Tracking attention in space and time : the dynamics of human visual attention

Attention is essential to everyday life: without some selective function to guide and limit the processing of incoming information, our visual system would be overwhelmed. A description of the spatiotemporal dynamics of attention is critical to our understanding of this basic human cognitive function and is the primary goal of this dissertation. In particular, the research reported here is aimed at examining two aspects of the spatiotemporal dynamics of attention: a) the rate at which the focus of attention is shrunk and expanded along with the factors that influence this rate, and b) the factors governing whether attention is deployed as either a unitary or a divided focus. The present research examines the spatiotemporal dynamics of focal attention by monitoring the pattern of accuracy that occurs when participants attempt to identify two targets embedded in simultaneously presented streams of items. By asking participants to monitor these streams simultaneously, with the spatial and temporal positions of the two targets in the streams being varied incrementally, it is possible to index the extent of focal attention in both space and time. Chapter 2 develops this behavioural procedure and assesses the rate at which the focus of attention is contracted. A qualitative model is put forward and tested. Chapter 3 examines factors that modulate the temporal course of attentional narrowing in young adults who presumably can exercise efficient control of attentional processes. In contrast, Chapter 4 examines the effect of reduced attentional control by examining the same process in older adults. The second goal of this thesis was to examine whether focal attention is deployed as a unitary or a divided focus. These two perspectives are generally viewed as mutually exclusive. The alternative hypothesis pursued in Chapter 5 is that focal attention can be deployed as either a single, unitary focus or divided into multiple foci, depending on the observers mental set and on the task demands. The final chapter then combines and compares the findings across all experiments and evaluates how they fit in with current theories of visual attention.Arts, Faculty ofPsychology, Department ofGraduat

Dynamic spatial tuning of attentional focus in the attentional blink

When two sequential targets (T1, T2) are inserted in an RSVP stream of distractors, perception of T2 is impaired at inter-target lags shorter than 700 ms. Paradoxically, this deficit disappears when T2 is presented directly after T1 (Lag-1 sparing). Visser et al. (1999) found that Lag-1 sparing occurs only when T1 and T2 are presented in the same stream. In contrast, Shih (2000) obtained Lag-1 sparing with targets in separate streams. Four experiments addressed this inconsistency and revealed Lag-1 sparing with targets in different streams, but only when observers had no foreknowledge of T1' s location. We hypothesized that when T1 location is known, the attentional spotlight is focused narrowly on that stream; if T2 then appears in the other stream it is missed, and Lag-1 sparing does not occur. When T1 location is not known, the spotlight is focused broadly, encompassing both streams, and Lag-1 sparing ensues.Arts, Faculty ofPsychology, Department ofGraduat