Color and luminance contrasts attract independent attention

Paying attention can improve vision in many ways, including some very basic functions such as contrast discrimination [1, 2], a task that probably reflects very early levels of visual processing. Electrophysiological [3, 4], psychophysical [1, 5], and imaging [6-9] studies on humans as well as single recordings in monkey [10-13] show that attention can modulate the neuronal response at an early stage of visual processing, probably by acting on the response gain. Here, we measure incremental contrast thresholds for luminance and color stimuli to derive the contrast response of early neural mechanisms [14-16] and their modulation by attention. We show that, for both cases, attention improves contrast discrimination, probably by multiplicatively increasing the gain of the neuronal response to contrast. However, the effects of attention are highly specific to the visual modality: concurrent attention to a competing luminance, but not chromatic pattern, greatly impedes luminance contrast discrimination; and attending to a competing chromatic, but not luminance, task impedes color contrast discrimination. Thus, the effects of attention are highly modality specific, implying separate attentional resources for different fundamental visual attributes at early stages of visual processing.