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The most common form of synaesthesia is grapheme–colour synaesthesia. However, rarer forms of synaesthesia also exist, such as word–gustatory and olfactory–gustatory synaesthesia, whereby a word or smell will induce a specific. In this study we describe a single individual (LJ) who experiences a concurrent olfactory stimulus when presented with congruent visual images. For some visual stimuli, he perceives a strong and automatic olfactory percept, which has existed throughout his life. In this study, we explore whether his experiences are a new form of synaesthesia or simply vivid imagery. Unlike other forms of synaesthesia, the concurrent odour is congruent to the visual inducer. For example, a photograph of dress shoes will elicit the smell of leather. We presented LJ and several control participants with 75 images of everyday objects. Their task was to indicate the strength of any perceived odours induced by the visual images. LJ rated several of the images as inducing a concurrent odour, while controls did not have any such percept. Images that LJ reported as inducing the strongest odours were used, along with colour-matched control images, in the context of an fMRI experiment. Participants were given a one-back task to maintain attention. A block-design odour localizer was presented to localize the piriform cortex (primary olfactory cortex). We found an increased BOLD response in the piriform cortex for the odour-inducing images compared to the control images in LJ. There was no difference in BOLD response between these two stimulus types in the control participants. A subsequent olfactory imagery task did not elicit enhanced activity in the piriform cortex in LJ, suggesting his perceptual experiences may not be based on olfactory imagery.

The McGurk-effect (McGurk and MacDonald, 1976) is a robust illusion which is broadly studied in the context of audiovisual integration. In the illusion, auditory speech perception is modified by discrepant visual lip-movements when presented synchronously, leading to a third, not physically present percept. There is an ongoing debate as to whether the percept is integrated early in the sensory system or if it is susceptible to cognitive intervention. Here, the McGurk-effect was studied using gender congruency (face/voice stimuli) as the cognitive intervention to address the question of integration. We first investigated changes in prevalence and reaction times due to perceived gender discrepancy during the McGurk-percept. In the second experiment, neural correlates of the gender discrepant McGurk-percept were studied using fMRI. We did not find differences in reaction times or accuracy between gender congruent or incongruent stimuli. Thus it appears the McGurk-percept itself is unaffected by gender incongruent stimuli on a behavioural level. In the fMRI experiment, we found the superior temporal gyrus to reveal a significantly increased activity when the McGurk-effect is perceived as compared to non-McGurk trials, but no effect in this area was found for gender congruency. However, a 2 × 2 whole brain ANOVA revealed a significant interaction in the face identity processing area (fusiform gyrus) as well as inferior parietal gyrus and superior colliculus. We suggest a mechanism, allowing a stable illusory percept in the gender discrepant McGurk-percept to be facilitated. To our knowledge this is the first study to demonstrate on a neural level that gender congruency affects processing of the McGurk-effect.

The processing of audiovisual information is an important aspect of object recognition since many objects are characterized by visual and auditory features. The images and sounds of everyday life objects are often associated with positive or negative emotions. There is evidence that emotional valence modulates the neural processing of object images and sounds presented in isolation. However, the impact of emotional valence on the neural processing of audiovisual objects is unknown. The aim of the present study was to investigate the influence of evaluative conditioning on cortical plasticity in human audiovisual object processing. We used functional magnetic resonance imaging (fMRI) to analyze changes in cortical audiovisual processing of unfamiliar artificial objects and sounds in naive subjects (Pre fMRI) and after a behavioral evaluative conditioning session in which subjects acquired positive and negative emotional associations with the artificial stimuli (Post fMRI). Results from the Post fMRI showed that additional brain regions were recruited for the processing of positively and negatively conditioned stimuli compared to the Pre fMRI data. The processing of positive stimuli additionally involved the left caudate nucleus and the right medial prefrontal cortex whereas the processing of negative stimuli additionally recruited the left insula. Furthermore, a congruence effect for matching emotional stimulus pairs was found in the right transversal temporal gyrus. These results indicate that emotional valence modulates object-related audiovisual processing. Moreover, we were able to show that emotional valence can be induced by a behavioral evaluative conditioning procedure leading to plastic changes in the processing of audiovisual stimuli.