Natural Contrast Statistics Facilitate Human Face Categorization

Abstract

The ability to detect faces in the environment is of utmost ecological importance for human social adaptation. While face categorization is efficient, fast and robust to sensory degradation, it is massively impaired when the facial stimulus does not match the natural contrast statistics of this visual category, i.e., the typically experienced ordered alternation of relatively darker and lighter regions of the face. To clarify this phenomenon, we characterized the contribution of natural contrast statistics to face categorization. Specifically, 31 human adults viewed various natural images of non-face categories at a rate of 12 Hz, with highly variable images of faces occurring every eight stimuli (1.5 Hz). As in previous studies, neural responses at 1.5 Hz as measured with high-density EEG provided an objective neural index of face categorization. Here, when face images were shown in their naturally experienced contrast statistics, the 1.5 Hz face categorization response emerged over occipito-temporal electrodes at very low contrast (5.1%, or .009 root-mean-square, RMS, contrast), quickly reaching optimal amplitude at 22.6% of contrast (i.e., RMS contrast of .041). Despite contrast negation preserving an image's spectral and geometrical properties, negative contrast images required twice as much contrast to trigger a face categorization response, and three times as much to reach optimum. These observations characterize how the internally stored natural contrast statistics of the face category facilitate visual processing for the sake of fast and efficient face categorization. Significance statementHuman faces share a universal property: the strict alternation of contrast, with the darker main features against the more uniform, lighter skin. The ability to categorise faces depends critically on the presence of these natural contrast statistics in the input stimulus. However, it is not yet known how natural contrast statistics facilitate the visual processing leading to categorisation. Using frequency tagging and high-density electroencephalography, we show that access to internally stored natural statistics reduces the amount of sensory input necessary for human face categorization.

Backward masking reveals coarse-to-fine dynamics in human V1

“…We replicate past findings that negation substantially hampers the accumulation of sensory input necessary at high-level stages of processing (Liu-Shuang et al, 2022; see also Bex & Makous, 2002).…”

“…The coarse representation initially formed in highlevel visual regions has been proposed to involve some prior knowledge about the natural statistics of the category best matching input (e.g., Bar, 2004;Lee, 2015;Mumford, 1992). We thus expected contrast negation to hamper coarse-to-fine processing in V1 as this simple and reversible image manipulation disrupts the natural statistics of the human face stimulus (George et al, 1999;Liu-Shuang et al, 2022;Yue et al, 2013).…”

A new ‘CFS tracking’ paradigm reveals uniform suppression depth regardless of target complexity or salience

“…Face images may therefore be salient at lower contrasts than other images such as random noise for high-level reasons specific to face processing. However, given there is a parsimonious account of low bCFS thresholds for faces in terms of low-level image properties, this would need to be established experimentally, perhaps using face inversion ( Matsuyoshi et al, 2015 ) or contrast reversal ( Liu-Shuang et al, 2022 ), manipulations that differentially impact visual processing of faces compared to other object categories This matter is still open and ripe for further experimental evidence, but our current findings add support to the argument that faces may become visible in CFS at lower contrasts for reasons unrelated to special access to awareness or partial processing during suppression.…”