Michael P. Ewbank

Distinct representations for facial identity and changeable aspects of faces in the human temporal lobe

The neural system underlying face perception must represent the unchanging features of a face that specify identity, as well as the changeable aspects of a face that facilitate social communication. However, the way information about faces is represented in the brain remains controversial. In this study, we used fMR adaptation (the reduction in fMRI activity that follows the repeated presentation of identical images) to ask how different face- and object-selective regions of visual cortex contribute to specific aspects of face perception. We report that activity in the face-selective region of the fusiform gyrus (FG) was reduced following repeated presentations of the same face. Adaptation in this area was not sensitive to changes in image size, but was sensitive to changes in viewpoint. In contrast, face-selective regions in the superior temporal lobe failed to adapt to identical presentations of the same face, but showed an increased response when the same face was shown from different viewpoints and with different expressions. These results reveal a largely size-invariant neural representation in the inferior temporal lobe that could be involved in the recognition of facial identity, and a separate face-selective region in the superior temporal lobe that could be used to detect changeable aspects of faces. The absence of fMR-adaptation in object-selective regions of visual cortex challenges the idea that a more distributed network of areas is used to represent information about faces.

Personality Predicts the Brain's Response to Viewing Appetizing Foods: The Neural Basis of a Risk Factor for Overeating

Eating is not only triggered by hunger but also by the sight of foods. Viewing appetizing foods alone can induce food craving and eating, although there is considerable variation in this “external food sensitivity” (EFS). Because increased EFS is associated with overeating, identifying its neural correlates is important for understanding the current epidemic of obesity. Animal research has identified the ventral striatum, amygdala, hypothalamus, medial prefrontal and premotor cortices as key interacting structures for feeding. However, it is unclear whether a similar network exists in humans and how it is affected by EFS. Using functional magnetic resonance imaging, we showed that viewing appetizing compared with bland foods produced changes in connectivity among the human ventral striatum, amygdala, anterior cingulate and premotor cortex that were strongly correlated with EFS. Differences in the dynamic interactions within the human appetitive network in response to pictures of appetizing foods may determine an individuals risk of obesity.

Autism Spectrum Traits in the Typical Population Predict Structure and Function in the Posterior Superior Temporal Sulcus

Autism spectrum disorders (ASDs) are typically characterized by impaired social interaction and communication, narrow interests, and repetitive behaviors. The heterogeneity in the severity of these characteristics across individuals with ASD has led some researchers to suggest that these disorders form a continuum which extends into the general, or “typical,” population, and there is growing evidence that the extent to which typical adults display autistic traits, as measured using the autism-spectrum quotient (AQ), predicts performance on behavioral tasks that are impaired in ASD. Here, we show that variation in autism spectrum traits is related to cortical structure and function within the typical population. Voxel-based morphometry showed that increased AQ scores were associated with decreased white matter volume in the posterior superior temporal sulcus (pSTS), a region important in processing socially relevant stimuli and associated with structural and functional impairments in ASD. In addition, AQ was correlated with the extent of cortical deactivation of an adjacent area of pSTS during a Stroop task relative to rest, reflecting variation in resting state function. The results provide evidence that autism spectrum characteristics are reflected in neural structure and function across the typical (non-ASD) population.

Differential sensitivity for viewpoint between familiar and unfamiliar faces in human visual cortex

People are extremely proficient at recognizing faces that are familiar to them, but are poor at identifying unfamiliar faces. We used fMR-adaptation to ask whether this difference in recognition might be reflected in the relative viewpoint-dependence of face-selective regions in the brain. A reduced response (adaptation) to repeated images of unfamiliar or familiar faces was found in the fusiform face area (FFA), but not in the superior temporal sulcus (STS) face-selective region. To establish if the neural representation of faces was invariant to changes in viewpoint, we parametrically varied the viewing angle of successive images using 3-dimensional models of unfamiliar and familiar faces. We found adaptation to familiar faces across all changes in viewpoint in the FFA. In contrast, a release from adaptation was apparent in the FFA when unfamiliar faces were viewed at increasing viewing angles. These results provide a neural basis for differences in the recognition of familiar and unfamiliar faces.

Connectivity from the ventral anterior cingulate to the amygdala is modulated by appetitive motivation in response to facial signals of aggression

For some people facial expressions of aggression are intimidating, for others they are perceived as provocative, evoking an aggressive response. Identifying the key neurobiological factors that underlie this variation is fundamental to our understanding of aggressive behaviour. The amygdala and the ventral anterior cingulate cortex (ACC) have been implicated in aggression. Using functional magnetic resonance imaging (fMRI), we studied how the interaction between these regions is influenced by the drive to obtain reward (reward–drive or appetitive motivation), a personality trait consistently associated with aggression. Two distinct techniques showed that the connectivity between the ventral ACC and the amygdala was strongly correlated with personality, with high reward–drive participants displaying reduced negative connectivity. Furthermore, the direction of this effect was restricted from ventral ACC to the amygdala but not vice versa. The personality-mediated variation in the pathway from the ventral anterior cingulate cortex to the amygdala provides an account of why signals of aggression are interpreted as provocative by some individuals more than others.

Personality influences the neural responses to viewing facial expressions of emotion

Cognitive research has long been aware of the relationship between individual differences in personality and performance on behavioural tasks. However, within the field of cognitive neuroscience, the way in which such differences manifest at a neural level has received relatively little attention. We review recent research addressing the relationship between personality traits and the neural response to viewing facial signals of emotion. In one section, we discuss work demonstrating the relationship between anxiety and the amygdala response to facial signals of threat. A second section considers research showing that individual differences in reward drive (behavioural activation system), a trait linked to aggression, influence the neural responsivity and connectivity between brain regions implicated in aggression when viewing facial signals of anger. Finally, we address recent criticisms of the correlational approach to fMRI analyses and conclude that when used appropriately, analyses examining the relationship between personality and brain activity provide a useful tool for understanding the neural basis of facial expression processing and emotion processing in general.

Changes in "top-down" connectivity underlie repetition suppression in the ventral visual pathway.

Repetition of the same stimulus leads to a reduction in neural activity known as repetition suppression (RS). In functional magnetic resonance imaging (fMRI), RS is found for multiple object categories. One proposal is that RS reflects locally based “within-region” changes, such as neural fatigue. Thus, if a given region shows RS across changes in stimulus size or view, then it is inferred to hold size- or view-invariant representations. An alternative hypothesis characterizes RS as a consequence of “top-down” between-region modulation. Differentiating between these accounts is central to the correct interpretation of fMRI RS data. It is also unknown whether the same mechanisms underlie RS to identical stimuli and RS across changes in stimulus size or view. Using fMRI, we investigated RS within a body-sensitive network in human visual cortex comprising the extrastriate body area (EBA) and the fusiform body area (FBA). Both regions showed RS to identical images of the same body that was unaffected by changes in body size or view. Dynamic causal modeling demonstrated that changes in backward, top-down (FBA-to-EBA) effective connectivity play a critical role in RS. Furthermore, only repetition of the identical image showed additional changes in forward connectivity (EBA-to-FBA). These results suggest that RS is driven by changes in top-down modulation, whereas the contribution of “feedforward” changes in connectivity is dependent on the precise nature of the repetition. Our results challenge previous interpretations regarding the underlying nature of neural representations made using fMRI RS paradigms.

Why are you angry with me? Facial expressions of threat influence perception of gaze direction

Gaze direction can influence the processing of facial expressions. Angry faces are judged more angry when displaying a direct gaze compared to an averted gaze. We investigated whether facial expressions have a reciprocal influence on the perception of gaze. Participants judged the gaze of angry, fearful and neutral faces across a range of gaze directions. Angry faces were perceived as looking at the observer over a wider range than were fearful or neutral faces, which did not significantly differ. This effect was eliminated when presenting inverted faces, suggesting these results cannot be accounted for by differences in visible eye information. Our findings suggest the existence of a reciprocal influence between gaze direction and angry expressions.

Different Neural Mechanisms within Occipitotemporal Cortex Underlie Repetition Suppression across Same and Different-Size Faces

Repetition suppression (RS) (or functional magnetic resonance imaging adaptation) refers to the reduction in blood oxygen level–dependent signal following repeated presentation of a stimulus. RS is frequently used to investigate the role of face-selective regions in human visual cortex and is commonly thought to be a “localized” effect, reflecting fatigue of a neuronal population representing a given stimulus. In contrast, predictive coding theories characterize RS as a consequence of “top-down” changes in between-region modulation. Differentiating between these accounts is crucial for the correct interpretation of RS effects in the face-processing network. Here, dynamic causal modeling revealed that different mechanisms underlie different forms of RS to faces in occipitotemporal cortex. For both familiar and unfamiliar faces, repetition of identical face images (same size) was associated with changes in “forward” connectivity between the occipital face area (OFA) and the fusiform face area (FFA) (OFA-to-FFA). In contrast, RS across image size was characterized by altered “backward” connectivity (FFA-to-OFA). In addition, evidence was higher for models in which information projected directly into both OFA and FFA, challenging the role of OFA as the input stage of the face-processing network. These findings suggest “size-invariant” RS to faces is a consequence of interactions between regions rather than being a localized effect.

It is all in the face: carotenoid skin coloration loses attractiveness outside the face

Recently, the importance of skin colour for facial attractiveness has been recognized. In particular, dietary carotenoid-induced skin colour has been proposed as a signal of health and therefore attractiveness. While perceptual results are highly consistent, it is currently not clear whether carotenoid skin colour is preferred because it poses a cue to current health condition in humans or whether it is simply seen as a more aesthetically pleasing colour, independently of skin-specific signalling properties. Here, we tested this question by comparing attractiveness ratings of faces to corresponding ratings of meaningless scrambled face images matching the colours and contrasts found in the face. We produced sets of face and non-face stimuli with either healthy (high-carotenoid coloration) or unhealthy (low-carotenoid coloration) colour and asked participants for attractiveness ratings. Results showed that, while for faces increased carotenoid coloration significantly improved attractiveness, there was no equivalent effect on perception of scrambled images. These findings are consistent with a specific signalling system of current condition through skin coloration in humans and indicate that preferences are not caused by sensory biases in observers.