory. The dissociations in short term memory tasks exhibited by these patients corresponded to a dissociation in their performance on sentence processing tasks. On a sentence anomaly judgment task in which the memory load had to be maintained before the judgment could be made CS performed similarly to the Control subjects, while CB showed a deficit which was related to memory load. The opposite pattern was observed for a verbatim sentence repetition task on which CB's performance was within the normal range, but CS was very impaired. The results support models of short term memory that postulate separable components of semantic and phonological short term memory and the differential contribution of the two components to sentence comprehension.

Three experiments are presented comparing the effectiveness of several parameters of sound for the auditory presentation of statistical data. The dimensions of pitch, loudness, panning, and time were used alone and redundantly to map the values of a box plot to an auditory display. Temporal mappings resulted in better performance than mappings using pitch, panning, or loudness. In the first two experiments, there was no benefit when the mapping condition used two dimensions redundantly over mappings using one dimension. However, for the third experiment, there was a benefit of a redundant design when the dimensions of sound used were integral whereas there was no benefit when they were separable. This third experiment used a task more closely approximating a real-life application of auditory displays. Its results suggest that sonification can be used effectively in situations requiring the monitoring of more than one source of information.

Two experiments investigated Shelton and Martin's (1992) claim that automatic priming effects in the lexical decision task appear for associatively related items only, but not for semantically related, unassociated items. Experiment 1 attempted to induce or suppress subject strategies by emphasizing either accuracy or speed. While inducing a main effect in overall response times, no interpretable facilitation effects were obtained. Experiment 1b clarified these findings by using the same stimuli, but without the procedural manipulation. Here, Shelton and Martin's findings were basically replicated, suggesting that the procedure employed in Experiment 1 prevented facilitation effects from arising. Experiment 2 used a masked paradigm that presumably prevents strategic influences. Primes were presented for 20, 30, 40, or 50 msec. Facilitation was obtained for a prime duration of 40 and 50 msec for both associatively related and semantically related, unassociated items, indicating an early rise of semantic information during single word processing. These experiments suggest that semantic activation develops quite rapidly at the initial stage of word recognition, but quickly fades away and shows no influence on lexical decision tasks with longer SOA's.

Causal reasoning is an important and complex process, in which individuals have multiple sources of information available to inform their judgments. An enduring tension exists between what cues to causality people use and focus on in acquiring causal knowledge and making causal judgments. Much research on causal reasoning has focused on how people use information about covariation in this process. More recently, research has shown that people seek and use information about causal mechanisms to inform their causal inferences. Consequently, an important theoretical question is how people combine knowledge about causal mechanisms, that is, how a candidate cause works to bring about or produce a given effect, with information about covariation, which is the empirical relation between these two variables. / Very little research has investigated how these two sources of information are integrated in determining people's causal judgments. Two general models of how these sources of input are combined currently exist: covariation and mechanism precedence models. Both these models account for people's causal judgments on the basis of the primacy of one source of information over the other. / The research presented in this thesis investigates several variables that are hypothesized to be key in the integration of covariation and mechanism information. It is hypothesized that the scope of the covariation information available, as well as the strength of the covariation present between possible cause and effect are important dimensions of covariation input. It is also hypothesized that the nature and structure of mechanism information available to the reasoner is an important variable influencing the integration, specifically the detailedness of the representation of mechanism information. In four experiments the effects of these different variables on judgments of causality were assessed in combination. Overall it was found that how information about covariation strength is used depends on the detailedness of mechanism information and the scope over which covariation information is defined. The results indicate that one source of information does not have primacy over the other. Thus, an interactive model of how these sources of input are integrated is proposed.

With the impending aging of the Canadian population, the need for an assessment tool that can accurately measure cognitive ability in the elderly and monitor changes in cognition over time is rapidly gaining importance. The objective of the present study was to contribute evidence to the interpretability of scores from a novel cognitive assessment tool (GRACE) designed to be administered adaptively in the elderly population. Responses to items from two cognitive screening tests administered to patients attending a Geriatric Cognitive Disorders Clinic in Montreal were calibrated onto an interval scale using Rasch analysis. The hierarchy of items, organized by level of cognitive difficulty, was administered in a pilot adaptive format to a new cohort of patients, followed by administration of the remaining items to calculate total test scores. The reliability and validity of the GRACE method were demonstrated by comparing scores obtained from different orders of item administration (i.e. across cohorts) and by comparing scores with validated measures used in the clinic (i.e. across test methods), respectively. Additionally, demonstration of the validity of administering only a subset of items provided sound justification for developing and prospectively evaluating an optimal algorithm for adaptively administering test items. In addition to reducing test burden, the GRACE method provides a quantitative estimate of cognitive ability across the range of ability levels from normal to severe dementia and may be useful to clinicians as single tool to rapidly quantify and monitor cognitive ability. / Avec le vieillissement imminent de la population canadienne, le besoin d'un outil d'évaluation pouvant mesurer précisément la capacité cognitive chez les personnes âgées et surveiller les changements qui surviennent avec le temps gagne rapidement de l'importance. L'objectif de la présente étude était de fournir des données probantes pour l'interprétation des points résultants d'un nouvel outil d'évaluation (GRACE) conçu pour être administré de manière adaptative dans la population âgée. Deux tests de dépistage cognitifs ont été administrés aux patients d'une clinique gériatrique de désordres cognitifs à Montréal. Les réponses aux items ont été calibrées sur une échelle d'intervalle par la méthode d'analyse Rasch. La hiérarchie des items, organisée par niveau de difficulté cognitive, a été administrée dans un format adaptatif à une nouvelle cohorte des patients, suivie de l'administration des items restants pour calculer les totaux pour les deux tests originaux. La fiabilité et la validité de la méthode de GRACE ont été démontrées en comparant les points obtenus à partir de différents ordres d'administration d'items (c.-à-d. à travers des cohortes de patients) et en comparant les points à ceux des autres mesures validées utilisées dans la clinique, respectivement. Ensuite, la démonstration de la validité d'administrer seulement un sous-ensemble d'items a fourni la justification pour développer et évaluer un algorithme optimal pour administrer les items de manière adaptative. En plus de réduire le fardeau d'examen, la méthode de GRACE fournit une évaluation quantitative de la capacité cognitive à travers la gamme des niveaux de capacité, du niveau normal jusqu'à la démence avancée. Cette méthode peut être utile aux cliniciens pour mesurer et surveiller la capacité cognitive rapidement.

The role of age of acquisition in determining level of ultimate second language (L2) attainment is one of the most longstanding and controversial issues in the field of L2 acquisition. In particular, it is unclear whether there are limits to the brain's ability to process a L2 learned after puberty. The research for this dissertation used event related brain potentials (ERPs) to examine the neuro-cognitive processes that late (i.e., post-puberty) L2 learners use when processing L2 morpho-syntax and phonology. It investigated: (1) how the neuro-cognitive bases of L2 processing change as late learners become more proficient in their L2 (Study 1); (2) to what extent high proficiency late L2 learners recruit similar neuro-cognitive processing mechanisms as native speakers (Study 2); and (3) the role of the first language in determining L2 processing at various stages of L2 proficiency (Study 1 & 2). The results of these studies suggest that a certain degree of plasticity remains in the neural systems supporting L2 grammar and phonological processing, even in adult learners. Native-like neuro-cognitive processing appears to be available to late L2 learners at relatively high levels of proficiency when processing certain aspects of their L2. Results are discussed with respect to the neuro-cognitive changes that are associated with L2 acquisition and whether there are maturational constraints that limit L2 acquisition and processing for late L2 learners. / L'importance du rôle de l'âge d'acquisition comme facteur déterminant du niveau de maîtrise d'une langue seconde (L2) figure parmi les questions les plus débattues dans la recherche sur l'acquisition d'une L2. Plus particulièrement, la question de savoir si le cerveau est limité dans sa capacité de traiter une L2 après la puberté demeure irrésolue. La recherche conduite dans le cadre de la présente thèse a utilisé les potentiels évoqués (PÉs) dans le but d'examiner les processus neurocognitifs utilisés par les apprenants tardifs (post-puberté) d'une L2 dans le traitement de la morphosyntaxe et la phonologie de cette même L2. Cette recherche a comme but de répondre aux questions suivantes: (1) comment les bases neurocognitives du traitement d'une L2 changent à mesure que les apprenants tardifs atteignent des niveaux supérieurs de maîtrise de L2 (étude 1); (2) dans quelle mesure les apprenants tardifs de L2 de niveau avancé recrutent des mécanismes neurocognitifs similaires à ceux des locuteurs natifs (étude 2); et (3) quel est le rôle joué par la langue maternelle dans le traitement de la L2 selon le niveau de maîtrise atteint (études 1 et 2). Les résultats de ces études suggèrent qu'un certain niveau de plasticité demeure au sein des systèmes neuraux sous-tendant le traitement grammatical et phonologique d'une L2, y compris à l'âge adulte. Un traitement neurocognitif similaire à celui retrouvé chez des locuteurs natifs semble être accessible à des apprenants tardifs d'une L2 lorsqu'ils atteignent des niveaux relativement élevés de compétence en L2. Ces résultats sont examinés dans le cadre de la question des changements neurocognitifs associés à l'acquisition d'une L2 chez des apprenants tardifs et des contraintes développementales limitant l'acquisition et le traitement d'une L2 chez des apprenants tardifs.