Our brain constantly makes decisions shaped by scents and behavior, relying on both direct and indirect associations. While some links are obvious, others form in ways that aren’t immediately visible, influencing choices in surprising ways. Mistakes in these associations can even alter how we react to familiar experiences.
A new investigation by the Cellular Mechanisms in Physiological and Pathological Behavior Research Group at the Hospital del Mar Research Institute explores how these indirect associations work. By testing mice in different scenarios, the researchers uncovered the brain mechanisms that allow an unrelated event to change later decisions and reactions.
How scents create indirect links in the brain
In the study published in PNAS by the Hospital del Mar Research Institute, mice were trained to associate one smell—banana—with a sweet taste, and another smell—almond—with a salty taste. Later, a negative stimulus was linked to the banana smell. From that point on, the animals rejected the sweet taste, even though it had not been paired directly with anything unpleasant.
This shift revealed that the brain had formed an indirect association: the sweet taste became connected to the negative experience through its link with the banana scent. As Dr. Arnau Busquets, supervisor of the research, explained, the project aimed to show how the brain enables decisions based not only on obvious links but also on subtle relationships between stimuli.
The experiment highlighted how powerful sensory cues like scents can be in shaping behavior. Even when the connection is indirect, the brain adapts decisions by reinterpreting past experiences. This mechanism demonstrates why environmental signals can have such a strong effect on choices and emotional responses.
The role of the amygdala
To identify which brain regions made these associations possible, researchers used genetic techniques delivered through viral vectors. These methods allowed them to monitor brain activity during the learning and consolidation phases. The results pointed directly to the amygdala, a region tied to fear, anxiety, and emotional memory.
When the mice linked smells and tastes, the amygdala showed strong activation. Additional imaging revealed that other brain areas also participated, forming a circuit that connected with parts of the cerebral cortex. This network enabled the storage and retrieval of indirect associations.
The team confirmed the amygdala’s key role by inhibiting its activity while the animals were exposed to the stimuli. When the amygdala was silenced, the mice were unable to form new indirect connections, proving that this region is essential for the process.
What this means for human behavior
Although the research was carried out on mice, the findings may also apply to humans. Dr. Busquets noted that the brain circuits involved in these decisions are thought to be similar across species. If confirmed, the results could have major implications for mental health research and treatment.
Indirect associations influence how people respond to everyday experiences, but when they malfunction, they can contribute to psychological disorders. For example, in conditions such as post-traumatic stress disorder (PTSD) or psychosis, faulty associations can distort how stimuli are perceived, reinforcing negative patterns of thought and behavior.
By understanding the brain circuits behind these processes, scientists may be able to design new therapies. Future approaches could involve brain stimulation or modulation of activity in the amygdala and connected cortical regions. Such strategies might help patients rebuild healthier associations, reducing the weight of harmful memories or distorted links.
