Our current study uses virtual reality to probe the function of the monkey hippocampus, an area of the brain critical for memory. We identify neurons that respond to the events experienced while the monkey maneuvers through a virtual maze. The activity of these neurons suggests that tracking experiences as they occur may be important to the role of the hippocampus in creating new memories.
The hippocampus and surrounding cortex of the medial temporal lobe are necessary for the formation of episodic memories, which are the autobiographical memories of experienced events. Individuals who have suffered hippocampal damage are unable to remember new experiences or recount the events of daily life, despite having otherwise normal intelligence. The hippocampus is one of the first and most affected brain structures in Alzheimer’s disease, and the resulting atrophy is central to the memory impairments observed in patients. Our long-term goal is to gain a better understanding of the neural mechanisms that underlie memory formation. This knowledge may bring us closer to developing new therapies for memory-impaired patients.
To investigate how the circuitry of the hippocampus processes information about experiences in order to form new memories, we train monkeys to use a joystick to traverse a visually rich virtual environment while we record the activity of neurons in the hippocampus. For our recordings, we use newly-developed techniques that enabled large scale chronic recordings throughout the full extent of the monkey hippocampus - allowing us to develop a simultaneous picture of the activity across this structure for the first time in a monkey. We find neurons with distinct, reliable responses to the monkey’s location in virtual space, similar to the “place cells” commonly observed in rodents. We also observe neurons that respond to non-spatial elements of our task, including neurons that respond in anticipation of reward and neurons that maintain activity during the delay period between trials. In addition, the use of virtual reality allows us to test the monkey in multiple visually distinct virtual mazes within a single recording session, while keeping all other aspects of the task the same. This manipulation revealed that the responses of the neurons were specific to particular environments, as might be expected if hippocampal neurons were able to distinguish the contexts in which different experiences occur.
These results suggest that the monkey hippocampus is capable of continuously tracking immediate experiences in our virtual task. This finding echoes decades of research in rodents characterizing the spatial responses of hippocampal neurons during exploration of an environment. Our study shows that the primate hippocampus is similarly capable of demonstrating location-specific activity, but reveals that hippocampal neurons are also monitoring non-spatial features of our virtual task. It is possible that the role of the hippocampus in supporting memory comes from linking events into unified experiences, and without this tracking ability, new episodic memories cannot be formed.