ScienceMade Influence Blog
June is Alzheimer’s Awareness month: Alzheimer’s is an untreatable cognitive disease that affects tens of millions of individuals worldwide. Hopefully this article will inspire you to learn more about the disease and join us in supporting the cause.
By Sonali Dasari
The aroma of a specific home-cooked dish might unlock forgotten memories of spending time with your grandparents as a child. The scent of charred wood might bring you back to your first experience of going camping. All of these complex memories and emotions can be awakened by a simple smell, but what exactly gives smells this unique ability to take us down memory lane?
Smell is the oldest sense, evolving first in single-celled organisms such as bacteria to detect harmful chemicals in their environment. While sight relies on four main visual receptors, smell has at least one thousand regenerative receptors, which give information to our brain and allow us to differentiate between the complex smells that we encounter on a daily basis. Adding to the unique characteristics of smell is the way we describe and interpret them. While we are able to directly describe a color by names like "yellow" or "blue", smells must be described indirectly through association. For example, one could say that it “smells like freshly cut grass” or “it smells like mangos”.
Specific structures of the brain reveal why smells are linked to emotional learning and memories (see figure). When a smell enters the nose, it is first processed at the smell processing center, the olfactory bulb, located in the front of the brain. This smell processing center has direct connections to two brain areas in the limbic system, a system which regulates emotional responses. These two brain areas are the memory center of the brain known as the hippocampus, and the emotion center of the brain known as the amygdala. The communication between the memory center, the emotion center, and the smell processing region gives smell the unique ability to evoke powerful memories or emotions almost instantaneously.
The emotional response system, known as the limbic system, is in charge of regulating whether we exhibit a fight-or-flight response (which increases heart rate, blood pressure, and breathing), or a relaxing response. This gives scents, such as essential oils, the ability to interact with this system in a way that can have significant effects on our breathing rate, relaxation, and even stress levels.
In addition to the structures of the brain, a process known as associative learning is extremely crucial for linking smells with memories. When first introduced to a new scent, we link it to a person, event, place, or moment. For instance, many people associate the smell of funnel cakes to fairs and carnivals. When this specific smell is encountered again, we are able to recall memories, feelings and emotions tied to the initial event.
These powerful and unique abilities of smell have been a subject of growing interest, particularly for researchers who are working to understand more about Alzheimer’s and other diseases related to the progressive loss of brain functions. A damaged sense of smell is one of the earliest symptoms of Alzheimer’s (and Parkinson’s) disease, and researchers hope to use this particular symptom to aid in the early diagnosis of Alzheimer’s.
Alzheimer’s is a brain disease that causes memory loss, the progressive worsening of thinking abilities, and a gradual decrease in the ability to complete many other everyday tasks. It is the sixth leading cause of death in the United States, affecting over five million Americans (typically 65 years and older) and disproportionately impacting women and communities of color.
As a result of this disease, the neural pathways that allow the smell processing center (olfactory bulb) to send information from the nose to the brain are impaired. Given this finding, people often experience a decreased ability to identify smells even before problems with memory arise. A recent study investigated the predictive potential of the smell test, which tests an individual's ability to identify smells. Two groups were evaluated: those without memory problems and those who had mild memory problems. The results revealed that those who performed well on the smell test did not have Alzheimer’s disease. However, those who were found to have odor impairment were three times more likely to experience memory decline and thus be at a higher risk for Alzheimer’s disease.
The incredible link between smells and memory shows just how fascinating and interconnected the brain truly is. Who knows what different odors will be able to tell us in the future?
Cover is from the National Cancer Institute. All images are sourced from the public domain.