An objective and reliable electrophysiological marker for implicit trustworthiness perception

Abstract

Trustworthiness is assumed to be processed implicitly from faces, despite the fact that the overwhelming majority of research has only involved explicit trustworthiness judgements. To answer the question whether or not trustworthiness processing can be implicit, we apply an electroencephalography fast periodic visual stimulation (FPVS) paradigm, where electrophysiological cortical activity is triggered in synchrony with facial trustworthiness cues, without explicit judgements. Face images were presented at 6 Hz, with facial trustworthiness varying at 1 Hz. Significant responses at 1 Hz were observed, indicating that differences in the trustworthiness of the faces were reflected in the neural signature. These responses were significantly reduced for inverted faces, suggesting that the results are associated with higher order face processing. The neural responses were reliable, and correlated with explicit trustworthiness judgements, suggesting that the technique is capable of picking up on stable individual differences in trustworthiness processing. By demonstrating neural activity associated with implicit trustworthiness judgements, our results contribute to resolving a key theoretical debate. Moreover, our data show that FPVS is a valuable tool to examine face processing at the individual level, with potential application in pre-verbal and clinical populations who struggle with verbalization, understanding or memory.

Keywords: EEG; SSVEP; fast periodic visual stimulation; implicit perception; trustworthiness impressions.

Fig. 1

Example (A) trustworthy and (B) untrustworthy oddball FPVS sequences. Images were shown at a frequency of 6 Hz, with oddball images shown at a frequency of 1 Hz. Thus, the sequence of five untrustworthy base (UT) face images was followed by one trustworthy oddball (T) (and vice versa for trustworthy oddball sequences). Faces were shown using a square wave function with a 100% duty cycle such that the next face appeared as soon as the previous face disappeared. There were equal numbers of trustworthy and untrustworthy oddball sequences in each block of the task.

Fig. 2

Oddball response amplitude spectra and scalp topographies for the (A) upright and (B) inverted conditions. Top row: baseline subtracted amplitude spectra, collapsed across both trustworthy and untrustworthy oddball face stimuli at the right occipito-temporal ROI. This ROI consists of electrodes P8, PO8 and P10 (z-scores for each electrode are in Supplementary Table S2). Bottom row: scalp topographies for the overall trustworthiness oddball response (sum of fundamental oddball frequency and its significant harmonics), grand averaged across participants (see Supplementary Figure S1 for topographies for each harmonic). Note that the 6 Hz response reflects the generic face detection response rather than the trustworthiness discrimination response and is not included in the sum of harmonics. * P < 0.05. All z-value tests report one-tailed P values, which are appropriate here as the signal is only ever meaningful above zero.

Fig. 3

Scalp topographies of the normalized sum of the harmonics for each participant (McCarthy and Wood, 1985). Of each set, the top row represents Block 1 and the bottom row represents Block 2, both for upright faces. * P < 0.05, ** P < 0.01, *** P < 0.001 (one tailed). The color bar represents the magnitude of neural activation, with 0 μV as the minimum, and the participants’ individual largest magnitude response as the maximum. Hotter colors represent stronger activation.