Comparing apples and oranges in priming of attentional selection?

Priming of attentional selection involves speeded selection of task-relevant visual search items when search stimuli remain constant from one search to the next. In the literature, diverse attentional priming effects have often been assumed to reflect the same mechanisms. Priming effects in various different paradigms (from lower-level to higher-level features) have been used interchangeably to study the nature of priming, even when tasks differ vastly in difficulty and neural mechanisms involved. Another view is that priming is a characteristic of all perceptual mechanisms, that operate at different processing levels. Here, this issue was addressed by contrasting time courses and relative sizes of priming effects for repetition of a lower-level and higher-level feature (colour vs. facial expression). Attentional priming was tested in two odd-one-out search tasks, one involving discrimination, the other a present/absent judgment. The main research question was whether the sizes and temporal profiles of priming for the two features would be similar or not. Firstly, the sizes of the priming effects were very different for colour and expression and secondly, colour priming effects lasted for much longer than expression priming, as measured with memory kernel analyses, suggesting that the mechanics behind the effects differ. These two forms of priming should therefore only be compared with great caution. Generally, the results suggest that priming occurs at many levels of processing and can take many forms. This view is highly consistent with research on the neural mechanisms of priming. Attentional priming should be thought of as a general principle of perceptual processing.