A body detection inversion effect revealed by a large-scale inattentional blindness experiment

As a social species, humans preferentially attend to the faces and bodies of other people. Previous research revealed specialized cognitive mechanisms for processing human faces and bodies. For example, upright person silhouettes are more readily found than inverted silhouettes in visual search tasks. It is unclear, however, whether these findings reflect a top-down attentional bias to social stimuli or bottom-up sensitivity to visual cues signaling the presence of other people. Here, we tested whether the upright human form is preferentially detected in the absence of attention. To rule out influences of top-down attention and expectation, we conducted a large-scale single-trial inattentional blindness experiment on a diverse sample of naive participants (N = 13.539). While participants were engaged in judging the length of a cross at fixation, we briefly presented an unexpected silhouette of a person or a plant next to the cross. Subsequently, we asked whether participants noticed anything other than the cross. Results showed that silhouettes of people were more often noticed than silhouettes of plants. Crucially, upright person silhouettes were also more often detected than inverted person silhouettes, despite these stimuli being identical in their low-level visual features. These results were replicated in a second experiment involving headless person silhouettes. Finally, capitalizing on the exceptionally large and diverse sample, further analyses revealed strong detection differences across age and gender. These results indicate that the visual system is tuned to the form of the upright human body, allowing for the quick detection of other people even in the absence of attention.