单项选择题

In patients with Huntington" s disease, it" s the part of the brain called the basal ganglia that" s destroyed. While these victims have perfectly intact explicit memory systems, they can"t learn new motor skills. An Alzheimer" s patient can learn to draw in a mirror but can" t remember doing it: a Huntington" s patient can" t do it but can remember trying to learn. Yet another region of the brain, an almond-size knot of neural tissue seems to be crucial in forming and triggering the recall of a special subclass of memories that is tied to strong emotion, especially fear. These are just some of the major divisions. Within the category of implicit memory, for example, lie the subcategories of associative memory—the phenomenon that famously led Parlov" s dogs to salivate at the sound of a bell which they had learned to associate with food and of habituation, in which we unconsciously file a-way unchanging features of the environment so we can pay closer attention to what" s new and different upon encountering a new experience. Within explicit, or declarative memory, on the other hand, there are specific subsystems that handle shapes, textures such as faces, names—even distinct systems to remember nouns vs. verbs. All of these different types of memory are ultimately stored in the brain" s cortex, within its deeply furrowed outer layer—a component of the brain dauntingly more complex than comparable parts in other species. Experts in brain imaging are only beginning to understand what goes where, and how the parts are reassembled into a coherent whole that seems to be a single memory is actually a complex construction. Think of a hammer, and your brain hurriedly retrieves the tool" s name, its appearance , its function, its heft and the sound of its clang, each extracted from a different region of the brain. Fail to connect person" s name with his or her face, and you experience the breakdown of that assembly process that many of us begin to experience in our 20s and that becomes downright worrisome when we reach our 50 s. It was this weakening of memory and the parallel loss of ability to learn new things easily that led biologist Joe Tsien to the experiments reported last week. " This age-dependent loss of function. " he says, "appears in many animals, and it begins with the onset of sexual maturity. " What" s happening when the brain forms memories—and what fails with aging, injury and disease—involves a phenomenon known as "plasticity". It" s obvious that something in the brain changes as we learn and remember new things, but it" s equally obvious that the organ doesn" t change its overall structure or grow new nerve cells wholesale. Instead, it" s the connections between new cells—and particularly the strength of these connections that are altered by experience. Hear a word over and over, and the repeated firing of certain cells in a certain order makes it easier to repeat the firing pattern later on. It is the pattern that represents each specific memory. The word "extract" in the second paragraph means______.