The amygdala is a small, almond-shaped structure deep in the brain, located on the medial surface of the temporal lobe, which processes both positive and negative emotions. Brain scanning studies show that the amygdala is activated in response to fearful faces, even when they are not consciously perceived.

Previous studies did not measure brain activity in real-time, however, and so direct evidence for rapid fear processing in the amygdala was lacking.

A rare opportunity
Yingying Wang of Zhejiang University and her colleagues had the rare opportunity to record neuronal activity directly from the brains of 18 patients undergoing presurgical evaluation for drug-resistant epilepsy.

While neurosurgeons monitored their brain activity to identify the source of debilitating seizures, the researchers implanted microelectrodes into their amygdalae, visual cortices, and various other brain regions, and recorded the responses of individual cells to images of happy, fearful, and neutral facial expressions.

The researchers used low- and high-resolution images of the faces of 96 actors that were rendered invisible by a process called backward masking, in which each image is shown briefly, and then quickly followed by another image of the same color that does not contain a face.

Low-resolution images of fearful faces, but not of happy or neutral ones, evoked rapid cellular responses in the amygdala, but not in the visual cortex or other regions. The earliest responses of amygdala neurons occurred within one-tenth of a second, even though the patients were not consciously aware of having seen the images.

https://bigthink.com/neuropsych/brain-fear-unconscious-awareness/