Can recordings of place cells reveal processes of memory consolidation during sleep?

Unfortunately we cannot give a rat an exam to ask what it remembers. However, we can look in other brain areas for electrophysiological patterns of activity that correlate with the hippocampal place-cell patterns. Wilson and colleagues described the fast replay of the hippocampal place-cell patterns as a unique EEG signal called “ripples.” They observed that these ripples were tightly coupled to slower EEG brainwaves that spread to cortical areas of the brain. When they recorded in some of those other areas—the frontal cortex and the visual cortex—they found similar patterns of firing (i.e., ripples) in those areas that were synchronized to the hippocampal ripples. They hypothesized that the ripples represent memory transcripts that are transferred to and stored in areas of the cortex.

Future directions

The neurophysiological studies of the acquisition and consolidation of declarative memories bring us closer to understanding the physical nature of a memory. The synaptic strengthening known as LTP (long-term potentiation) lasts a long time in contrast to action potentials, but not nearly as long as a memory that can virtually last a lifetime. We can think of a memory as a piece of information that is stored in the brain. We have used the word “information” a lot in this chapter, but we really do not know what “information” is in the brain. It is not simply action potentials; they are brief, transient events. It is not patterns of synapses; they are constantly being altered. It must involve patterns of connectivity in and between neural networks in the brain; but what maintains those networks over years? We have much to learn about how our brains work.