Section Summary

Memory has many different categories that are supported by distinct brain regions. These categorizations can be based on time (short vs. long-term memory) or content of the memory (explicit vs. implicit). Once memories are encoded, they need to be stabilized in a time-sensitive process known as consolidation, but even then, the memories are not solidified; memories are vulnerable to disruption every time they are recalled and require reconsolidation to become stable once again. Much of what we know about memory comes from the unfortunate but common cases when memory fails, including amnesia and neurological disorders such as Alzheimer’s disease. Although memory abilities are more likely to decline with aging, it is now known that severe memory loss like that associated with dementia is not an inevitable consequence of aging.

Implicit memories are memories that are formed without our conscious awareness. Implicit memories can be categorized into associative memories, including classical and operant conditioning, and nonassociative memories, including habitation and sensitization. In classical conditioning, two previously unrelated stimuli (one neutral, one inherently meaningful) are associated with one another, such that the previously neutral stimulus evokes the same species-typical response that the inherently meaningful one does. In operant conditioning, a stimulus is associated with a response, making responses that follow reward more likely to happen in the future and responses that follow punishment less likely to happen in the future. This type of learning is supported primarily by the cortical-striatal system, aided by structures such as the medial prefrontal cortex, amygdala, and mesolimbic dopamine system.

The hippocampus is an essential structure for encoding episodic memory and aiding in spatial navigation in rodents and humans. Firing patterns of neurons recorded from the hippocampus and related structures predict both of these functions, helping to explain how they are linked to one another. One striking feature of hippocampal neurons is their tendency to dramatically increase their firing rate when a rodent occupies a specific location in the environment. We call these neurons place cells. A class of neurons recorded from medial entorhinal cortex also show spatial encoding, but with one important difference: these cells, termed grid cells, increase their firing rate in multiple locations of the environment, with the firing fields arranged in a regular pattern. Other classes of neurons within this network are sensitive to environmental boundaries, head direction, and speed. Together, this brain network of place cells is thought to provide an accurate representation of location in space, allowing for accurate navigation to goals. These spatially-tuned cells are likely to participate in other mental representations such as episodic memories, thereby integrating spatial information with formation of new episodic memories.

Up until the early 1970’s there was only speculation that the neural mechanism of memory might be a change in the strength of previously-active synapses. Empirical support for this notion came from Bliss and Lomo’s groundbreaking discovery that high frequency stimulation of the presynaptic input to the dentate gyrus led to a long-lasting increase in the postsynaptic response, a phenomenon known as LTP. We now know that the mechanism of this increased postsynaptic response comes from changes in the sensitivity of the postsynaptic membrane to transmitter release. This increased sensitivity arises from AMPA receptors being added to the postsynaptic membrane through activation of the CAMKII pathway. Synapses can also be weakened through LTD via mechanisms similar to LTP.