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Joseph Le DouxSynaptic Self Joseph Le Doux INTRODUCTION: This book is a good and readable account of a range of brain processes, notably the relationship between the working memory and executive activities of the prefrontal and the regions of the brain involved in emotional processing, the formation of memory and the synaptic plasticity and cell processes that underlie the latter. As is predictable with a neuroscience book of this kind, the attempt to extend the discussion to consciousness is not successful. Consciousness is viewed mainly as a rather circumscribed aspect of the working memory, but there is no attempt to suggest how it might arise, or what function it performs that could not be achieved by unconscious processing. Le Doux emphasises the plasticity of the brain. If the brain did not have plasticity, it would not be capable of being modified by experience. There are two main receptors for the excitatory neurotransmitter, glutamate, the AMPA receptor involved in normal synaptic transmission, and the NMDA receptor involved in synaptic plasticity The latter allows the cell to record which pre-synaptic sites were active when the post-synaptic site fired. This process is called long-term potentiation (LTP). This comes in two forms, early and late LTP. The former lasts only about an hour, while the latter is concerned with longer term memories. Calcium, referred to as a 'second messenger', in contrast to the 'first messenger' neurotransmitters, flows into the cell from the NMDA receptor to direct internal chemical reactions. These involve enzymes called protein kinases that render other proteins active by adding a phosphate group (phosphorylation). In early LTP, this process works to increase the number of AMPA receptors. Late or long lasting LTP, however, requires the creation of new proteins. Activation of particular kinases permits them to move inside the cell nucleus, and activate a gene transcription factor that allows the creation of new proteins that are then transferred to the synapse. There is a widespread assumption that LTP is involved in learning as well as memory. Another influence is the action of neurotrophins that diffuse backwards from the post-synaptic to the pre-synaptic site, where they influence the development of new synaptic connections. The parahippocampal area and the hippocampus together make up what is known as the medial temporal lobe memory system. Output from the sensory areas converges in the parahippocampal, before being transferred to the hippocampus. The parahippocampal integrates material from different modalities, and is described as a convergence zone. Vision, sound etc. are here put together into a global memory. More abstract concepts can also be formed in convergence zones. Convergence zones are few, except in the brains of primates, and they may be viewed as a marker of cognitive development in a species. The hippocampus is needed for initial memory storage, but over time, the longer term storage in the neocortex becomes independent of the hippocampus. Many researchers think that long-term memories are stored in the parts of the cortex where the initial sensory processing occured. Studies also support the hypothesis that consolidation of memories occurs during sleep, with the hippocampus feeding new memories to the cortex. There appears to be a link between the activity of the hippocampus and consciousness. Amnesiac patients with damage to the hippocampus can be conditioned to make a response, for instance to a tone that has previously been paired to a puff of air to the eye, but at the same time, they have no conscious memory of the conditioning process itself. There is an apparent distinction between the processing of conscious memories and of unconscious conditioning. On the basis of various studies of brain damaged patients, it has been determined that the frontal lobes are involved with executive functions, such as planning and control of behaviour, as well as with working memory. Le Doux emphasises the importance of working memory, sometimes referred to as short-term memory, which is involved in thinking and problem solving. The working memory, which can also be regarded as a workspace, can hold only a limited amount of material for a limited length of time, but has the important feature of being able to integrate different types of information and sensation. The frontal lobes are also involved with movement. In primates, there is a greater development of the prefrontal, which lies in front of the movement areas. The prefrontal is regarded as another convergence zone receiving inputs from the sensory cortex, such as the separate 'what' (temporal lobe) and 'where' (parietal lobe) pathways for vision and similarly from the auditory pathways, from the long-term memory and from the hippocampus. The prefrontal is responsible for integrating this information. Reciprocal pathways from the prefrontal back to the sensory cortex can focus attention on particular stimuli. The prefrontal is used in decision taking, including planning several steps ahead. This decision taking may have to be based on imperfect information. The prefrontal is thought to function as a series of interconnected circuits. The lateral prefrontal is particularly involved with working memory. The anterior cingulate is also involved with working memory. These two areas are closely connected, and form part of the frontal lobe connectional area involved in decision taking and movement control. The ventral prefrontal and especially the orbital prefrontal area are also involved with working memory and especially with emotional information. P. In discussing emotion, this is seen as the process by which the brain assesses the value of stimuli. Signals with emotional content may further create involuntary bodily responses, such as changes in heart rate and blood pressure. Le Doux looks at emotion particularly with respect to fear, because it is the best researched emotion. The amygdala, functioning as a fear centre in the brain, can serve to amplify emotions by triggering responses in the body that send hormones back into the brain. The amygdala can also modulate the flow of neuromodulators, such as dopamine, from the brain stem to the working memory area. The amygdala also has a role in modulating the formation of explicit memories, which may be more vivid as a result. However, the amygdala interacts with the medial prefrontal cortex and notably with the anterior cingulate and orbital cortex, and these areas have some power to regulate the amygdala and its fear reactions. The relationship between these prefrontal areas and the amygdala is reciprocal. Emotional arousal is seen as being important in the coordination of brain states, including decision making, and can also modulate sensory processing. The orbital region is connected with the anterior cingulate, and receives information from both the amygdala and the hippocampus. Damage to the orbital cortex results in impairment of social responses and decision taking. The anterior cingulate and orbital cortex are closely connected with one another and with the lateral prefrontal, and are involved with processing both sensory inputs and the short-term working memory. The anterior cingulate and orbital cortex thus form a type of junction between emotional, sensory and memory inputs, on the one hand, and the short-term working memory on the other. Patients with damage to the medial and ventral prefrontal areas show impaired decision taking in emotional situations. Correspondingly, damage to the amygdala impairs the ability to judge emotions in faces and voices. The medial prefrontal is suggested to be an interface between the brains cognitive and emotional systems. In looking at the brain's reward and pleasure system, the release of dopamine is now seen as more important relative to the anticipation of pleasure, rather than the pleasure itself. The focus is instead on an area known as the nuclear accumbens, which is located in front of the amygdala, and receives sensory inputs via the amygdala. The hippocampus has connections both to the amygdala and the accumbens. The anterior cingulate is also connected to the accumbens. The orbital cortex is also stressed to be important in the processing of motivational, or reward and punishment, information. The orbital, the anterior cingulate, the amygdala and the hippocampus all appear to be closely involved in emotional and motivational processing. The anterior cingulate and the orbitofrontal area in particular are viewed by Le Doux as a single system for integrating emotional and cognitive information and relating it to the working memory. Le Doux discusses the role of synchronous activity in the brain. This activity is suggested to have a double role, firstly, in binding together processes in different modalities, such as vision and sound, in response to an immediate stimulus, and secondly, in altering synapses in cells in two or more modalities, so that they will respond together, if the same stimulus is presented again. It is emphasised that this suggestion is hypothetical rather than evidence based at the moment. |
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