The Psychotherapist’s Essential Guide to the Brain – Part 5
Fear & Anxiety
Last month’s “Guide to the Brain” examined the neural substrates of OCD. Over the next two months we introduce some of the neuroscience behind other anxiety disorders such as phobias, panic attacks, post-traumatic stress disorder (PTSD) and generalised anxiety disorder (GAD). These anxiety disorders can become debilitating, and as clinicians we encounter them often among the problems clients come to us for help with. The opportunity presents itself to learn the neural mechanisms involved in anxiety disorders and to evaluate interventions against a sound understanding of what is taking place in our client’s nervous system. In our survey this month we begin with the neurological bases of fear and anxiety, and we will continue in more detail next month.
The emotional response of fear and anxiety is, of course, adaptive. It is a natural response designed to keep us safe from a perceived threat, be it physical or a more complex social/emotional threat. On one level we can appreciate that apprehension about failing an exam can motivate us to study a little harder, or that fear of losing our job spurs us to get to work on time. But when these adaptive and useful mechanisms become exaggerated and go into overdrive, we have a problem: rather than achieving the goal of keeping us safe, fear cripples us, robbing us of a normal life. So what constitutes our fear mechanism, how does it become maladaptive, and what can we do about it?
The Rats of Fear
Joseph LeDoux has devoted his career to mapping out the fear network of the brain. By conditioning rats in order to elicit a fear response associated with a certain stimulus (a tone), LeDoux was able to trace the neural pathways of fear. His early experiments involved lesioning the auditory cortex to prevent the rats from “hearing” the feared stimuli—the assumption was that without auditory processing the rats would not respond with fear. Surprisingly, the rats continued to respond to the tone with a fear response after the lesion. How could the animals know to fear the tone without being able to process it in their auditory cortex? The fear network was unaffected, meaning that its mechanism must be operational before auditory information reaches the auditory cortex; that is, there must be a pathway other than that of the thalamus–cortex. And so there is. LeDoux developed the neuroscience of fear based on a non-cortical pathway he called the “low road”—a fast, direct pathway that requires no higher-order thought to activate a physiological response to a threat.
The implicit emotional learning of the amygdala is unlike the explicit memory of places, events, and facts that is mediated by the hippocampus. The explicit learning of the contextual details of a fear-related incident is hippocampal-dependent. The memories of context by themselves are not emotional; emotional aspects of memory require amygdala activity and the pairing of contextual elements with the emotional representation of those elements. Unlike the rats in LeDoux’s lab, we can make emotional associations just by recognising a dangerous situation from context, our amygdala coming into play to encode an emotional element to the learning—a helpful skill to have when the feared stimuli are oncoming trucks, exposed power lines, guns, or strange men offering a ride.