The Functional Neuroanatomy of Memory Systems in the Human Brain

National Institute Of Mental Health

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Abstract

Our research program is concerned with elucidating the neural structures associated with the acquisition, storage, and retrieval of information, and on how activity in these structures is modulated by experience. We have continued to concentrate on two domains of memory. The first domain, semantic memory, is concerned with how conceptual knowledge - including facts, ideas, and the meaning of objects and words - is represented in the brain. These studies have provided a window into a broad set of questions on how information about specific object concepts is stored and organized in cerebral cortex. The second domain, repetition priming, is an implicit form of learning that has provided us with a model system for studying functional cortical plasticity. Specifically, how neural activity is modulated by experience with objects and other types of stimuli. The results of our previous functional brain imaging studies on semantic memory using positron emission tomography (PET), suggested that object knowledge is stored in distributed networks of discrete cortical regions. In addition, the location of these regions parallels the organization of sensory and motor systems in the brain. These findings have been extended by a recent series of investigations of object processing using functional magnetic resonance imaging (fMRI). These studies have revealed fine-grained differences between a number of object categories (animals, tools, faces, houses) along both the ventral (especially the fusiform gyrus), and lateral (middle temporal gyrus, superior temporal sulcus) regions of the temporal lobes. The results of these studies have been consistent with the idea that information about object form, and form-related features such as color, is stored in ventral regions of the temporal cortex, anterior to sites in occipital cortex that mediate perception of these features, whereas information about object motion is stored in lateral temporal regions, anterior to areas in occipital cortex that mediate perception of motion. In addition, activity in the left premotor cortex and left intraparietal sulcus - both of which are active when objects are manually manipulated - was enhanced during recognition and naming of pictures of tools, relative to other objects that are not typically manipulated by hand. These results provide additional evidence that information about the defining features of objects is stored near regions of the brain that were active when we first learned about those objects. Our studies of perceptual priming have continued to explore how neural activity is modulated by experience. Previous behavioral studies have shown that prior exposure to an object facilitates the time needed to name that object the next time it is encountered. In addition, this facilitation has been found when as much as a year intervenes between the first and second occurrence of the object, suggesting a permanent change. In a recent series of studies we showed that a single, brief (200 msec), presentation of an object leads to fast developing (within 30 sec) and long-lasting (up to 3 days) decreases in neural activity in posterior cortex, coupled with slowly developing (over a 3 day period) decreases in left inferior prefrontal cortex and increases in insular cortex. These findings suggest that time-dependent changes in posterior and anterior cortices may underlie perceptual and lexical/semantic forms of learning that occur whenever objects are identified and named.