Inversion improves the recognition of facial expression in thatcherized images

Detecting implicit and explicit facial emotions at different ages

Emotions are processed in the brain through a cortical route, responsible for detailed-conscious recognition and mainly based on image High Spatial Frequencies (HSF), and a subcortical route, responsible for coarse-unconscious processing and based on Low SF (LSF). However, little is known about possible changes in the functioning of the two routes in ageing. In the present go/no-go online task, 112 younger adults and 111 older adults were asked to press a button when a happy or angry face appeared (go) and to inhibit responses for neutral faces (no-go). Facial stimuli were presented unfiltered (broadband image), filtered at HSF and LSF, and hybrids (LSF of an emotional expression superimposed to the HSF of the same face with a neutral expression). All stimuli were also presented rotated on the vertical axis (upside-down) to investigate the global analysis of faces in ageing. Results showed an overall better performance of younger compared to older participants for all conditions except for hybrid stimuli. The expected face-inversion effect was confirmed in both age groups. We conclude that, besides an overall worsening of the perceptual skill with ageing, no specific impairment in the functioning of both the cortical and the subcortical route emerged.

Gauging response time distributions to examine the effect of facial expression inversion

Introduction

We used images of facial expressions (FEs) of emotion in a speeded Same/Different task to examine (i) distributional characteristics of response times (RTs) in relation to inter-stimulus similarity and (ii) the impact of inversion on FE processing.

Methods

Stimuli were seven emotion prototypes, posed by one male and one female, and eight intermediate morphs. Image pairs (N = 225) were presented for 500 ms, upright or inverted, in a block design, each 100 times.

Results

For both upright and inverted FEs, RTs were a non-monotonic function: median values were longest for stimulus pairs of intermediate similarity, decreasing for both more-dissimilar and more-similar pairs. RTs of "Same" and "Different" judgments followed ex-Gaussian distributions. The non-monotonicity is interpreted within a dual-process decision model framework as reflecting the infrequency of identical pairs, shifting the balance between the Same and Different processes. The effect of stimulus inversion was gauged by comparing RT-based multidimensional scaling solutions for the two presentation modes. Solutions for upright and inverted FEs showed little difference, with both displaying some evidence of categorical perception. The same features appeared in hierarchical clustering solutions.

Discussion

This outcome replicates and reinforces the solutions derived from accuracy of "Different" responses reported in our earlier companion paper. We attribute this lack of inversion effect to the brief exposure time, allowing low-level visual processing to dominate Same/Different decisions while elevating early featural analysis, which is insensitive to face orientation but enables initial positive/negative valence categorization of FEs.

The role of emotion in the dyad inversion effect

When observing two individuals, people are faster and better able to identify them as other people if they are facing each other than if they are facing away from each other. This advantage disappears when the images are inverted, suggesting that the visual system is particularly sensitive to dyads in this upright configuration, and perceptually groups socially engaged dyads into a single holistic unit. This dyadic inversion effect was obtained with images of full bodies. Body information was sufficient to elicit this effect even when information about head orientation was absent. However, it has not been tested whether the dyadic inversion effect occurs with face images and whether the emotions displayed by the faces modulate the effect. In three experiments we obtained robust dyadic inversion with face images. Holistic processing of upright face pairs occurred for neutral, happy, and sad faces but not for angry and fearful face pairs. Thus, perceptual grouping of individuals into pairs appears to depend on the emotional expressions of individual faces and the interpersonal relations they imply.