More than once you will have noticed that a smell (that of freshly cut grass as Leon Tolstoy did) or a taste (that of a tea with muffins as Marcel Proust narrates) bring you from oblivion a vivid memory of your childhood.
Popular culture suggests that certain smells are powerful reminders of autobiographical experiences of our early years. This has been called the Proust phenomenon, in honor of the French writer Marcel Proust. Let’s see what is known about him from a neuroscientific perspective.
Thanks to the senses of smell and taste we can perceive a very high number of molecules present in the outside world. These are related not only to food substances, but also to other potentially dangerous ones.
The memory of its effects allows accepting or rejecting some of them in the long term. In addition, in many species (although not in ours) the smell facilitates the detection of pheromones, which produce important behavioral changes.
How do we perceive smells?
In a series of momentous experimental works, recognized in 2004 with a Nobel Prize, Linda B. Buck and Ricard Axel they demonstrated that olfactory receptors are proteins sensitive to the presence of certain odors.
These proteins are located in the sensitive endings of receptor neurons located in the nasal passages. In humans there are about 350 different proteins, but other species, such as the mouse, express more than a thousand.
They seem few, but every odorous substance activates a combination of these receptors. For this reason, the number of different smells that can be perceived is enormous in practice.
On the contrary, in the taste buds located mainly on the tongue There are only receivers for five flavors: sweet, salty, bitter, acid and umami (the one produced by amino acids such as glutamate).
In reality, the taste of food does not depend exclusively on the activation of these taste receptors. It also depends on the volatile substances that the chewing of those sends from the oral cavity to the olfactory receptors through the retronasal route.
The same happens with other factors such as the texture and temperature of what you eat, its visual presentation and its smell. That is, the one that reaches the olfactory receptors from the outside through the antennasal route.
There is a fundamental difference in the way that information from different types of sensory modalities (vision, touch, pressure, pain, hearing, balance, taste and smell) reaches the brain.
All these nerve pathways, except for olfaction, reach the cerebral cortex. There they reach a conscious level, through the thalamus. The latter is so named because it is like a bed on which the entire cerebral cortex sits.
Where in the brain odors are stored
On the contrary, hehe nerve pathways that transmit olfactory information directly reach nerve centers related to our inner world. That is, the one in which our emotional heritage, both conscious and unconscious. Among these nervous structures, the following stand out:
- First, the core of the amygdala. This is mainly related to negative or unpleasant emotions and aversive-type learning aimed at avoiding stimuli that evoke these situations.
- Second is the hippocampus. In it, the memories that make up our autobiography not only cognitive but also sentimental are processed or reactivated. These are the so-called episodic memories, which allow, even in the very long term, the conscious recollection of personal and precise moments from our past.
- And finally, a portion of the cortex located in the most anterior region of the brain, the orbitofrontal cortex. This is related to decision making. That is, with our ability to choose between different alternatives.
In summary, the above-mentioned argumentation, based mainly on neuroanatomical considerations, is the best foundation available to date to justify why olfactory stimuli associated with important experiences that occurred during our childhood have so much evocative power.
In any case, other sensory stimuli, such as music, also have easy access to our emotional world. For example, it is easy to feel sad when hearing the famous aria A bel di, vedremo by Madama Butterfly.
How memories are stored
For this reason, neuroscientists have been trying to elucidate for some years not where, but also how the reactivation of memories with a strong emotional tonality takes place and that are associated with olfactory sensory stimuli or other sensory modalities.
From an experimental psychological point of view it seems true that stimuli of olfactory origin are kept in memory longer than, for example, other of visual origin.
Now, one thing is the memorization in the laboratory of tasks and situations for its experimental study and another the recall of situations lived by the subjects in their recent or remote past.
Regarding the second point, it has been experimentally shown that an autobiographical memory related to a certain smell is more easily evoked by using that smell as a trigger than using the name of the odor, an unrelated odor or an image also associated with said memory.
Some of these studies have been done in people with brain imaging techniques, such as positron emission tomography or functional magnetic resonance imaging. These allow to specify in detail which brain structures (such as those indicated above) are activated during the evocation of autobiographical memories.
It will surely take us a while to find the right answer to these questions. It is not a question of determining whether smell is the best and most powerful stimulus to associate with memories that we want to keep from our emotional past. There is an elementary and preliminary question to answer: Where and how does the fruit that melts in the mouth transform in our brain first in flavor and finally in memory? I leave the answer pending.
* By José María Delgado Garcia, for The Conversation
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Why do we remember the smells of the past?