Over the past century or so, we've learned a lot about the mental processes of producing, perceiving and learning language. This knowledge is detailed and extensive, but in most cases, we do not know how these processes are actually implemented in the brain. Over the same period, we've learned a great deal about the localization of different linguistic abilities in different regions of the brain, and also about how neural computation works in general. However, our understanding of how the brain creates and understands language remains relatively crude. One of today's great scientific challenges is to integrate the results of these two different kinds of investigation of the mind and of the brain with the goal of bringing both to a deeper level of understanding.
As a concrete example of this mind/brain dichotomy, consider the following. From literally thousands of studies, “we know that word frequency has a large effect on mental processing of both speech and text: in all sorts of tasks commoner words are processed more quickly than rare ones, other things equal” (Zull, 2002). However, we don't know for sure how this is implemented in the brain. Is neural knowledge of more common words stored in larger or more widespread chunks of brain tissue? Are the neural representations of common words more widely or strongly connected? Are the resting activation levels of their neural representations simply higher? Are they less efficiently inhibited? Amazingly enough, there is no clear evidence about the relative contributions of these four different kinds of brain mechanisms to the phenomenon of word frequency effects.
Again, psychological research tells us that there is also a strong recency effect: in all sorts of tasks, words that we've heard or seen recently are processed more quickly........
