INTRODUCTION: This chapter discusses the interaction of the prefrontal cortex, the ventral subiculum in the hippocampal region and the dopamine system. These interact in particular in the nucleus accumbens area of the basal ganglia. This is important with respect to their role in decision making and behavioural responses. Dopamine also helps to establish long-term potentiation (LTP) or long-term depression (LTD) in both the prefrontal and the subiculum. This two-way pull could be reflected in fluctuations in what external stimuli receive attention at any particular point in time. The interaction between the subiculum and the dopamine system suggests the influence of the limbic or emotional system on the decision making process in combination with rational processing in the prefrontal. The author considers that dopamine is not itself the reward. This is quite possible but it is unfortunate that he does not give us much guidance as to where the reward does arise. Dopamine it is suggested might provide the motivation or expectation of reward but not the reward itself. The reward is here suggested to come from something mysteriously referred to as ‘the system’ which is tantamount to saying that we don’t know where it happens or else that it comes from some undiscussed combination of the influences within the brain. This is possibly true but doesn’t need to be left in the air like this. The orbitofrontal, amygdala, hippocampal region, basal ganglia and the dopamine system are all interactive in the reward predicting and reward receiving system, in which subjective emotion is involved. Neuroscience needs to spend more time considering where in this processing conscious experience is involved. This is as likely to be in all or several areas as in only one. The orbitofrontal cortex is shown to represent predicted rewards, and its activity is shown to be calibrated according to the desirability of external stimuli. This would seem to be a prime candidate for the experiencing of subjective emotion. The basal ganglia look to act as a kind of mixer tap, where inputs from the orbitofrontal are blended with inputs from many other parts of the brain. Subjective emotion could be seen as a common neural currency in which to trade these different inputs. The role of dopamine is complicated and relationships are not one-to-one. It could be involved predicting or giving reward or in synaptic plasticity to change responses if stimuli are repeated.
The dopamine system is involved in both the control of emotional expression and cognitive processing. Decision making is argued to be modulated by the inputs of the dopamine system to the nucleus accumbens. Dopamine has also been shown to be involved with working memory in the prefrontal cortex. Its input into areas such as the amygdala is known to influence emotional learning, and it has a powerful influence on the ventral striatum including the nucleus accumbens. These areas are strongly connected to the cognitive areas of the brain and to areas involved in assessing expectation of reward or punishers. This brain region is not directly involved with cognitive processing, its connection give it an influence on cognition and on goal-directed behaviour. Neurons within the ventral striatum receive inputs from the amygdala, the hippocampus and the prefrontal. The ventral striatum has outputs that influence the dopamine system and also projects to the thalamus which projects onwards to the cortex. The loops running between the prefrontal and the limbic areas to the ventral striatum and back again are indicated to have important influences on reward assessment, attention and goal-directed behaviour.
The entorhinal cortex receives inputs to the hippocampal area from several brain regions, while output from the area comes from the ventral subilicum. Inputs to the hippocampus modulate other signals in terms of emotional value. At the stage of the ventral subilicum information has been expanded to include the emotional significance of the original signal. This emotional value is here described as ‘context’. Moreover information about memories can be transferred to the prefrontal in a manner that is also influenced by dopamine. The ventral subilicum appears to have a gating function, based on reinforcing prefrontal activity that is relevant to the context of processing and blocking inputs which are not. The ventral subilicum also projects to the ventral striatum. This area can be seen as a neural cross roads, receiving excitatory input from the prefrontal and the amygdala.
The prefrontal cortex has been demonstrated to be important with respect to the flexibility of behaviour. It can override stimuli that previously predicted reward, but have more recently failed to do so. The inputs of both the prefrontal cortex and the subiculum to the nucleus accumbens in the basal ganglia are modulated by dopamine. The prefrontal is highlighted as being important for behavioural flexibility. When actions do not produce the expected reward, the prefrontal can allow the selection of a new strategy. The prefrontal to activate to ignore a strongly reinforced behaviour that is no longer working.
The dopamine system is based in the midbrain. A relatively small number of neurons have very widespread connections in the brain, but its activity is controlled by the ventral subiculum. A pathway originating in the ventral subiculum determines the level of firing in the dopamine system. The interaction of the subiculum, the prefrontal and the dopamine system is suggested to play an important role in behavioural responses and decision making. The ventral striatum receives excitatory inputs from the subiculum and the prefrontal. The dopamine system acts on these inputs, reducing that from the prefrontal and strengthening that from the subiculum. The decision making process is argued to be driven in a reward-based manner, with the subject feeling the consequence of actions and being provided with motivation and drive. The modulation of dopamine works to reduce the glutamate based excitatory inputs from the prefrontal, but to enhance the inputs from the subiculum.
The dopamine system is based in the ventral tegmenta area of the midbrain. The dopamine system is seen to fire when there is a relevant stimulus such as the prediction of a reward. The ventral subiculum is suggested to help stimulate the dopamine neurons to fire. The subiculum will also tend to regulate the alertness of the dopamine system according to the background environment, with a low level of activation in a neutral environment but a high level in an environment with a higher propensity for risk or reward. The ventral subiculum determines how many dopamine neurons are set to fire, but the midbrain area triggers the actual firing in response to a predictions of a reward or punisher. Dopamine firing is also seen as drawing a subject’s attention towards the object that has triggered the firing.
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