Think back to your first kiss, your soccer championship game, or hearing about the death of a loved one. Do you remember what you were wearing? Do you remember who was there with you and specifically where you were? You might even remember the exact words from what people said around you. These crystal-clear memories are called flashbulb memories, and are processed by the amygdala, a region of the brain associated with regulating emotions and emotional memory.
A new study now reveals that directly stimulating the amygdala can result in improved memory without a combined emotional experience. Participants in a study at Emory University Hospital received brief, low-amplitude electrical stimulation to the amygdala and demonstrated improved declarative memory the next day without any subjective emotional feelings or involuntary emotional responses, such as increased heart rate or faster breathing.
The study, published online in December 2017 in the journal PNAS, took place in conjunction with the Emory University School of Medicine. Epilepsy patients with electrodes already implanted in their brains were recruited to participate, and fourteen individuals took part in the study. Participants were shown numerous neutral images (i.e. a picture of a basketball or a key), and then either given a short stimulation of the amygdala or no stimulation. Immediately afterwards and again the following day, participants were shown more neutral images in a recognition-memory test. The patients who received the stimulation exhibited greater memory retention of images after one day, compared to the control group.
While the participants in the study were simultaneously receiving treatment for epilepsy, they showed substantial memory enhancement due to the amygdala stimulation. One patient, who suffered from brain damage and memory impairment and rarely recognized researchers and physicians, displayed the most memory improvement. Other patients who experienced seizures in between the initial stimulation and the test the following day showed improved memory, presenting evidence that the amygdala stores memories in spite of other neurologically debilitating disorders. None of the patients reported being able to feel the stimulation.
Researchers speculate that the amygdala plays a role in delegating non-emotional declarative memory to other structures, namely the hippocampus and the perirhinal cortex. Specific stimulation to the hippocampus and perirhinal cortex to improve memory has been erratic in prior studies, and the amygdala might be the missing link. According to co-author Joseph Manns, “the long-term goal of this research program is to understand how modulation of the hippocampus by the amygdala can at times lead to memory enhancement and at times lead to memory dysfunction, such as that observed in post-traumatic stress disorder (PTSD).”
Regarding the targeted amygdala stimulation, co-author Cory Inman explained, “One day, this could be incorporated into a device aimed at helping patients with severe memory impairments, like those with traumatic brain injuries or mild cognitive impairment associated with various neurodegenerative diseases.” Small deep-brain stimulation implants are already being used to treat Parkinson’s disease. This study may be utilized in future research to develop similar clinical treatments for patients with memory disorders, so that their non-emotional memories like what they ate for last night’s dinner or what they read in a good book, can be remembered the next day.
Inman, C.S., Manns, J.R., Bijanki, K.R., Bass, D.I., Hamann, S., Drane, D.L., Fasano, R.E., Kovach, C.K., Gross, R.E., and Willie, J.T. 2018. Direct electrical stimulation of the amygdala enhances declarative memory in humans. PNAS 115: 98-103.
Emory Health Sciences. 2017. Direct amygdala stimulation can enhance human memory for a day: Preliminary study of time-specific electrical stimulation. ScienceDaily. Retrieved January 31, 2018 from www.sciencedaily.com/releases/2017/12/171218151808.htm.