Role of ordinal contrast relationships in face encoding

Abstract

What aspects of facial information do we use to recognize individuals? One way to address this fundamental question is to study image transformations that compromise facial recognizability. The goal would be to identify factors that underlie the recognition decrement and, by extension, are likely constituents of facial encoding. To this end, we focus here on the contrast negation transformation. Contrast negated faces are remarkably difficult to recognize for reasons that are currently unclear. The dominant proposals so far are based either on negative faces' seemingly unusual pigmentation, or incorrectly computed 3D shape. Both of these explanations have been challenged by recent results. Here, we propose an alternative account based on 2D ordinal relationships, which encode local contrast polarity between a few regions of the face. Using a novel set of facial stimuli that incorporate both positive and negative contrast, we demonstrate that ordinal relationships around the eyes are major determinants of facial recognizability. Our behavioral studies suggest that destruction of these relationships in negatives likely underlies the observed recognition impairments, and our neuro-imaging data show that these relationships strongly modulate brain responses to facial images. Besides offering a potential explanation for why negative faces are hard to recognize, these results have implications for the representational vocabulary the visual system uses to encode faces.contrast negation ͉ face perception ͉ fMRI ͉ neural representation ͉ object recognition I n principle, a contrast negated image is exactly as informative as its positive counterpart; negation perfectly preserves an image's 2D geometric and spectral structure. However, as anyone who has had to search through a roll of negatives for a snapshot of a particular person knows, this simple operation has dramatically adverse consequences on our ability to identify faces, as illustrated in Fig. 1 (1-6). Exploring the causes of this phenomenon is important for understanding the broader issue of the nature of information the visual system uses for face identification.Several researchers have hypothesized that negated face-images are hard to recognize because of the unnatural shading cues in negatives, which compromise shape from shading processes (7-11). The resulting problems in recovering veridical 3D facial shape are believed to impair recognition performance. Although plausible, it is unclear whether this explanation is a sufficient one, especially in light of experimental results showing preserved recognition performance in the absence of shading gradients (12), and theories of face recognition that are based on the use of 2D intensity patterns rather than recovered 3D shapes (13,14). Another prominent hypothesis is that negation causes faces to have unusual pigmentation (15, 16). However, the adequacy of this ''pigmentation hypothesis'' has been challenged by data showing that hue negation, which also results in unnatural pigmentation (making the entire face l...

Reduced sensitivity to contrast signals from the eye region in developmental prosopagnosia

“…Because the eye region contains several important contrast-related signals (e.g. the boundaries between the sclera, iris and pupil of the eye, and contrast differences between the eyes and surrounding regions including eyebrows), inverting the contrast polarity of this region is particularly detrimental for face recognition, and restoring this region to normal contrast eyes in an otherwise contrast-inverted face leads to dramatic improvements in recognition performance (Gilad et al, 2009;Gandhi et al, 2012;Sormaz et al, 2013).…”

“…These images were generated by selectively inverting the contrast of the eye region within an otherwise normal-contrast face (negative-eyes chimeras) or by leaving the eye region unchanged and contrast-inverting the rest of the face (positive-eyes chimeras; see Figure 1A). Previous studies have shown that restoring 7 the normal contrast of the eye region in an otherwise contrast-inverted face improved face recognition to approximately 90% of the level observed for normal-contrast faces (Gilad et al, 2009;Sormaz, Andrews, & Young, 2013), and also strongly reduced the effects of contrast inversion on the N170 component (Fisher, Towler, & Eimer, 2015;Gandhi, Suresh, & Sinha, 2012). Such observations demonstrate that contrast signals from the eye region are particularly important during early perceptual stages of face processing.…”

“…Such observations demonstrate that contrast signals from the eye region are particularly important during early perceptual stages of face processing. These stages appear to be specifically sensitive to deviations from the typical contrast within this region, presumably because contrast-related signals from this region are particularly important for face detection and face recognition processes (e.g., Gilad et al, 2009;Peterson & Eckstein, 2012;Sormaz et al, 2013). Participants with intact face processing abilities tend to fixate the area between both eyes during face perception and recognition tasks (Hsiao & Cottrell, 2008).…”

“…In addition to normal-contrast and fully contrast-inverted faces, we also employed two types of partially contrast-inverted face images ("contrast chimeras"; see Gilad, Meng, & Sinha, 2009). These images were generated by selectively inverting the contrast of the eye region within an otherwise normal-contrast face (negative-eyes chimeras) or by leaving the eye region unchanged and contrast-inverting the rest of the face (positive-eyes chimeras; see Figure 1A).…”

Face identity matching is selectively impaired in developmental prosopagnosia

“…Previous ERP studies of DP that focused on the N170 component have found evidence that DPs are less sensitive to the prototypical spatial configuration of upright faces and to contrast signals from the eye region (Fisher et al, 2016b). Such spatial-configural and contrast-related related signals, in particular from the eyes, provide important cues to identity (e.g., Gilad, Meng, & Sinha, 2009), because they remain invariant across changes in expression and other image changes (e.g., Burton, 2013). If the perceptual analysis of such image-invariant visual identity cues was selectively impaired, DPs would have to rely more strongly on low-level image-dependent features.…”