Biology 202

2006 Third Web Paper

On Serendip

Vladimir Nabokov
Interview to BBC-2, 1969

It is a normal day on Bryn Mawr Campus. I am walking to class, my reading is done, it is the beginning of spring and the air is filled with the scent of magnolias. Suddenly I stop, shell-shocked. It is too early in the season for the daily ritual of grass-mowing, so a few brave weeds recklessly hasten to reach their full height. Nothing has changed: the buildings, the trees, the green are as they have been a moment ago. And yet I am in Russia, I feel it within every cell of my body. Even the tangible evidence of the gray stone walls seems insignificant in comparison; in fact, the buildings around me look as if they have been cut out of cardboard – reality feels artificial, while the holistic memory within me has an air of truth. How does my mind make that leap? I can logically trace the development of that thought: they don't mow the grass all that often in Russia, so it is natural to make the connection; but how can I explain the sheer intensity of the memory, the feeling of its immediate reality?

The phenomenon of nostalgia is complex and fascinating. Although the term is usually used for the particular shade of immigrant homesickness, for the desire to return to one's home country, it can also be applied to our common longing for "the old times," for the flavor of jam that our grandmother used to make or for the song that they used to play on the radio twenty years ago. Despite its widespread character, nostalgia is not well-studied. The neurobiology of memory and emotion is still in its infancy, and nostalgia is, perhaps, much too complex to consider meaningfully at this time. Yet it seems that the current understandings of emotion and memory can be applied to it. In this paper, I will attempt to do so by trying to unpack my own experiences of and with nostalgia in light of contemporary neurobiological research. Naturally, this is a laywoman's attempt and bound to be wrong as such, but I think it will help me understand something about myself that, until now, I have taken for granted.

1. What Is Memory?

When most of us think about memory, we imagine a grand neural library, with complete records filed away at particular locations, ready to be pulled off the shelf when the correct stimulus is introduced. Current neurobiological research, however, challenges this model of memory and suggests that there are no complete "records" stored anywhere in the brain. Instead, both long- and short-term memories arise from the synaptic interactions between neurons. When a short-term memory is created (for instance, when you hear a name of a new acquaintance), the initial firing of the neuron makes the synapse temporarily more sensitive to similar subsequent signals: that is, the synapse is "strengthened." Over time, however, this sensitivity wears out. In order to acquire a long-term memory of the event (for instance, in order to remember the name of a new friend), the synapse must be strengthened permanently; that is, it must remain sensitive to or familiar with the particular neural firing pattern. (6) This permanent strengthening constitutes a memory "trace" in the brain, a path that, once made, is followed easily by subsequent signals. Complex memories of events constitute patterns of traces formed over large areas of the brain. When a familiar signal (such as the sight of an unmowed weed) is introduced, our brain facilitates its communication to other neurons, which respond with their own firing patterns. A particular memory, then, is not "retrieved," in the sense of being pulled off the shelf. It is, in fact, being recreated in "real time" by complex neural interactions, using the existing trace pathways. (1) The task of remembering is essentially a creative and multivalent process, not a direct "input-and-output" connection.

Such a model of memory stresses the importance of the "rules of storytelling" (1), of the larger interrelated brain structures required for the creation of particular memories. In order for me to connect the sight of the weeds to my earlier Russian experiences, a complex pattern of neural interactions, which constitutes my "story" about the event, must be played out. What is that pattern and where is it located in the brain?

2. The Geography of Memory

Memory is not unitary. There are "subgenres" of it, each constituting a separate way of telling stories. Declarative memory, for instance, is conscious. We normally consider declarative memory as the phenomenon par excellence, since it is responsible for our lasting knowledge of events, facts, etc. and makes it possible for us to access this knowledge at will. Declarative memory can be subdivided into episodic memory (which contains autobiographical, and especially spatiotemporal, information) and semantic memory (our knowledge of general facts about the world). Procedural memory, on the other hand, is unconscious and is responsible for our ability to retain information about action patterns, such as driving a car or making some coffee.

Classically, different areas of the brain are thought to be responsible for different subgenres. Thus, short-term declarative memory is believed to be connected to the cortical regions of the brain, with the left prefrontal cortex involved in episodic memory formation and prefrontal and parietal cortical areas involved in episodic (and especially spatiotemporal) memory retrieval (7). At some stage of the process of long-term declarative memory formation, the hippocampal regions become involved. Studies show that if these areas are damaged, patients can no longer form lasting memories, although they function well in situations calling for short-term retrieval. After some time, however, long-term declarative memory becomes independent of the hippocampus, since patients with damage to this area are able to retain memories of events prior to the injury (7). Nevertheless, it is difficult to pinpoint the exact brain regions responsible for particular memory functions, and it may be suggested that the processes responsible for memory formation and retrieval are not localized but, rather, exist in widely distributed brain structures. Moreover, it is difficult to separate the processes and structures involved in memory formation and retrieval (7) and it may be that these processes are, in fact, interrelated and perhaps even simultaneous. It is also hard to separate memory-related processes from other directly related cognitive processes, such as the evaluation of particular memories in terms of the current situation.

3. Emotion and Memory

While the above structures and processes are involved primarily in memory formation due to repetition, it has long been established that emotions can have a strong effect on memory even in the absence of repetitive situations. Studies exploring the interactions between fear and memory shed some light on the processes and structures implicated.

It has been suggested that there exist at least two distinct memory systems that can act independently of each other. The first system is linked to the amygdala and specializes in forming memories directly related to emotional experiences. The second system is connected to the hippocampus and regulates declarative and episodic memory unrelated to emotion (such as repetition-based memory and recollection of events at will). Although these systems can theoretically stand on their own, in practice they continually influence each other.

The amygdala, which is connected to human emotion, can influence the encoding and storage of hippocampal-dependent episodic memories. We all have experience with the fact that memories of emotional events are often more vivid and persistent than the ones acquired by mere repetition. One reason for this may be that, when the emotional stimulus is encountered for the first time, the amygdala enhances our ability to perceive it and the attention that we pay to it, which facilitates the formation of the memory trace. The amygdala has direct connections with sensory cortical processing regions (for instance, with the visual cortex). Therefore, it receives signals from them before the hippocampus does and can increase our sensitivity to them without our being consciously aware of it. It has been suggested that the events "tagged" by the amygdala as emotionally significant receive priority in memory encoding. (4)

Moreover, the amygdala can influence retention of emotional memories. It is known that hippocampal-dependent memories undergo a period of fragility, when the synapses can be easily desensitized if the signal is not repeated. This has been called the process of memory consolidation. Emotional reactions, mediated by the amygdala (such as arousal and release of hormones), can "tag" these memories as necessary for survival and, therefore, worthy of retention. Brain imaging studies show a correlation between amygdala activity at the moment of encoding and later persistent memory of emotional stimuli. Moreover, it is thought to influence perceptual systems as well, possibly affecting the very strength of the signals we receive. (4)

Hippocampus, in turn, can influence the amygdala function. When a subject learns by verbal instruction to associate a signal (such as, for instance, a blue square) with an aversive even (a mild shock to the wrist), there is activity in the amygdala even when the signal is never actually accompanied by the aversive event. That is, the amygdala "experiences" a painful emotion every time a blue square is produced, even if this experience is merely "imaginary." The acquisition and possibly retrieval of hippocampal-dependent episodic memory can, thus, significantly influence the amygdala, which, in turn, has an affect on physiological expression of emotion when an appropriate signal is encountered. (4)

The interaction between the amygdala and the hippocampus may also play an important role in our self-regulation of social behavior. For instance, when subjects are taught specific strategies (which require hippocampal-dependent memory for acquisition and application) for reappraisal of negative events in a positive or a non-emotional light, both subjective reports about emotional reaction to negative stimuli and the amygdala response to them are diminished. Hippocampus, thus, plays a role in modulating the amygdala function (4), which is connected to such emotions as fear, sadness, and anticipation of pain, especially those with autobiographical and personal value, and to the generation of response to these emotions (5).

4. Memory and the Self

While it is evident that memory is one of the components making up the complex thing we call the self, the mechanisms for its participation in this formation are not clear. It has long been noted that patients with damage to the hippocampus could still "remember," that is identify as theirs, specific personality traits. Therefore, something other than hippocampus must be able to reconstruct personal memories. Todd Heatherton, a psychologist at Dartmouth University, suggests that medial prefrontal cortex – located in the cleft between the hemispheres of the brain – plays an important role in consolidating and recreating perceptions and memories to produce a unitary sense of self. Just as the hippocampus does not "store" but creates memories by combining the trace patterns, medial prefrontal cortex creates (at least in part) the sense of self. (3)

Matthew Lieberman of the UCLA suggests that there may be two systems that connect memories and the sense of self in the human brain. His experiments imply that when we are confronted with words or experiences that have become deeply entrenched in us whether by repetition or emotion (for instance, athletes strongly associate the word "athletic" with their sense of self), what he calls the reflexive memory system is at work. It mediates not memories but intuitions; bypassing the regions of the brain that require explicit reasoning, it taps into the regions that produce quick emotional response. These types of connections take a long period of time to form, but once formed, allow athletes to identify themselves as "athletic" without the mechanism of memory retrieval. The reflective memory system, on the other hand, mediates our responses to neutral or ambiguous personal stimuli (for instance, the association of the word "performer" with the sense of self in an athlete). Lieberman argues that this system utilizes the hippocampus and other areas of the brain already known to be involved in memory retrieval/recreation; when faced with such signals (especially in new circumstances), the reflective system is responsible for thinking consciously about our experiences and for connecting them meaningfully with the already existent memory traces. (3)

Overall, the lower regions of the brain are responsible for the retention of memory traces, while the higher regions (such as the neocortex) provide frameworks of stories for the meaningful recreation and recombination of these traces. When a new input is received, the neocortex attempts to fit it into the already existing frameworks or pattern structures (1). Our brains are conservative, they place a value (often an emotional value) on the coherence and stability of old stories and do not easily change them. Humans want to preserve things the way they were, they feel comfortable when the new experiences fit their previous stories (2).

It should also be noted that the mechanisms of memory reconstruction are so complex and interrelated that sometimes there can be no understanding of what has affected us and, especially, of what will affect us in the future. We like to preserve our past stories, but the brain is also a creative organ that can change its frameworks from one moment to another. Therefore, memory can be productive and generative, with the new sensory inputs (or sensory inputs resembling the old ones but appearing in new situations) altering our stories of self. Precisely because memory is not a complete record that can be pulled off the shelf, precisely because it is never "real" or "faithful," it can play an important role in creating new stories (2).

5. What All of This Means for My Nostalgia

What do these understandings about memory, emotion, and the self have to do with my nostalgia? Everything. They provide me with a framework for exploring its mechanisms and the effects it has on my emotional and rational well-being. Over time, complex interrelated patterns of traces have been formed in my brain. Partially, they were formed by repetition: for example, I have learned to associate unmowed weeds with the summers spent at my grandmother's country house. Partially, they have been affected by the emotions associated with them, including the sense of belonging, comfort with my surroundings, love for my country, the happiness of my childhood, etc. After moving to America, the positive value of these emotions has been subjectively strengthened by the fact of my remembering or, perhaps, reminding myself that I am deprived of them. Moreover, the existence of these set memory patterns, of stories already used by the brain to account for certain signals (such as the unmowed weed associated with Russia, summer, and childhood), is comfortable in its own right. Therefore, when the signal (such as the weed) is presented to my conservative and emotional brain, the latter recreates the familiar story. The problem is that this story clashes with the current reality: I am in America, not in Russia. Instead of accommodating the story to the signal, my memory and emotional systems attempt to fit the signal into the existing schemes. When they are successful, I experience the feeling of being back in Russia and of the irreality of the American landscape. When they fail, the emotional pain of nostalgia enters as a reaction to the mismatch of the brain's expectations and the outside signals. The feelings of irreality and pain can be simultaneous, since they are both involved in the processes related to my story of self in the present moment.

Most painful and disruptive nostalgia episodes can, in fact, be self-induced. If the hippocampus and the amygdala do exist in a highly reciprocal relationship, I can actually "teach" myself that certain events or experiences (such as eating my mother's pies, for example) are pleasant, while others (such as American cuisine) are not, even if there is no "objective" emotional value to these experiences. When I encounter them, they will elicit strong positive or negative nostalgic emotions.

On the other hand, some of my memories and their emotional associations have migrated into my unconscious and became deeply interwoven with my enduring sense of self. Perhaps this can account for the fact that I nostalgically react to certain stimuli without consciously understanding or acknowledging them. Sometimes a sight or a smell makes me happy or sad, and only later I realize that it was because it made me feel like I was back home. And I am not certain that I can (or want to) change these, since such an alteration will modify some parts of my self that I consider fundamental.

Although nostalgia is usually associated with painful and negative emotions, with the inability to adapt oneself to new surroundings and situations, with the insistence on continuously reliving the past, this need not be true in all cases. There can be a creative and positive way of experiencing the feelings of nostalgia – whether it involves happiness or pain. If the memory retrieval triggers new memory formation – like my particular experience with the weeds in front of Denbigh – it can result in creative re-appraisal of my current stories of self. I can consciously teach myself to re-evaluate these experiences in a positive and generative light – for instance, as providing me with new knowledge or understandings – and by doing this diminish the negative (in the sense of debilitating and uncreative) emotional responses. But I don't even have to transform painful emotional memories into happy ones. The sadness and pain of nostalgia is not necessarily incapacitating; it can (and does) provide me with a different way of looking at the world, with a more complete sense of my being. Often, it is a prompting for creative work par excellence – and this paper can serve as an example. As with all elements of our stories of the self, nostalgia can be useful to our well-being, emotional and rational. And I believe that this usefulness is enhanced by a clearer understanding of the neural mechanisms involved in it.

WWW Sources

1)A summary of the 2004-2005 Brown Bag Discussion in BMC Center for Science in Society on Memory, History, and the Brain, Part II., Transcribed by Anne Dalke. Presenters: Elliott Shore, Paul Grobstein, and Paula Viterbo.

2)A summary of the 2004-2005 Brown Bag Discussion in BMC Center for Science in Society on Memory, History, and the Brain, Part I., Presenters: Elliott Shore and Paul Grobstein.

3)Zimmer, Carl. "The Neurobiology of Self." Scientific American, November 2005.

4)Phelps, Elizabeth. "Human emotion and memory: interactions of the amygdala and hippocampal complex." Current Opinion in Neurobiology, 2004.

5)Phillips, M. L. "Understanding the Neurobiology of Emotion Perception: Implications for Psychiatry." British Journal of Psychiatry, 2003.

6)Fields, Douglas R. "Making Memories Stick." Scientific American, 2005.

7)Roskies, Adina. "Mapping Memory with Positron Emission Tomography." Proceedings of the National Academy of Sciences of the United States of America, March 1994.


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