Bottom-up biases in feature-selective attention.

Previous studies of feature-selective attention have focused on situations in which attention is directed to one of two spatially superimposed stimuli of equal salience. While such overlapping stimuli should maximize stimulus interactions, it is still unknown how bottom-up biases favoring one or the other stimulus influence the efficiency of feature-selective attention. We examined the integration of bottom-up contrast and top-down feature-selection biases on stimulus processing. Two fully overlapping random dot kinematograms (RDKs) of light and dark dots were presented on a gray background of intermediate luminance. On each trial, human participants attended one RDK to detect brief coherent motion targets, while ignoring any events in the unattended RDK. Concurrently, through changes in background luminance, stimulus contrast could be set to five different levels: the stimuli could either be equal, or one of the two stimuli could have twice or four times the contrast of the other stimulus. This manipulation introduced a bottom-up bias toward the stimulus with the higher contrast while keeping the difference between the stimuli constant. Stimulus processing was measured by means of steady-state visual evoked potentials (SSVEPs). SSVEP amplitudes generally increased with higher contrast of the driving stimulus. At earlier levels of processing, attention increased the slope of this linear relation, i.e., attention multiplicatively enhanced SSVEP amplitudes. However, at later levels of processing, attention had an additive effect. These effects of attention can be attributed to the differential integration of gain enhancement and inhibitory stimulus competition at different levels of the visual processing hierarchy.