Author Archives: Elliot Freeman

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Our new tACS paper is out!

 

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

New electrical brain stimulation study shows evidence that disinhibition of latent neural connections may underpin visual-ear experiences, promoting cooperation between vision and audition.

Fassnidge, C., Ball, D., Kazaz, Z., Knudsen, S., Spicer, A., Tipple, A. and Freeman, E. D. (2019).  Journal of Cognitive Neuroscience, https://www.mitpressjournals.org/earlyaccessmit
Free version: http://openaccess.city.ac.uk/21725/

Highlights

  • Some people report hearing auditory sensations evoked by visual movement and flashes. We report the first neurostimulation investigation of causal brain mechanisms underlying this visual-evoked auditory response (vEAR).
  • Results support distinct inhibitory versus integrative functions of alpha and gamma oscillations, respectively
  • Alpha-frequency transcranial Alternating Current Stimulation reveals that vision and audition normally compete, but less so in people who report ‘hearing’ visual movement and flashes
  • Gamma-frequency stimulation to temporal areas disrupts performance of a visual sequence-memory task, specifically in people who benefit from being able to recode visual events as sounds
  • Results suggest that vEAR depends on disinhibition of normally-occurring latent inter-cortical connections, not just on abnormally increased cross- connectivity

Abstract

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.