Attentional interference at small spatial separations.

The spatial characteristics of attention were studied by measuring the accuracy with which two target letters could be identified from a circular display of 24 characters. Traditional notions of spatially-limited regions of attentional enhancement predict that performance should be best when the pair of targets fall within the boundaries of a single attentional 'window'. The results were opposite to this expectation: performance was poorest when the targets were close together and improved with increasing target separation. The effects were not due to lateral sensory masking or to sensory transients and were replicated with several different types of attentional cues. Two possible models are proposed to account for the observed effects of target separation. The first model assumes that attending to one location necessarily reduces processing in the local surround. The second model proposes that the poorer performance observed at small target separations results from imprecise targeting when attention is directed to a pair of nearby locations. Both models illustrate spatially-local limits on processing capacity that attention is unable to circumvent. Enhancement at one location is achieved primarily at the expense of the immediate surround. Such spatially-local tradeoffs in processing capacity could have the useful consequence of making the attended target stand out even more against the immediate background.