Seeing Is Remembering
The link between your senses and your past
Having grown up in the mountains, the distinctive scent of sun-baked soil has all but defined my childhood. It is difficult to describe to someone who has never smelled it; the best words I can think of are an earthy scent, the fragrance of living, growing things. This aroma was often accompanied by the near-constant chorus of an assortment of insects and birds inhabiting the dense tangle of bushes and forests surrounding my hometown. At this time of year, the wind blows cold and sharp, a harbinger of winter. When it nips at my skin, I recall outdoor Christmas celebrations at school.
I recount these scenes in such detail because, whenever I visit home, my senses reacquaint themselves with familiar scents and sounds, and instantly a montage flashes across my mind’s eye: high school corridors, winding hill roads, voices I last heard many moons ago. This film reel is rendered in my brain with startling clarity, as though I experienced these events only yesterday.
Our five senses are what allow us to experience the world. Our memories are created and stored by special structures deep within the brain. But how are they connected? Why does the routine task of gathering information through your sense organs act like hitting the Play button on a movie you didn’t know you owned? Why can this process often be so much more potent than merely looking at old photographs or talking about past events?
How Do You Actually ‘Make’ Memories?
Neurons that fire together, wire together.
The neurons in your brain are wildly dynamic. They remain in a constant cycle of forging, severing, rebuilding, pruning and nourishing connections every second you spend alive. Neuronal communication is at the heart of everything you think, plan, imagine, believe, dream, feel…and remember. Your nervous system, and subsequently, your entire body, can thrive only if neurons can efficiently and reliably relay signals to each other and ultimately, to your brain and other organs. They are the middlemen through which the brain and the organs can interact and compare notes about the world without and the body within. From your first breath to your last, they play a perpetual game of Chinese whispers where they can’t possibly afford to mix up the message. The stronger the connections between neurons, the greater the ease with which they can talk to each other.
Over time, a certain neuron, say neuron A, can grow more likely to generate a signal following another neuron’s signal, which we’ll call neuron B. In other words, neuron A can be said to be triggered by its companion. Thus, every time neuron B fires off a signal, neuron A becomes increasingly likely to respond in kind. Now, extend this image across an entire group of neurons, a close-knit collection of brain cells all activating each other, like a bundle of matches packed tightly together. When one lights up, you can count on the rest doing the same. All for one and one for all. This underlies the central philosophy of Hebbian theory, which has been quoted at the beginning of this section. This theory, named after neuropsychologist Donald Hebb, who proposed it in 1949, states that neurons that fire simultaneously become better at communicating with each other and are more likely to fire as a single unit. These neuron cliques are known as ‘ensembles’. And that, in its most basic form, is precisely what a memory is: a neuronal ensemble.
Each memory is encoded by a different ensemble. So, recalling your tenth birthday party will trigger a specific ensemble, while the recollection of your high school sweetheart will activate a different one. In a nutshell, the act of bringing a memory to mind is simply the decision to reactivate the particular neuronal ensemble that has been assigned to that memory.
It is important to note that these ensembles are not immutable. Each time you recall a memory, the connections between the ensemble’s neurons shift a little, in light of the life experience and other memories you have acquired since the original memory was created. In this way, your memories are actually modified very slightly with every recollection. This is especially common with emotionally charged memories, with the result that later reminiscences can heighten the positive or negative emotions associated with past events, regardless of how intense your feelings actually were at the time the event occurred.
The brain has entire departments devoted to the storage and processing of different categories of memories. The most widely known of these is the hippocampus, which enables us to recall episodes and events from our own lives. Yet, despite hogging most of the limelight, this structure is far from the only one involved in memory and learning. The amygdala, which is the brain’s emotional processing hub, plays a role in fear memories, thus helping us remember which situations, triggers or neighbours’ Chihuahuas to avoid. This region is also the reason why memories with a strong emotional component are far more likely to stick with you. On the other hand, if you need to remember the steps to perform tasks such as playing the guitar or driving your car, it is the cerebellum to which you must appeal, as this is the seat of a class of memory known as procedural memory. The prefrontal cortex is the structure that holds all remembrances of general knowledge and impersonal facts and is primarily responsible for your sinking or swimming during an exam or pop quiz.
As you may have deduced from this section, not all memories are created equal; the way they are processed and stored depends entirely on the unique event or piece of information they seek to encode.
Eyes And Ears
“Anytime you are forming memories that you want to stay with you, you want them to be as multisensory as possible.” — Andrew Budson, MD ’93, professor of neurology, Boston University Chobanian & Avedisian School of Medicine.
Your five sense organs are your gateway to the outside world, and as such, they have an enviable impact on your quality of life. It only makes sense that such vital entities should remain in constant, active communication with the brain. Each of these organs hosts its own dedicated team of specialised cells called receptors, appropriately named, as they function like radio antennae that receive information about your environment, while also being able to detect changes that may require action. Like army scouts on a recon mission, they report back to the brain with any noteworthy news.
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But wait. Who exactly do they report to? The brain is a complex, multifaceted beast, after all, with dedicated areas for different purposes. Their primary point of contact is the cerebral cortex. There are departments here devoted to the initial processing of raw sensory data. Subsequently, the processed input is shuttled off to the hippocampus for further investigation. This name seems familiar. We have encountered it in the previous section; recall that the hippocampus is the storage vault for your memories of your life experiences (or ‘episodic’ memories). The truth is that this structure is more than a mere warehouse; it is responsible for consolidating the information your senses take in. It is comparable to a master analyst that collects and sorts sensory inputs and, crucially for this story, integrates that information with the memories it houses. It builds a kind of index that nearly instantly connects a highly specific sensory trigger — a scent, a song, a tingle on your skin — with the memory to which it is bound. The world’s most efficient lookup table is built into your brain.
This brings us to Dr. Budson’s quote that introduces this section. The strongest memories are those in which multiple senses are involved; the more, the better. This helps the hippocampus to forge stronger connections between sensory triggers and memories.
Where Does Nostalgia Fit Into All This?
Fun Fact: The word originated as a name for the deep sorrow common among homesick soldiers in the late 17th century, born from the Greek words nostos (desire to return) and algos (pain). Until roughly the end of the 20th century, nostalgia was even viewed as a psychiatric disorder, featuring anxiety, insomnia, sadness and pessimism as its symptoms.
In today’s world, fortunately, nostalgia has taken on a far more positive connotation and unites multiple cultures and age groups in a shared longing and fondness for the good old days. But what exactly is going on in your brain every time you feel that distinctive pang of happiness sharpened by sorrow?
Based on the specific qualities of this bittersweet emotion, researchers have hypothesised the particular brain regions that they consider likely to be involved.
Two of nostalgia’s key components are autobiographical memory retrieval and emotional regulation. The former lies within the domain of the hippocampus, which we have explored in detail previously, while the latter is the role of the amygdala, another structure we have already encountered.
There is some evidence suggesting that the hippocampus is involved in nostalgia; in one study, participants were shown nostalgic images from their past, such as a pencil case from their childhood, along with neutral images. fMRI scans of their brains revealed that the nostalgic pictures stimulated stronger responses from the hippocampus. Participants were also asked to rate the emotional and personal significance linked with each picture. The highest-rated pictures were associated with correspondingly greater activity in the hippocampus.
Intriguingly, researchers have also proposed an interplay between the brain’s memory and reward centres. Nostalgia appears to make your brain, and you, feel good, and consequently, the brain wants more of it.
Odour-activated autobiographical memories have been found to be more emotionally significant than other sensory cues. Our sense of smell, which happens to be humanity’s oldest sense, is arguably the most powerful trigger of the sensory quintet, forming the tightest links to our memories. This is because odour-related information takes a shorter, more direct path to the hippocampus and amygdala than the other senses, requiring less preprocessing before further analysis can be conducted. As a result, there is less of a time lag between your nose’s receptors perceiving a smell and the hippocampus making sense (pun intended) of that smell and connecting it to a relevant memory. This explains the instant, uniquely vivid recollections that can only be sparked by a scent, rather than an image or sound.
It is common knowledge that taste and smell are intricately linked; this is the reason why even your favourite dish can disappoint you during a bad head cold. It also explains why taste, like smell, can rapidly activate old memories in sharp detail. The distinctive abilities of these two senses have been termed the Proust effect, named for French author Marcel Proust, who wrote about a character biting into a tea-dunked madeleine and immediately recalling childhood memories of his aunt.
Music is also known for being a strong trigger, likely due to its simultaneously stimulating effect on several brain areas involved in memory and emotional processing. As most people will agree, a single song can inspire a complex blend of emotions. One study suggested that those who are less predisposed to nostalgia by nature exhibit increased activity in the amygdala and other brain regions on hearing nostalgia-evoking music.
Does Nostalgia Have Potential Therapeutic Effects?
Nostalgia has been shown to have several benefits for mental health, having been found to reduce depression, stress, anxiety and loneliness, boost self-esteem and increase the chances of someone feeling that they have lived a meaningful life.
Research also suggests that sensory cues could be used to help Alzheimer’s patients recall lost memories. Scents have proven particularly promising, for the reasons explained above, as has music. In fact, a study of 63 patients with mild to moderate Alzheimer’s disease suggested that nostalgic music therapy can boost cognitive function, reduce negative emotions and improve sleep in such patients.
Nostalgia has cemented itself as one of humanity’s most universal experiences. The next time a long-forgotten song or characteristic aroma sparks a reminiscence of long ago, it is also worth remembering the intricate mechanisms whirring tirelessly inside your head to shape your reality, your identity and your unique perception of the world.
Further Reading:
Nostalgia Doesn’t Need Real Memories — An Imagined Past Works Too
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