A recent paper in Neuropsychologia got a lot of attention on Twitter and at the Cognitive Neuroscience Society meeting in Boston over the weekend. It discusses what can happen when the amygdala, a region of the brain known for regulating emotional responses, receives direct electrical stimulation. A thrill ride – but for only one study participant. Two of nine people noticed the electrical stimulation. One individual reported (a video is included in the paper):

“It was, um, it was terrifying, it was just…it was like I was about to get attacked by a dog. Like the moment, like someone unleashes a dog on you, and it’s just like it’s so close…

He also spontaneously reported “this is fun.” He further explained that he could distinguish feelings in his body that would normally be associated with fear recognized and the absence of an actual threat, making the experience “fun”.

But wait, why were Emory neuroscientists Cory Inman, Jon Willie and Stephan Hamann and colleagues doing this?

At the end of 2017, the same group published another paper in PNAS about direct electrical stimulation of the amygdala and its effects on visual recognition memory in humans. This was a preliminary step toward having a memory enhancing device for people with impaired memory, caused by traumatic brain injury or neurodegenerative diseases, for instance.

The newly published research was performed first. The goal: to help investigators refine exactly where, how long, and how strongly they should stimulate the amygdala for future memory-related applications. In fact, their objective was to determine whether there was a threshold of stimulation over which patients would have an emotional experience, so they could stay under that limit in their memory enhancement study. Both research studies were performed with the help of epilepsy patients undergoing intracranial monitoring. That is, they had electrodes in their brains for diagnosis of seizures.

For reference: The 2018 paper used a voltage range of 1–12 V, corresponding to expected currents of 1–1.25 milliamps for 1 V and 12–15 milliamps for 12 V. The 2017 PNAS paper kept the current at half a milliamp, and the authors reported that the epilepsy patients did not notice amygdala stimulation and did not show signs of autonomic activation, such as altered heart rate.

In the new paper, 9 patients received amygdala stimulation on one side of the brain, and only one (the person mentioned above) said that he noticed it. All the others experienced reduction in heart rate, as well as increases in skin conductance, a measure of changes in sweat on the hands that is a standard measure of autonomic arousal.

Upon bilateral stimulation only, another person reported a smell of “burnt peppermint” and at higher voltages described feelings using words such as “weird” and “joy.”

Why did only the one person report such strong fear-related sensations? Although all the people in the study had electrodes placed within the basolateral amygdala, the differences in what they experienced may have to do with precise electrode placement or possibly idiosyncratic brain wiring.

The authors do point out:

“… our method of assessing subjective emotional responses may not have been sufficiently sensitive to exhaustively probe whether patients were experiencing subtle changes in emotional experience. For instance, although we asked patients to report any change in emotional experience, they may have elected to not report subtle or transient emotional responses.”

The research helps further define the role of the amygdala in anxiety and autonomic responses. The experiences of the patients in the Emory study line up well with previous studies in which people received electrical stimulation to the amygdala. Here’s another example from co-author Kelly Bijanki and colleagues from Iowa, in which amygdala stimulation resulted in changes in how someone viewed facial expressions.

On Twitter, one scientist remarked: “I’m definitely using this for the next time I teach intro psych.” Inman responded: “Make sure to point out that though this ‘fear’ effect is interesting, it is also just as interesting that it is rare to generate this emotional effect, even when other patients received stimulation at the same amplitudes.”