还记得朱自清《荷塘月色》中的“微风过处，送来缕缕清香，仿佛远处高楼上渺茫的歌声似的”吗？这名句常被用来举例说明“通感”这种不同感觉在大脑中互相沟通交错的现象。英国一项最新研究说，这种现象并非文学家的比喻，而有其生理基础。
牛津大学研究人员在新一期《当代生物学》杂志上报告了相关成果。据介绍，通感现象有多种表现，与朱自清“听见”香味相似，有的人可以“看见”声音，即在听到某些声音时眼前会出现某种颜色，还有些人在看到某些特别的数字或字母时眼前会浮现出某种颜色。本次研究的对象就是一些有最后这一种通感表现的人。
研究中使用了一种磁刺激手段，即对大脑特定部位施加微弱的磁场，观察大脑皮层的反应。结果显示，与普通人相比，那些有通感表现的受试者大脑中的视觉皮层更加敏感和活跃，只需很小的刺激，他们的视觉神经元就会发出相应信号。
研究人员德温·特休恩说，大脑中与视觉功能相关的皮层特别敏感可能是造成通感现象的原因，相关受试者虽然只看到一些字母或数字，但其视觉皮层被激起了关于颜色的反应。
本次研究还显示，这种通感能力的大小是可调节的。通过对大脑相应区域施加特殊的电流，可以改变这些区域中大脑皮层的敏感程度。如果把大脑皮层敏感度调得更高，受试者的通感表现就更为明显；而如果降低大脑皮层的敏感度，甚至可以完全消除通感现象。（生物谷 Bioon.com）
doi:10.1016/j.cub.2011.10.032
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Enhanced Cortical Excitability in Grapheme-Color Synesthesia and Its Modulation
Devin Blair Terhune, Sarah Tai, Alan Cowey, Tudor Popescu, Roi Cohen Kadosh
Synesthesia is an unusual condition characterized by the over-binding of two or more features and the concomitant automatic and conscious experience of atypical, ancillary images or perceptions. Previous research suggests that synesthetes display enhanced modality-specific perceptual processing, but it remains unclear whether enhanced processing contributes to conscious awareness of color photisms. In three experiments, we investigated whether grapheme-color synesthesia is characterized by enhanced cortical excitability in primary visual cortex and the role played by this hyperexcitability in the expression of synesthesia. Using transcranial magnetic stimulation, we show that synesthetes display 3-fold lower phosphene thresholds than controls during stimulation of the primary visual cortex. We next used transcranial direct current stimulation to discriminate between two competing hypotheses of the role of hyperexcitability in the expression of synesthesia. We demonstrate that synesthesia can be selectively augmented with cathodal stimulation and attenuated with anodal stimulation of primary visual cortex. A control task revealed that the effect of the brain stimulation was specific to the experience of synesthesia. These results indicate that hyperexcitability acts as a source of noise in visual cortex that influences the availability of the neuronal signals underlying conscious awareness of synesthetic photisms.
