Speech intelligibility can vary depending on the characteristics of background sound in which it is presented. Along the auditory pathway interference may occur due to a physically degraded representation of speech at the peripheral level and/or a perceptually degraded representation at higher central cognitive levels. By manipulating background sounds the level at which interference occurs can be considered. In the presence of an interfering single-talker speech background, intelligibility is sometimes improved compared to performance in a stationary noise background thought to be due to an improved physical representation of the target speech. In children, this improvement is often found to be smaller than that compared to adults, although such findings have not always been reported. What this could suggest however, is that children may be more detrimentally affected by speech backgrounds than adults. This generally has been understood to reflect the maturation of central cognitive processes in children, where speech backgrounds may interfere with speech intelligibility at higher cognitive levels. Recent research however, proposes that this measured improvement may be subject to a signal-to-noise-ratio (SNR) confound which relates to differences in performance with the baseline stationary noise condition from which such improvement is compared. Thus previous findings could have been misinterpreted. The first of three experiments within this thesis aimed therefore to quantify such speech intelligibility improvement amongst children and adults and to investigate the developmental trajectory over a one year period. Contributing data from large samples, it was found that the intelligibility improvement with speech backgrounds was significantly smaller in children aged 5-6 years compared to adults, and although performance got better amongst children one year later the improvement remained significantly smaller. The second experiment aimed to further understand the effect speech backgrounds have on speech intelligibility in children by exploring the validity of found child/adult differences. This concerned the SNR confound and used analyses inspired by previous research to take into account baseline stationary noise differences. It was concluded that the difference between children and adults’ intelligibility improvement with speech backgrounds lessened, yet a difference remained suggesting fundamental differences in the effects speech backgrounds have on children compared to adults. Such results may have implications for children listening in noisy classrooms. Since speech backgrounds may interfere with speech intelligibility at higher cognitive levels, the final experiment attempted to further understand the mechanisms involved by aiming to tap into cognitive demands arising from communication in realistic listening situations. In order to do this, a reading paradigm was utilised to track eye movements during reading in the presence of differing background sounds. Whilst input from visual and auditory modalities may not cause either any physical degradation at peripheral levels, interference might occur at higher cognitive levels when processing language. Only one other study to date has used eye tracking technology to provide online insights into cognitive processing difficulty when reading amongst background speech. Therefore the final aim was to determine how different background sounds disrupt the reading process. It was found in stark contrast to speech intelligibility findings, a single-talker speech background caused more interference to the reading task compared to a stationary noise background in adult participants, suggesting that speech intelligibility measurements may not provide any information about cognitive load in everyday communication conditions. It is concluded that children are more detrimentally affected by speech backgrounds in speech intelligibility tasks compared to adults and that speech backgrounds interfere at higher cognitive levels invoking complex cognitive processes. Further research is needed to establish how speech backgrounds affect children during reading which could have important implications for noise levels in classroom settings.

Jointly attending to a shared referent with other people is a social attention behaviour that occurs often and has many developmental and ongoing social impacts. This thesis focused on examining the online, as well as later emerging, impacts of being the gaze leader of joint attention, which has until recently been under-researched. A novel social orienting response that occurs after viewing averted gaze is reported, showing that a gaze leader will rapidly orient their attention towards a face that follows their gaze: the gaze leading effect. In developing the paradigm necessary for this illustration a number of boundary conditions were also outlined, which suggest the social context of the interaction is paramount to the observability of the gaze leading effect. For example, it appears that the gaze leading effect works in direct opposition to other social orienting phenomenon (e.g. gaze cueing), may be specific to eye-gaze stimuli, and is associated with self-reported autism-like traits. This orienting response is suggested as evidence that humans may have an attention mechanism that promotes the more elaborate social attention state of shared attention. This thesis also assessed the longer term impacts of prior joint gaze interactions, finding that gaze perception can be influenced by prior interactions with gaze leaders, but not with followers, and further there is evidence presented that suggests a gaze leader’s attention will respond differently, later, to those whom have or have not previously followed their gaze. Again, this latter finding is associated with autism-like traits. Thus, the current work opens up a number of interesting research avenues concerning how attention orienting during gaze leading may facilitate social learning and how this response may be disrupted in atypically developing populations.

A number of attempts have been made to understand which physical image properties are important for the perception of different facial characteristics. These physical image properties have been broadly split in to two categories; namely facial shape and facial surface. Current accounts of face processing suggest that whilst judgements of facial identity rely approximately equally on facial shape and surface properties, judgements of facial expression are heavily shape dependent. This thesis presents behavioural experiments and fMRI experiments employing multi voxel pattern analysis (MVPA) to investigate the extent to which facial shape and surface properties underpin identity and expression perception and how these image properties are represented neurally. The first empirical chapter presents experiments showing that facial expressions are categorised approximately equally well when either facial shape or surface is the varying image cue. The second empirical chapter shows that neural patterns of response to facial expressions in the Occipital Face Area (OFA) and Superior Temporal Sulcus (STS) are reflected by patterns of perceptual similarity of the different expressions, in turn these patterns of perceptual similarity can be predicted by both facial shape and surface properties. The third empirical chapter demonstrates that distinct patterns of neural response can be found to shape based but not surface based cues to facial identity in the OFA and Fusiform Face Area (FFA). The final experimental chapter in this thesis demonstrates that the newly discovered contrast chimera effect is heavily dependent on the eye region and holistic face representations conveying facial identity. Taken together, these findings show the importance of facial surface as well as facial shape in expression perception. For facial identity both facial shape and surface cues are important for the contrast chimera effect although there are more consistent identity based neural response patterns to facial shape in face responsive brain regions.

Faces provide information critical for effective social interactions. A face can be used to determine who someone is, where they are looking and how they are feeling. How these different aspects of a face are processed has proved a popular topic of research over the last 25 years. However, much of this research has focused on the perception of facial identity and as a result less is known about how facial expression is represented in the brain. For this reason, the primary aim of this thesis was to explore the neural representation of facial expression. First, this thesis investigated which regions of the brain are sensitive to expression and how these regions represent facial expression. Two regions of the brain, the posterior superior temporal sulcus (pSTS) and the amygdala, were more sensitive to changes in facial expression than identity. There was, however, a dissociation between how these regions represented information about facial expression. The pSTS was sensitive to any change in facial expression, consistent with a continuous representation of expression. In comparison, the amygdala was only sensitive to changes in expression that resulted in a change in the emotion category. This reflects a more categorical response in which expressions are assigned into discrete categories of emotion. Next, the representation of expression was further explored by asking what information from a face is used in the perception of expression. Photographic negation was used to disrupt the surface-based facial cues (i.e. pattern of light and dark across the face) while preserving the shape-based information carried by the features of the face. This manipulation had a minimal effect on judgements of expression, highlighting the important role of the shape-based information in judgements of expression. Furthermore, combining the photo negation technique with fMRI demonstrated that the representation of faces in the pSTS was predominately based on feature shape information. Finally, the influence of facial identity on the neural representation of facial expression was measured. The pSTS, but not the amygdala, was most responsive to changes in facial expression when the identity of the face remained the same. It was found that this sensitivity to facial identity in the pSTS was a result of interactions with regions thought to be involved in the processing of facial identity. In this way identity information can be used to process expression in a socially meaningful way.