Shall we be Frightened or not?

Alan Challoner MA MChS

You encounter a rabbit whilst walking along a path in the woods. Light reflected from the rabbit is picked up by your eyes. The signals are then transmitted through the visual system to your visual thalamus, and then to your visual cortex, where a sensory representation of the rabbit is created and held in a short-term visual object buffer. Connections from the visual cortex to the cortical long-term memory networks activate relevant memories (facts about rabbits stored in memory as well as memories about past experiences you may have had with rabbits). By way of connections between the long-term memory networks and the working memory system, activated long-term memories are integrated with the sensory representation of the stimulus in working memory, allowing you to be consciously aware that the object you are looking at is a rabbit.

A few strides later down the path and you come across a snake coiled up next to a log. Your eyes also pick up on this stimulus. Conscious representations are created in the same way as for the rabbit by the integration in working memory of short-term visual representations with information from long-term memory. However, in the case of the snake, in addition to being aware of the kind of animal you are looking at, long-term memory also informs you that this kind of animal can be dangerous and that you might be in danger.

According to cognitive appraisal theories, the processes described so far would constitute your assessment of the situation and should be enough to account for the fear that you are feeling as a result of encountering the snake. The difference between the working memory representation of the rabbit and the snake is that the latter includes information about the snake being dangerous. These cognitive representations and appraisals in working memory are not enough to turn the experience into a full-blown emotional experience. Something else is needed to turn cognitive appraisals into emotions, to turn experiences into emotional experiences. That something, of course, is the activation of the system built by evolution to deal with dangers; and that crucially involves the amygdala.

Many people, but not all, who encounter a snake in a situation such as the one described will have a full-blown emotional reaction that includes bodily responses and emotional feelings.1 This will only occur if the visual representation of the snake triggers the amygdala. A whole host of connections will then be activated. Activation of these is what makes the encounter with the snake an emotional experience, and the absence of activation is what prevents the encounter with the rabbit from being one. 2

Neurons in the area of the thalamus that project to the primary auditory cortex are narrowly tuned they are very particular to what they will respond. However, cells in the thalamic areas that project to the amygdala are less particular they respond to a much wider range of stimuli and are said to be broadly tuned. Music will sound the same to the amygdala by way of the thalamic projections but quite different by way of the cortical 1 Erdelyi, M H. The recovery of unconscious (inaccessible) memories: Laboratory studies of

hypermnesia. In The psychology of learning and motivation: Advances in research and theory, G. Bower, ed. (New York: Academic Press), pp. 95-127; 1984.

2 If, in your past, you have experienced rabbits in association with some trauma or stress, then the rabbit too could serve as a trigger stimulus that would turn on the amygdala and its outputs.

projections. So when two similar stimuli are used in a conditioning study, the thalamus will send the amygdala essentially the same information, regardless of which stimulus it is processing, but when the cortex processes the different stimuli it will send the amygdala different signals. If the cortex is damaged, the animal has only the direct thalamic pathway and thus the amygdala treats the two stimuli the same both elicit conditioned fear.

Although the thalamic system cannot make fine distinctions, it has an important advantage over the cortical input pathway to the amygdala. That advantage is time. In a rat it takes about twelve milliseconds (twelve one-thousandths of a second) for an acoustic stimulus to reach the amygdala through the thalamic pathway, and almost twice as long through the cortical pathway. The thalamic pathway is thus faster. It cannot tell the amygdala exactly what is there, but can provide a fast signal that warns that something dangerous may be there. It is a quick and dirty processing system.

Imagine walking in the woods. A crackling sound occurs. It goes straight to the amygdala through the thalamic pathway. The sound also goes from the thalamus to the cortex, which recognizes the sound to be a dry twig that snapped under the weight of your boot, or that of a rattlesnake shaking its tail. But by the time the cortex has figured this out, the amygdala is already starting to defend against the snake. The information received from the thalamus is unfiltered and biased toward evoking responses. The cortex's job is to prevent the inappropriate response rather than to produce the appropriate one. Alternatively, suppose there is a slender curved shape on the path. The curvature and slenderness reach the amygdala from the thalamus, whereas only the cortex distinguishes a coiled up snake from a curved stick. If it is a snake, the amygdala is ahead of the game. From the point of view of survival, it is better to respond to potentially dangerous events as if they were in fact the real thing than to fail to respond. The cost of treating a stick as a snake is less, in the long run, than the cost of treating a snake as a stick. (LeDoux, 1998, Idem)

So we can begin to see the outline of a fear reaction system. It involves parallel transmission to the amygdala from the sensory thalamus and sensory cortex. The subcortical pathways provide a crude image of the external world, whereas more detailed and accurate rep-resentations come from the cortex. While the pathway from the thalamus only involves one link; several links are required to activate the amygdala by way of the cortex. Since each link adds time, the thalamic pathway is faster. Interestingly, the thalamo-amygdala and

cortico-amygdala pathways converge in the lateral nucleus of the amygdala. In all likelihood, normally both pathways transmit signals to the lateral nucleus, which appears to play a pivotal role in coordinating the sensory processes that constitute the conditioned fear stimulus. Once the information has reached the lateral nucleus it can be distributed through the internal amygdala pathways to the central nucleus, which then unleashes the full repertoire of defensive reactions.3

3 Johnson-Laird, PN. The computer and the mind: An introduction to cognitive science

(Cambridge: Harvard University Press; 1988).