The effect of facial expression on contrast sensitivity: A behavioural investigation and extension of Hedger, Adams & Garner (2015).

Abigail L M Webb,Paul B Hibbard,Thomas Suslow

doi:10.1371/journal.pone.0205621

Abstract

It has been argued that rapid visual processing for fearful face expressions is driven by the fact that effective contrast is higher in these faces compared to other expressions, when the contrast sensitivity function is taken into account. This proposal has been upheld by data from image analyses, but is yet to be tested at the behavioural level. The present study conducts a traditional contrast sensitivity task for face images of various facial expressions. Findings show that visual contrast thresholds do not differ for different facial expressions We re-conduct analysis of faces’ effective contrast, using the procedure developed by Hedger, Adams and Garner, and show that higher effective contrast in fearful face expressions relies on face images first being normalised for RMS contrast. When not normalised for RMS contrast, effective contrast in fear expressions is no different, or sometimes even lower, compared to other expressions. However, the effect of facial expression on detection in a backward masking study did not depend on the type of contrast normalisation used. These findings are discussed in relation to the implications of contrast normalisation on the salience of face expressions in behavioural and neurophysiological experiments, and also the extent that natural physical differences between facial stimuli are masked during stimulus standardisation and normalisation.

Highlights

Fearful facial expressions are salient to the human visual system, receiving preferential allocation of attentional resources, and inhibiting this attention from relocating to different stimuli [1,2,3,4]
Adams and Garner [15] recently showed that the visibility, or salience, associated with fear expressions is predicted by their effective contrast content; the extent that the Fourier amplitude of fear expressions, compared to neutral faces, exploits the contrast sensitivity function
We conducted a traditional contrast sensitivity task to test whether higher effective contrast purported for fear expressions is associated with lower visual contrast thresholds at the behavioural level

Summary

Introduction

Fearful facial expressions are salient to the human visual system, receiving preferential allocation of attentional resources, and inhibiting this attention from relocating to different stimuli [1,2,3,4]. When fearful expressions compete with salient noise stimuli for visual awareness, with the face and noise presented to different eyes, they break suppression faster compared to neutral faces [7,8], and are associated with increased activity in subcortical threat-processing regions even when observers report not having observed a face [5,6, 9]. These findings converge on the notion that the human visual system has evolved specific visual neural mechanisms that enable rapid identification of fearful expressions. This concept is reminiscent of LeDoux’s [10] ‘quick and dirty pathway’ for processing environmental information necessary for successful threat-avoidance.

Methods

Results

Conclusion