The integration of paralinguistic information from the face and the voice

We live in a world which bombards us with a huge amount of sensory information, even if we are not always aware of it. To successfully navigate, function and ultimately survive in our environment we use all of the cues available to us. Furthermore, we actually combine this information: doing so allows us not only to construct a richer percept of the objects around us, but actually increases the reliability of our decisions and sensory estimates. However, at odds with our naturally multisensory awareness of our surroundings, the literature addressing unisensory processes has always far exceeded that which examines the multimodal nature of perception. Arguably the most salient and relevant stimuli in our environment are other people. Our species is not designed to operate alone, and so we have evolved to be especially skilled in all those things which enable effective social interaction – this could be engaging in conversation, but equally as well recognising a family member, or understanding the current emotional state of a friend, and adjusting our behaviour appropriately. In particular, the face and the voice both provide us with a wealth of hugely relevant social information - linguistic, but also non-linguistic. In line with work conducted in other fields of multisensory perception, research on face and voice perception has mainly concentrated on each of these modalities independently, particularly face perception. Furthermore, the work that has addressed integration of these two sources by and large has concentrated on the audiovisual nature of speech perception. The work in this thesis is based on a theoretical model of voice perception which not only proposed a serial processing pathway of vocal information, but also emphasised the similarities between face and voice processing, suggesting that this information may interact. Significantly, these interactions were not just confined to speech processing, but rather encompassed all forms of information processing, whether this was linguistic or paralinguistic. Therefore, in this thesis, I concentrate on the interactions between, and integration of face-voice paralinguistic information. In Chapter 3 we conducted a general investigation of neural face-voice integration. A number of studies have attempted to identify the cerebral regions in which information from the face and voice combines; however, in addition to a large number of regions being proposed as integration sites, it is not known whether these regions are selective in the binding of these socially relevant stimuli. We identified firstly regions in the bilateral superior temporal sulcus (STS) which showed an increased response to person-related information – whether this was faces, voices, or faces and voices combined – in comparison to information from objects. A subsection of this region in the right posterior superior temporal sulcus (pSTS) also produced a significantly stronger response to audiovisual as compared to unimodal information. We therefore propose this as a potential people-selective, integrative region. Furthermore, a large portion of the right pSTS was also observed to be people-selective and heteromodal: that is, both auditory and visual information provoked a significant response above baseline. These results underline the importance of the STS region in social communication. Chapter 4 moved on to study the audiovisual perception of gender. Using a set of novel stimuli – which were not only dynamic but also morphed in both modalities – we investigated whether different combinations of gender information in the face and voice could affect participants’ perception of gender. We found that participants indeed combined both sources of information when categorising gender, with their decision being reflective of information contained in both modalities. However, this combination was not entirely equal: in this experiment, gender information from the voice appeared to dominate over that from the face, exerting a stronger modulating effect on categorisation. This result was supported by the findings from conditions which directed to attention, where we observed participants were able to ignore face but not voice information; and also reaction times results, where latencies were generally a reflection of voice morph. Overall, these results support interactions between face and voice in gender perception, but demonstrate that (due to a number of probable factors) one modality can exert more influence than another. Finally, in Chapter 5 we investigated the proposed interactions between affective content in the face and voice. Specifically, we used a ‘continuous carry-over’ design – again in conjunction with dynamic, morphed stimuli – which allowed us to investigate not only ‘direct’ effects of different sets of audiovisual stimuli (e.g., congruent, incongruent), but also adaptation effects (in particular, the effect of emotion expressed in one modality upon the response to emotion expressed in another modality). Parallel to behavioural results, which showed that the crossmodal context affected the time taken to categorise emotion, we observed a significant crossmodal effect in the right pSTS, which was independent of any within-modality adaptation. We propose that this result provides strong evidence that this region may be composed of similarly multisensory neurons, as opposed to two sets of interdigitised neurons responsive to information from one modality or the other. Furthermore, an analysis investigating stimulus congruence showed that the degree of incongruence modulated activity across the right STS, further inferring neural response in this region can be altered depending on the particular combination of affective information contained within the face and voice. Overall, both behavioural and cerebral results from this study suggested that participants integrated emotion from the face and voice.