Hearing through your eyes: neural basis of audiovisual cross-activation, revealed by transcranial alternating current stimulation

Some people experience auditory sensations when seeing visual flashes or movements. This prevalent synaesthesia-like ‘visual-evoked auditory response’ (vEAR) could result either from over-exuberant cross-activation between brain areas, and/or reduced inhibition of normally-occurring cross-activation. We have used transcranial alternating current stimulation (tACS) to test these theories. We applied tACS at 10Hz (alpha-band frequency) or 40Hz (gamma-band), bilaterally either to temporal or occipital sites, while measuring same/different discrimination of paired auditory (A) versus visual (V) 'Morse code' sequences. At debriefing, participants were classified as vEAR or non-vEAR depending on whether they reported 'hearing' the silent flashes. In non-vEAR participants, temporal 10Hz tACS caused impairment of A performance, which correlated with improved V; conversely under occipital tACS, poorer V performance correlated with improved A. This reciprocal pattern suggests that sensory cortices are normally mutually inhibitory, and that alpha-frequency tACS may bias the balance of competition between them. vEAR participants showed no tACS effects, consistent with reduced inhibition, or enhanced cooperation between modalities. In addition, temporal 40Hz tACS impaired V performance, specifically in individuals who showed a performance advantage for V (relative to A). Gamma-frequency tACS may therefore modulate the ability of these individuals to benefit from recoding flashes into the auditory modality, possibly by disrupting cross-activation of auditory areas by visual stimulation. Our results support both theories, suggesting that vEAR may depend on disinhibition of normally-occurring sensory cross-activation, which may be expressed more strongly in some individuals. Furthermore, endogenous alpha and gamma-frequency oscillations may function respectively to inhibit or promote this cross-activation

A deafening flash! Visual interference of auditory signal detection

In some people, visual stimulation evokes auditory sensations. How prevalent and how perceptually real is this? Of 40 neurotypical adults, 22% responded 'Yes' when asked whether they heard faint sounds accompanying flash stimuli, and showed significantly better ability to discriminate visual ‘Morse-code’ sequences. This benefit might arise from an ability to recode of visual signals as sounds, thus taking advantage of superior temporal acuity of audition. In support of this, those who showed better visual relative to auditory also had poorer auditory detection in the presence of uninformative visual flashes, though this was independent of awareness of visually-evoked sounds. Thus a visually-evoked auditory representation may occur subliminally and disrupt detection of real auditory signals. The frequent natural correlation between visual and auditory stimuli might explain the surprising prevalence of this phenomenon. Overall, our results suggest that learned correspondences between strongly correlated modalities may provide a precursor for some synaesthetic abilities

Sounds from seeing silent motion: raw datasets from massive on-line survey of the visually-evoked auditory response (vEAR)

<div>Raw data from large on-line survey of visually-evoked auditory response (vEAR), at http://tinyurl.com/vEARsurvey<br></div><div><div><br></div><div>Results are published in Fassnidge, C. & <a href="http://openaccess.city.ac.uk/view/creators_id/elliot=2Efreeman=2E1.html">Freeman, E. D.</a> (2018). Sounds from seeing silent motion: Who hears them, and what looks loudest?.<em>Cortex</em>, doi: <a href="http://doi.org/10.1016/j.cortex.2018.02.019" target="_blank">10.1016/j.cortex.2018.02.019</a></div></div><div><br></div><div>Two files are provided. The one dated 'June' contains results from a large sample of volunteers who clicked through to the survey from online news articles reporting our previous research. The file dated 'June-Dec' contains data from naive participants recruited from an on-line participant pool. </div><div><br></div><div>For full details of methodology please refer to the published paper. </div><div><br></div><div>Note: Column entitled <i>'Introductory text' </i>contains responses to the question <i>'Have you previously been aware of experiencing this type of auditory sensation when viewing visual movement?'</i></div><div><i><br></i></div><div>For more information please email author [email protected]</div><div><i><br></i></div><div>Files:</div><div>Hearing-Motion+Survey_June+30,+2017_03.29_merged: main sample of volunteers <br></div><div>Hearing-Motion+Survey_Prolific_JuneDec.xlsx: naive paid participants<br></div