Wednesday, February 20, 2008

Neural fate of ignored stimuli

Yi, D. et al. (September 2004). Neural fate of ignored stimuli: dissociable effects of perceptual and working memory load. Nature Neuroscience, Volume 7, Number 9, 992-996.

After some debate over whether attention acts a filter early in the processing hierarchy ("early selection") or whether it blocks awareness of perceptually encoded stimuli at a late stage of processing ("late selection"), the field has accepted both accounts to some degree. When task difficulty is low, late selection may be more likely, whereas aggressive early selection may be more common when task difficulty is high. However, task difficulty can be defined in several ways and this paper predicted and confirmed that increasing perceptual demands properly constitutes task difficulty and leads to this early selection behavior, whereas increasing working memory loads does not.

Experimentally, subjects were shown faces foveally, surrounded by background images depicting scenes. They were told to fixate on the faces and ignore the background images. Faces were cycled, but so were the background images. In the EASY block, subjects were asked to play a "one-back" game, discriminating between the current face image and face presented prior. Subject performance was high. However, in addition to this primary task, the parahippocampal place area (PPA) showed activity that indicated it was indeed processing the background image changes successfully as well. But as the perceptual load was adjusted, with the HARD block requiring more difficult facial discriminations, it was clear that the PPA was not processing the background scenes to the same extent as in the low perceptual load condition. By comparison, the MEMORY block, in which "two-back" rules required subjects to maintain more information in working memory for longer, did not demonstrate these attenuation effects on background processing, i.e. the PPA was still successfully detecting background changes. This indicated, as hypothesized, that perceptual demands and working memory load result in differential attentional effects.

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