Early top-down attentional modulation in visual processing

An established view of attention is that it acts by biasing the competition between rival representations at various stages in visual processing (e.g., Desimone & Duncan, 1995). The bias brings about a processing advantage for attended items over unattended items, so that attended items have greater neural activity in associated representations, are better perceived, remembered, and eventually are more likely to be acted on. There is extensive evidence that the bias can originate in one of two sources. It can be data driven and result from input properties, in which case more salient items will be biased over less salient ones (e.g., Theeuwes, 1991). Conversely, the bias can be internally driven and result from the current behavioural goals of the observer (e.g., Folk, Remington & Jonhnston, 1992). Both types of bias have the effect of resolving the competition between processing for perceived items. A common view is that for all types of input, and at all processing stages, the competition between rival representations is resolved by means of a combination of bottom-up and top-down biases. In this way, attention optimally leads to selection of items that may be relevant because of their objective appearance (bottom-up selection), while at the same time allowing selection of items that are currently task-relevant (top-down selection)