On Emotion and Memory

from reference 1


Why is it that experiences imbued with emotion crystalize into easily recollected memories? Our memory is quite limited, so we need a system for deciding what to remember and what to forget. Emotions may thus act as a filter, marking certain experiences as being of particular importance. In this way, we have templates of states we felt were positive or negative, examples of the consequences of our behaviors, with significantly happy or sad outcomes featuring as the most poignant reminders.

None of this gestalt psychological explanation is informative as to the neurophysiological mechanisms underlying this phenomenon. However, some recent research does address what mechanisms may be at work on a molecular level. Joseph Ledoux and Robert Malinow have been working on memory for a quite a while, and they are the two most senior authors on a paper published in Cell concerning AMPA receptors, emotion, and memory (ref. 1). AMPA receptors are one of the major glutaminergic receptors in the brain. Glutamate is the neurotransmitter they recognize, and it is the major excitatory neurotransmitter in the brain. So if one neuron wants to send a signal to turn on another, it will almost invariably release glutamate at it's axon terminal, and that glutamte will likely be recognized by an AMPA receptor on the post-synaptic cell (the target of the excitation). The major finding of this paper is that norepinephrine (more commonly known as adrenaline) facilitates the incorporation of AMPA receptors into the membranes of cells, during periods of high activity.

It is commonly known that adrenaline is released during times of emotional distress and happiness, these researchers have found that one specific effect of the adrenaline is to increase the number of receptors being incorporated into a synapse, again during periods of high activity. Let's imagine a scenario where this might apply. An animal is being chased by a predator. His motor planning and execution areas are blasting away action potentials, they're highly activated. He makes a decision about some route to take during his escape, activating a specific subset of pathways. It is these connections that will be strengthened by the application of adrenaline. Because more receptors are being integrated into the synapses in these circuits, they will be more likely to be activated the next time he is in the same situation. In this sense, he has formed a memory of the experience which is modulated by the amount of adrenaline, and by extension the intensity of the emotion experienced.

This is essentially what these researchers observed. While it is impossible to directly modulate the emotional state of the animal, they can apply norepinephrine during a learning task. What they found was that animals who received larger doses of applied norepinephrine were more likely to remember the task. The figure at the top of this piece illustrates the finding. The authors compared genetically altered (GA) mice - which lack the effects of adrenaline on AMPA receptor trafficking - to "normal" or wild-type (WT) mice. The graph on the left displays the responses of the WT mice, with the GA mice on the right. The key is that two data points are significantly different (marked with an asterisk) on the left, but not on the right.

While this work doesn't do much to help understand the biosychological basis of Proust, it does illuminate one more minuscule thread in the web of conscious experience.


References

1. Hailan Hu, Eleonore Rea, Kogo Takamiya, Myoung-Goo Kang, Joseph Ledoux, Richard L. Huganir and Roberto Malinow, (2007) Emotion Enhances Learning via Norepinephrine Regulation of AMPA-Receptor Trafficking, Cell 131,1, pp 160-173 [doi:10.1016/j.cell.2007.09.017]