Phase noise reveals early category-specific modulation of the event-related potentials

Abstract

Previous studies have found that the amplitude of the early event-related potential (ERP) components evoked by faces, such as N170 and P2, changes systematically as a function of noise added to the stimuli. This change has been linked to an increased perceptual processing demand and to enhanced difficulty in perceptual decision making about faces. However, to date it has not yet been tested whether noise manipulation affects the neural correlates of decisions about face and non-face stimuli similarly. To this end, we measured the ERPs for faces and cars at three different phase noise levels. Subjects performed the same two-alternative age-discrimination task on stimuli chosen from young-old morphing continua that were created from faces as well as cars and were calibrated to lead to similar performances at each noise-level. Adding phase noise to the stimuli reduced performance and enhanced response latency for the two categories to the same extent. Parallel to that, phase noise reduced the amplitude and prolonged the latency of the face-specific N170 component. The amplitude of the P1 showed category-specific noise dependence: it was enhanced over the right hemisphere for cars and over the left hemisphere for faces as a result of adding phase noise to the stimuli, but remained stable across noise levels for cars over the left and for faces over the right hemisphere. Moreover, noise modulation altered the category-selectivity of the N170, while the P2 ERP component, typically associated with task decision difficulty, was larger for the more noisy stimuli regardless of stimulus category. Our results suggest that the category-specificity of noise-induced modulations of ERP responses starts at around 100 ms post-stimulus.

Keywords: N170; P1; P2; category effect; phase noise.

FIGURE 1

Procedure and sample stimuli. Timeline depicts some examples of faces and car test stimuli at different levels of phase coherences (100, 30, or 24%) and with different ages (young, middle-aged, or old).

FIGURE 2

Behavioral results. Effect of added noise on the accuracy (A) and response times (B) in the age discrimination task for faces (black columns) and for cars (gray columns). Just noticeable differences (JND; ±SD) were calculated (see Materials and Methods) to characterize the performance of the subjects. The x-axis denotes different levels of phase coherences (*p < 0.05).

FIGURE 3

Grand average ERPs displayed between -100 and 500 ms of the pooled posterior-occipito-temporal recording sites of the N170 for the left (LH) and for the right hemisphere (RH). 100% phase coherence: thick line, 30% phase coherence: dashed line, 24% phase coherence: dotted line; for faces (black) and cars (red), respectively. Insets depict the category-specificity of noise-induced modulations on the P1 component. Topographical voltage maps of ERP differences between faces and cars at different phase coherence levels (100%-upper, 30%-middle, 24%-lower) show hemispheric asymmetries in the P1 time window. Positivity is red.

FIGURE 4

Mean (±SD) of the amplitudes and latencies of the (A,D) P1, (B,E) N170, and (C,F) P2 components for faces (black columns) and cars (gray columns) at different levels of phase coherences.

FIGURE 5

The effect of stimulus ambiguity reflected on the N170 component. Mean (±SD) of the amplitudes of the N170 for faces (black columns) and cars (gray columns) at different level of age (old vs. middle-aged vs. young).