The neural representation of facial expression and viewpoint in the human brain

The human face conveys a wealth of information to the perceiver. Not only can we make judgments on the identity of the person, but we can also make finer interpretations about the emotional state of the individual, and what they are currently attending to. The ability to process and act upon this information effectively, facilitates successful social interactions. The key information that indicates to us how a person is feeling and what they are attending to, is their facial expression and facial viewpoint. Because of their dynamic properties, facial expression and viewpoint are described as changeable aspects of faces. The human brain contains a core and extended network of face-responsive regions. One region in the posterior superior temporal sulcus (pSTS) is thought to have a central role in the processing of these changeable aspects of faces. An outstanding question about the neural representation of faces is whether changeable aspects such as expression and viewpoint, have distinct representations within these regions, or whether there are overlapping representations. This thesis aimed to further the understanding of the neural representation of facial expression and facial viewpoint, focussing on the neural representation in the pSTS. First, this thesis investigated how expression is represented in the pSTS. A variety of behavioural evidence has shown that face images (in contrast to other objects) are processed holistically. In contrast to these findings, this chapter demonstrated that the pSTS represents facial expressions in a feature-based way, showing changes in response to any change in facial expression. However this chapter was also able to demonstrate that a region considered to be part of the extended face processing system, the inferior frontal gyrus (IFG), has a holistic representation of facial expression that reflects behavioural holistic processing. The second experimental chapter asked whether there are distinct neural representations for processing changes in expression and changes in viewpoint, across the core and extended face-responsive regions. Distinct patterns of response were found for changes in expression and viewpoint in the core regions. These representations were largely invariant to changes in identity, supporting the idea of distinct processing pathways for invariant and changeable aspects of faces. In contrast to the core regions, regions of the extended system (IFG and amygdala) were predominantly selective to changes in facial expression rather than changes in viewpoint. The third experimental chapter asked whether there was a more fine-grained representation of facial viewpoint. Distinct patterns of response to different viewpoints were found. Interestingly, there was also a strong similarity for symmetrical viewpoints in the fusiform face area (FFA) and pSTS. This similarity in the patterns of response to symmetric viewpoint directions suggests that these regions represent an intermediate step towards full viewpoint invariance. The final experimental chapter aimed to determine the relative dominance of expression and viewpoint in the neural representation of the core regions. The patterns of response were largely dominated by viewpoint, perhaps reflecting a neural coding that is linked to the visual properties of the face. Overall, these findings support the existence of distinct patterns of response to expression and viewpoint, whereby these changeable aspects of faces are represented by an overall pattern across the core face-responsive regions, rather than as discreet modules

Power contours : optimising sample size and precision in experimental psychology and human neuroscience

When designing experimental studies with human participants, experimenters must decide how many trials each participant will complete, as well as how many participants to test. Most discussion of statistical power (the ability of a study design to detect an effect) has focussed on sample size, and assumed sufficient trials. Here we explore the influence of both factors on statistical power, represented as a two-dimensional plot on which iso-power contours can be visualised. We demonstrate the conditions under which the number of tri- als is particularly important, i.e. when the within-participant variance is large relative to the between-participants variance. We then derive power contour plots using existing data sets for eight experimental paradigms and methodologies (including reaction times, sensory thresholds, fMRI, MEG, and EEG), and provide example code to calculate estimates of the within- and between-participant variance for each method. In all cases, the within-participant variance was larger than the between-participants variance, meaning that the number of trials has a meaningful influence on statistical power in commonly used paradigms. An online tool is pro- vided (https://shiny.york.ac.uk/powercontours/) for generating power contours, from which the optimal combination of trials and participants can be calculated when designing future studies

Symmetrical viewpoint representations in face-selective regions convey an advantage in the perception and recognition of faces

Learning new identities is crucial for effective social interaction. A critical aspect of this process is the integration of different images from the same face into a view-invariant representation that can be used for recognition. The representation of symmetrical viewpoints has been proposed to be a key computational step in achieving view-invariance. The aim of this study was to determine whether the representation of symmetrical viewpoints in face-selective regions is directly linked to the perception and recognition of face identity. In Experiment 1, we measured fMRI responses while male and female human participants viewed images of real faces from different viewpoints (-90⁰, -45⁰, 0⁰, 45⁰, 90⁰ from full-face view). Within the face regions, patterns of neural response to symmetrical views (-45⁰ & 45⁰ or -90⁰ & 90⁰) were more similar than responses to non-symmetrical views in the FFA and STS, but not in the OFA. In Experiment 2, participants made perceptual similarity judgements to pairs of face images. Images with symmetrical viewpoints were reported as being more similar than non-symmetric views. In Experiment 3, we asked whether symmetrical views also convey an advantage when learning new faces. We found that recognition was best when participants were tested with novel face images that were symmetrical to the learning viewpoint. Critically, the pattern of perceptual similarity and recognition across different viewpoints predicted the pattern of neural response in face-selective regions. Together, our results provide support for the functional value of symmetry as an intermediate step in generating view-invariant representations