Synaesthesia or Vivid Imagery? A Single Case fMRI Study of Visually Induced Olfactory Perception

in Multisensory Research
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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.

Multisensory Research

A Journal of Scientific Research on All Aspects of Multisensory Processing



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  • A subset of the images presented in the behavioural rating task. Images were presented for 2500 ms. These images were also used in the one-back task. One-word descriptions of the indicated images were used in the olfactory imagery task. This figure is published in colour in the online version.

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  • Results from the behavioural rating task. Participants indicated the level of perceived odour on a scale of 1–7, with 1 representing no odour and 7 representing a strong odour. This figure is a subset of the total set of images presented. This figure is published in colour in the online version.

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  • Results from the smell localizer, averaged across all participants. BOLD activity was contrasted between the smell and the no-smell blocks. The right piriform cortex is indicated within the circle (Talairach coordinates: 19, 6, −12). The piriform region-of-interest based on the activity from this localizer was used in the subsequent analyses of the one-back and olfactory task. This figure is published in colour in the online version.

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  • Activity in the piriform cortex during the one-back task. BOLD activity in the SYN and NON-SYN conditions were contrasted against baseline. There was increased BOLD activity in the piriform cortex when LJ viewed the SYN images compared to the NON-SYN images. However, there was little change in BOLD activity for each of the control participants when they perceived the SYN and NON-SYN images. Each of the bars represents the condition > baseline (SYN > baseline and NON-SYN > baseline).

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  • Activity in the piriform cortex during the imagery task. BOLD activity in the three conditions (SYN, NON-SYN and NON-ODOUR) were contrasted against baseline. There was little difference in BOLD activity in the piriform cortex between the SYN, NON-SYN, and NON-ODOUR conditions compared to baseline, in the control participants. There was deactivation in piriform cortex when LJ tried to imagine the SYN words compared to baseline. There was little change in the NON-SYN and NON-ODOUR conditions compared to baseline. Each of the bars represents the condition > baseline (SYN > baseline, NON-SYN > baseline and NON-ODOUR > baseline).

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