From Neurons to Nostalgia

Joe Reynolds

Some memories burn brighter than others, that is a certainty. I can (just about) remember my childhood friend’s phone number from thirteen years ago, yet I struggle to remember my WiFi password despite typing it into my phone only three days ago. Both of these strings have around the same number of digits, yet the phone number appears with an instinctive clarity. Did I simply spend more time committing the phone number to memory? This is one possibility. However, the equations of motions I memorised relentlessly for two years of A-Level maths are a foggy blur in my mind. My ability to reflexively recite the phone number is due to its inextricable link to previous emotional states. Calling my friend was more than just an action – it was the excited anticipation of hanging out, the nervousness of his parents answering and the disappointment of nobody picking up. Often our clearest memories are the most painful or the most embarrassing, the ones that sneak up on you in the shower. They don’t just play episodically in your head like a stilted school drama, they feel as if they transport your whole body back to the time when they were formed. You can still feel that pain in your chest. Nostalgia is often discussed in terms of a subjective feeling – but what can neuroscience tell us about the processes behind it?

Short term memories are processed in the frontal cortex, where they linger for approximately thirty seconds, and after they are replicated in the hippocampus to be stored long term. The hippocampus connects strongly to an area of the brain concerned with emotional processing, the amygdala. When a memory is stored at a time of emotional activity, the imprint it leaves is stronger, possibly due to reciprocal neurotransmitter release between these regions. Neuroscientists have used light sensitive proteins called channelrhodopsins (ChR) to label specific neurons that encode a memory, known as an engram. When these labelled neurons are exposed to a specific frequency of light, they are reactivated. Dr Tonegawa and his team at MIT used this technique in mice which were placed in an environment, and after a few minutes received a mild foot shock, causing them to learn to fear that particular environment. The neurons which activated during this experience were tagged with ChR. Later in a completely different environment the tagged neurons were exposed to triggering light pulses – and the mice froze in fear. It was demonstrated that memories could be reactivated, and with them a very specific emotional state. Not only are memories stored in specific neurons, they are deeply connected to the emotional centres of our brains. Often, nostalgia feels good, and it has been shown that the reward centres of the brain, the substantia nigra and the ventral tegmental area, are activated upon hearing nostalgic music.

Nostalgia is a powerful expression of humanity, one of longing, sadness and hope, and it seems counterproductive to reduce it to a series of action potentials, reciprocal connections and neurotransmitters. The question brings neuroscientists to the hard barrier of consciousness, the precipice of what can be explained by biochemical phenomena – we can observe what matter does, not what it is in itself – and this can be applied to consciousness. Descartes believed the mind could not be explained in this way, that ‘I think, therefore I am’, yet advances in biochemical techniques have allowed us to pry into the mechanisms behind cognition in ever more minute detail. Whatever science does reveal about our mind, I don’t believe that it detracts from the experiences we have, and the meaning they bring to our own lives. Nostalgia itself simultaneously reminds us who we are, and who we were.