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Flexibility in the brainDynamic coordination in brain and mind William Phillips, Christoph von der Malsburg & Wolf Singer In:- Dynamic Coordination in the Brain MIT Press (2010) In their preface the authors admit that they are still in the dark as to how the brain flexibly applies knowledge and situation awareness to the achievement of goals, although dynamic coordination of different and widely distributed parts of the brain by means of neural synchronies is viewed as having a role in this. Dynamic coordination is seen as dealing with the unpredictable aspects of the external world. The unpredictable nature of the external world favours organisms with brains that are flexible enough to deal with the novel. This means that any type of neural code must be reliable, but must also be able to sometimes code for different things in differing situations. Neural activity has a wide spatial distribution across the brain, so dynamic coordination is needed to produce responses to novel situations. Dynamic coordination cannot be prespecified because the circumstances that the system has to respond to are unknown until they arise. However, even novel patterns of coordination are built up from familiar components. Neural synchronies could have two messages. Firstly that a particular feature was present, and secondly that they were communicating with other parts of the brain to produce an overall representation. It is suggested that there might be an interactive relationship between signal correlations changing synaptic strength, and synaptic strengths modulating signal correlations. The authors think that the coordination dynamics are distributed across the brain rather than being an executive function of the prefrontal. They also suggest that there may be dynamic coordination within the prefrontal cortex made necessary by the different functions of different parts of the prefrontal. Ambiguities in signals from the external world can be reduced by using the broader context of the environment from which the signals come. Dynamic grouping into subsets may occur as a result of conscious attention, but can also be unconscious. Cognitive functions require flexibility in communication between brain regions . NMDA receptors are suggested to mediate coordination within the cognitive system. These receptors have a large and immediate impact on ongoing activity. They amplify what is relevant in the current context, and suppress what is not relevant. It is suggested that their role in learning might be secondary to their role in processing, because synaptic changes are a record of amplified patterns of neural activity. The authors discuss the role of interneurons and GABA receptors. These are important in generating and coordinating rhythmic activities. These are seen as acting in conjunction with NMDA receptors. Pyramidal cells receive inhibitory input from interneurons that temporarily prevents their spiking, after which there is a recovery phase. Synchronisation of the inhibitory interneurons can in its turn synchronise the phase of recovery. This phase of synchronised disinhibition is thought to play a major role in attention and perception. This process is particularly effective at gamma frequencies. In terms of neural assemblies the rapid formation, change and dissolution of assemblies is stressed. The authors make an analogy with Bayesian techniques. The feed forward pyramidal neurons transmit the data to be interpreted, while other neurons carry information about probabilities that disambiguate perceptions. The authors think that a Bayesian approach matches the distinction between initial input and modulatory inputs. The latter may be seen as representing prior information. Synchronisation of neuronal spiking is seen as being important in this process. It is suggested that the frequency of spikes and the synchronisation of the spiking of cells could act in a complimentary manner. Studies suggest that synchronisation is important in the segregation of an object or figure from its background. Both synchrony and the higher frequency beta and gamma rhythms have been connected to learning, attention and consciousness. Conscious perception of objects is assisted by attention, and synchrony is suggested to play a role in attention, with synchronised disinhibition playing a role in modulating the context that is to be dealt with. Context appears to be important in reducing the ambiguity of a local view, such as whether for instance an object is a tent or a book with its central ridge uppermost. Thus in object recognition, the process of recognition involves not only the features of the object but its relationships in time and space, and its relation to stored patterns. One central suggestion in this book is that there is a coordination of transmitting and receptivity between different brain regions. In discussing executive functions the consensus of the authors is that cognitive groupings are important in coordinating other activities and are themselves a function of dynamic components. |
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