4.3 General Discussion of the Attentional Blink studies
4.3.3 Implications of the findings on theories of the AB
Utilising the RSVP procedure, which results in an AB when two targets must be identified, has not only provided information regarding the carry-over effect, it has also provided additional evidence for the AB effect. Therefore it is important to assess how the main findings of the AB experiments in this thesis fit with theories of the AB. The results show that when T1 is substantially different from T2, and always precedes T2 it can be used as a cue to this second target in a single target RSVP. In addition, there was evidence for individual variation in AB magnitude, and crucially, AB magnitude was related to carry-over, with carry-over occurring when the blink was more substantial. Finally the studies have shown that increasing the similarity between RSVP items (including T1 and T2) increases the blink.
The cuing effect is not necessarily useful in supporting one theory over another; however it questions the suitability of using a single target block as a control condition. AB studies should allow for the possibility that an irrelevant T1 may alert attention to T2 and measures should be taken to avoid the effect. One way that this author attempted to eliminate any cuing was by increasing the similarity between items in the RSVP; if T1 is more similar in size and appearance to the distracters it may not capture attention in a single target block. Of course increasing the similarity between T1 and the distracters also increased the similarity between T1 and T2. It may be argued that making the target more similar would lead to contingent capture in a single target block; however the pattern of performance in the no-set-priming group is not consistent with this, and instead fits with the assumptions of Di Lollo,
Kawahara, Ghorashi, & Enns (2005) that when faced with a set of highly similar stimuli the attentional set will be more specific in order for targets to be selected and distracters to be inhibited. The lack of contingent capture when T1 is task-irrelevant
shows the importance of top-down control, and also shows that the orienting system can be configured to a specific level on the basis of task requirements.
In addition to including a negative lag in which T2 was presented before T1, raising the similarity of the stimuli effectively removed the cuing effect. It also increased the magnitude of the blink (from Experiment Two to Experiment Three). This increase in the size of the blink is consistent with previous studies showing that if RSVP stimuli are similar blink magnitude increases (e.g., Isaak et al., 1999; Olivers & Watson, 2006). This is assumed to be because the processing system has to work harder to select targets from distracters (e.g., Bundesen, 1990; Duncan & Humphreys, 1989; Visser et al., 2004). The finding is also consistent with limited capacity theories of the AB (e.g., Chun & Potter, 1995; Jolicœur & Dell’Acqua, 1998; Raymond et al., 1992). According to such theories, T2 processing is impaired at early lags because the system is busy processing T1, preventing T2 from being detected (Raymond et al., 1992), or causing the decay of T2 by post T2 distracters (Chun & Potter, 1995). As similarity increases more resources will have to be given to the detection and
identification of T1, leaving fewer resources for T2 and causing a larger blink. This is contrary to other findings which show that the task difficulty of T1 does not alter blink magnitude (Shapiro et al., 1994).
One interesting finding however is that in Experiment Three T1 accuracy was higher for nonblinkers (
x
¯ = 93%) than for blinkers (x
¯ = 85%). It may be presumed that if more resources were allocated to T1 (improving performance) this would have a greater impact on T2 accuracy, in which case blinkers should show higher T1 accuracy. Arnell, Howe, Joanisse, and Klein (2006) have also found that T1 accuracy correlates negatively with AB magnitude; the higher the accuracy the smaller the blink. They favour a limited-capacity account of the AB and propose that if anindividual can encode T1 effectively the system can move on to processing T2. This means that T2 will suffer from interference from T2+1 to a lesser extent, reducing the size of the blink. Better T1 performance will therefore coincide with a smaller blink. Consequently it seems that although increasing similarity does increase processing demand of the RSVP task (enhancing the AB); some participants are still better able to encode T1 than others. Despite the individual differences found these results fit well with resource depletion explanations.
These explanations may be less suitable for adequately explaining why blinkers were more likely to suffer from carry-over than nonblinkers (a finding of Experiment Four). It has been concluded that the carry-over effect found in the present AB experiments is due to the investment of resources into an attentional set, and the perceived costs of changing this set. When participants put more effort into completing the RSVP task (and suffer from a larger AB as a result of this) the set strengthens and the costs of switching set increase, leading to carry-over. Although the carry-over effect is therefore attributed to a balance of resources (which would fit with a resource depletion account), it stems from the influence of top-down control; more control leads to more resources. Resource depletion theories do not attribute the AB effect to top-down control, other than stating that attention is allocated to the RSVP using top-down processing. It is possible to explain the present findings using these theories because processing resources appear to be key to both the AB effect and the carry-over effect, however limited-capacity theories may benefit by attributing a greater influence to top-down control.
The TLC model (Di Lollo, Kawahara, Ghorashi, & Enns, 2005) does attribute the AB to top-down control. The model states that the endogenous filter which allows targets to be selected requires constant feedback and when T1 enters the processing
system these signals are interrupted. If T1+1 is a distracter it will trigger an exogenous set and the top-down set will have to be reconfigured to regain control. The reconfiguration is time consuming and effortful and it is this which causes an impairment to T2 identification. When more resources are allocated to the RSVP task the attentional set may strengthen; this will mean that reconfiguration of this set following a T1+1 distracter will be more costly (increasing blink magnitude) and it will be more likely to persist to a second task. This model can therefore effectively account for the link between AB magnitude and carry-over.
The present finding that increased similarity of RSVP items increases the magnitude of the AB cannot be so easily explained using the TLC model. Although the model attributes the AB to a loss of top-down control, this loss is directly influenced by the characteristics of T1+1, not the characteristics of T1. Blink magnitude should therefore not be affected by the processing resources allocated to T1. Di Lollo, Kawahara, Ghorashi, & Enns (2005) have based their model on findings showing that when T2 is presented immediately after T1, at lag 1, and a third target is situated at lag 2, there is no AB on the third target (T3). The model accounts for this by stating that T2 triggered the original attentional set, therefore there was no set switch and no associated processing costs. However, Dux, Asplund, and Marois (2008) completed a similar study and found that when greater attentional resources are given to T1 a blink will occur on T3, showing that the characteristics of T1 do have a role to play in the AB. Moreover they report that the findings of Di Lollo and colleagues were paired with low T1 accuracy, suggesting that T2 identification did not suffer because fewer resources were allocated to T1. The TLC model would predict no correlation between AB magnitude and T1 accuracy (even though it can explain the link between carry-over and AB magnitude), yet this has been found in the present
set of results (Experiment Three) and by Arnell et al. (2006). This provides further support for a resource depletion account.
There are of course several other models to account for the AB effect; however the primary aim of the present AB studies was not to evaluate these models and as such focus has been given to the most prominent accounts. The findings of the five AB experiments completed for this thesis appear to lend more support to resource depletion theories; however the relationship between AB magnitude and carry-over cannot be sufficiently accounted for at present. This highlights the benefits of utilizing the AB paradigm to measure other aspects of attentional control; it can provide more varied support for particular theories. A study by Zhang, Shao, Nieuwenstein, and Zhou (2008) demonstrates this. They measured the effects of the AB on an individual’s ability to orient attention to different spatial locations, and found that when a cue was presented in the time window between T1 and T2 it could effectively orient attention to the spatial location of T2. Zhang et al. state that their results support resource depletion accounts of the AB by showing that top-down control is not lost following the appearance of a distracter at T1+1. In addition, although an AB effect does occur, this does not prevent items presented at lags 2-5 from being processed to a certain extent (e.g., Vogel et al., 1998), or from acting as a cue to a later target (e.g., Nieuwenstein, Chun, van der Lubbe, & Hooge, 2005; Shapiro et al., 1997).
It may therefore be concluded that although the present findings cannot definitively support a resource depletion account of the AB, the evidence is most in favour of such an account. Moreover, by using the RSVP methodology and the AB effect to study a different aspect of attentional control, the experiments in this thesis have provided more varied evidence for the theory.