Contrast normalisation masks natural expression-related differences and artificially enhances the perceived salience of fear expressions.

Fearful facial expressions tend to be more salient than other expressions. This threatbias is to some extent driven by simple low-level image properties, rather than thehigh-level emotion interpretation of stimuli. It might be expected therefore that differentexpressions will, on average, have different physical contrasts. However, studies tendto normalise stimuli for contrast, potentially removing a naturally-occurring difference insalience. We assessed whether images of faces differ in both physical and apparentcontrast across expressions. We measured physical contrast and the Fourieramplitude spectra of 5 emotional expressions prior to contrast normalisation. We alsomeasured expression-related differences in perceived contrast. Fear expressions havea steeper Fourier amplitude slope compared to neutral and angry expressions, andconsistently significantly lower contrast compared to other faces. This effect is morepronounced at higher spatial frequencies. With the exception of stimuli containing onlylow spatial frequencies, fear expressions appeared higher in contrast than a physicallymatched reference. These findings suggest that contrast normalisation artificiallyboosts the perceived salience of fear expressions; an effect that may account forperceptual biases observed for spatially filtered fear expressions.