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The amygdalaThe Human Amygdala Eds: Paul Whalen & Elizabeth Phelps This book gives coverage of the amygdala, with regard to a wide spread of neuroscience, medicine and psychology. We summarise those chapters that are more relevant to consciousness. In that we are conscious of emotions, and recent research has led to an increased emphasis on the function of emotion in determining brain processing and behaviour, the best possible understanding of emotions seems relevant to any study of consciousness. Chapter 1 Neuroanatomy of the Primate Amygdala Jennifer Freese & David Amaral The amygdaloid complex is located in the medial temporal lobe just in front of the hippocampal region. Studies in recent decades show that the amygdala has a network of connections, both inward and outward, with many parts of the brain, beyond its traditionally recognised connections with the hypothalamus and the brain stem. Its nuclei can influence diverse regions including the spinal cord, frontal, cingulate, temporal and occipital cortices. Projections to the amygdala from the frontal lobes come mainly from the orbitofrontal and parts of the media prefrontal. These areas are related to social activity. The amygdala has substantial connections with the temporal lobe, and reciprocal connections with a variety of subcortical regions. The lateral nucleus of the amygdala, occupies a larger proportion of the total amygdala in primates including human than in other mammals. This is related to this nucleus receiving many inputs from the neocortex. The upper parts of the lateral nucleus receive inputs from the sensory cortex. The primate amygdala is involved with all types of sensory information, but is most heavily influenced by the visual. The largest proportion of visual input comes from the ventral 'What?' pathway indicating that the amygdala is to a good extent a danger detector that can orchestrate a whole body response. Chapter 2 The Human Amygdala: Insights from Other Animals Joseph LeDoux & Daniella Schiller The word amygdala denotes an almond shaped structure in the medial temporal lobe, although the almond shaped area is now recognised as a sub-division of the full amygdala. The evolutionary newer part of the amygdala is associated with the cortex. Output connections from the central amygdala to the brainstem are involved in controlling emotional reactions, while outputs from the basal amygdala to the striatum are involved in controlling physical actions. Most of the inputs to the amygdala involve excitatory pathways that use glutamate as a neurotransmitter. The amygdala is subject to three categories of neuromodulators. Peptides are released locally from axons within the amygdala; amine transmitters are transmitted from distant parts of the brain; hormones reach the amygdala via the bloodstream. The amines, noradrenalin, dopamine serotonin and acetylcholine are released in the amygdala and influence how excitatory and inhibitory neurons interact. Output connections of the amygdala's central nucleus terminate on modulatory networks in the brainstem. Activation of the amygdala leads to the release of modulatory chemicals throughout the forebrain. The various neuromodulators have more diffuse effects than excitatory and inhibitory transmitters, which mostly act at specific synapses. The positioning of receptors in the amygdala determines which areas respond to which neuromodulators. Damage to the amygdala produces changes in fear reactivity, feeding and sexual behaviour. The amygdala is also implicated in reward learning, motivation and drug addiction. It has been implicated in aggressive, maternal, sexual and eating/drinking behaviour. It is also involved in the modulation of various cognitive functions such as attention, perception and memory. The amygdala has extensive connectivity with areas of the brain involved in cognitive functions. Once the amygdala has detected an emotional stimulus, it can also influence the processing of that stimulus. The amygdala also projects to association areas in the temporal lobe. Cognitive functions are able to influence the amygdala, and prefrontal executive areas have some influence. Cognitive activity such as reappraisal can alter activity in the amygdala. The amygdala can also influence cortical functions indirectly. When the amygdala detects something emotionally significant, it directs the release of neuromodulators, such as noradrenaline, dopamine and serotonin that influence cognitive processing in cortical areas. Amygdala activity also releases hormones into the bloodstream that later feed back from the body to the brain. Activity in the basal amygdala influences hippocampal processing of memory. Chapter 4 Amygdala Function in Positive Reinforcement Elizabeth Murray, Alicia Izquierdo & Ludise Malkova The amygdala can link neutral sensory cues such a light or sound signals with an aversive experience such as an electrical shock. The amygdala is also involved in positive reinforcement. The authors view the amygdala as endowing cognitive constructs, such as words, rules and concepts with emotional valence. This works partly by linking neutral representations with innate response mechanisms. The amygdala is essential for associations that are needed for survival, such as food seeking, reproductive, parental and defensive behaviours. The amygdala appears to link sensory inputs with autonomic reflexes. It can assign positive or negative values to neural representations of sensory inputs. The authors point to the connection between the orbitofrontal and the amygdala, concerning the relative value of objects and actions stored in the orbitofrontal. This accounts for the emotionally laden nature of particular words or images. One function of the amygdala is to assign value to object representations, and this could also extend to abstract concepts. Damage to the amygdala can lead to deficits in recognising fear or trustworthiness. Amygdala based values may guide social interactions. The preference for particular objects can be unconsciously acquired via the amygdala. Patients with amygdala damage do not acquire preferences in this way. Chapter 8 The Human Amygdala and Memory Stephan Hamann Research points to emotion as being important in influencing the strength and subjective quality of memories. The emotional quality of memories is seen as adaptive, because positive or aversive memories of events have more influence on behaviour than neutral memories. The evidence suggests that emotionally arousing events are better remembered than neutral events. The brain mechanism involved is the amygdala located within the temporal lobe and close to the hippocampus. There are plentiful connections between the amygdala and the hippocampus. The amygdala is seen as facilitating the encoding of emotional memories in other parts of the brain, rather than storing the memories itself. In emotional events, the attention is regarded as narrowing or focusing on the central emotional issues. Emotional arousal involves increased activity in the amygdala, leading to increased activity in the medial temporal lobe, including the hippocampus, entorhinal cortex and related structures. Emotional arousal has been demonstrated to increase synchrony between neuronal firing in the amygdala and the hippocampus at the theta frequency. New episodic memories are thought to undergo a process of consolidation that converts them into a more permanent form. Hormonal influences, for instance those related to stress can play a role in the consolidation process. The author claims that there is growing evidence for consolidation of emotional memories taking place during sleep, particularly REM sleep. (Holland & Lewis, 2007: Wagner et al, 2006). Cognitive processes, such as thinking about an event, can also reinforce emotional memories. The hippocampus is seen as having a special role in the recollection of memories, although they are stored more generally throughout much of the cortex. Activity in the ventral striatum, a brain region involved in reward, is correlated with memories of positive stimuli. Many of the same structures that are involved during emotional memory coding are also involved in retrieval, but in this case they seemed to be more involved with the subjective experience than the historical accuracy of the memories. The amygdala's interaction with both the medial temporal lobe regions and the prefrontal cortex is enhanced during the retrieval of emotional memories. Retrieval of emotional memories involves enhanced bidirectional activity between the amygdala and the hippocampus. The amygdala facilitates access to earlier subjective or emotional states, and preserves current emotional states for future access. It is also involved in the imagining of possible future emotional events. For non-declaritive or non-conscious memories traces are stored in the amygdala, but for declarative or conscious memories of incidents or facts the amygdala only modulates the activity of the hippocampus and related structures. Chapter 10 The Role of the Human Amygdala in Perception and Attention Patrick Vuilleumier The amygdala is associated with the motivational value of environmental events. The amygdala projects to many output systems such as the autonomic, motor, memory and cognitive systems. It is said to resemble a Grand Central Station of the brain. Emotions are here seen as the appraisal of and response to events. The amygdala is positioned to modulate cortical pathways involved in perception and attention, which may in turn affect memory and cognitive functions. This can lead to an influence on goal directed behaviour. Attention is seen as a selection mechanism for deciding what to respond to in the environment. Unattended events do not usually enter consciousness. Emotion appears to have a parallel selection process in deciding what should be processed. The authors states that there is evidence that attention is directed towards emotionally significant stimuli. This is regarded as having evolutionary advantages. Attention and emotion are therefore not entirely separate systems, although some of the mechanisms involved may be distinct. Visual scenes with emotional content are shown to produce greater activation in the occipital cortex than neutral scenes. The same applies to the auditory cortex relative to sounds. Studies show increased activation correlated with amygdala responses. The amygdala is not just responsive to fear cues, but also to positive cues and to ambiguous cues. Emotional as opposed to neutral targets are more quickly picked out from among distracters. This is related to the amygdala function. The amygdala projects to widespread regions in the cortex, including all stages of the perceptual pathways. The basal forebrain receives dense inputs from the amygdala, and in turn projects to the frontal, parietal and sensory cortices, where neural responses can be amplified or sustained. Interactions between attention and emotion may involve projections from the amygdala to the orbitofrontal and cingulate cortices. Emotional signals from the amygdala to the cortices may act in parallel to other top-down signals to the cortices, particularly those due to voluntary attention that are under frontal and parietal control. Emotional information may be monitored, even when it is outside the current focus of attention. This is supported by a number of studies. Attention and emotion are here regarded as interactive. Emotional influences are seen to arise outside voluntary control and awareness, but may be either amplified or reduced in the amygdala that may in turn be influenced by inputs from the orbitofrontal. On the other hand, activity in the orbitofrontal and cingulate may be increased, when emotional cues have to be overridden. Emotional modulation may also produce lasting changes in perceptual pathways. Thus emotional processing may not just appraise sensory inputs, but may regulate perceptions and organise attention. |
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