Sleep And Memory Consolidation

obviously has a great number of secondary effects on disposition, attention, and motivation—further complicating attempts to approach the problem experimentally.

Nevertheless, there are a number of intriguing observations consistent with a role for sleep-associated neuronal activity in memory consolidation. Work in this area has come primarily from Bruce McNaughton, Carol Barnes, Matt Wilson, Gyorgi Buzsaki, and their respective colleagues. These investigators have shown sleep-associated reproduction of specific patterns of hip-pocampal pyramidal neuron firing: firing patterns that mimic firing patterns that the animal had established while awake and learning. In other words, it appears that the hippocampus and cortex are "replaying" episodic events while asleep as part of a process of consolidation of memory. In addition, there are a number of correlative studies suggesting that loss of these types of replay episodes causes memory dysfunction.

On the other hand, in the human literature there is not as much support for the idea of a necessity for sleep per se in memory consolidation. For example, patients with specific types of brain lesions or on certain types of medication never sleep or have profound disruptions of their sleep pattern (see reference 37). There also are a few examples of individuals who spontaneously lose the desire to sleep and are essentially insomniac for their entire remaining lifetime. These phenomena do not lead to any profound memory disruption, dissociating sleep from memory formation. However, given the ambiguity in defining sleep, it certainly is possible that insomniac individuals may have certain sleeplike patterns of CNS activity that functionally substitute for the lack of sleep.

Obviously, resolution of the issue of the role of sleep in memory formation will require much additional study. At this point, however, it looks like the answer could be fascinating.

This is the fundamental quandary of the cognitive neurobiologist, and it shows up over and over again in the contemporary literature in experiments involving monitoring of cellular firing (or functional Magnetic Resonance Imaging (fMRI) signals) in real time in response to environmental stimulation. Where does sensory processing end and cognition begin? How does cellular firing get translated into an abstract construct in the brain? The real potential of beginning to answer these types of questions, which really are the modern reformulation of the philosophical mind-body problem that has intrigued mankind for millennia, is one of the best reasons I can think of to be particularly excited about being a neuroscientist in the contemporary era.

B. Time

The hippocampus is involved not only in processing of spatial information but also in what I will refer to, for lack of a better term, as processing temporal information. I do not necessarily mean that the hippocampus is involved in encoding time itself (which could also be true), but rather I am referring to the hippocampus being involved in temporally dependent learning such as trace associative conditioning and the ability to remember the order of events. Also, the hippocampus exhibits time- and experience-dependent alterations in its cellular firing properties. In this section, we will discuss a few examples of time-dependency of hippocampal function and information processing. We will start with an example of experience-dependent alterations in the behavior of hippocampal place cells as an example of changes in the hippocampus that occur with repeated environmental signals over time. In the next section, we will discuss the important role of the hippocampus in the formation of time-dependent associations, by and large using trace associative conditioning as our example.

We already discussed the impressive, rapid formation of hippocampal place fields when an animal is introduced into a new environment. These place fields are, of course, manifest as a burst of action potential firing in a CA1 pyramidal neuron when an animal enters a particular spatial location (or more precisely when an animal enters what it perceives to be a particular spatial location). What happens to place cell firing over time when the animal re-enters that same location? Are there time-dependent changes in place cell firing properties?

Recent work from the laboratories of Matt Wilson, Gyorgi Buzsaki, and Carol Barnes and Bruce McNaughton, along with several others, has given us clear answers to these two questions. There clearly are time-dependent changes in place cell firing properties that depend on the animal's experience (see Figure 9). Reentering the same place field repetitively over time leads to several pronounced effects on cellular firing patterns, at the level of the individual neuron. These changes are a clear example of experience-dependent changes in the firing properties of hippocampal pyramidal neurons, and, as described earlier, I use them to illustrate "time"-dependent information processing by the hippocampus.

Three specific examples of such cellular changes follow. One, with repeated reentry into the place field of a pyramidal neuron, there is an increase in the place cell's firing rate upon successive reexposure (11). Two, with reentry into a place field, there is a decrease in the latency of the time required for firing the first action potential in a place cell's burst of action potentials (11). Three, the extent of dendritic action potential attenuation decreases over time with experience in the place field (12, we will return to back-propagating action potentials in much more detail in later chapters). Thus, the first entry into a place sets up a hippocampal neuronal place field, but there are subsequent time- and experience-dependent changes in place cell action potential firing properties as well.

The mechanisms underlying this experience-dependent alteration in hip-pocampal pyramidal neuron properties is a subject of active investigation. In fact, we will return in later chapters to many details of the cellular and molecular mechanisms likely mediating these types of changes. For now, suffice it to say that intriguing mechanisms that could be contributing to these changes include activity-dependent changes in sodium or potassium channels in place cell neurons, changes in neuro-modulatory inputs (e.g acetylcholinergic or noradrenergic inputs) to the hippocampus, or activity-dependent synaptic plasticity within the hippocampus itself (12).

Memory for Real Time—Episodic Memory, Ordering, and the CS-US Interval

Experience-dependent changes in hip-pocampal place cell firing patterns only begin to scratch the surface of the involvement of the hippocampus in time-dependent encoding of information and temporal information processing. The hippocampus is necessary for a wide variety of different time-dependent learning tasks. I will briefly highlight a few examples here, but a common theme that is emerging in modern studies of hippocampal function is that the

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