The Section on the Neurobiology of Learning & Memory of the Laboratory of Neuropsychology, NIMH, studies the neural mechanisms underlying perception, learning, memory, and other cognitive processes. To this end, we have examined the effects of selective cerebral ablations and disconnections on different types of learning and memory in macaque monkeys. Overview. The """"""""ventral visual stream"""""""" is a group of striate, prestriate, and inferior temporal cortical fields devoted to processing visual information about objects. While the processing of visual information is carried out by these visual cortical fields, the actual storage of visual representations in monkeys appears to depend in large part on a strip of cortex lying at the base of the medial temporal lobe. This region is comprised of the entorhinal cortex and perirhinal cortex, abbreviated here as """"""""rhinal cortex"""""""". The rhinal cortex appears to be critical for accurate stimulus identification, stimulus recognition, and stimulus-stimulus association, including both intramodal stimulus-stimulus associations and crossmodal ones. Furthermore, the rhinal cortex can mediate these types of information storage in the absence of the medial temporal lobe limbic structures, the amygdala and hippocampus. The rhinal cortex interacts with other brain regions to link stored stimulus representations with emotion and action. Accomplishments of the past year. Recently we have hypothesized that the perirhinal cortex contains neurons that represent complex conjunctions of visual stimulus features, and, as a consequence, that this region is important for discriminating objects under conditions of high feature overlap. An example of feature overlap is when a given visual feature or set of features is part of two different objects that need to be differentiated. We have tested this idea and have found that the perirhinal cortex is indeed important for discriminating objects when we increase """"""""feature overlap"""""""" experimentally, either by using software to morph or """"""""blend"""""""" two different stimuli or by specially constructing visual stimuli that have a number of features in common. By contrast, the perirhinal cortex is not necessary for visual discrimination objects when there is little or no feature overlap. Thus, the perirhinal cortex operates as part of the ventral visual stream or """"""""what"""""""" pathway, in helping to process and store information about objects. We suggest that its special contribution to this kind of processing is the representation of complex conjunctions of features, as opposed to any special computational function it might possess. Once a visual item is identified, it can be associated with other kinds of information, for example with emotions (or affective valence), spatial locations and responses. Recent work from this and other laboratories indicates that the amygdala is important for associating a visual stimulus with its affective valence, and further, that the amygdala must interact with the orbital prefrontal cortex to enable animals to choose advantageously under conditions in which the outcome of their actions may be changing for the better or worse. Additional studies have extended the work on visual information processing and storage by examining the role of various prefrontal cortical fields in associating visual stimuli with specific actions. This kind of learning probably underlies the acquisition of language, especially the association between the written word and the motor programs necessary to produce speech. At least two prefrontal cortical areas, the ventral prefrontal cortex and the orbital prefrontal cortex, are important for establishing this kind of rapid, arbitrary linking of visual stimuli with actions. As far as we can discern, the necessary roles for the prefrontal areas is the association between vision and action, and the acquisition of rules guiding those kinds of behavior, as opposed to the visual identification of the object or the production of the response, per se. In the past year we have published 13 reports, including 4 peer-reviewed primary research articles together with several refereed review and encyclopedia articles. Significance to biomedical research and mental health. Memory loss typically accompanies strokes and viral infections of the brain, and is a central feature of certain diseases such as Alzheimer's Disease and AIDS dementia complex. Our research has been dedicated to understanding the functions of the key brain areas involved in information storage, including the rhinal cortex, amygdala, and hippocampus. By gaining a better understanding of the normal functional organization of these regions, we hope to reveal ways in which the disorders of memory can be understood and, potentially, ways in which the disorders can be ameliorated.
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