The Visually-Evoked Auditory Response

In synaesthesia a sensation in one modality triggers a consciously perceived sensation in another sensory modality or cognitive domain. In this thesis we investigate auditory sensation that are induced by dynamic visual stimuli, akin to hearing-motion synaesthesia (Saenz and Koch, 2008). We term this the Visually-Evoked Auditory Response (vEAR). We first establish the prevalence of vEAR in a random sample, with questionnaire responses indicating a higher prevalence (as many as 1 in 5) than canonical synaesthesias. We report that those who experience vEAR showed better performance compared to controls when discriminating between ‘Morse-code’ style rhythmic sequences in the visual domain, as did Saenz and Koch (2008). We also demonstrate that vEAR is perceptually real enough to interfere with hearing real world sounds. We then demonstrate that in control subjects Transcranial Alternating Current Stimulation (TACS), when applied over the temporal versus the occipital lobes, impairs auditory versus visual sequence discrimination respectively. However, temporal TACS improved visual and occipital TACS improved auditory sequence discrimination performance. This suggests the presence of normally-occurring mutual alpha-mediated competitive inhibition of the two cortices. This TACS effect was not seen in individuals with vEAR, indicating that their auditory and visual cortices are able to cooperate to perform the task despite disruption from TACS. Finally, we investigate the types of visual stimuli that best evoke vEAR, and the types of people who tend to experience it. We conducted a large online survey in which respondents rated the amount of vEAR evoked by a series of silent videos depicting types of motion. The predictiveness of a real-world sound was identified as a major contributor to ratings in all respondents, while motion energy (raw changes in light over space and time) specifically influenced ratings in those who experience vEAR. We also report demographic and trait questions relating to auditory perception that predict higher ratings, including the frequency one experiences music imagery in their head, or whether they have tinnitus or types of synaesthesia. We conclude that vEAR results from both high and low-level connectivity between the visual and auditory cortices and an atypical inhibition of these connections