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Neuronal synchronisation and consciousnessNeuronal Synchronisation and Consciousness Lucia Melloni & Wolf Singer, Max Planck Institute In:- New Horizons in the Neuroscience of Consciousness – Eds. Elaine Perry, Daniel Collerton, Fiona LeBeau & Heather Ashton INTRODUCTION: Lucia Melloni and Wolf Singer discuss studies that demonstrate that conscious percepts produce different types of brain activity from unconscious percepts. In particular consciousness is demonstrated to produce long-range synchronisation of gamma oscillations in widely separated neural assemblies. What is not mentioned in this chapter is the extent to which these studies undermine some 20th century views of consciousness. The argument that consciousness is just a brain state now comes up against the question as to which aspect of brain functioning it is the same as, and what is special about this as opposed to other brain states. These findings similarly put greater demands on functionalism, which specifies that any system that does what the brain does will be conscious. This was based on the idea that the brain did little more than a conventional computer, whereas functionalism now has to embrace a mechanism not just for computing as in unconscious brain processing, but a mechanism for the additional conscious processing demonstrated here. It is suggested in this chapter that the synchronisation of widely distributed neuronal activity meets some of the requirements for explaining how conscious experience arises in the brain. Synchrony is proposed to be at the least an important correlate of consciousness. However, the crucial distinction between an occurrence that is correlated with conscious experience, and the actual description of some process that is causal of consciousness is admitted by the authors. The unity of consciousness is one of its notable properties, but in contrast to this the brain comprises a number of specialised although connected processing areas. Only a small part of the brain's processing is conscious suggesting the existence of a gating process for access to consciousness. In order for consciousness to become unified it has to overcome the problem of being represented in different modalities. It is generally agreed that there is no single central processing area, and also that much of the brain supports both conscious and unconscious processing. A great deal of effort seems to have been wasted in consciousness studies under the aegis of Dennett and other in decrying the possibility of such a centre, as if this negating this would of itself somehow solve the problem of consciousness. The lack of a single centre seems to have been obvious to most researchers for a good time, and the constructive thing is to move on and look for what it is that does create consciousness. As a constructive alternative to the homunculus and the Cartesian theatre of Dennett, recent neuroscience has suggested that the processing of spatially separated neuronal assemblies is bound together by signalling between them. Neurons are synchronised into coherent assemblies, and these assemblies signal the presence or absence of particular features in them to other neural assemblies. This process is suggested to give rise to a distributed representation of an object. Neuronal assemblies are self-organising and form and dissolve rapidly, which could account for the easy shifting of consciousness from one focus to another.Synchronisation also allows better control of interactions between neurons. Excitatory inputs are effective if they arrive at the depolarising slope of an oscillation cycle and ineffective at other times. This means that groups of neurons that oscillate in synchrony will be able to signal to one another, and groups that are out of synchrony will be ignored. This mechanism can function both within neural assemblies or between separated assemblies. The frequency and phase of oscillation can alter so as to influence signalling. Studies suggest that local processing is unconscious, whereas large scale activity such as reciprocal signalling between separate neural assemblies is a correlate of consciousness. This is argued to be a so-called 'small worlds' system, where there is a coexistence between local and long range networks. In the brain it is suggested that the local networks are between neurons only a few hundred micrometers apart within layers of the cortex, while the long distance networks run mainly through the white matter and link spatially separated areas of the cortex. It is these latter that can establish global coordination that is related to consciousness. The authors suggest that masking is a good way of studying consciousness, because this allows the same stimuli to be either conscious or unconscious. In a study run by the authors words could be perceived in some trials but not in others (1. Melloni et al, 2007). Local synchronisation was similar in both cases, but with consciously perceived words there was a burst of long distance gamma synchrony between the occipital, parietal and frontal cortices. Also subsequent to this burst there was activity that could have indicated a transfer of information to working memory, while an increase in frontal theta operations may have indicated material being held in working memory. Words processed at the unconscious level could lead to increase in power in the gamma frequency range, but only conscious stimuli produced increases in long distance synchronisation. Long distance synchronisation, plus possibly the theta oscillation look to be a requirement for consciousness. In another study long distance synchronisation in the beta as well as the gamma range was observed. Recent studies suggest a nesting of different frequencies of theta and gamma oscillations where there is conscious processing. Further to this some neurons in the medial temporal lobe respond only to conscious perceptions. This brain region is linked to the hippocampus and the formation of memory. In general the researchers have difficulty distinguishing between actual consciousness and the consequences of consciousness within the brain. The authors suggest the need for further research into the individual effects that appear to distinguish conscious from unconscious processing. P. What is not mentioned in this chapter is the extent to which these studies undermine some 20th century views of consciousness. The argument that consciousness is just a brain state now comes up against the question of which aspect of brain functioning it is the same as, and what is special about this as opposed to other brain states. These findings similarly put greater demands on functionalism, which specifies that any system that does what the brain does will be conscious. This view was on the idea that the brain did little more than a conventional computer, whereas functionalism now has to embrace a mechanism not just for computing as in unconscious processing, but a mechanism for the additional conscious processing demonstrated here. Reference:- Melloni, Lucia (2007) - Synchronisation of neural activity across cortical areas correlates with conscious perception - Journal of Neuroscience, 27, pp. 2858-65 |
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