9/01/2006

Selection and Updating Efficiency in the Attentional Blink

In the attentional blink paradigm, two targets are serially displayed in rapid succession; astonishingly, participants show a brief temporal window in which they cannot identify the second target, while on either side of that window recognition proceeds normally. It is as though the proverbial mind's eye must "blink" in order to attend to two temporally distinct meaningful items. Imaging research shows that the second incoming item is still processed by higher visual areas, even if subjects are unable to report seeing that item - so, what happens during this time, when participants are functionally blind?

No one really knows. One explanation is that this "attentional blink" is actually the switch cost associated with task switching between looking for target 1 and looking for target 2 (also known as switching the "attentional set"). Accordingly, several studies implicate the areas responsible for working memory: right posterior parietal, cingulate, and left temporal/frontal regions. Further, when the second target is detected, subjects tend to show a large area of phase coherence in the gamma range (30-80 Hz) throughout the task, suggesting that this synchronized activity might reflect differences in working memory updating or attentional focus, which subsequently translate into improved target detection.

What other kinds of individual differences can be observed in this paradigm? As reviewed in the Martens et al. paper in the current issue of the Journal of Cognitive Neuroscience, patients with neurological damage to inferior parietal, superior temporal gyrus, or frontopolar cortex show a prolonged attentional blink, as do those with dyslexia or ADHD. And yet there is also a minority in normal populations who show no attentional blink whatsoever. In an fMRI study comparing "blinkers" to "nonblinkers," nonblinkers showed more anterior cingulate activity (presumably engaged by some form of conflict when both targets appear close together), more medial prefrontal cortex activity (which Martens et al interpret as reflecting self-directed attentional focus) and frontopolar activity (engaged by the updating of working memory and sometimes also thought to reflect subject preparedness).

Unfortunately, these results could originate from the fact that nonblinkers detect the second target, rather than inherent cognitive differences. Therefore, Martens et al fine-tuned this comparison of blinkers and non-blinkers by measuring ERPs on 11 subjects of each type, each performing 600 attentional blink trials. Their analysis time-locked the ERPs to the onset of the stream of visual stimuli, controlled for any baseline differences in EEG mean amplitude and amplitude within standard power bands (there were no differences in either, interestingly), downsampled the data to 250 Hz, low-pass filtered at 20 Hz, corrected for eye movements, and "DC detrended" the data, with all unusually large amplitude changes removed as artifacts.

As in previous work with ERPs of the attentional blink, targets that were not detected showed no parietally-centered positive deflection at 300 msec after target presentation, further confirming that this "P300" wave reflects updating, or (to use Martens' et al's term) "target consolidation." However, this P300 wave was slower on average for blinkers than for nonblinkers for both targets (but even more so for the second target on trials in which blinkers happened to correctly detect it), suggesting that "nonblinkers" may not blink simply because their WM updating processes are faster than those of blinkers. Furthermore, nonblinkers showed a much stronger bilateral ERP wave known as frontal selective positivity (FSP) than blinkers, who tended to show a weaker FSP only in the left hemisphere.

Martens et al also carried out a second experiment to test the hypothesis that the attentional blink is caused only by the amount of time available to process stimuli. According to this hypothesis, shortening the time between target 1 presentation and target 2 presentation by an amount equivalent to the difference between P3 waves in blinkers and nonblinkers should essentially make many of the nonblinkers experience the attentional blink. Such an effect was not found, which led the authors to propose that the attentional blink is more related to the efficiency of neural processing rather than simply the speed with which stimuli are processed.

The authors conclude that "selection efficiency" is a primary determinant of the attentional blink, as reflected in the fact that blinkers also tended to show more prefrontal activity in response to distractors. Martens et al. also suggest that their results are incompatible with some theoretical models of the attentional blink, one of which (the "two stage model") states that bandwidth-limited or serial attentional processes must select from the results of a first bandwidth unlimited (or parallel) processing stage that transforms visual information into conceptual representations. According to the authors, although the two-stage model correctly predicts slower processing of the second target at lag 3, it doesn't provide any reason to think that detection of the first target would be slower, which was indeed found behaviorally. Therefore Martens et al endorse what they call an "interference model," which posits that the targets compete for representation with distractors.

Although these results are certainly interesting, I don't think that they clearly rule out two-stage models (in fact, since no one would actually specify a two-stage connectionist model of the AB that excludes competition for representation, connectionist implementations of the interference and two-stage models are not really distinct from one another). These data do seem to offer additional constraints on modeling of the attentional blink, particularly with regard to the role of processing time constraints, which have often featured prominently in these models. Instead, these data suggest that processing time is only indirectly important, while processing or selection efficiency may be the more critical variable determining whether an attentional blink will take place.

Related Posts:
The Mind's Eye: Models of the Attentional Blink
Selection Efficiency in Updating Working Memory
Selection Efficiency and Inhibition
Anticipation and Synchronization
Attention: The Selection Problem

3 Comments:

Blogger onclepsycho said...

This is fascinating research, thanks for reporting it, I completely overlooked it while reading the TOC of the journal issue. Although it is clearly too early to say at this stage of research, do you think it makes a robust difference in real life to be a blinker or a nonblinker? Would nonblinkers be more immune to clumsiness or perceptive illusions? Are they better readers? etc.

9/02/2006 07:01:00 AM  
Blogger Chris Chatham said...

good question.. i really think it's too early to say. there may be advantages to being a "blinker" too. for example, it's possible that nonblinkers are more "tuned in" to specifics and spend less time monitoring non-focal items in their environment, but this is pure speculation.

9/02/2006 12:23:00 PM  
Blogger MoonShadow said...

i think so too.

non-blinkers may have much more attention span and more attention to detail?

perhaps blinkers have the advantage of being more aware of their surroundings because the attentional blink causes them to be able to turn their attention to other things in the environment?

9/03/2006 05:26:00 AM  

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