A common problem in vision research is explaining how humans perceive a coherent, detailed and stable world despite the fact that the eyes make constant,jumpy movements and the fact that only a small part of the visual field can beresolved in detail at any one time. This is essentially a problem of integrationover time - how successive views of the visual world can be used to create theimpression of a continuous and stable environment. A common way of studyingthis problem is to use complete visual scenes as stimuli and present a changedscene after a disruption such as an eye movement or a blank screen. It is found inthese studies that observers have great difficulty detecting changes made duringa disruption, even though these changes are immediately and easily detectablewhen the disruption is removed. These results have highlighted the importance ofmotion cues in tracking changes to the environment, but also reveal the limitednature of the internal representation. Change blindness studies are interestingas demonstrations but can be difficult to interpret as they are usually applied tocomplex, naturalistic scenes. More traditional studies of scene analysis, such asvisual search, are more abstract in their formulation, but offer more controlledstimulus conditions. In a typical visual search task, observers are presented withan array of objects against a uniform background and are required to report onthe presence or absence of a target object that is differentiable from the otherobjects in some way. More recently, scene analysis has been investigated bycombining change blindness and visual search in the ‘visual search for change’paradigm, in which observers must search for a target object defined by a changeover two presentations of the set of objects. The experiments of this thesis investigate change blindness using the visual search for change paradigm, but alsouse principles of design from psychophysical experiments, dealing with detectionand discrimination of basic visual qualities such as colour, speed, size, orientationand spatial frequency. This allows the experiments to precisely examine the roleof these different features in the change blindness process. More specifically, theexperiments are designed to look at the capacity of visual short-term memory fordifferent visual features, by examining the retention of this information acrossthe temporal gaps in the change blindness experiments. The nature and fidelityof representations in visual short-term memory is also investigated by manipulating (i) the manner in which featural information is distributed across space andobjects, (ii) the time for which the information is available, (iii) the manner inwhich observers must respond to that information. Results point to a model inwhich humans analyse objects in a scene at the level of features/attributes ratherthan at a pictorial/object level. Results also point to the fact that the working representations which humans retain during visual exploration are similarlyfeature- rather than object-based. In conclusion the thesis proposes a model ofscene analysis in which attention and vSTM capacity limits are used to explainthe results from a more information theoretic standpoint.
Prioritization of new objects during visual search is limited by the capacity of visual short-term memory