Ever wondered why you get chills down your spine each time you sit in the dentist’s chair or strap yourself into a rollercoaster? They're produced by a conditioned fear response, which can be learned following a bad experience and recalled whenever certain stimuli relating to that event present themselves. While this concept is certainly nothing new, scientists have for the first time identified a particular brainwave that appears to regulate this response, potentially opening the door to new treatments for conditions such as post-traumatic stress disorder.
The brain’s ability to learn and recall conditioned behaviors was famously illustrated by Ivan Pavlov and his slobbery dogs in the 19th century, and numerous subsequent studies have revealed how this phenomenon can be manipulated to elicit fear as well as hunger. Rodents, for instance, can easily be trained to freeze upon hearing a tone that they have come to associate with an aversive experience such as pain.
Despite this, the mechanisms by which the brain recalls these conditioned memories had, until now, eluded scientists. However, a new study in the journal Nature Neuroscience may have finally provided an answer, suggesting that brainwave oscillations of a particular frequency may serve to create a "fearful" brain state, generating conditioned behaviors associated with fear.
Previous studies have shown that such responses to conditioned terror depend on an interaction between brain regions called the dorsal medial prefrontal cortex (dmPFC) and basolateral amygdala (BLA). To obtain a deeper understanding of this process, scientists trained a group of mice to develop a fear response to a noise, while measuring their neurological activity using electrodes that had been implanted into these regions of their brains.
They found that, slightly prior to freezing in response to the tone, the neurons of the dmPFC began to show electrical activity that oscillated at a frequency of 4 Hertz. Shortly afterwards, the neurons of the BLA adopted the same frequency, thereby becoming synchronized with the dmPFC. When this oscillation ceased in the dmPFC, the mice became mobile again, and the oscillation was then terminated in the BLA as well.
This led the researchers to conclude that the generation of 4-Hertz oscillations in prefrontal-amygdala circuits serves to regulate conditioned fear responses, and that this process is driven by the dmPFC.
Synchronization of the prefontal-amygdala circuits appears to be the main driver of conditioned fear responses. Nikolas Karalis et al
To confirm this, the researchers conducted a second test using mice that had been genetically engineered to produce certain light-sensitive proteins which, when activated by a flashing light, caused the neurons of the dmPFC to fire at 4 Hertz. Interestingly, even when mice had not been conditioned to fear a certain stimulus, the oscillations in the BLA still became synchronized with those of the dmPFC, causing the mice to freeze.
Brainwaves of 4 Hertz fall into the category of theta waves, which range from 3 to 8 Hertz and have previously been associated with the processes involved in learning and memory. However, by identifying the specific frequency and circuit involved in eliciting conditioned responses to fear, the study authors hope to have unlocked a key pathway for treating those with a range of disorders related to fear.