As a first step toward the current goals and objectives of this project, we have begun to study hippocampal-prefrontal interaction with diffusion tensor imaging (DTI), a method that can reveal connections between brain regions, as well as alterations in connections that occur with normal development and following brain damage. We have proposed that prefrontal areas create the kind of cross-domain, analogical, and metaphorical knowledge that is thought to underlie advanced cognition. When that knowledge interacts with the hippocampal complex, what emerges is the human ability to encode and recollect knowledge consciously and to embed ourselves in these facts and events, both real and imagined, in both space and time, thereby creating a coherent, conscious life experience. We know that the prefrontal cortex and hippocampus interact via both direct and indirect connections. The indirect connection involves entorhinal cortex. The prevailing model of object memory emphasizes the role of the entorhinal cortex as a sensory gateway through which the neocortex sends information to the medial temporal lobe. But it is well known that the connections between these brain structures run both ways. As we focus more on the role of the hippocampal complex in object memory, the entorhinal cortex takes on increased importance as an indirect route connecting the hippocampus to the prefrontal cortex. This idea finds support in recent findings from another laboratory that damage to either direct hippocampal outputs or indirect outputs (entorhinal cortex) causes a large deficit on an object-in-scene memory task. It is in this context that we have begun collaborative work using DTI. DTI provides a valuable tool for assessing presumptive white matter alterations in human disease and in animal models. Although the brain substrates for object memory are frequently studied with lesion techniques, the distal effects of such lesions on other memory-related brain regions remains unknown. In the current project, we used DTI to examine the effects of selective neurotoxic lesions of the hippocampus on major white matter tracts and the brain regions receiving inputs from those tracts. First, we evaluated the extent of damage to the hippocampal complex, defined as the dentate gyrus, CA1-3 fields, presubiculum, subiculum, and parasubiculum. Next, DTI analysis was conducted on the corpus callosum, fornix, the white matter of the temporal stem, the cingulum bundle, the subcortical white matter of the ventromedial prefrontal cortex, and the optic radiations. The lesions caused a 72% decrease of hippocampal volume in the lesion group compared to the controls, without any apparent inadvertent damage in adjacent regions. DTI analysis showed that, of the fiber tracts examined, only the fornix and ventromedial prefrontal white matter were affected by the lesion. The findings show that hippocampal damage leads to alterations in other brain regions involved in object memory, including portions of the prefrontal cortex. This part of the project provides a new tool for examining hippocampal-complex function and hippocampal-prefrontal interactions in both clinical settings and in animal models. A second part of this project examines the possibility of studying the role of the hippocampal complex and hippocampal-prefrontal interactions in second-order assessments of object memory. Memory awareness, also called metamemory, can be assessed by measuring how often subjects collect information. When they lack any memory of an object, subjects will seek out information, but when they have a clear memory, they will not. In our study (Basile et al., 2009), subjects searched for an object after either watching the experimenter put it into of four opaque tubes or when they had not been allowed to watch the object being put into the tube. In this circumstance, the behavior of subjects can reveal what they know about the contents of their own object memories. This second part of the project should provide a new method for examining memory awareness in animal models. A third part of this project involves an exploration of the role of the hippocampal complex in the memory for object-object associations and object-response associations. We have previously shown that the hippocampal complex is necessary for rapid learning of object-response associations. However, we have also obtained some evidence that the hippocampal complex is not necessary for learning object-object associations, also known as paired-associate learning. Our work on object-response associations notwithstanding, the results from object-object learning could mean that the hippocampal system is uninvolved in associative learning when neither component of the association is spatial in nature. Later work on this project, however, showed that lesions of the hippocampal complex cause a deficit in learning object-response associations when neither the object nor the response is spatial in nature. The deficits seen in that experiment were virtually identical to those observed when the response was spatial. This finding has led us to reexamine the role of the hippocampal complex in the memory for object-object associations. In our previous study of object-object associations, subjects learned the associations slowly, so it remains an open question whether hippocampal-complex damage would cause deficits on rapid learning of this type. In the past year, we have trained subjects to learn both object-object and object-response associations rapidly. In the next year, we will examine the effects of reversible inactivations of the two main parts of the hippocampal complex, the hippocampus proper and the subiculum, separately and in combination with inactivation of the entorhinal cortex. We predict that a dramatic deficit will occur only when both the direct and indirect outputs of hippocampus to the prefrontal cortex are blocked. This means that large deficits should occur only when both subiculum and entorhinal cortex are inactivated, disrupting the direct and indirect pathways, respectively. We predict that this inactivation will show that the hippocampal complex is necessary for the rapid learning of new object-object associations, but not for the recollection of previously learned object-object associations.
