Neurobiological theories of anxiety
Theories of anxiety
When it comes to detecting and responding to danger, the [vertebrate] brain just hasn’t changed much. In some ways we are emotional lizards.
What happens in our brains when we feel anxiety? Until the relatively recent development of neuroimaging technology, which allows biochemical activity in the brain to be recorded and pictured, scientists could only conjecture. But remarkable advances have been made in recent years, as we’ll see in a moment.
First, however, a word of caution. Neuroscience has come a long way in a short space of time. But even were we to understand exactly how our brains function — and we are still a very long way indeed from that end point — we wouldn’t thereby possess a complete explanation for our experiences. For example, though scientists can now be much more certain than ever about which parts of the brain are involved in anxiety, it is understood that no emotion can be reduced merely to a set of brain events and structures. There are always other levels of explanation, including the behavioural and cognitive aspects we’ve discussed already in this chapter.
The way in which such levels work has been nicely captured by the neurobiologist Steven Rose:
The language of mind and consciousness relates to the language of brains and synapses as English does to Italian; one may translate into the other, though always with some loss of cultural resonance. But we do not have to assign primacy to either.
It’s the same with anxiety; scientists approach the issue from different perspectives, but none of those perspectives has priority and all are interrelated. The best theories join up the different levels, and cognitive neuroscience has begun to do that, as we’ll now see.
Long before the advent of neuroimaging, scientists had suspected that the brain’s limbic system plays a major role in the production of emotions. The limbic system in humans closely resembles that found in the first mammals around two hundred million years ago. It is part of the forebrain, a relatively recent part of the brain in evolutionary terms, and is arranged in an approximate circle around the much more ancient brainstem (’limbic’ is derived from the Latin for ’border’). Its job is to make a rapid and pre-conscious appraisal of a situation in order to help determine which emotion (and therefore reaction) is appropriate.
Also located within the forebrain are two other key components of our emotional system. The frontal lobes of the cerebral cortex lie directly behind the eyes, and handle many of the tasks we tend to regard as quintessentially human, such as planning, decision-making, language, and conscious thought. It’s the frontal lobes that consciously think through and regulate our emotional responses.
3. Joseph LeDoux (b. 1949) is a US neuroscientist and Director of New York University’s Center for Neural Research. LeDoux’s ground-breaking research has highlighted the central role played by the brain’s amygdala in the experience of anxiety and other emotions. LeDoux is also vocalist and guitarist with The Amygdaloids, a rock band who specialize in ’songs about love and life peppered with insights drawn from research about mind and brain and mental disorders’
In this, the frontal lobes are assisted by the hippocampus, which helps form and store contextual memories — vital benchmarks as the frontal lobes figure out how best to react in a given situation.
Joseph LeDoux has been foremost in identifying one particular region of the limbic system as the brain’s ’emotional computer’, and as especially important in relation to fear and anxiety. That region is the amygdala, two small pieces of tissue shaped, in the view of early scientists, like almond seeds (amygdala is the Latin for ’almond seed’). The amygdala seems to be responsible for fear reactions in all species that have one, including reptiles and birds as well as mammals. It houses a store of unconscious fear memories, meaning that we can become anxious without knowing why. And it is extremely well connected to other parts of the brain. LeDoux has written:
The amygdala is like the hub of a wheel. It receives low-level inputs from sensory-specific regions of the thalamus [another area of the forebrain], higher level information from sensory-specific [areas of the cerebral] cortex, and still higher level (sensory independent) information about the general situation from the hippocampal formation. Through such connections, the amygdala is able to process the emotional significance of individual stimuli as well as complex situations. The amygdala is, in essence, involved in the appraisal of emotional meaning.
The amygdala’s connections don’t end there. Through the hypothalamus, it can influence the basic processes that comprise the autonomic nervous system (for example, breathing, blood pressure, and body temperature). As we saw in Chapter 1, changes to the autonomic nervous system when we’re anxious can lead to a wide range of physical effects including elevated heart rate, dilated pupils, and altered breathing. Prefrontal cortex
4. The brain, showing the location of the amygdala
The amygdala is able to make an appraisal of a potentially threatening situation extremely rapidly — so rapidly, in fact, that we may not realize why we’re suddenly feeling afraid. LeDoux has suggested that the amygdala offers a ’low road’ to fear responses, supplying a ’quick and dirty’ reaction to events that is designed to save our life first and ask questions later. The ’high road’, by contrast, involves sensory information being processed by the frontal lobes (the part of the brain responsible for thinking things through) before it reaches the amygdala. The high road is more accurate, but slower. As you might imagine, both routes have their advantages and disadvantages.
Important though the amygdala seems to be, we shouldn’t forget that anxiety — just like any other emotion — is the result of an extremely complex process involving multiple regions of the brain. As we’ve mentioned, these regions include the frontal lobes and the hippocampus; also involved is the insula, a part of the cerebral cortex that helps us become aware of internal feelings, and several neurochemicals. Among the most significant of these neurochemicals are:
• Corticotropin-releasing hormone (CRH), which is triggered when the amygdala detects danger and in turn sparks the release of stress hormones to ensure that we’re ready for action in the face of danger; and
• Gamma aminobutyric acid (GABA), which calms us down when we’re anxious.
Given that anxiety is the result of a system rather than one element, what happens when that system malfunctions? LeDoux and others have speculated that people with anxiety disorders may possess:
• An overactive amygdala, and/or:
• insufficiently active frontal lobes, and/or:
• a hippocampus that doesn’t pinpoint exactly which elements in a situation on the basis of past experience signal danger, meaning that they may become anxious unnecessarily.
The amygdala, as we’ve seen, is a kind of rapid-response unit, triggering ’just in case’ fear reactions that are then appraised by the more deliberative areas of the brain. But if the frontal lobes, for example, can’t make themselves heard over the noise emanating from the amygdala, we’re likely to experience unnecessary anxiety over what are essentially false alarms.
There’s evidence that persistent anxiety (through the effects of stress hormones) can alter the way in which the brain functions, for example, by impairing short-term memory or even shrinking the size of the hippocampus. These effects are usually reversible, but in the long term they can become permanent.
In the next chapter, we’ll look at two more perspectives on anxiety. How much of a role do life experiences play in making us vulnerable to anxiety, and how significant are genetic factors?