City University London.
King's College London.
J Cogn Neurosci. 2019 Jun;31(6):922-935. doi: 10.1162/jocn_a_01395. Epub 2019 Mar 18.
Some people experience auditory sensations when seeing visual flashes or movements. This prevalent synaesthesia-like visually evoked auditory response (vEAR) could result either from overexuberant 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 10 Hz (alpha band frequency) or 40 Hz (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 10-Hz 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 40-Hz 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.
有些人在看到视觉闪光或运动时会体验到听觉感觉。这种普遍存在的类联觉视觉诱发听觉反应（vEAR）可能是由于大脑区域之间过度活跃的交叉激活，或者正常发生的交叉激活的抑制减少所致。我们使用经颅交流电刺激（tACS）来检验这些理论。我们在颞部或枕部双侧施加 10 Hz（alpha 频段频率）或 40 Hz（gamma 频段频率）的 tACS，同时测量成对的听觉（A）与视觉（V）莫尔斯码序列的相同/不同辨别。在汇报时，根据参与者是否报告“听到”无声闪光，将其分类为 vEAR 或非 vEAR。在非 vEAR 参与者中，颞部 10 Hz tACS 导致 A 表现受损，这与 V 的改善相关；相反，在枕部 tACS 下，较差的 V 表现与 A 的改善相关。这种相互关联的模式表明，感觉皮层通常是相互抑制的，alpha 频率 tACS 可能会影响它们之间竞争的平衡。vEAR 参与者没有显示出 tACS 效应，这与抑制减少或模态之间的增强合作一致。此外，颞部 40 Hz tACS 损害了 V 的表现，特别是在那些 V（相对于 A）表现出优势的个体中。因此，伽马频率 tACS 可能通过干扰视觉刺激对听觉区域的交叉激活，调节这些个体将闪光重新编码到听觉模式的能力。我们的结果支持这两种理论，表明 vEAR 可能取决于正常发生的感觉交叉激活的抑制，而这种抑制在某些个体中可能表现得更强。此外，内源性 alpha 和 gamma 频率振荡可能分别起到抑制或促进这种交叉激活的作用。
