The Functional Neuroanatomy Of Memory Systems In The Hum

National Institute Of Mental Health

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Abstract

Our research program is concerned with elucidating the neural structures associated with memory acquisition, storage, and retrieval. We have continued to concentrate on three domains of memory. The first domain focuses on semantic memory, the part of long-term memory composed of general information, such as facts, ideas, and the meaning of objects and words. We are particularly interested in characterizing the neural substrate mediating object and word meaning and its role in object perception. We are also interested in understanding how abstract forms of knowledge, such as information about of social interactions, are represented. The second domain focuses on repetition priming, a form of implicit learning not dependent on conscious retrieval of prior events. These studies concentrate on characterizing changes in neural activity associated with perceptual skill learning to provide a model system for studying cortical plasticity. The third domain, episodic memory, is an explicit form of memory that involves conscious recollection of a specific past experience. These studies concentrate on identifying the brain structures unique to this form information acquisition and retrieval.[unreadable] Our functional brain imaging studies on semantic memory have shown that information about salient properties of an object - such as what it looks like, how it moves, and how it is used - is stored in the sensory and motor systems active when that information was acquired. As a result, objects belonging to different categories such as animate entities (people, animals) and manmade manipulable objects (tools, utensils) are represented in partially distinct neural circuits. This work has been extended to show that these networks underpin our ability to understand more abstract events such as social and mechanical interactions. Building on these findings, investigations are underway to evaluate the integrity of these neural systems in subjects with development disorders of social understanding and communication (autism spectrum disorders).[unreadable] Our studies of implicit (perceptual priming) and explicit (recognition) memory processes have continued to explore how behavior and neural activity are modulated by experience. Our investigations of perceptual priming have shown that learning measured by increased ability to identify and name objects with repeated experience is extremely long-lasting (over one month between the first and second presentation of an object) and impervious to aging; similar levels of behavioral improvement were found in young adults (20-30 year olds) and aged individuals (> 75 years old). On the neural level, we have found that this form of implicit learning is associated with both fast developing (within 30 sec) and long-lasting (3 days) decreases in neural activity in posterior cortex, coupled with slowly developing changes in more anterior regions of the brain (over several days). 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. Explicit recognition of an item is also associated with reduced activity in posterior cortices, suggesting that this repetition-related reduction in neural activity is an automatic response to the repeated presentation of an object. In contrast to priming, explicit recognition memory is associated with increased activity in several regions of prefrontal cortex that are not active during performance of implicit memory tasks. Finally, using newly-developed, high-resolution imaging technology, we have found that the acquisition of object and spatial information is subserved by different regions of the medial temporal lobe memory system. This finding adds to an increasing body of literature on the neuroanatomical substrate of learning and memory.