Timing the path to perception

Study shows how brain events in 'attentional blink' relate to consciousness

By | September 12, 2005

The sooner a person becomes conscious of an image, the more likely it is that a second image shown shortly thereafter will be seen, according to a study this week in Nature Neuroscience.

The research takes "new ideas about how consciousness might work in the brain and show[s] that it actually works" by "very closely link[ing]… behavior to brain activity at different points in time in the brain," James Enns at the University of British Columbia in Vancouver, who did not participate in the study, told The Scientist.

The French research team used these methods to develop a complete cascade of brain events associated with attentional blink—the difficulty of perceiving the second of two targets presented within a half-second of the first. The paradigm addresses the mechanism of how much can enter your awareness, said Justin Feinstein at the University of Iowa, who did not participate in the study.

Among various explanations for attentional blink, a common view is that both targets enter a first stage of nonconscious processing, then the second target may be denied access to a subsequent, conscious processing stage with limited capacity, coauthor Claire Sergent at Institut National de la Santé et de la Recherche Médicale in Orsay, France, told The Scientist.

In their model of attentional blink, the authors used two word targets (T1 and T2) inserted in short succession into a stream of distracting images on a screen. Their previous work with this model suggested that seeing the second target in attentional blink is an all-or-none phenomenon; subjects either saw it fully or did not see it at all, even though they were asked to rate visibility.

In order to investigate the neurological basis of this bimodal distribution, the team measured event-related potentials (ERPs) to chart the time course of the brain's response. To isolate those ERP readings invoked specifically by the second word, they subtracted the readings obtained when T2 was replaced by a blank screen. They found that waves correlated with early visual processing, P1 and N1, were preserved whether or not the subject had reported seeing the second stimulus.

It was only after 170 ms that the researchers observed differences in waveforms between trials in which T2 was and was not seen. Even after 270 ms, some waveforms were present for unseen T2s, showing that nonconscious processing does not stop immediately after being denied access to the second stage.

Three hundred to 500 ms after T2, the researchers found that the P300 waveform occurred in all-or-none fashion based on whether or not T2 had been seen. The next step was "to look at what determines this drastic difference," said Sergent. "Was it the time or the efficiency in the processing of the first target?" They found that in trials when T2 was seen, the P300 T1-evoked waveform tended to reach its peak earlier and decrease faster than when T2 was not seen.

According to the paper, this result provided direct evidence that stochastic variations in the strength or duration of the T1-induced wave affected the degree of competition between T1 and T2 processing at the later stage, often preventing T2 from entering this stage. "The bottleneck in perception, the limitation on being able to do two things at once…is at the second stage. You're only going to get one target in at a time," Enns explained.

The authors' assessment of cerebral localization using ERP was generally consistent with previous studies that associated the visual cortex with stage-one processing and the frontoparietal cortex with stage two. "An obvious next step would be to get more precision as to where in the brain these things are happening," Enns said. According to Sergent, while ERP is effective in visualizing the brain's response to the stimuli over time, functional MRI is a much better tool for tracking these events spatially. Feinstein said the "best way to do it is to integrate the two methods," though "it's very hard to simultaneously gather ERP data and fMRI data."

Last year, when Enns published a textbook on this topic: The Thinking Eye, The Seeing Brain, he said he "could have only dreamed about" the "details" offered by this paper.

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