Investigation of Fear Conditioning

Investigation of Fear Conditioning
To understand what fear conditioning is, we must first understand what
the term conditioning means. Classical conditioning is a type of
learning, in which a neutral stimulus, which initially elicits no
response in a subject (conditioned stimulus), is paired with a
stimulus that provokes a strong response (unconditioned stimulus). If
the conditioned and unconditioned stimuli are presented consecutively
and repeatedly, the subject will associate the first stimulus with the
second, and will react to the first stimulus alone. Fear conditioning
is simply classical conditioning where the unconditioned stimulus
evokes fear and thus a fear response is elicited on presentation of
the conditioned stimulus. It is one of the mechanisms of survival, so
that dangerous situations can be avoided after learning that they have
unpleasant effects.Pavlov?s experiment in 1901 with dogs is perhaps the most famous of classical conditioning experiments. He found that when dogs were presented with food, the dog salivates. The sound of a bell evoked no similar response. However, after the bell is paired with the food on several trials, the bell alone will generate salivation. The bell is now a conditioned stimulus, and salivation to it is a conditioned response. The dog had been conditioned to respond to the bell. To be able to measure whether the subject is responding fearfully, changes in the autonomic nervous system and patterns of movement are usually examined. LeDoux in 1994 demonstrated fear conditioning by pairing a sound, with an electrical shock to the foot of a rat. The blood pressure of the rat and change in movement patterns was monitored and a rise in blood pressure and cessation or reduction in movement was taken to be a response to fear. Initially, the rat heard the sound alone. This had little effect on the blood pressure of the rat, or its movement patterns. The next set of trials involved playing the sound to the rat, and immediately afterwards giving the rat an electric shock. After several trials the sound alone was enough to trigger a physical reaction in the rat, a raised blood pressure and cessation of movement for several seconds. The rat had now been fear conditioned. Animals are a convenient way of studying fear conditioning, as parts of the brain in animals can be removed or lesioned and the results looked at and compared with the results of a normal rat. This is a good method of testing theories as to which part of the brain is responsible, for example if you believe one part of the brain is responsible, and then remove and it is then impossible to fear condition a subject it helps support the theory. The study of human patients with damage to the brain is less useful in that the damage is less specific when damage to the brain is caused, and of course it is not possible to lesion parts of human subject?s brains for an experiment. Parts of the brain that are involved in the interpretation of fear are the amygdala and cerebral aqueduct. The amygdala has been shown to have a role in the control of both innate and acquired emotional responses, including fear. It is situated within the anterior parts of the temporal lobes, and it consists of several groups of nuclei that each have different inputs and outputs. It is part of the limbic system of the brain, which has been traditionally associated with emotion. I am going to look at the role of the amygdala by considering its anatomical connections, activity of parts of the amygdala in fearful situations and effects when there is lesioning of the amygdala, as well as the effects when the amygdala is stimulated. The amygdala consists of approximately twelve regions (each with subregions), with 4 major regions ? the medial, lateral, central and basal nuclei. The medial nucleus receives sensory information and relays it to basal forebrain and hypothalamus; the lateral nucleus which receives sensory information and has multiple projections (nerves projecting) to the cortex, hypothalamus, and hippocampus as well as projections to the central nucleus. The central nucleus projects to the lower brain structures that control emotional expression. This is the most important part of the amygdala for emotional expression related to negative and unpleasant stimuli. Learning and reacting to the stimuli that cause fear and warn of danger involve neural impulses sending information to the amygdala, which then causes the body to respond in various ways. The lateral nucleus plays an important role in fear conditioning. Unpleasant stimuli cause neuronal response across a dual pathway ? both lead from sensory organs to the thalamus. The first pathway leading from organs such as the ear to the thalamus. The nerve impulses are then sent to the section of the sensory cortex concerned with the relevant signals in this case the auditory part. This sensory cortex analyses the inputs and then sends impulses to the amygdala. The second pathway is simply from the thalamus directly to the amygdala. This is a much faster way of getting information to the amygdala, but it sends less detailed information about the stimulus. When the impulses reach the amygdala, signalling a threat to the subject, it sends out signals that cause a series of reactions in the organism. This includes reactions within the autonomic nervous system such as an increase in heart rate and blood pressure, within the endocrine system causing the release of stress hormones as well as behavioural changes. The first pathway prepares the body for immediate reaction to a stimulus, which could be invaluable in survival even if it is not very precise. As LeDoux said "You?re better off mistaking a stick for a snake than a snake for a stick.? The second pathway is more precise, and can then analyse whether there is an actual threat. This means that if what has been perceived as a threat is in fact harmless, the subject will not continue to be afraid of it. The central nucleus plays a key role in the emotional response to unpleasant stimuli. It has been shown that activity in the central nucleus is greatly increased when the subject is presented with unpleasant/aversive stimuli. If this central nucleus is removed then the capacity for fear conditioning is abolished. Animals will no longer show signs of fear when presented with a stimulus that has previously been paired with an aversive stimulus (investigated by LeDoux in 1995). Lesions to the central nucleus also abolish other types of behaviour valuable to survival for example animals with such lesions are more confident and less aware of danger than they were before (and so would be more susceptible to predators). Experiments in which this nucleus has been stimulated with electricity or excitatory chemicals such as certain amino acids have shown that the animal will exhibit behavioural (e.g cessation of movement) and physiological signs of fear (e.g a raised heart beat) as shown by experiments such as that if Davis in 1992. Bilateral lesions of the basolateral complex of the amygdala prevent a subject from being able to be fear conditioned or able to express previous fear responses. This is true of both animals and humans. In this way the amygdala is not only important but essential to fear conditioning ? conditioning cannot take place independently of the amygdala. In human patients with amygdala lesions, autonomic responses are not impaired, but the subject is no longer able to associate the pairs of stimuli and learn to avoid a stimulus that causes them to react fearfully physiologically. Part of the cerebral aqueduct, the periaqueductal gray is believed also to have a role in all fear responses ? controlling protection as well as defensive mechanisms. Electrical stimulation of parts of the amygdala suppresses the effects of the periaqueductal gray and causes the subject to freeze. In this way freezing, or total cessation of movement is a good way of measuring whether a subject has been fear conditioned. In 1988 LeDoux et al lesioned two parts of the brain (animal not human) that the central nucleus projected to ? the lateral hypothalamus and the caudal aqueductal grey. I have stated that amygdala stimulation causes the subject to freeze, and conversely a lesion of the caudal aqueductal grey interfered with freezing as a response to fear. A change in a subject?s blood pressure as a response to fear is also interfered with by lesioning of the lateral hypothalamus. These two parts of the brain cause the response to fear behaviourally and physiologically when a subject has been fear conditioned, and are under the control of the amygdala. Chemicals such as certain tranquillising drugs (e.g opiates) decrease the expression of conditioned responses and the ability to acquire new such responses. Studies into the effect of electrical stimulation on the brains of people have been made, and it was found that stimulation of certain parts of the brain (for example the hypothalamus) result in a physiological response consistent with a fear response but no awareness of being afraid. Only when the amygdala was stimulated did subjects report feelings of fear, (studies concerned with this were White 1940; Halgren et al 1978; Gloor et al 1982) Studies such as that of LaBar et al in 1995 showed that a lesion of the amygdala in humans has a similar effect as that of lesions on a rat ? the reduction or elimination of the ability to acquire new conditioned responses. Imaging is a more recent technique that allows the activity of parts of the brain to be measure. This has been used in some studies in relation to the amygdala, and it was found that the amygdala is involved in emotional responses (fear or otherwise). An example of this is the experiment of Cahill et al when subjects watched both emotionally neutral and emotionally arousing films (for example films about death). It was found that when the subjects were asked to recall the the neutral films activity in the right amygdala stayed the same but increased for the emotionally arousing films. The films that caused the highest activity levels in this part of the amygdala were the films that the subjects first remembered. Words that are associated with fearful situations also cause increased activity in the amygdala for example ?torture? and ?murder? would elicit such a response as opposed to ?grape? and walking? which would not. The relationship of the amygdala and emotion makes it unsurprising that it has a key role in fear conditioning. In conclusion the amygdala plays a key role in fear conditioning, and if it is lesioned or removed then such conditioning cannot take place. The different functions of the two pathways sending impulses to the amygdala play different roles in animals, but both are necessary for survival. Quick reaction to a potential threat must be possible, but also there is a need for a more detailed analysis of the potential threat in order to establish if whether or not danger is present.

Investigation of Fear Conditioning 9 of 10 on the basis of 1208 Review.