Proximity to danger may dictate how fear is formed in the human brain
The human brain reacts completely differently to a threat that invades your personal space compared to those that are further away, according to a new study that combined MRI scans with VR technology. Distant threats engage circuits in the brain linked to problem-solving, while closer dangers trigger parts the brain that deal with defensive, less reasoned animal instincts, and are more likely to induce longer lasting fear reactions, including PTSD.
Like it or not, fear is a major factor that dictates how we live our lives. However, not all fears are equal in their ability to leave a lasting effect on our mental health. This is because the human brain uses different circuits to deal with different types of threatening scenarios.
A new Duke University study published in Proceedings of the National Academy of Sciences has sought to shed light on how the brain handles threats when perceived at distance and close by. More specifically, it highlighted how the proximity of a threat influences the learning and memory duration of fear associations.
Traditionally, this type of proximity-based fear testing has been difficult to monitor in a laboratory setting, as participants are usually sat a fixed distance from a computer screen. In order to get around that issue, the study authors got imaginative with their equipment.
Forty-nine healthy study participants were put inside the tube of an MRI scanner that had been outfitted with a virtual reality system made up of elements that included a 3D television and 3D glasses. While in the MRI tube, test subjects were shown a first-person view of two settings, while two human avatars – one friendly and one threatening – appeared either at long range or close proximity.
During the first day of testing, the volunteers were shown a long dark alleyway through which they were passively guided forward until an avatar appeared. Whenever the threatening avatar appeared either at a distance of 10 ft (3 m) or at the closer range of 2 ft (0.6 m), the subject was given an electric shock to teach fear. The friendly avatar appeared at the same distances but without the shock.
On the second day, the volunteers were treated to the same scenarios, but were only given three shocks at the beginning of the session in order to remind them of the yesterday’s threat, instead of every time the threatening avatar appeared. At this stage, the researchers were looking to see the extent to which the scenarios reinstated the fear responses learnt on the previous day, without the real-time electrical shocks.
MRI data captured on the first day revealed that different areas of the brain were triggered by the threats that came within arm’s length, when compared to those that were perceived 10 feet away. The more distant threats engaged sections of the brain connected to higher-order thinking (including problem-solving, planning and visual processing), such as the amygdala, hippocampus and ventral medial prefrontal cortex.
Threats that were perceived within arm’s length were more frightening, and triggered a more animal, reactionary response in an evolutionarily older part of the brain. These defensive survival circuits are located in the limbic and mid-brain regions.
The team discovered that the distant threats left less of an impression, and that the fears induced by these encounters were more easily extinguished. Conversely, the fear created by threatening avatars that invaded a test subject’s personal space were much harder to extinguish, even when, on the second day, there was no associated electric shock administered.
This is reflected in earlier research which has highlighted that traumatic events that involve the invasion of personal space – like rape and assault – are more strongly associated with post traumatic stress disorder than threatening encounters that happen at distance.
According to the team, a better understanding of the human brain’s reaction to proximity-based trauma could inform future treatments for PTSD.
"We think that the cerebellum might be an interesting place to intervene," comments Kevin LaBar, senior author of the new study, and a professor of psychology and neuroscience at Duke. "Clinically, it's a new interventional target. If you can somehow get rid of that persistent threat representation in the cerebellum, you might be less likely to reinstate (the fear) later on."
A paper on the study has been published in the Proceedings of the National Academy of Sciences.
Source: Duke University