Neuroanatomy Of Fear

Discuss the role played by defensive conditioning in our understanding of the neuroanatomy of fear in animals.

Although animals have some innate, unconditioned fears (LeDoux, 1996), it isdifficult to observe these as we cannot tell what the specific fear stimulus is when they are showing signs of anxiety. For this reason, a learned fear must be created in the animal in order to study thebrain areas responsible for fear. To create a learned fear, defensive conditioning is used. This technique usually involves pairing a somatosensory unconditioned stimulus, such as pain, with an auditoryneutral stimulus, such as a tone, which then becomes a conditioned stimulus. For the animal, the tone has a weak connection with fear, but after being repeatedly presented at the same time as a painfulstimulus which has a strong connection with fear, the connection between the tone and fear is strengthened. The animal then displays fear responses to the conditioned stimuli, the tone.
Theexistence of this conditioned fear then allows for studies into its neural correlates. The assumed pathway of the auditory information (the tone) would be from the cochlea to the auditory midbrain, to theauditory thalamus and finally to the auditory cortex. However, lesion studies in rats have shown that defensive conditioning still occurs when the auditory cortex is removed, but not when the auditorythalamus or auditory midbrain are removed (LeDoux, 1994). This demonstrates that fear conditioning occurs at a subcortical level, before any information is processed by the cortex. Further evidence ofthis comes from studies of cortically blind humans, who can still be conditioned to fear a visual stimulus without making use of their visual cortex (Zald, 2003).
With no cortical processinginvolved, the auditory thalamus must project to other subcortical structures. The Anterograde Tracing WGA-HRP technique had demonstrated that information from the auditory thalamus travels along axons...