The work described within this thesis will further investigate the three key areas o f research outlined within this chapter. That is, the experiments will study issues relating to Tnattentional Blindness’, ‘Change Blindness’ and transaccadic processing. As discussed in detail earlier, much research into these three areas has appeared to reveal that little information can be perceived or brought forward without attention and/or across saccades. However, much of this evidence is derived from explicit and often retrospective measures. Therefore, a unifying theme within this thesis will concern the use of indirect measures, in an attempt to examine whether implicit processing can occur in the absence of explicit perception. Furthermore, the role of attention in explicit change detection will be thoroughly examined.
The first two experimental chapters examine whether an indirect and on-line method can reveal implicit processing of Gestalt visual grouping under conditions that appear to conform to Rock, Mack and their colleagues’ (e.g.. Mack, Tang, Tuma,
Kahn & Rock, 1992) criteria for ‘inattention’. Within these experiments, subjects
complete a central task while an ignored pattern of grouped elements is presented as surrounding background to the central stimulus. Background grouping can either be congruent or incongruent with subjects’ responses in the task; thus the paradigm will examine whether congruency or otherwise o f background grouping can effect behavioural responses to the primary task. It is predicted that if visual grouping can be implicitly processed, subjects should be less accurate and/or slower to respond
when background grouping is incongruent to the task response. Furthermore, a
surprise question block at the end o f each study will examine whether the stimuli meet
attempting to answer these retrospective explicit questions regarding the background grouping.
Within the third experimental chapter (Chapter 4) the paradigm utilised in these earlier experiments will be adapted and extended in order to assess whether the Gestalt grouping can be implicitly extracted and compared across a saccadic eye movement. That is, subjects will complete a similar task to that previously utilised whilst an ignored grouping pattern is simultaneously displayed on the screen. However, in order to complete the task subjects must now make a saccade during each trial. The same congruency measure utilised earlier will examine whether there is implicit processing of any change in the grouping across eye movements. Additionally, as in the first two experimental chapters, a retrospective surprise question block will also be presented to the subjects at the end o f the study.
During Chapter 5 the studies shift topic slightly to address ‘Change Blindness’ (CB). The first experiments on this issue examine the role of attention in a standard CB paradigm. As outlined earlier, the posited role of attention in change detection has been suggested by the faster detection of changes in areas of ‘Central’ rather than ‘Marginal’ interest (e.g., Rensink, O ’Regan & Clark, 1997). This appears to be a rather circuitous way in which to assert a causal role for attention; thus the experiments within this chapter will utilise attentional cues to assess their effect on change detection more directly. These cues will either occur before the change (pre cues), after the change (post-cues) or at the end of the trial (delayed-cues) in an attempt to delineate any differential effect of the timing o f attentional allocation to the locus of change. It is expected that pre-cues will substantially improve change detection. Moreover, it is possible that post-cues could also facilitate detection of
changes, which would demonstrate that internal visual representations are unlikely to be as sparse as proposed by many CB researchers (e.g. see O'Regan, 1992).
Chapter 6 continues to examine the role of attention within ‘Change Blindness’. In an attempt to further substantiate the claims of Rensink et al (1997) concerning facilitated change detection at areas o f ‘Central Interest’, it is hypothesised that areas described as being of ‘Central Interest’ could correspond to the ‘figure’ of the image whereas areas of ‘Marginal Interest’ could be described as the ‘ground’, within the terms of the classical figure versus ground dichotomy (e.g., Rubin, 1915). Using simplified displays, this chapter attempts to investigate whether changes are preferentially detected in the foreground (or ‘figure’) o f the image as compared to the background (or ‘ground’), and whether this is due to a spontaneous bias to attend to foreground regions. In order to provide a strong test of this proposal, changes to the background are physically stronger than those occurring in the foreground. It was predicted that background changes will nevertheless be consistently missed in conditions that allow spontaneous allocation of attention. However, these background changes should become easily detectable once subjects are informed that they should attend to the background of the scene, not just the foreground.
The experimental approach in Chapter 7 returns to the examination of implicit processing outside the focus of attention using indirect methods. Utilising similar displays to those in the previous chapter, the experiment examines whether undetected background changes can nevertheless undergo implicit processing. To this end, a simultaneous contrast illusion is exploited (e.g.. Rock, 1975), whereby a smaller item resting upon a larger background appears to change its own brightness when just the background undergoes a change of luminance. This illusion will be manipulated such that, in some conditions, when the background changes it can induce the illusory
impression that a foreground item has changed in brightness. It is predicted that subjects will continue to miss the background changes under spontaneous attention conditions as in the experiments of Chapter 6. However, implicit extraction of the background change would be demonstrated if subjects detect illusory change in the foreground, as this illusion could not be perceived without the computation of the
luminance values of the background. This would suggest that the background
luminance must be extracted despite changes to it being undetected.
In order to confirm that any results of Chapter 7 are not specific to the luminance domain, the final experimental chapter attempts to refine and extend these findings to the case of visual motion. Chapter 8 investigates whether background motion can be implicitly extracted to produce illusory foreground motion, despite an explicit failure to detect the effective background motion. Thus, this experiment will take advantage of an induced motion illusion within which observers generally perceive a vertically moving foreground dot to move diagonally if the background behind it undergoes a horizontal or diagonal shift (e.g. Bacon, Gordon & Schulman,