Attention biases decisions but does not alter appearance

Recently, M. Carrasco, S. Ling, and S. Read (2004) reported that transient visual attentional cues could increase the perceived contrast of Gabor grating targets. We replicated their study using their exact stimuli and procedures. While we were able to reproduce their results, we discovered that the reported attentional effects vanished when we changed the type of decision that subjects performed from a comparative judgment ("which target has higher contrast?") to an equality judgment ("are the two targets equal in contrast?") that is resistant to bias. To ensure that the difference between the judgments was not due to a difference in attentional strategies, we also performed a control experiment in which subjects were instructed on a trial-by-trial basis which judgment to perform only after the stimuli had disappeared. In this experiment, the magnitude of attentional effect for the comparative judgment was diminished but still significant and the equality judgment still measured no effect. We conclude that the reported effects of attention upon appearance can be entirely explained by decision bias, and that attention does not alter appearance.