Inattentional blindness : the role of perceptual load, effects of stimulus type and position, and development over childhood
Inattentional blindness refers to a failure to detect visible objects when attention is engaged in a task. Despite the central role for attention implied by its name, there is surprisingly little evidence that inattentional blindness indeed results from inattention. In this thesis I provide such evidence in demonstrating that rates of inattentional blindness critically depend on the extent to which a relevant task exhausts attentional capacity (under high perceptual load) or leaves spare capacity (under low perceptual load) for determining awareness of task-irrelevant stimuli. This was found when load was increased by requiring a more subtle line-length judgment in the traditional inattentional blindness cross-task, or by increasing the number of items in a visual search task. Further experiments generalised the effects of perceptual load on awareness across simple shapes and meaningful objects, and for irrelevant stimuli appearing in the periphery and at fixation. By contrast, upright (but not inverted) faces reached awareness regardless of the level of perceptual load in the relevant task. These findings are consistent with previous behavioural perceptual load studies using reaction time (RT) measures of task-irrelevant processing (Lavie, 1995 Lavie, Ro & Russell, 2003 see Lavie, 2005 for review) and support the conclusion that perceptual load determines conscious awareness. The experiments also found no advantage for awareness at fixation versus awareness at the periphery, highlighting a potential dissociation between awareness measures and distracter effects on RTs (which have previously shown such an advantage, Beck & Lavie, 2005). Finally, this thesis presents a preliminary investigation of the development of awareness as measured by rates of inattentional blindness under different levels of task load in children and in adults. Results demonstrated a clear pattern of increasing awareness with increasing age, and lend partial support to the notion that the development of attentional capacity underlies this trend in awareness.