Perceptual load and enumeration: Distractor interference depends on subitizing capacity

Attention is limited, both in processing capacity (leading to phenomena of “inattentional blindness”) and in the capacity for selective focus (leading to distraction). Load theory (e.g., Lavie, 1995) accounts for both limitations by proposing that perceptual processing has limited capacity but proceeds automatically and in parallel on all stimuli within capacity. Here we tested these claims by applying load theory to the phenomenon of “subitizing”: the parallel detection and individuation of a limited number of items, established in enumeration research. We predicted that distractor interference will be found within but not beyond a person’s subitizing capacity (measured as the transition from parallel to serial slope). Participants reported the number of target shapes from brief displays while ignoring irrelevant cartoon-image distractors. As predicted, distractor cost on enumeration performance was found within subitizing capacity and eliminated in larger set sizes. Moreover, individual differences results demonstrated that distractor effects depended on an individual’s capacity (i.e., their serial-to-parallel transition point), rather than on set size per se. These results provide new evidence for the load theory hypotheses that perceptual processing is automatic and parallel within its limited capacity, while extending it to account for selective attention during enumeratio

Establishing gaze markers of perceptual load during multi-target visual search

Highly-automated technologies are increasingly incorporated into existing systems, for instance in advanced car models. Although highly automated modes permit non-driving activities (e.g. internet browsing), drivers are expected to reassume control upon a 'take over' signal from the automation. To assess a person's readiness for takeover, non-invasive eye tracking can indicate their attentive state based on properties of their gaze. Perceptual load is a well-established determinant of attention and perception, however, the effects of perceptual load on a person's ability to respond to a takeover signal and the related gaze indicators are not yet known. Here we examined how load-induced attentional state affects detection of a takeover-signal proxy, as well as the gaze properties that change with attentional state, in an ongoing task with no overt behaviour beyond eye movements (responding by lingering the gaze). Participants performed a multi-target visual search of either low perceptual load (shape targets) or high perceptual load (targets were two separate conjunctions of colour and shape), while also detecting occasional auditory tones (the proxy takeover signal). Across two experiments, we found that high perceptual load was associated with poorer search performance, slower detection of cross-modal stimuli, and longer fixation durations, while saccade amplitude did not consistently change with load. Using machine learning, we were able to predict the load condition from fixation duration alone. These results suggest monitoring fixation duration may be useful in the design of systems to track users' attentional states and predict impaired user responses to stimuli outside of the focus of attention

Individual differences in visual perception capacity and related brain morphology

Limited capacity for perception results in various phenomena of inattentional blindness in task conditions that load perceptual capacity. These effects have been extensively studied under the load theory framework, with numerous demonstrations spanning a wide variety of perceptual load manipulations. Research also established contrasting effects of loading perception versus cognitive control functions (e.g. working memory). The convergence of findings across different manipulations of perceptual load together with the contrasting effects of cognitive control load suggests a generalised capacity for perception, which is distinct from general cognitive capacity. The purpose of this thesis was to examine this hypothesis further, using an individual differences approach and relating traditional visual perception and awareness paradigms to the phenomenon of subitizing: the ability to detect a limited number of items in parallel from a brief exposure that has traditionally been studied within the enumeration literature. The research first extended perceptual load effects to measures of unattended processing in an enumeration paradigm, demonstrating that distractor effects are only found within subitizing capacity but not in set sizes that exceed capacity (Chapter 2). A series of individual differences experiments then revealed significant correlations between tasks involving subitizing, motion tracking, ‘change blindness’ and ‘inattentional blindness’. These relationships were furthermore established to withstand controls for non-perceptual factors, establishing perceptual capacity as distinct from working memory capacity or general cognitive effort (Chapters 3-4). Finally, voxel-based morphometry analyses of structural brain images established distinct correlates of grey matter density for perceptual capacity across tasks (Chapter 5). Taken together, the results of this thesis establish individual differences in perceptual capacity across a diverse range of paradigms and stimuli, demonstrating a common, general capacity limit for perception which correlates with individual differences in performance and grey-matter density and is independent from other cognitive constructs such as number estimation abilities and executive working memory

Establishing Individual Differences in Perceptual Capacity

Limited capacity for visual perception results in various “inattentional blindness” phenomena across a wide variety of manipulations that load perception. Here, we propose that these phenomena are mediated by an underlying generalized capacity for visual perception, which also underlies subitizing: the ability to enumerate a limited number of items in parallel from a brief exposure. We tested this proposal by examining whether individual differences reveal common intraindividual variance between measures of visual perception as well as of subitizing capacity. Visual perception was measured in change blindness (Rensink, O’Regan, & Clark, 1997), load-induced blindness (Macdonald & Lavie, 2008), and multiple object tracking tasks. Subitizing capacity was measured as the number of items that could be reported in parallel in an enumeration task. Perceptual capacity as indexed by subitizing was consistently a unique predictor of performance in change blindness, load-induced blindness, and motion tracking beyond any general factors that apply to both subitizing and estimation of larger set sizes. Moreover, when measures of working memory were included, factor analysis indicated two orthogonal factors: perceptual and working memory. Overall, the results support the hypothesis of a generalized capacity for visual perception, and establish subitizing capacity as a predictor of individual susceptibility to inattentional blindness under load

Moving towards more responsible fishing practices in Australia’s northern prawn fishery

Bycatch in Australia s Northern Prawn Fishery (NPF) is extremely diverse in both species and size composition. This makes developing responsible fishing practices to reduce the amount of bycatch in the NPF, a complex process. During this study we assessed the performance of Bycatch Reduction Devices (BRDs) that could be used in the NPF. We tested 16 different BRDs which can be grouped into one of 3 categories: (1) inclined grids (2) fish exclusion devices and (3) square-mesh codends. Four inclined grids were tested during our study. All the grids were extremely effective at excluding large animals such as sharks, stingrays and sea turtles. Three different fish exclusion devices were tested. Fish exclusion ranged between 0 and 39 percent, depending on the device. Prawn retention rates for both inclined grids and fish exclusion devices varied between devices. Two sizes of square-mesh codend (38 mm and 45 mm) were also tested. More than 95% of market-sized prawns were retained. Fish exclusion varied greatly between species. Optimum exclusion of bycatch for the NPF could be provided by a combination of devices from each of the three categories

Summary of data from productivity experiments in the York River, Virginia : June 1960 - June 1961

Also part of series: Virginia Fisheries Laboratory special scientific report ; no. 22