A new study published in the Journal of Vision has unveiled how our brains detect faces even when presented with limited and ambiguous visual information. Using a specialized technique to suppress conscious awareness, the researchers found that stimuli resembling faces are processed more quickly, even when visual details are minimal. These findings suggest that the human brain is particularly sensitive to facial features, responding rapidly even under conditions where conscious perception is absent.
Faces play a pivotal role in human communication and social interaction, making face perception an essential cognitive skill. Our brains have specialized mechanisms for detecting and processing faces, allowing us to recognize individuals and interpret emotions with remarkable speed and accuracy. However, much of what is known about face perception comes from studies conducted under conscious viewing conditions. Less is understood about how the brain processes incomplete or ambiguous visual information when we are unaware of it. The researchers sought to address this gap, particularly how the brain identifies face-like stimuli under unconscious conditions.
“Facial recognition is essential to human interaction, and we were curious about how the brain processes ambiguous or incomplete facial images—especially when they’re hidden from conscious awareness. We believe understanding these mechanisms can shed light on subconscious visual processing,” said study author Makoto Michael Martinsen, a PhD student conducting research under the Visual Perception and Cognition Laboratory and the Cognitive Neurotechnology Laboratory at the Toyohashi University of Technology.
To investigate how the brain processes face-like stimuli unconsciously, the researchers used a method called Continuous Flash Suppression (CFS). In this technique, participants were presented with a dynamic series of high-contrast masking images in one eye while a target image—such as a face-like stimulus—was shown to the other eye. The rapid flashing of the mask suppressed the perception of the target image, rendering it temporarily invisible to the participant. By measuring the time it took for the target image to “break through” the suppression and reach conscious awareness, the researchers could infer how efficiently the brain processed the image.
The study included 24 participants, all university students aged 20 to 24, with normal or corrected-to-normal vision. They were exposed to two types of visual stimuli: grayscale images of faces and binary images resembling faces. These binary images were created using black-and-white contrasts to simulate minimal facial features, such as contours and the general arrangement of facial elements. Each image was presented in both upright and inverted orientations to assess the impact of orientation on recognition.
During each trial, participants were tasked with identifying the location (left or right) of the target image as soon as it became visible. Following the main experiment, participants also rated the “face-likeness” and “object-likeness” of the binary images on a scale, providing additional data on how the visual system categorized these ambiguous stimuli.
The results showed that the brain responds more rapidly to face-like stimuli, even when they are presented unconsciously. For grayscale images of faces, upright stimuli were detected significantly faster than inverted ones, confirming the “inversion effect,” a well-established phenomenon in face perception research.
“The inversion effect in face perception is the difficulty people have recognizing upside-down faces compared to upright ones,” Martinsen explained. “This happens because our brains are wired to process faces holistically, and flipping them disrupts this ability.”
Interestingly, the inversion effect was not observed for the binary face-like stimuli. The researchers hypothesized that the lack of detailed facial information in these images might have hindered the brain’s ability to process them holistically.
“We didn’t see the expected ‘inversion effect’ with binary face images, which was surprising because it’s a common result with grayscale faces,” Martinsen told PsyPost. “This suggests that binary images don’t provide enough visual detail for the brain to fully engage in holistic facial processing.”
However, the study found a significant correlation between the perceived “face-likeness” of the binary images and the speed at which they were detected. Binary images judged to be more face-like broke through suppression more quickly, suggesting that even minimal facial cues are sufficient to activate the brain’s face-processing mechanisms.
In contrast, no significant relationship was found between the perceived “object-likeness” of the stimuli and detection speed. This indicates that the brain’s sensitivity to facial features is distinct from its response to other types of visual information. The study also ruled out low-level visual factors, such as pixel density, as a confounding influence on the results.
“Our study shows that even vague, face-like images can trigger subconscious processing in the brain, demonstrating how deeply rooted facial recognition is in our visual system,” Martinsen said. “This ability likely evolved to help us prioritize faces, which are critical for social interaction, even when visual information is scarce.”
But as with all research, there are some caveats to consider. “We didn’t consider factors like emotion or attractiveness, which can affect facial perception,” Martinsen noted. “Additionally, participants might have interpreted inverted faces inconsistently, and the masking method we used could have influenced results.”
Despite this, “our study highlights the brain’s incredible ability to extract important information from minimal cues, especially when it comes to faces,” Martinsen said. “It emphasizes the importance of facial features in both conscious and subconscious perception and raises interesting questions about how this mechanism evolved.”
“Our next step is to incorporate techniques like eye-tracking to help identify which facial features participants focus on during unconscious processing, which could potentially provide further insights into how attention is distributed across different face parts. Additionally, we hope to better understand how the brain identifies faces under challenging conditions and how minimal visual cues can activate specific brain areas.”
The study, “Facial ambiguity and perception: How face-likeness affects breaking time in continuous flash suppression,” was authored by Michael Makoto Martinsen, Kairi Yoshino, Yuya Kinzuka, Fumiaki Sato, Hideki Tamura, Tetsuto Minami, and Shigeki Nakauchi.
