Since the beginning of applying rigorous scientific methods to investigate psychological phenomenon, we have known that sleep is beneficial for learning and memory stabilization. Over the past century, many efforts have been made to understand this phenomenon more thoroughly. Although some illuminating insights have been made in this endeavor, the cognitive and neural mechanisms underlying this fascinating phenomenon remain outside the realm of our understanding. However, based on both theoretical evidence developed several decades ago and experimental evidence gathered more recently, it seems that a process colloquially known as “memory replay” is important for this strengthening of our memories that occurs while we sleep.
Memory replay refers to an event in the brain in which patterns of neural activity that were apparent during, and characteristic of, a particular experience are reactivated during a subsequent period of rest. If one just takes a minute to think about sleep, this concept will be almost self-evident: our dreams typically reflect what has been happening recently in our lives. Clearly, these thoughts are still present within the brain and “on our mind” during sleep! In that light, it makes intuitive sense that these patterns of neural activity that were active during learning are then reactivated during sleep. Experimental evidence also provides support for the theory that replay is important for memory. Not only have multiple experiments shown strong correlations between the neural activity occurring during learning and then during a subsequent sleep, a few studies have also demonstrated that disrupting replay leads to learning deficits in the task being performed. All of this points to memory replay as a candidate mechanism to explain the benefits effects that sleep has on learning and memory.
Figure: A rat with unilateral hippocampal lesion performing the spatial sequence task from memory. Neural activity is recorded with high density electrode arrays
Although all this experimental evidence supports the theory that memory replay is involved in learning, the theory needs to be tested even more rigorously and the details of this phenomenon need to be discovered and explained. One prevalent idea is that a structure called the hippocampus is capable of uniquely coding our experiences which can then be indexed and the patterns of neural activity can be reactivated in other brain areas, specifically the neocortex. My interest is in understanding the dynamics of this potential indexing and hierarchical reactivation. To explore this interest experimentally, the hippocampus is lesioned unilaterally and the rat is trained on a spatial sequence memory task on a circular maze (see Figure). In this task, the animal learns that reward is given at particular locations on the maze in a particular sequence. In order to complete this task, the rat must remember where the locations are and in what order they are rewarded or what action sequence to execute. The rat then is placed in a quiet, comfortable location so it can rest quietly and sleep. Neural activity is recorded in the neocortex bilaterally throughout. Because the hippocampus has primarily ipsilateral direct connections with the neocortex, this experimental preparation allows us to investigate the effects that the hippocampus has on neocortical replay during sleep within a single animal. Our hypothesis is multifaceted: we expect to see stronger replay in the neocortex of the hemisphere with a healthy hippocampus than its contralateral counterpart. We also expect that replay of recent maze sessions (neural activity that went on during a recent behavioral session) will be stronger. A separate, but not exclusive, line of research suggests that memories are replayed in an interleaved order. If this is true, and the hippocampus is involved in the organization of this process, we might see that remote and recent memories are not interleaved or that only recent memories are replayed and remote memories cannot be properly indexed in the lesioned hemisphere.