Bottom-up biases in feature-selective attention

Abstract

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.

Figure 1.

Illustration of contrast ratio manipulation and trial sequence. A, The luminance of the background was manipulated over five different levels on different trials. The luminance, and hence the difference, between light and dark dots was held constant. This resulted in conditions in which either the contrasts of both stimuli could be equal, or one stimulus had approximately two times or four times the contrast of the other stimulus. B, Trial sequence. Each trial began with the presentation of a fixation cross on a background of the luminance for that trial. Subsequently, a stationary field of either light or dark dots indicated the attended color. After a short presentation of fixation cross only, the moving and flickering dots (dark dots, 10 Hz; light dots, 12 Hz) were presented for 8500 ms. Any combination of one to five coherent motion targets or distractors could occur within this duration. Before the beginning of the next trial, a fixation cross only was presented for another half second.

Figure 2.

Topographical maps and phase coherence of SSVEPs. A, Grand mean scalp current density map for SSVEP amplitudes for both stimuli (i.e., stimulation frequencies) averaged over all experimental conditions. Maximal amplitudes are located at central occipital (black circles), lateral parietal left (gray triangles), and right (gray squares) electrodes. B, Grand mean scalp map of SSVEP phase for the condition with the highest overall amplitudes (attended, highest contrast ratio). Central occipital electrodes differ clearly in signal phase from the two lateral parieto-occipital electrode clusters, with the signals almost being counterphase for the conditions shown here. To facilitate comparison between the two frequencies, all phases were rotated so as to yield a phase of −π/2 for electrode Oz. C, Phase coherence for all pairs of electrodes from the three clusters averaged across all subjects and conditions. Phase coherence was quantified as the cosine of the phase difference between electrodes, i.e., a value close to +1 corresponds to almost identical phase of both electrodes of the pair for all conditions and subjects. Note: the diagonal is the pairing of each electrode with itself and hence equals +1.

Figure 3.

Behavioral results and SSVEP amplitudes. A, Reaction time of target detection responses and response bias both decrease with higher contrast ratio, indicating more and faster responses to stimuli with higher contrast Observer sensitivity d′ show a slightly V-shaped pattern with lowest sensitivity at equal contrast of both stimuli. B, Normalized grand mean SSVEP amplitudes at medial occipital and left and right lateral parieto-occipital electrode clusters. Amplitudes increase linearly with higher contrast ratio for all 3 clusters for both attended and unattended stimuli. Attention has a multiplicative effect at central occipital electrodes and an additive effect at lateral parieto-occipital electrodes. C, Average of slope and offset parameters of the regression analysis over all subjects. All three clusters showed equal attentional enhancement of the offset (i.e., at contrast ratio 1); however, only the central occipital cluster showed an attention effect on the slope of the regression. Error bars in A–C correspond to within-subjects 95% confidence intervals which in A and C were calculated by subtracting individual mean values over conditions for variance estimation.