The Brain That Changes Itself (17)

The Question for Merzenich was, how does this topographic order emerge in the brain map? The answer he and his group came to was ingenious. A topographic order emerges because many of our everyday activities involve repeating sequences in a fixed order. When we pick up an object the size of an apple or baseball, we usually grip it first with our thumb and index finger, then wrap the rest of our fingers around it one by one. Since the thumb and index finger often touch at almost the same time, sending their signals to the brain almost simultaneously, the thumb map and the index finger map tend to form close together in the brain. (Neurons that fire together wire together.) As we continue to wrap our hand around the object, our middle finger will touch it next, so its brain map will tend to be beside the index finger and farther away from the thumb. As this common grasping sequence --- thumb first, index inter second, middle finger third --- is repeated thousands of times, it leads to a brain map where the thumb map is next to the index finger map, and so on. Signals that tend to arrive at separate times, like thumbs and pinkies, have more distant brain maps, because neurons that fire apart wire apart.

但是这个例子并不能解释topographic的概念。只有触觉和和人的身体结构有关的，所以存在空间上的相关性，但是很多其他的感官不存在空间性，那么这些感官在中枢里使如何排列的呢？再以味觉和嗅觉为例。人吃东西的时候同时受多种味道的刺激，大脑是如何分辨不同的味道的呢？大脑皮层上是不是有不同的区域对应不同的味觉？那些味觉在大脑中是相邻的？为什么它们相邻？等等。这些问题都不能用简单的topographic来解释。