Attention to faces modulates early face processing during low but not high face discriminability

Abstract

In the present study, we investigated whether attention to faces results in sensory gain modulation. Participants were cued to attend either to faces or to scenes in superimposed face-scene images for which face discriminability was manipulated parametrically. The face-sensitive N170 event-related potential component was used as a measure of early face processing. Attention to faces modulated N170 amplitude, but only when faces were not highly discriminable. Additionally, directing attention to faces modulated later processing (~230-300 msec) for all discriminability levels. These results demonstrate that attention to faces can modulate perceptual processing of faces at multiple stages of processing, including early sensory levels. Critically, the early attentional benefit is present only when the "face signal" (i.e., the perceptual quality of the face) in the environment is suboptimal.

Figure 1

Examples of stimuli from the three stimulus categories used in the experiment. The high face discriminability stimulus consisted of a face at 70% opacity superimposed on a scene at 30% opacity. The medium face discriminability stimulus consisted of a face at 50% opacity superimposed on a scene at 50% opacity. The low face discriminability stimulus consisted of a face at 30% opacity superimposed on a scene at 70% opacity.

Figure 2

Topographic distribution of the N170 and LN components. The topographic maps depict the grand average waveform elicited by overlay stimuli. The N170 (A) was most robust in right parieto-occipital electrodes. A second response of smaller amplitude, the LN, (B) was observed in parietal and occipital electrodes. The effect of attention (attend to face – attend to scene) on the LN was most pronounced in right parieto-occipital electrodes (C). Note the similarity between the topographic distributions of the LN attention effect (C) and the N170 (A).

Figure 3

The main effects of attention and face discriminability on the N170 and LN components. All plots depict the grand average waveforms from the EOI electrodes (see Method section). Both the N170 (p < 0.005) and the LN (p < 0.001) were larger in amplitude when subjects directed their attention to the face, relative to the scene, in the overlay (A). (B) Depicts the effect of face discriminability averaged across attention conditions. As face discriminability increased, N170 amplitude increased (p < 0.001), and LN amplitude decreased (p < 0.005).

Figure 4

The interaction between attention and face discriminability. For the high face discriminability stimulus (A), we observed no difference in N170 amplitude as a function of attention (p > 0.94). Both the medium (B) and low (C) face discriminability stimuli elicited a larger N170 when subjects attended to the face relative to the scene (p < 0.05 for medium face discriminability; p < 0.005 for low face discriminability). The effect of attention on the LN component was significant at all levels of face discriminability (ps < 0.005), and did not interact with face discriminability to influence LN amplitude (p > 0.25).