Adults' awareness of faces follows newborns' looking preferences

Abstract

From the first days of life, humans preferentially orient towards upright faces, likely reflecting innate subcortical mechanisms. Here, we show that binocular rivalry can reveal face detection mechanisms in adults that are surprisingly similar to inborn face detection mechanism. We used continuous flash suppression (CFS), a variant of binocular rivalry, to render stimuli invisible at the beginning of each trial and measured the time upright and inverted stimuli needed to overcome such interocular suppression. Critically, specific stimulus properties previously shown to modulate looking preferences in neonates similarly modulated adults' awareness of faces presented during CFS. First, the advantage of upright faces in overcoming CFS was strongly modulated by contrast polarity and direction of illumination. Second, schematic patterns consisting of three dark blobs were suppressed for shorter durations when the arrangement of these blobs respected the face-like configuration of the eyes and the mouth, and this effect was modulated by contrast polarity. No such effects were obtained in a binocular control experiment not involving CFS, suggesting a crucial role for face-sensitive mechanisms operating outside of conscious awareness. These findings indicate that visual awareness of faces in adults is governed by perceptual mechanisms that are sensitive to similar stimulus properties as those modulating newborns' face preferences.

Figure 1. Stimuli and procedure.

(A) Top row: Example test stimuli from Experiments 1–3. In Experiment 1, we compared inversion effects for faces with normal and reversed contrast polarity. In Experiment 2, we used faces that were either lit from above or from below (5). In Experiment 3, we presented faces with normal contrast polarity and “chimeric” faces with reversed contrast polarity but normal contrast polarity in the eye regions . Middle row: Masks created to highlight the key features for face detection (eyes, mouth) in dark gray. Bottom row: Example stimuli with superimposed masks (at 25% transparency) illustrating the differences between test stimuli regarding the contrast relations of the key features. In (top-lit) faces with normal contrast polarity, the eyes and the mouth are dark, whereas in faces with reversed contrast polarity and in bottom-lit faces, these key features are lighter. In chimeric faces, the eyes are dark as in faces with normal contrast polarity, whereas the mouth is lighter as in faces with reversed contrast polarity. (B) Schematic of an example trial. To induce interocular suppression, high contrast CFS masks flashing at 10 Hz were presented to one eye, while a face was gradually introduced to the other eye. Participants indicated in which quadrant the test stimulus or any part of the test stimulus became visible. The contrast of the face was linearly increased over the first second of a trial, while the contrast of the CFS masks was slowly ramped down over the course of a trial.

Figure 2. Inversion effects for all test stimulus conditions.

For each subject and each condition, normalized inversion effects were obtained by dividing the difference between mean RTs for upright and inverted test stimuli by the mean RT for upright stimuli . Thumbnails depict upright and inverted example stimuli for each experiment. Positive and negative error bars denote 95% confidence intervals for the comparison against zero. Asterisks indicate a significant difference between the inversion effects for normal polarity or top-lit test stimuli and reversed polarity or bottom-lit test stimuli, p<0.05.