There have been increased efforts to develop methods for improving attention across a range of tasks including those assessing sustained attention. Using a variety of techniques, researchers have reported modest reductions in errors on sustained attention tasks. However, published reports often have not documented changes in response times (RTs) that might accompany error reductions, which is problematic given that the error reductions could be mediated by a slowing strategy (i.e., speed-accuracy trade-off). In three studies, I explored the effects of speed-accuracy trade-offs in a sustained attention task (The Sustained Attention to Response Task; SART). In Study 1, I examined the effects of changing SART instructions from the double-edged "be fast and accurate" to the more conceptually accurate goal of maintaining high accuracy by responding slowly and carefully, and found that instructions to respond slowly and accurately resulted in both significantly longer RTs and fewer SART errors. In Studies 2 and 3, I developed a modified version of the SART that allowed me to experimentally manipulate RTs and found that errors were a systematic function of manipulated differences in RT independent of individual differences in response strategies. The results of these experiments indicate that it is possible that any technique that alters RT might indirectly alter error rates independently of improvements in sustained attention. I therefore conclude that investigators need to carefully attend to, control for, and report any changes in RT that accompany improvements in accuracy of performance, or alternatively employ tasks controlling for RT.

There have been increased efforts to develop methods for improving attention across a range of tasks including those assessing sustained attention. Using a variety of techniques, researchers have reported modest reductions in errors on sustained attention tasks. However, published reports often have not documented changes in response times (RTs) that might accompany error reductions, which is problematic given that the error reductions could be mediated by a slowing strategy (i.e., speed-accuracy trade-off). In three studies, I explored the effects of speed-accuracy trade-offs in a sustained attention task (The Sustained Attention to Response Task; SART). In Study 1, I examined the effects of changing SART instructions from the double-edged "be fast and accurate" to the more conceptually accurate goal of maintaining high accuracy by responding slowly and carefully, and found that instructions to respond slowly and accurately resulted in both significantly longer RTs and fewer SART errors. In Studies 2 and 3, I developed a modified version of the SART that allowed me to experimentally manipulate RTs and found that errors were a systematic function of manipulated differences in RT independent of individual differences in response strategies. The results of these experiments indicate that it is possible that any technique that alters RT might indirectly alter error rates independently of improvements in sustained attention. I therefore conclude that investigators need to carefully attend to, control for, and report any changes in RT that accompany improvements in accuracy of performance, or alternatively employ tasks controlling for RT.

A decision process can be conceptually separated into a perceptual process and a decision strategy. The former includes all the different mechanisms that contribute to accumulate information relevant to the decision, whereas the decision strategy determines when enough information has been accumulated and a decision can be taken. Although perceptual processes have been extensively investigated in the last decades, decision strategies have received comparatively little attention. The main aim of this work is to fill this gap by analysing four decision strategies with two different experimental paradigms. We also focus on ancillary decision-making topics, such as the effect of stimulus intensity, foreperiod duration, payoff manipulation, and the response distributions in the rate domain. We initially performed a qualitative analysis of decision strategies by using a classic reaction time tasks on human participants while assuming the Drift Diffusion Model, one of the many models used for simple and fast decisions, as the perceptual process. We found that increasing the time of the trial does not have a relevant effect on the response, which is in contrast with some of the decision rules considered here. However, this approach is limited by the implicit assumption of a perceptual model that would result in different prediction for the decision strategies. We suggest the use of a different experimental design, called the EXACT Paradigm, which allows us to analyse decision strategies without having to assume any perceptual process. We tested the feasibility of such approach and applied it to several experimental studies, including a direct comparison with a classic reaction time task. Overall, two of the four decision strategies (modified Reward Rate and Reward/Accuracy) appeared to model the data satisfactorily. We discuss several ways in which the EXACT Paradigm can be used for expanding our knowledge in the field of decision-making.

Yes / Huntington’s disease (HD) is associated with impairments in dual-task performance. Despite that, only a few studies have investigated dual-tasking in HD. We examined dual-task performance in 15 participants in the early stages of HD and 15 healthy controls. Participants performed direct circle tracing (able to view arm) and indirect circle tracing (arm obscured) either on their own (single tasks) or paired with serial subtraction by twos or threes (dual tasks). Overall, our results suggested that HD participants were significantly slower and less accurate than controls. Both groups were slower and less accurate when performing indirect circle tracing compared with direct circle tracing. HD participants experienced greater dual-task interference in terms of accuracy when performing direct circle tracing compared with indirect circle tracing. Despite that, controls were more inclined to speed–accuracy trade-offs compared with HD participants. Importantly, unlike controls, HD participants were not disproportionately faster when performing direct circle tracing as a single task compared with the dual-task conditions. Our results suggest that simple tasks place greater attentional demands on HD participants compared with controls. These findings support that impaired automaticity may be responsible for some of the attentional deficits manifested in HD. / Supported by the School of Psychological Sciences, Monash University.

Yes / The aim of this study was to investigate age-related differences in dual task performance by using an upper limb proprioceptive task. Twenty-eight younger (18–30 years) and 28 older (>60 years) healthy adults performed circle tracing and serial subtraction tasks separately and concurrently. The tasks had two levels of difficulty: easy and hard. The circle tracing task included direct (easy) and indirect (hard) visual feedback conditions, and it was paired with serial subtraction by twos (easy) or threes (hard). We found that older adults were significantly slower than younger adults across all conditions and had significantly greater dual task costs when they performed circle tracing with easy serial subtraction. Higher levels of task difficulty were associated with slower speed in both groups. We found no age differences in accuracy. Participants either traded speed for accuracy or accuracy for speed regardless of age group. Overall, the findings suggest that speed and accuracy may be affected differently during dual tasking. In addition, older adults may rely more extensively on proprioceptive feedback to guide upper limb movement compared with younger adults. / Financial support for this study was obtained from the School of Psychology and Psychiatry, Monash University.

Human movements are remarkably adaptive, such that we are capable of completing movements in a novel environment with similar accuracy to those performed in a typical environment. Our ability to perform in these environments involves accurate processing of sensory feedback for online and offline control. These processes of control have been widely studied for well learned actions, but not for actions in a novel visuomotor environment. In two experiments, we examined control processes underlying reaches when participants were first introduced to a visuomotor rotation (Experiment 1) and then following visuomotor adaptation (Experiment 2). All participants completed 150 reach training trials when (1) a cursor accurately represented their hand motion (i.e., aligned cursor) and (2) a cursor was rotated 45 degrees clockwise relative to their hand motion (i.e., rotated cursor). In Experiment 1, we sought to determine if the control processes underlying movements in typical and novel visuomotor conditions were comparable. Participants (n = 16) received either continuous visual feedback or terminal visual feedback regarding movement endpoint during reach training. Analyses revealed that participants were able to demonstrate similar outcomes (i.e., movement time and endpoint errors) regardless of visual or cursor feedback, but also demonstrated more offline control (i.e., took more time planning and were less consistent in initiating their movements) when reaching with a rotated cursor compared to an aligned cursor, even at the end of training. Together, the results suggest a greater contribution of offline control processes and less effective online corrective processes when reaching in a novel environment compared to when reaching in a typical environment. In attempt to promote online corrective processes, participants (n = 16) in Experiment 2 first completed the training trials with continuous visual feedback and then completed an additional 45 reaches under (1) slow movement time (i.e., Slow MT: 800-1000 ms) and (2) fast movement time (i.e., Fast MT: 400-500ms) constraints. Results showed a shift to online control (i.e., greater endpoint accuracy) when reaching with an aligned and rotated cursor, when sufficiently more time was provided (i.e., Slow MT). Specifically, participants were able to more effectively utilize visual feedback for online control under the Slow MT constraint compared to when reaching quickly (i.e., Fast MT). Together, these experiments demonstrate a flexibility in control processes underlying reaches with rotated visual feedback of the hand. In that reaches first engage in offline control processes during adaptation to a visuomotor rotation, and then shift to online corrective processes following visuomotor adaptation.

La loi de Fitts, qui relie le temps de mouvement MT dans une tache de pointage aux dimensions de la cible visée D et W est usuellement exprimée à partir d’une imitation de la formule de la capacité de Shannon MT = a + b log 2 (1 + D/W). Toutefois, l’analyse actuelle est insatisfaisante: elle provient d’une simple analogie entre la tache de pointage et la transmission d’un signal sur un canal bruité sans qu’il n’y ait de modèle explicite de communication.Je développe d’abord un modèle de transmission pour le pointage, où l’indice de difficulté ID = log 2 (1 + D/W) s’exprime aussi bien comme une entropie de source et une capacité de canal, permettant ainsi de réconcilier dans un premier temps l’approche de Fitts avec la théorie de l’information de Shannon. Ce modèle est ensuite exploité pour analyser des données de pointage récoltées lors d’expérimentations contrôlées mais aussi en conditions d’utilisations réelles.Je développe ensuite un second modèle, focalisé autour de la forte variabilité caractéristique du mouvement humain et qui prend en compte la forte diversité des mécanismes de contrôle du mouvement: avec ou sans voie de retour, par intermittence ou de manière continue. À partir d’une chronométrie de la variance positionnelle, évaluée à partir d’un ensemble de trajectoires, on remarque que le mouvement peut-être découpé en deux phases: une première où la variance augmente et une grande partie de la distance à couvrir est parcourue, est suivie d’une deuxième au cours de laquelle la variance diminue pour satisfaire les contraintes de précision requises par la tache.Dans la deuxième phase, le problème du pointage peut-être ramené à un problème de communication à la Shannon, où l’information est transmise d’une“source” (variance à la fin de la première phase) à une “destination” (extrémité du membre) à travers un canal Gaussien avec la présence d’une voie de retour.Je montre que la solution optimale à ce problème de transmission revient à considérer un schéma proposé par Elias. Je montre que la variance peut décroitre au mieux exponentiellement au cours de la deuxième phase, et que c’est ce résultat qui implique directement la loi de Fitts. / Fitts’ law, which relates movement time MTin a pointing task to the target’s dimensions D and Wis usually expressed by mimicking Shannon’s capacityformula MT = a + b log 2 (1 + D/W). Yet, the currentlyreceived analysis is incomplete and unsatisfactory: itstems from a vague analogy and there is no explicitcommunication model for pointing.I first develop a transmission model for pointing taskswhere the index of difficulty ID = log 2 (1 + D/W) isthe expression of both a source entropy and a chan-nel capacity, thereby reconciling Shannon’s informa-tion theory with Fitts’ law. This model is then levera-ged to analyze pointing data gathered from controlledexperiments but also from field studies.I then develop a second model which builds on thevariability of human movements and accounts for thetremendous diversity displayed by movement control:with of without feedback, intermittent or continuous.From a chronometry of the positional variance, eva-luated from a set of trajectories, it is observed thatmovement can be separated into two phases: a firstwhere the variance increases over time and wheremost of the distance to the target is covered, follo-wed by a second phase where the variance decreasesuntil it satisfies accuracy constraints. During this se-cond phase, the problem of aiming can be reduced toa Shannon-like communication problem where infor-mation is transmitted from a “source” (variance at theend of the first phase), to a “destination” (the limb ex-tremity) over a “channel” perturbed by Gaussian noisewith a feedback link. I show that the optimal solution tothis transmission problem amounts to a scheme firstsuggested by Elias. I show that the variance can de-crease at best exponentially during the second phase,and that this result induces Fitts’ law.

Although research has highlighted the importance of decisions when learning and performing motor actions, few studies have focused on individuals’ ability to choose between potential motor actions. To help bridge this gap, this thesis presents a series of studies that investigate the behaviour of able-bodied individuals when attempting to choose movements based on a speed-accuracy tradeoff. In the first study, a two-alternative forced-choice task was used to determine whether people are consistent with Fitts’s law when choosing the movement they perceive to require the least movement duration. Participants performed almost perfectly when clear visual cues were available—when one of the targets was closer, wider, or both. Contrary to Fitts’s law, however, participants showed a preference for closer targets when visual cues were not informative—when one of the targets was closer and narrower. This study demonstrates that motor decisions are not always optimal, especially when participants are naïve at the task. To determine the basis of individuals’ preference for closer targets, a pair of studies explored the relation between motor decisions, imagined movements, and visual perception. Participants showed a similar deviation from Fitts’s law when imagining movements—believing that movement duration increased with distance within the same index of difficulty. Participants did not behave similarly, however, in a perceptual version of the decision task. These results suggest that imagined movements and motor decisions are linked, but they are not always based on veridical representations of actual movement. To further probe the origin of individuals’ erroneous belief about movement duration, the final study of this thesis measured movement duration for movements made at speeds other than ‘as fast as possible’. Movements made at more natural movement speeds shared important similarities with decisions and imagined movements. This study suggests that the biases seen in naïve motor decisions might originate from participants considering movements for which they have more experience, such as target-directed movements made at a naturally-selected pace. Together, the findings presented in this thesis may help to identify the ways that motor decisions can deviate from optimal, suggesting how those decisions must change with practice to better accomplish a task.

Although research has highlighted the importance of decisions when learning and performing motor actions, few studies have focused on individuals’ ability to choose between potential motor actions. To help bridge this gap, this thesis presents a series of studies that investigate the behaviour of able-bodied individuals when attempting to choose movements based on a speed-accuracy tradeoff. In the first study, a two-alternative forced-choice task was used to determine whether people are consistent with Fitts’s law when choosing the movement they perceive to require the least movement duration. Participants performed almost perfectly when clear visual cues were available—when one of the targets was closer, wider, or both. Contrary to Fitts’s law, however, participants showed a preference for closer targets when visual cues were not informative—when one of the targets was closer and narrower. This study demonstrates that motor decisions are not always optimal, especially when participants are naïve at the task. To determine the basis of individuals’ preference for closer targets, a pair of studies explored the relation between motor decisions, imagined movements, and visual perception. Participants showed a similar deviation from Fitts’s law when imagining movements—believing that movement duration increased with distance within the same index of difficulty. Participants did not behave similarly, however, in a perceptual version of the decision task. These results suggest that imagined movements and motor decisions are linked, but they are not always based on veridical representations of actual movement. To further probe the origin of individuals’ erroneous belief about movement duration, the final study of this thesis measured movement duration for movements made at speeds other than ‘as fast as possible’. Movements made at more natural movement speeds shared important similarities with decisions and imagined movements. This study suggests that the biases seen in naïve motor decisions might originate from participants considering movements for which they have more experience, such as target-directed movements made at a naturally-selected pace. Together, the findings presented in this thesis may help to identify the ways that motor decisions can deviate from optimal, suggesting how those decisions must change with practice to better accomplish a task.

Neste estudo foi analisado o efeito de distância, velocidade, tamanhos do disco e do alvo, e massas do disco e da manopla, sobre o desempenho motor em movimentos requisitando rapidez e precisão. Para tanto, foram analisadas as características cinemáticas da tarefa de projetar um disco a um alvo com movimento de contato balístico, empunhando uma manopla. Os movimentos foram desempenhados sobre uma base plana e filmados com câmera optoeletrônica de alta freqüência. O estudo foi conduzido em seis experimentos com um grupo único de participantes. Os resultados indicaram que os modelos de controle motor que têm sido empregados para analisar a relação velocidade-precisão em tarefas mais simples não foram apropriados para explicar o comportamento da tarefa utilizada. O controle motor na tarefa demonstrou ser dinâmico e flexível, frente às diferentes restrições de movimento. As seguintes estratégias de controle foram sugeridas na explicação dos resultados: sincronização da maior velocidade e do instante de contato manopla-disco, manutenção na proporção entre as fases aceleração-desaceleração, maior inércia e menor impacto para aumentar a estabilidade de movimento, e o controle da velocidade e da precisão em dimensões independentes. A análise das ações articulares demonstrou as particularidades das estratégias utilizadas pelo sistema no movimento em função das variáveis manipuladas. Em conjunto, esta seqüência de experimentos permitiu uma compreensão mais ampla das estratégias de controle motor empregadas em movimentos com alta demanda de velocidade e de precisão / In this study it was analyzed the effect of distance, velocity, disc and target width, disc and manipulandum mass, over motor control of a rapid and accurate movement. For such, it was analyzed the kinematic characteristics of the task of launching a disc to a target using a ballistic movement, performed with a manipulandum. Movements were performed on a flat surface and filmed with a high frequency optoelectronic camera. The study was conducted through six experiments with a single group of participants. Analysis of results indicated that models of motor control that has been used to analyze speed-accuracy tradeoff in simpler tasks were not appropriate to explain the observed behavior in the task used. Motor control in the task showed to be dynamic and flexible, regarding the several constraints manipulated. The follow strategies of control were suggested to explain the results: synchronization of the peak velocity and the instant of disc-manipulandum contact, maintenance of the proportion between the acceleration-deceleration phases, greater inertia and minor impact to increase movement stability, and control of velocity and accuracy of independent dimensions. The prediction of Analysis of joint actions showed the particularities of the strategies used by the system on movement as a function of the manipulated variables. Together, this study sequence of experiments allowed for a deeper comprehension of the control strategies used in the control of rapid and accurate movements