Uncovering the mechanisms of conscious face perception: a single-trial study of the n170 responses

Abstract

When a face is flashed to an observer, a large negative component is elicited in the occipitotemporal cortex at ∼170 ms from the onset of presentation (N170). Previous studies have shown that the average N170 is correlated with conscious face perception; however, the single-trial mechanisms underlying such modulation remain largely unexplored. Here, we studied in human subjects the average and the single-trial N170 responses to briefly flashed faces, coupled with backward masking and varying degrees of Gaussian noise. In the average evoked responses we observed that, at fixed levels of noise, supraliminal faces exhibited significantly larger N170 amplitudes than subliminal faces. Moreover, the average N170 amplitude decreased with noise level both for the perceived and the nonperceived faces. At the single-trial level, the N170 amplitude was modulated by conscious recognition, which allowed predicting the subjects' perceptual responses above chance. In contrast, the single-trial N170 amplitudes were not modulated by the amount of noise and the effect found in the average responses was due to different latency jitters, as confirmed with latency-corrected averages. Altogether, these results suggest that conscious face perception is correlated with a boost in the activity of face-selective neural assemblies, whereas the stimulus uncertainty introduced by the added noise decreases the timing consistency (but not the amplitude) of this activation.

Figure 1.

Structure of a trial. A fixation cross was shown for 500–700 ms, followed by the presentation of a picture containing either a face or a car with varying degrees of Gaussian noise, flashed for 57 ms. Each stimulus was backward masked for 443 ms, and, after the mask presentation, the participant was requested to report whether the picture was a face or not.

Figure 2.

Neural correlates of conscious versus unconscious face perception. a, Grand average ERPs recorded at electrode PO8 for the sub-threshold, threshold, and supra-threshold conditions. For the threshold condition, we also display the car evoked potential (black line). Bands around mean values denote SEM. b, Scalp topography of activation for seen and unseen trials in the threshold condition, 170 ms after stimulus onset. Units of the bottom colorbars are microvolts.

Figure 3.

Decoding of the conscious reports with the single-trial N170. a, Average ERP and 15 denoised single-trial ERPs at electrode PO8 for a typical subject. The identification of the single-trial peaks is shown with the markers. b, Distribution of single-trial N170 responses in a typical participant used for decoding. Each blue (red) dot represents the single-trial N170 amplitudes measured at electrodes PO7 and PO8 in a seen (unseen) trial. The black line represents the Fisher's linear discriminant. The blue and red lines show the normalized distributions for seen and unseen trials projected along the axis perpendicular to the linear discriminant. c, Black dots show in a logarithmic scale the individual p values of the decoding performance with different components. The red lines indicate the median of the distribution of p values for each component.

Figure 4.

Single-trial analysis of ERP responses. a, Mean and SD of the single-trial N170 amplitude at channel PO8. b, Mean and SD of the N170 latency jitter at channel PO8. c, The noise-level effect (electrode site: PO8) measured at the average ERP level (top-left panel) vanished after latency-correction (top-right panel). In contrast, the conscious-report effect (bottom-left panel) persisted after latency correction (bottom-right panel). Bands around mean values denote SEM.