Facial ambiguity and perception: How face-likeness affects breaking time in continuous flash suppression

Previous studies have elucidated that humans can implicitly process faces faster than they process objects. However, the mechanism through which the brain unconsciously processes ambiguous facial images remains unclear. In our experiment, upright and inverted black-and-white binary face stimuli were presented in a two-alternative forced-choice location discrimination task combined with continuous flash suppression, a technique that suppresses visual stimuli perception using rapidly changing masks. The breaking time (BT) or the time required for a stimulus to be perceptually recognized was recorded for each face stimulus. The results showed that the BT for inverted grayscale images was significantly longer than that for upright grayscale faces, whereas the BT for upright and inverted binary faces did not reach statistical significance. A significant correlation between face likeness and BT was established after evaluating face likeness for each binary face stimulus, with high-face-like binary faces exhibiting shorter BT and low-face-like stimuli resulting in a more prolonged BT. Our results suggest that even an ambiguous object rated highly in face likeness can reduce the BT under implicit processing, indicating the possibility that facial parts such as the eyes and nose are subconsciously detected in ambiguous facial stimuli, enabling facial perception.