Abstract
Dogs are looking at and gaining information from human faces in a variety of contexts. Next to behavioral studies investigating the topic, recent fMRI studies reported face sensitive brain areas in dogs' temporal cortex. However, these studies used whole heads as stimuli which contain both internal (eyes, nose, mouth) and external facial features (hair, chin, face-outline). Behavioral studies reported that (1) recognition of human faces by dogs requires visibility of head contour and that (2) dogs are less successful in recognizing their owners from 2D pictures than from real human heads. In contrast, face perception in humans heavily depends on internal features and generalizes to 2D images. Whether putative face sensitive regions in dogs have comparable properties to those of humans has not been tested so far. In two fMRI experiments, we investigated (1) the location of putative face sensitive areas presenting only internal features of a real human face vs. a mono-colored control surface and (2) whether these regions show higher activity toward live human faces and/or static images of those faces compared to scrambled face images, all with the same outline. In Study 1 (n = 13) we found strong activity for faces in multiple regions, including the previously described temporo-parietal and occipital regions when the control was a mono-colored, homogeneous surface. These differences disappeared in Study 2 (n = 11) when we compared faces to scrambled faces, controlling for low-level visual cues. Our results do not support the assumption that dogs rely on a specialized brain region for processing internal facial characteristics, which is in line with the behavioral findings regarding dogs inability to recognize human faces based on these features.
Highlights
While object perception is based on thedistributed response from neurons coding different aspects of an object, the neural response to faces in humans (Andrews et al, 2010) and in some other primate species (Burke and Sulikowski, 2013) seems to be somewhat exceptional
In Study 1 we found multiple, bilateral brain regions in the temporal and occipital cortex which showed higher activity toward the inner parts of a single face than to a mono-colored control surface. This is in line with previous human results, reporting higher activity toward faces than surfaces/textures in humans, showing that even contrasts with such low level controls activate these higher-level regions in the human fusiform face areas (Puce et al, 1996)
We found no such effect in Study 2 when the stimuli contained images from multiple individuals and we controlled for color and brightness
Summary
While object perception is based on thedistributed response from neurons coding different aspects of an object, the neural response to faces in humans (Andrews et al, 2010) and in some other primate species (Burke and Sulikowski, 2013) seems to be somewhat exceptional. The composite effect, which means slower identification of half of a chimeric face if it is aligned with an inconsistent other half-face than if the two half-faces are misaligned (Tsao and Livingstone, 2008). These effects show that in the human brain, faces are represented as a combination of the parts (eyes, nose, mouth), but as a non-decomposable whole. These effects are disproportionately present for faces in contrast to e.g., everyday objects
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