Face detection mechanisms: Nature vs. nurture

Abstract

For many animals, faces are a vitally important visual stimulus. Hence, it is not surprising that face perception has become a very popular research topic in neuroscience, with ca. 2000 papers published every year. As a result, significant progress has been made in understanding the intricate mechanisms underlying this phenomenon. However, the ontogeny of face perception, in particular the role of innate predispositions, remains largely unexplored at the neural level. Several influential studies in monkeys have suggested that seeing faces is necessary for the development of the face-selective brain domains. At the same time, behavioural experiments with newborn human babies and newly-hatched domestic chicks demonstrate that a spontaneous preference towards faces emerges early in life without pre-existing experience. Moreover, we were recently able to record face-selective neural responses in the brain of young, face-naïve chicks, thus demonstrating the existence of an innate face detection mechanism. In this review, we discuss these seemingly contradictory results and propose potential experimental approaches to resolve some of the open questions.

Keywords: brain; face perception; innate; neuron; stimuli.

Figure 1

Face detection and face categorization seem to rely on different neural mechanisms. However, both processes happen simultaneously, which makes it difficult to disentangle them in the experiment. (upper part) Face detection might be an evolutionary ancestral trait of vertebrates, which relies on subcortical processing of a face-like pattern. Hence, stimuli with a schematic representation of facial features (two eyes and a mouth/beak) elicit a behavioural response in newborn animals. Two essential control stimuli could be used to describe properties of a face detector: (a) an inverted face-like pattern virtually identical to the face-like pattern in terms of spatial frequency and luminosity; (b) an asymmetric configuration to reveal a potential effect of symmetry and topheavyness (see main text) on the face detection. (lower part) In contrast to face detection, face categorization is less likely to have a common evolutionary origin among distantly-related species (such as mammals and birds). Therefore, the optimal face stimulus should be experimentally determined for each species. The control stimuli to describe the properties of a “cortical” face categorization mechanism should involve various modifications. (a) Inverted faces should affect holistic processing of facial features or face detection mechanism. (b) Scrambled faces that should preserve low-level visual characteristics (spatial frequency, luminosity) and simultaneously be semantically meaningless [e.g., through diffeomorphic transformation (Stojanoski and Cusack, 2014)]. (c, d) Faces without single facial elements or (e) with shuffled elements might reveal the feature-selectivity of neural responses. Importantly, hiding single facial elements behind the bars of a background colour is suboptimal, since it might also affect the perception of a head outline, as well as change overall spatial frequency, texture, and luminosity of a stimulus. (f, g) Chimeric faces consisting of facial elements of different individuals/species (in this case domestic chicken and pigeon) aim to investigate the selectivity of face categorization mechanism. In this case, the face detection mechanism should not be affected, since the face-like configuration is preserved.