Stimulus saliency modulates pre-attentive processing speed in human visual cortex

Abstract

The notion of a saliency-based processing architecture [1] underlying human vision is central to a number of current theories of visual selective attention [e.g., 2]. On this view, focal-attention is guided by an overall-saliency map of the scene, which integrates (sums) signals from pre-attentive sensory feature-contrast computations (e.g., for color, motion, etc.). By linking the Posterior Contralateral Negativity (PCN) component to reaction time (RT) performance, we tested one specific prediction of such salience summation models: expedited shifts of focal-attention to targets with low, as compared to high, target-distracter similarity. For two feature-dimensions (color and orientation), we observed decreasing RTs with increasing target saliency. Importantly, this pattern was systematically mirrored by the timing, as well as amplitude, of the PCN. This pattern demonstrates that visual saliency is a key determinant of the time it takes for focal-attention to be engaged onto the target item, even when it is just a feature singleton.

Figure 1. Stimulus displays used in the present visual pop-out binary localization task.

Participants were required to give a speeded forced-choice response indicating the position (left vs. right hemi-field) of the feature singleton, which was selected randomly from one of the six lateral positions on the middle circle.

Figure 2. Behavioural results.

(a) Reaction times (lines) and error rates (bars) as a function of Saliency (High, Middle, Low) for orientation-defined (pop-out) targets. (b) Reaction times (lines) and error rates (bars) as a function of Saliency (High, Middle, Low) for color-defined (pop-out) targets.

Figure 3. Grand averaged event-related brain potentials elicited in response to color-defined (pop-out) targets at electrodes PO7/PO8.

(a) Waveforms contra- and ipsilateral to the singleton location. (b) Topographical maps of PCN scalp distributions for each of the three Salience conditions (High, Middle, Low) at the point in time when the difference between contra- and ipsilateral waveforms reached its maximum. These maps were computed by mirroring the contra-ipsilateral difference waves to obtain symmetrical voltage values for both hemispheres (using spherical spline interpolation). (c) PCN difference waves obtained by subtracting ipsilateral from contralateral activity for each of the three Salience conditions (High, Middle, Low).

Figure 4. Grand averaged event-related brain potentials elicited in response to orientation-defined (pop-out) targets at electrodes PO7/PO8.

(a) Waveforms contra- and ipsilateral to the singleton location. (b) Topographical maps of PCN scalp distributions for each of the three Salience conditions (High, Middle, Low) at the point in time when the difference between contra- and ipsilateral waveforms reached its maximum. These maps were computed by mirroring the contra-ipsilateral difference waves to obtain symmetrical voltage values for both hemispheres (using spherical spline interpolation). (c) PCN difference waves obtained by subtracting ipsilateral from contralateral activity for each of the three Salience conditions (High, Middle, Low).