Abstract
Humans and non-human primates are able to detect threatening stimuli (e.g., snakes), faster than non-threatening stimuli. It is thought that this ability operates on low spatial frequency (LSF) information. However, a natural image has higher contrast at low spatial frequencies than at high spatial frequencies. This means that when an image is frequency filtered to retain either LSF or HSF information, the LSF images have higher contrast than HSF images, making the LSF images more visible to the eye. Thus, it is unclear whether rapid threat detection truly relies on LSF information or simply on the greater visibility of the higher contrast stimuli. Previous studies have failed to isolate the spatial frequency information completely by not contrast-normalizing the frequency filtered stimuli. In the current study, we ran a rapid threat detection experiment (N = 39) using HSF and LSF versions of threatening (snakes, wasps) and non-threatening (salamanders, flies) animal images that were either contrast-normalized after filtering, or not contrast-normalized. Results revealed higher accuracy for distinguishing between threatening and non-threatening animals with LSF images (accuracy = 62.8%) compared to HSF images (49.0%; p < 0.001) when the images were not contrast-normalized. Critically, when the images were contrast-normalized after being frequency filtered, there was no difference in accuracy between LSF and HSF images (accuracy = 64.1% vs. 64.4%, p = 0.92). This work has important implications for the field of threat detection, since much of the work in this field is based on the idea that LSF information is what allows people to make rapid threat judgements and other emotional appraisals. Our findings call this idea into question and are an important reminder that to make a true claim about the role of spatial frequency information, one must isolate spatial frequency from other low-level properties such as contrast and luminance.