Perceptual load affects spatial and nonspatial visual selection processes: an event-related brain potential study

One major question toward understanding selective attention regards the efficiency of selection. One theory contends that this efficiency in vision is determined primarily by the perceptual load (PL) imposed by the relevant stimuli; if this load is high enough to fill attentional capacity, irrelevant stimuli will be excluded before they interfere with task performance, but if this load is lower the spare capacity will be directed automatically to the irrelevant information, which will then interfere with task performance. The current study attempts to test and extend this theory in order to understand better the role of PL by examining its effects on event-related brain potentials (ERPs), voltage fluctuations recorded at the scalp that reflect underlying cognitive operations. Stimuli were presented one at a time, and subjects were instructed to respond to rare deviant stimuli that appeared within a relevant stimulus channel and to ignore stimuli in an irrelevant channel, where channel was defined by either spatial (left, right) or nonspatial (red, blue) attributes in separate tasks. PL was manipulated by varying the similarity between the target/deviant and standard stimulus, and increases in PL were found to increase the magnitude of the relevant-irrelevant difference waveforms in both tasks at predicted temporal windows. These findings suggest that PL affects attentional selection that is tonically maintained across many experimental trials, and does so not only when selection is spatially based but also when it is based upon nonspatial cues.