ENCABEZADO

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1st display with cue superimposed 50 msecs

1st Display 50 msecs

Figure 5.9. Schematic example of Pre-cue trials within E xperim ent 9.

The three cue conditions were: pre-cue, post-cue, delaved-cue. Post-cue and delayed-cue trials were identical to those in Experiment 8. However the new type of pre-cue trials differed. Each trial in the pre-cue condition began with a 50 msecs presentation of the grid with nine pictorial items. After this time a cue was introduced to the scene and the whole display (nine items, grid and cue) remained on screen for a

further 50 msecs. This cue thus appeared prior to any change, during the first display. The pictorial items then were removed, leaving just the grid and the cue for 100 msecs. Finally, nine items reappeared for 100 msecs, after which subjects were required to judge whether the item within the cue was the same as that presented in the first picture display. After 2000 msecs (or the subjects’ response if later) the next trial began. Again, see Figure 5.9 for a schematic example o f a pre-cue trial from the present experiment.

The presentation order of the six blocks were counterbalanced across subjects. It was never the case that the same type of cue-condition block (i.e., pre-cue, post-cue or delayed-cue) was presented consecutively. The screen was viewed from 50 cm with position and distance maintained with a chin rest.

Results and Discussion

See Figure 5.10 and Table 5.4 for mean accuracy scores across cue condition, split by change versus no-change trials. Accuracy data were first analysed in a within- subjects Friedman’s test to examine the effect of overall cue condition (pre-cue versus post-cue versus delayed-cue). This revealed that there was an overall effect of cue

condition: (2)= 22.167, p < .001.

Consequently, Wilcoxon signed-rank-tests were performed on the data. These revealed that pre-cue trials were significantly more accurate than both post-cue or delayed-cue trials: Wilcoxon T(N=10, 2 tied) = 0, p < .01; Wilcoxon T(N=11, 1 tied) = 0, p < .01 respectively. Additionally, subjects were significantly more accurate in post-cue rather than delayed-cue trials, replicating Experiment 8: Wilcoxon T(N=10, 2 tied) = 4.5, p < .05.

% Correct

Pre-Cue Post-Cue Delayed-Cue

Overall 85 70 62

Change Trials 80 69 51

No-Change Trials 89 71 72

Table 5.4. Mean accuracy (% ) for Experim ent 9.

C hange N o-C nan ge

Pre-Cue Post-Cue D elayed -C ue

Figure 5.10. Mean accuracy in E xperim ent 9, across cue condition (pre versus post versus delayed), split according to trial type (change versus no-change).

Data were then split aceording to whether they eorresponded to ehange versus no- ehange trials. A Friedman’s test for the six conditions (pre-cue/change, pre-eue/no- ehange, post-eue/ehange, post-eue/no-change, delayed-cue/ehange, delayed-eue/no- change) demonstrated that there was a significant interaction between cue type and

trial type; (2) = 29.916, p < .0001.

Wilcoxon signed rank tests were then used to further investigate this

interaction. Pre-cue/change was significantly more accurate than both post­

cue/change and delayed-cue/change : Wilcoxon T(N=11, 1 tied) = 12, p < .05; Wilcoxon T(N=12) = 0, p < .01 respectively. Furthermore, post-cue/change trials were responded to significantly more accurately than delayed-cue/change trials, replicating Experiment 8: Wilcoxon T(N=11, 1 tied) = 5, p < .01.

For no-change trials, pre-cue trials were again significantly more accurate than both post-cue and delayed-cue trials: Wilcoxon T(N=11, 1 tied) = 0, p < .01; Wilcoxon T(N=10, 2 tied) = 3, p < .01 respectively. There was no difference in accuracy between post-cue and delayed-cue no-change trials: Wilcoxon T(N=10, 2 tied) = 38, n.s..

Finally, data were analysed to examine whether change versus no-change trials varied in accuracy within each cue condition. There was no difference in accuracy during pre-cue and post-cue trials according to whether the trial involved a change or not: Wilcoxon T(N=10, 2 tied) = 21, n.s; Wilcoxon T(N=11, 1 tied) = 35, n.s.. However, subjects were significantly more accurate on no-change trials during delayed-cue blocks: Wilcoxon T(N=11, 1 tied) = 3.5, p < .01.

It appears that the pre-cue is most effective in helping subjects identify whether the stimulus has changed or not. This was expected due to the earlier results with complex visual scenes (see Experiment 6). However, the post-cue remains more effective than the delayed-cue, suggesting that subjects maintain some knowledge from the first scene for at least a short period of time that can be extracted with the help of a post-cue prior to onset of the second display.

General Discussion

The experiments outlined in this chapter have used attentional cues, to

investigate two issues. The first issue is whether Rensink and his colleagues (e.g.

O'Regan, Rensink & Clark, 1996: Rensink, O'Regan & Clark, 1997; O'Regan, Deubel, Clark & Rensink, 2000; Rensink, O'Regan & Clark, 2000) were correct in suggesting that visual attention is a vital component in change detection. The second issue concerns whether it is possible that the representation of the first experimental scene may be somewhat less sparse, and potentially longer lasting, than suggested by previous studies of ‘Change Blindness' (e.g. O'Regan, Rensink & Clark, 1996: Rensink, O'Regan & Clark, 1997; O'Regan, Deubel, Clark & Rensink, 2000; Rensink, O'Regan & Clark, 2000).

First, it has previously been suggested that without focused attention, changes

remain undetected (e.g. Rensink, O'Regan & Clark, 1997). However, the

methodology previously used to assess the importance of attention in ‘Change Blindness' may have been flawed due, for example, to its reliance on verbal descriptions. Therefore, attentional pre-cues were used during Experiment 6 in order to manipulate whether attention was directed to the locus of the change. Pre-cues greatly improved change detection for the complex visual scenes used during this experiment, thereby suggesting that Rensink et al (e.g., 1997) may be correct in proposing a fundamental role for attention in change detection across interruptions.

Experiment 7 investigated the second issue, examining whether some representation o f the first scene can be maintained by subjects to a somewhat greater

degree than the ‘Change Blindness' paradigm has been taken to suggest. Within this

respect to the first display). Studies of iconic memory (e.g., Sperling, 1960) have demonstrated that components of a brief visual display can outlast what one would expect from uncued whole-report performance, when a subset o f the display is cued shortly after its offset. Hence, it was hoped that a post-cue of this type could improve change detection through enabling access to the visual representation o f the relevant

part of the first scene. However, the post-cue did not improve performance on the

change detection task, at least when using the complex scenes of Experiment 7. This might have arisen because no iconic representation endures or alternatively because of the delay in the post-cue, or some difficulty in using it to interrogate the preceding complex scene.

Experiment 8 was carried out in order to investigate whether a post-cue could improve performance in more controlled simplified displays. Additionally, the timing of the post-cue was altered such that now it appeared immediately after the offset of

the first display. Results of this experiment suggested that these post-cues could

indeed aid change detection, as subjects were significantly better at detecting changes with a post-cue as opposed to a delayed-cue (or no cue at all). Finally, as the paradigm utilised had now altered so much since the first experiment in this chapter, which had initially demonstrated the effectiveness of pre-cues in change detection. Experiment 9 was carried out. This study compared subjects’ performance when cued with a pre-cue, a post-cue, or a delayed-cue, on displays identical to those used in

Experiment 8. This experiment confirmed that a pre-cue was most effective in

improving subjects’ detection of the changes. However, the benefit of a post-cue appearing immediately after the offset o f the first display was replicated, relative to the delayed cue.

Thus, Experiments 6 and 9 provide evidence in support of the role of attention within a ‘Change Blindness’ paradigm as suggested by Rensink and his colleagues (e.g. O’Regan, Rensink & Clark, 1996: Rensink, O’Regan & Clark, 1997). It was important to confirm this role for attention, as earlier descriptions o f the role of attention in change detection were somewhat unclear. It is important to note that eye movements are not likely to have played a large part in the present results, as the displays in which the cue is presented are too brief to allow a saccade to the cued item during the first displav. Thus, as processing of both displays is necessary to detect the change, and there is not sufficient time for the eyes to reach the cued item in the first display, the pre-cue benefit cannot be explained merely by eye movements.

Scholl (2000) has also attempted to investigate the role of attention in change detection with exogenous attentional cues, in work published subsequent to the completion of the present study. Just as the work within this chapter, he reasoned that established forms of exogenous cue would provide stronger evidence (than the comparison of Cl versus MI regions from the scene) that visual attention was at the site of the change, and thus should confirm whether the failure to detect changes was

attenuated at the locus o f attention. His experiments did not use full-colour

photographic images as Rensink et al (1997) had done; instead, his displays consisted of an array of black-and-white line drawings of everyday objects, selected from

Snodgrass & Vanderwart (1980). Twelve o f these drawings appeared in each

experimental display. There was a change in every trial and subjects were told to identify which item had changed as quickly as possible. Each change was either a replacement of one of the original items with a different line drawing, or a rotation of one item such that it turned 180 degrees. As in the original ‘Change Blindness’

studies, the displays (the original and the changed version) were iterated with an intervening blank until the subject responded.

Subjects were either presented with the experimental displays as described above, or they received one o f two forms of exogenous cue during their trials. These cues were either a Tate-onset’ or a ‘colour-singleton’. For the late-onset cue one of the items in the original display appeared 200 msecs later than the other 11 items in the scene. It has previously been shown that this form of cue can lead to exogenous attentional capture by the late-onset item (e.g., Yantis & Jonides, 1984). The colour- singleton condition involved one of the figures appearing in blue or red amongst the other uniform black items. It is also well established that colour-singletons can lead

to attentional capture by the unique items (e.g., Theeuwes, 1992). Importantly,

attentional cues were used no more frequently on items that changed than for items that did not change. Therefore, subjects were aware of the fact that attending to the

cued item would not help them in the change-detection task. However, results

demonstrated that change detection was greatly speeded if the change occurred at the

site of the attentional cue. Scholl’s result seems to provide further support for

Rensink et al’s (1997) assertion that attention is intimately involved in change detection across interruptions, as the spontaneous capture of attention to the site of the change ameliorates the poor detection otherwise observed.

It is difficult to judge whether the present Experiment 7 failed to obtain a post­ cue benefit due to the complexity of the scenes or due to the late presentation of the post-cue. Further studies are needed to clarify this issue. However, it does seem to be the case that a post-cue can support change detection when it is presented immediately the first experimental display, during displays that are ordered and simple (Experiments 8 & 9). It thus appears that some form o f iconic memory of the