She turns on the CD player in her car and “When I’m 64” streams out over the speakers. Mona starts to cry. She is on Highway 280, driving down to Palo Alto to see her brother Michael. She’s flooded by memories of her childhood with Michael. She can hear his voice and visualize him singing the song when he was 9 or 10 years old. Michael has been in a car accident and has been in a coma for several weeks. It all seems hard to believe. He’s only 24. How can this be happening?
Mel is walking around in the grocery store. He sees a ripe yellow mango with red spots on the top. He picks it up and sniffs it. “Singaporean,” he can hear his ex-girlfriend’s voice say. He can visualize her eating the mango, the juices running down her mouth as she bites into the skin. He wonders what she is doing now. Perhaps she is eating one of these; he thinks and puts it in his grocery cart.
Mina passes by Dosa on Valencia Street. She remembers the time she went in there with her aunt, who was visiting from India. She remembers that day and how much fun it was to have dosa in her own neighborhood with her favorite aunt from India.
It’s not difficult to guess that the music, the scent, the visual cues produce emotional reactions in the brain of these people. But, what exactly happens in the brain when we store and retrieve a memory?
“What seems to happen is that a piece of familiar music serves as a soundtrack for a mental movie that starts playing in our head,” according to Petr Janata, a cognitive neuroscientist at University of California, Davis. “It calls back memories of a particular person or place, and you might all of a sudden see that person’s face in your mind’s eye.”
Familiar with studies of brain areas activated during recall of autobiographical stories, Janata theorized that the brain area behind the forehead is responsible for recalling memories associated with music. This area is called the medial pre-frontal cortex. He conducted a study in which he had college students listen to top-40 music when the students would have been between the ages of 8-18. Using fMRI brain scan, he noted the music was associated with mental activity spikes in the medial pre-frontal cortex of the subjects’ brain. The subjects later wrote down the memories recalled by the music.The vividness of the memory recalled was proportional to the intensity of activity in the medial pre-frontal cortex.
This area of the brain is just one of the many areas involved in long-term memory and one of the areas associated with Mona’s recall of her childhood memories of her brother.
The hippocampus, a tiny structure that resembles a sea horse and curls near the center of the brain, is one of the primary regions involved in orchestrating the storage of information from short-term memory to long-term memory. As Mona and her brother listen to “The Beatles” and their eyes and ears register the event, the hippocampus harmonizes the visual and auditory sensations into one “event”. The event is stored as a pattern of biochemical changes in nerve cell networks. Later the hippocampus will manage the retrieval and re-play of the event.
Over time and with repetitions of the stored events in the brain, other parts of the brain eventually do not need the hippocampus to manage them and can recall the information on their own. In Mona’s case and in the case of the college students in the experiment, the memory has been in the brain for so long that the hippocampus is probably no longer required to manage the recall. Depending on how long ago it was that Mel’s ex-girlfriend told him about the Singaporean mango and how often he has replayed that memory in his head, his hippocampus might or might not still be involved in recalling this memory. Same applies to Mina’s memories of her aunt.
All the memories presented here (Mona’s of her brother, Mel’s of his ex-girlfriend, and Mina’s of her aunt) have emotional context. This suggests the involvement of a region of the brain called the limbic system and one particular organ called the amygdala. Often referred to as amygdala, there are actually two of them in the brain. They are almond shaped with a diameter of about an inch but the size can vary from person to person. The amygdala assists in deciding as to whether an experience has emotional significance and prioritizes the event in terms of importance for storing in long-term memory.
The memories discussed here are all associated with storage of emotionally significant events. In each of these three examples the memory can be triggered by sensual clues: the sound of the music, the feel and smell of a mango, the taste of Indian dosa. Mona and her brother listen to music as he sings the lyrics and they both activate their auditory senses. Mel’s girlfriend picks out a mango and tells him about it. He smells it. Later, she eats it in a way that’s unfamiliar to him, without peeling and cutting it. His auditory and olfactory senses are activated. Mina has an enjoyable dinner with her aunt. Her olfactory senses are activated and so are her taste buds. In all these case, the visual senses are also activated. The combination of activities creates an event that is prioritized by the amygdala and managed by the hippocampus for storage in long-term memory. Later, the network of brain cells in various locations in the brain will synchronize to retrieve the memory.
Although there have been tremendous advances in the biochemistry and anatomy of long-term memory, much remains unknown. There is a strong indication, however, that it’s the result of team-work amongst complex networks of nerve cells and organs that provides human beings with the ability to replay events from their life based on retrieval cues that are picked up by the senses.
Dr. Nicola Wolfe is the neuroscience consultant for this column. She earned her Ph.D. in Clinical Psychopharmacology from Harvard University and has taught neuroscience courses for over 20 years at various universities.