After studying this chapter, you should be able to:
Describe the various forms of memory.
Identify the parts of the brain involved in memory processing and storage.
Define synaptic plasticity, long-term potentiation (LTP), long-term depression (LTD), habituation, and sensitization, and their roles in learning and memory.
Describe the abnormalities of brain structure and function found in Alzheimer disease.
Define the terms categorical hemisphere and representational hemisphere and summarize the difference between these hemispheres.
Summarize the differences between fluent and nonfluent aphasia, and explain each type on the basis of its pathophysiology.
The understanding of brain function in humans has been revolutionized by the development and widespread availability of positron emission tomographic (PET), functional magnetic resonance imaging (fMRI), computed tomography (CT) scanning, and other imaging and diagnostic techniques. PET is often used to measure local glucose metabolism, which is proportional to neural activity, and fMRI is used to measure local amounts of oxygenated blood. These techniques provide an index of the level of the activity in various parts of the brain in completely intact healthy humans and in those with different diseases or brain injuries (see Clinical Box 15–1). They have been used to study not only simple responses but complex aspects of learning, memory, and perception. Different portions of the cortex are activated when hearing, seeing, speaking, or generating words. Figure 15–1 shows examples of the use of imaging to compare the functions of the cerebral cortex in processing words in a male versus a female subject.
Comparison of the images of the active areas of the brain in a man (left) and a woman (right) during a language-based activity. Women use both sides of their brain whereas men use only a single side. This difference may reflect different strategies used for language processing. (Used with permission of Shaywitz et al, 1995. NMR Research/Yale Medical School.)
Other techniques that have provided information on cortical function include stimulation of the exposed cerebral cortex in conscious humans undergoing neurosurgical procedures and, in a few instances, studies with chronically implanted electrodes. Valuable information has also been obtained from investigations in laboratory primates. However, in addition to the difficulties in communicating with them, the brain of the rhesus monkey is only one-fourth the size of the brain of the chimpanzee, our nearest primate relative; and the chimpanzee brain is in turn one-fourth the size of the human brain.
A characteristic of animals and particularly of humans is their ability to alter behavior on the basis of experience. Learning is acquisition of the information that makes this possible and memory is the retention and storage of that information. The two are obviously closely related and are considered together in this chapter.