Abstract
We studied how task constraints modulate the relationship between single-trial event-related potentials (ERPs) and image noise. Thirteen subjects performed two interleaved tasks: on different blocks, they saw the same stimuli, but they discriminated either between two faces or between two colors. Stimuli were two pictures of red or green faces that contained from 10 to 80% of phase noise, with 10% increments. Behavioral accuracy followed a noise dependent sigmoid in the identity task but was high and independent of noise level in the color task. EEG data recorded concurrently were analyzed using a single-trial ANCOVA: we assessed how changes in task constraints modulated ERP noise sensitivity while regressing out the main ERP differences due to identity, color, and task. Single-trial ERP sensitivity to image phase noise started at about 95–110 ms post-stimulus onset. Group analyses showed a significant reduction in noise sensitivity in the color task compared to the identity task from about 140 ms to 300 ms post-stimulus onset. However, statistical analyses in every subject revealed different results: significant task modulation occurred in 8/13 subjects, one showing an increase and seven showing a decrease in noise sensitivity in the color task. Onsets and durations of effects also differed between group and single-trial analyses: at any time point only a maximum of four subjects (31%) showed results consistent with group analyses. We provide detailed results for all 13 subjects, including a shift function analysis that revealed asymmetric task modulations of single-trial ERP distributions. We conclude that, during face processing, bottom-up sensitivity to phase noise can be modulated by top-down task constraints, in a broad window around the P2, at least in some subjects.
Highlights
Following the first reports of larger scalp responses to faces compared to objects (Bötzel and Grüsser, 1989; Jeffreys, 1989; Jeffreys and Tukmachi, 1992; Jeffreys et al, 1992; Seeck and Grüsser, 1992), there have been hundreds of studies on the early event-related potentials (ERPs) to faces and objects
The parametric ERP modulation by image noise can be better appreciated by looking at the time-course of the groupaveraged noise sensitivity, which peaked at the same electrode and time point as R2 did
Noise sensitivity was reduced in the color task compared to the identity task in a single cluster, between 139 and 277 ms after stimulus onset (Table 1)
Summary
Following the first reports of larger scalp responses to faces compared to objects (Bötzel and Grüsser, 1989; Jeffreys, 1989; Jeffreys and Tukmachi, 1992; Jeffreys et al, 1992; Seeck and Grüsser, 1992), there have been hundreds of studies on the early event-related potentials (ERPs) to faces and objects. The vast majority of these studies used (i) averaged ERP, (ii) group statistics, and (iii) categorical designs. Their findings can be summarized shortly: sometime between 100 and 200 ms after stimulus onset, ERPs to different object categories tend to differ from each other, and faces are most of the time associated with larger N170 peaks than other object categories (Rossion and Jacques, 2008). Individual subjects’ ERPs, show, not surprisingly, systematic differences between faces and objects consistent with group effects reported so far (Rousselet et al, 2008a). These studies have revealed inter-subject differences: despite coarse agreement between group and individual subject statistical analyses, individual subjects show reliable ERPs to faces and noise, which differ reliably across subjects, for reasons yet to be discovered (Rousselet et al, 2010; Gaspar et al, in press Reliability of ERP and single-trial analyses)
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