Our research projects, together, aim at exploring the declarative memory system. Declarative memory is memory for specific facts and events; generally thought of as the kind of memory that one can bring to conscious recollection and "declare". This system is based in the hippocampal region and involves three key structures: the neocortex, the parahippocampal region, and the hippocampus. We are using electrophysiological recordings to determine the flow of information through this system as rats perform an odor sequence task. Our individual honors projects will look at combinations of these structures.
Odor memory dependence on the hippocampus
For the next year, I will be focusing on two memory structures in particular - an olfactory neocortical structure called the orbitofrontal cortex (OF) and the hippocampus. Case studies have shown that damage to the hippocampus causes a specific type of amnesia - declarative memory loss for recent events (anterograde and some retrograde amnesia) with selective preservation of very remote memories. This preservation suggests that the hippocampus is involved in memory up to a certain point, after which memories are stored in the neocortex, independent of the hippocampus. In my project, I will be exploring this dependence on the hippocampus, by implanting electrodes into both structures and looking at their neuronal firing. I predict that if memories are formed through the hippocampus and stored in the OF, OF neurons will begin to exhibit "memory" firing (anticipatory firing before an event,) after hippocampal cells consolidate the memory.
Interactions between the Parahippocampal Region and Orbitofrontal Cortex
My project will focus on the interactions between the parahippocampal region (PHR) and the cortical association area known as the orbitofrontal cortex (OF) in the rat. These two structures are necessary for normal performance on a particular declarative memory task known as the delayed non-matching to sample task (DNMS). DNMS was developed as a task to test for declarative memory in animals, who, unlike humans, cannot verbally declare what they remember. In this task animals are presented with a sample object and then after a delay interval are asked to choose between the sample object and a novel object. The novel, non-matching object is the correct choice. Lesion studies have shown that removal of the hippocampus has no effect on performance in this task. However, upon removal of the PHR, animals have a difficult time remembering the stimuli involved in the task, whereas removal of the OF makes it difficult for the animals to learn the rules of the task. Over the next year, I will simultaneously record from electrodes implanted in the PHR and the OF of the rat to explore the temporal dynamics of the flow of information during a DNMS task. After observing neuronal firing and timing in these two areas, I will lesion the PHR and record from the OF to determine whether OF is dependent upon PHR performance in this task.
Anticipatory Firing in Orbitofronal Cortex
Claire Discenza and Alissa Waite
This summer we have surgically implanted rats with electrodes in the orbitofrontal cortex. We have been able to isolate and identify individual neurons and we have begun to identify their functions. We have found cells that fire preferentially to a reward stimulus, as well as neurons that fire to odor stimuli. In doing this, we have learned to build our own microelectrodes and have become proficient at performing surgeries. We trained the rats on a behavioral task requiring them to respond to different odors. We hope that with further investigation we will be able to identify both sensory and memory cells in the orbitofrontal cortex. We would expect that "memory" cells would be neurons that fire before the onset of a particular odor, therefore indicating an anticipatory response.