Have you ever heard the color blue?  Or tasted the musical note F-sharp?  For most people, in both cases the answer is no.  But for some people blue has a particular sound or shape, and F sharp has a slightly sour taste.  This unusual blending of the senses is called synesthesia, a rare neurological phenomenon occurring in roughly 1 in 100,000 people.

　　Synesthesia

　　People with this condition do not merely associate sound with color or taste with sound, or imagine hearing a sound when they see a certain color.  Rather, when a person with synesthesia encounters a particular sensory stimulus, say seeing the color red, she will hear a sound even when there is no outside source producing the sound.

　　In a similar manner, a particular sound may cause someone with synesthesia to see certain shapes or certain colors, even when there is no object in that person’s line of sight.  The same goes for taste and smell.

　　Causes

　　Scientists know very little about what causes synesthesia.  While it appears to be hereditary, the actual neurological processes that account for the phenomenon remain a mystery.  Although most scientists agree that the hippocampus, an area of the brain responsible for memory, plays a role, exactly how the hippocampus might cause synesthesia is not clear.

　　Otherwise, scientific knowledge about synesthesia is limited to several interesting observations.  For example, synesthetic perceptions remain consistent over time.  In other words, if a person with synesthesia sees blue and green flashes when he hears a C-sharp, he will always see those same colors upon hearing that note.

　　颜色有气味吗？

　　你曾听到过蓝色么？你曾经尝到过音符F调么？对于大多数人来说，答案是否定的。但是，对于某些特定人群来说，蓝色确实有一种特定的声音或形状，F调也确实有一种酸酸的味道。这种不寻常的感官混淆被称为“共同感觉”，一种鲜有的神经学现象，通常十万分之一的人有这种症状。

　　“共同感觉”

　　有这种症状的人不仅仅会将声音与颜色联系、味觉与声音联系，也可能会在看到某种特定颜色时设想听到了某种声音。更确切地说，一个有“共同感觉”的人如果受到了某种特殊的感官刺激，比如看到了红色，哪怕外界并未有发音源，她也依然会听到某种声音。

　　类似情况下，某种特定的声音也会引发拥有“共同感觉”的人能看到某种形状或特定的颜色，哪怕在他的视觉范围内并未有这样的物体出现。对于味觉和嗅觉，这种情况也同样存在。

　　原因

　　科学家们对于为何会引发“共同感觉”知之甚少。虽然看起来这是一种遗传疾病，但事实上，对于这种现象的神经发生过程如何解释依然成谜。尽管很多科学家们认为“海马体”-大脑中用于控制记忆的一片区域导致此种现象出现，但究竟它是如何影响并引发“共同感觉”的症状依然不得而知。

　　另外，目前对于“共同感觉”的科学认识也仅仅是基于几种有趣的现象观察。比如，这种连觉的感知并不会随着时间过去而消失。换句话说，如果当一个人在听到C大调时看到蓝色与绿色闪过，那么将来一旦他听到同样的声音，眼前就会浮现出蓝色和绿色。