Attentional Blink (AB, or ‘blink’) is the phenomenon that the second of two targets cannot be detected or identified when it appears close in time to the first.
The ‘attentional blink’ (AB) was first described by Raymond, Shapiro, & Arnell (1992), though reports published prior to this revealed the existence of the same outcome (e.g., Broadbent & Broadbent, 1987) but did not use this term. The basic AB paradigm employs a method known as rapid serial visual presentation (RSVP) where stimuli such as letters, digits, or pictures are presented successively at a single location at rates between 6 – 20 items per second. In the procedure derived by Raymond et al. (1992), participants were required to identify the only white letter (first target; T1) in the 10-item per second RSVP stream of black letters (non-targets or distractors), then to report whether the letter ‘X’ (second target; T2) occurred in the subsequent letter stream (see Figure 1 for a static example and Figure 2 for a slowed dynamic example). T2 was presented on only 50% of trials and when presented, occurred with an interval separating the two targets of between 100 to 800 ms. Report of both targets was required after the stimulus stream ended. The AB is defined as having occurred when T1 is reported correctly but report of T2 is inaccurate at short T1/T2 intervals, typically between ~100 to 500 ms, but recovers to the baseline level of accuracy at longer intervals (see Figure 3).
Given that the RSVP stream containing the two targets is flashing by at 10 items per second, one possible account of the AB is that the task is perceptually too demanding, with each item potentially masking the preceding item. To test this hypothesis Raymond et al. (1992) implemented a control condition where the stimuli were the same as in the experimental condition described above – thus equating the two conditions perceptually – but participants were not required to report T1. No AB occurred in this condition suggesting attention to T1 and not perceptual masking accounts for the AB outcome. This control also points out that the AB is not due simply to better performance at the end of the RSVP stream as opposed to the beginning, i.e., a recency effect. Another possible explanation is that the AB is not due to a failure of attention but to a failure to retain memory of the two targets until prompted for report at the trial end. The duration and capacity of visual short-term memory (VSTM), however, makes this account unlikely. Finally, task switching costs and spatial switching costs are also different from the AB. The AB occurs even when T1 and T2 involve the same task and are presented in the same location (e.g., Chun & Potter, 1995). Adding a task switch between T1 and T2 may add to the T2 deficit.
A few AB C’s
The question is often asked, “What are the requirements to produce an AB?” One issue that has been investigated is whether the full RSVP stream is required. To investigate this Ward, Duncan, and Shapiro (1997) presented masked targets, but without any other stimuli, in the centre of a visual display and obtained an AB outcome, though not revealing the typical AB function (see lag-1 sparing below). Although such a ‘skeletal’ procedure is far less often employed than the typical RSVP stream procedure, nevertheless, an AB does result. Another issue investigated has been the importance of masking both T1 and T2. Seiffert & Di Lollo (1997) concluded there are different requirements for the nature of the T1 vs. T2 mask, suggesting the role played by the mask is different for each target. In a related issue, the vast majority of published reports have employed a mask (typically the item in the RSVP stream succeeding each target) as Raymond et al. (1992) found that removing the T1 mask completely attenuated the AB. However, Visser (2007) discovered an AB can be obtained without the use of a T1 mask, though the T1 task was novel and thus likely more difficult. Moreover, Giesbrecht and Di Lollo (1998) showed that T2 must be appropriately masked; simply degrading T2 does not produce an AB. Finally, the issue of T1 difficulty and its effect on the AB has been heavily investigated with some investigators finding a relationship between T1 difficulty and the magnitude of the AB (e.g., Seiffert & Di Lollo, 1997; Ward Duncan, & Shapiro, 1997) and others not (McLaughlin, Shore, & Klein, 2001). Although it might be expected that the AB should be maximal immediately following T1, a phenomenon referred to as lag-1 sparing often arises, where T2 accuracy is preserved when T2 occurs immediately after T1. One final question is whether an AB exists within other modalities (audition, touch) such as exists in the visual modality and whether cross-modal (between modalities; e.g., visual T1, auditory T2) AB’s exist. Evidence suggests that both the above experimental situations can lead to an AB, although whether or not an AB occurs depends to a considerable extent on the method employed.
Temporal vs. spatial attention
The AB paradigm can be viewed as a method to study the availability of attention across time. More frequently, attention is studied from the perspective of how it is distributed across space. For example the method of visual search examines the ability to deploy attention across spatial locations to find a target. As all stimuli are presented in a single location in the canonical AB paradigm, the deficits observed cannot be attributed to a failure to appropriately distribute attention across space. A good way to think about the AB paradigm is that it enables one to chart the availability of attention for the second target task, after it has been deployed to the first target task.
What came before
Although the term ‘attentional blink’ originated with the first published report (Raymond et al., 1992), investigators employing different approaches found results similar to the AB. Notable among these is a finding by Weichselgartner and Sperling (1987) who required participants to report the identity of the only white letter in an RSVP stream of black letters then to report the next three letters. Similar to the outcome of the AB method, participants reported with a higher but equal probability the letter just following T1 and the letters in the 5th, 6th, and 7th positions. Letters in the 2nd – 4th positions were reported with a lower frequency indicative of an outcome like the AB. The AB procedure employed by Raymond et al. does not suffer from the possible alternative interpretation, i.e., a failure of memory, as does the study by Weichselgartner and Sperling. In another early experiment to examine the timecourse of attention to identify a second target after identifying a first, Broadbent and Broadbent (1987) required participants to report the identity of two words, defined by being uppercase in an RSVP stream of lowercase words. Similar to the outcome of Raymond et al.’s study, participants were able to report the target and the word just following it with a reasonably high frequency but were deficit in reporting words occurring between approximately 400 to 700 ms after the first target. The improvement of the Raymond et al. study over that by Broadbent and Broadbent was the finding of a similar outcome but using stimuli, i.e., letters, that did not require access to ‘meaning’, as well as the attentional control condition discussed previously.
Not be confused with the AB
Two experimental outcomes have been studied extensively that share a high degree of similarity to the AB. The first of these is known as ‘repetition blindness’(RB), where it has been determined that participants tend to omit the second occurrence of a repeated word when the interval separating the two occurrences is between ~ 100 to 700 ms (Kanwisher, 1987). Although the similarity between these two outcomes is obvious, various investigators (e.g., Chun, 1997; Ward, Duncan, and Shapiro, 1997) have shown the two effects to be dissociable. Another phenomenon similar to the AB is known as the psychological refractory period (PRP). In the PRP paradigm, two targets separated by a varying interval and often from different modalities are required to be identified as quickly as possible after each is presented. The targets do not occur as part of an RSVP stream and are not masked. The basic finding is that whereas RT to the first target remains unaffected by the temporal separation between the two targets, RT to the second target is substantially lengthened the closer the two targets occur in time to each other. The requirement to make a reaction time response to both targets differentiates the PRP from the AB in an important way that has implications for their respective underlying mechanisms (cf. Arnell, Helion, Hurdelbrink & Pasieka, 2004).
A diverse number of manipulations have been made to the basic AB paradigm. Experimenters have examined various aspects of the target task; including the target ‘category’ (letters, digits, symbols, colours, and pictures), perceptual manipulations to the target such as contrast, the number of targets (1 vs. 2 vs. 3), and the categorical and semantic relationship of the targets to the non-targets (distractors). Investigators have varied the duration of the stimuli in the RSVP stream yielding different rates of presentation. Typically such variations are held constant for a given experiment but recently, different rates of presentation within and between RSVP streams in a given experiment have been examined.
When does an AB not occur?
As previously mentioned, an AB may not be observed in some cross-modal experimental situations. Other examples where an AB may not be observed are when T2 is a highly salient target, such as when the second target is one’s own name (Shapiro, Caldwell, & Sorensen, 1997), a highly familiar face (Jackson & Raymond, 2006), one associated with reward (Raymond & O’Brien, in press), or an emotionally arousing word (Anderson, 2005. The AB is also absent when the entire RSVP stream including the targets is morphed from one object into another (Kellie & Shapiro, 2004) or when the T1 task requires detection of the appearance of a new feature of an object seen in the preceding non-target stream (Raymond, 2003). More recently, it has been shown that distracting stimuli, such as music playing in the background, also causes the AB to be significantly attenuated (Arend, Johnston, & Shapiro, 2006; Olivers & Nieuwenhuis, 2005). In a related issue, Luck, Vogel, & Shapiro (1996) found that even when T2 cannot be reported, it nevertheless evokes a semantic awareness of the second target as indexed by event-related potentials (ERPs).
In addition to the behavioural approach, upon which virtually all of the experiments discussed thus far have been based, a variety of other approaches have proved useful for studying the AB phenomenon. Cognitive neuropsychological investigations have revealed, for example, that individuals with lesions in the amygdala fail to show a reduced AB for highly emotional words, such as that shown by individuals without such lesions (Anderson & Phelps, 2001). Moreover, patients with hemineglect due to parietal or frontal lesions suffer from an abnormally prolonged AB (Husain et al., 1997). Electroencephalography (EEG) has provided evidence that, although T2 cannot be reported on a significant proportion of trials in the AB paradigm, participants nevertheless are processing T2 to the level of meaning (Luck, Vogel & Shapiro, 1996). Magnetoencephalography (MEG) has enabled researchers to conclude that the modulation of the long-range synchronisation of neuronal populations can predict when an AB will occur (Gross, Schmitz, Schnitzler, I., Kessler, Shapiro, Hommel, & Schnitzler, A., 2004). Functional imaging (fMRI; Marois, Yi, & Chun, 2004) and transcranial magnetic stimulation (TMS; Cooper, Humphreys, Hulleman, Praamstra, and Georgeson, 2004) have revealed the locus of brain areas involved in the AB and have shown different amounts of neural activation to correctly reported (T2) vs. incorrectly reported targets. Finally and more recently, neuropharmacological approaches have implicated the involvement of well-studied neurotransmitter systems, such as dopamine, and suggested ways in which such systems may play a role in mediating the AB (Nieuwenhuis, Gilzenrat, Holmes, and Cohen, 2005).
Although there are a number of different ideas to explain the AB, all current theories place the mechanism leading to it at a relatively late stage of processing, i.e., after sensory processing and at or after a stage related to selective attention. These theories fall into three main categories, though distinctions between these categories are blurred by neural mechanisms that do not necessarily respect such pigeonholing: (1) Filter-based accounts, where the AB reflects the workings of a mechanism designed to filter out non-relevant visual information (e.g., Di Lollo, Kawahara, Ghorashi, & Enns, 2005; Raymond et al., 1992); (2) Consolidation bottlenecks, where the AB arises from a bottleneck in the transfer of highly-processed information into a short term memory cache that eventually makes information available to consciousness (e.g., Chun & Potter, 1995; Sergent, Baillet, and Dehaene, 2005); and (3) Retrieval complications, where the AB reflects errors in the retrieval of information from a short term memory cache into consciousness where it is accessed for veridical report (e.g., Shapiro et al., 1994). Importantly numerous computational models have also been developed recently that more formally set out possible mechanisms underlying the AB. They too are generally based around either filter-based approaches (e.g., Olivers & Meeter, 2008) or consolidation bottlenecks (e.g., Bowman, & Wyble, 2007)
Recent investigations involving the AB paradigm have shifted emphasis from examining the results of parametric manipulations to using the paradigm to investigate high-level cognitive functions such as consciousness. There has also been a rise in the use of the AB paradigm to study deficits in cognitive function in specific clinical populations, e.g., schizophrenia. Finally, individual difference investigations of the AB have been used to examine whether the magnitude of an individual’s AB can be predicted by other cognitive/affective mechanisms, e.g., working memory, fluid intelligence, emotion, and learning.
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