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Neuroscience 5Neuroscience 5:- Includes reviews of recent work on brain rhythms by Miles Whittington, Nancy Kopell, Roger Traub, Wolf Singer, Mayank Mehta, Buszaki, Moser, E. and Catherine Tallon-Baudry. 1.) Vilayanur Ramachandran - In:- Conversations on Consciousness - Ed. Susan Blackmore 2.) Francisco Varela - In:- Conversations on Consciousness - Ed. Susan Blackmore 3.) Susan Greenfield - In:- Conversations on Consciousness - Ed. Susan Blackmore 4.) What are the local circuit features concerned with coordinating rhythms - Miles Whittington, Nancy Kopell, Roger Traub - Background to brain rhythms including axonal gap junctions. 5.) Coordination in Circuits - Mayank Mehta et al - neural synchronies, compression of information and gamma communication between neural assemblies. 6.) Neocortical Rhythms - Wolf Singer - Relates to gamma synchrony and consciousness 7.) Dynamic coordination in brain and mind - William Phillips, Christoph von der Malsburg & Wolf Singer - Neural synchronies, perception and attention 8.) Oscillation-supported information processing and transfer at the hippocampus-entorhinal-neocortical interface - Buzsaki, G. & Diba, K. - Oscillations and information transfer between the hippocampus and the neocortex 9.) Coordination in brain systems - Moser, E. I. et al - Studies of oscillation based brain synchronies 10.) Neural Coordination and Human Cognition - Catherine Tallon-Baudry - Brain oscillations and synchronies related to the psychological present and perceptual unity. 1.) Vilayanur Ramachandran In:- Conservations on Consciousness Susan Blackmore Oxford University Press Ramachandran sees the self and qualia as intertwined. Without the self, he thinks that there would be nothing that experiences the qualia, and without the experiencing of the qualia there would be nothing to identify as self. Blackmore raises the objection that in altered states such as Zen meditation the self disappears but there is still experience. Ramachandran is in denial on this, claiming it is not possible, although the evidence of many accounts from meditation and other experiences in varied cultures is that this is exactly what happens. The argument here seems to be quite straightforward, it doesn't fit the pet theory, so it can't be true. To justify this difficult position Ramachandran has to defend the now unfashionable view that animals including great apes are not conscious. His exacts views on this are a bit fuzzy. At one point he's prepared to concede a raw background awareness but not a self. In this he appears for a moment to be close to being in line with the modern consensus that animals can be conscious, but with the exception of great apes and perhaps a few other species not self-conscious. But later on this doesn't seem to be what he really thinks, because he says that animals don't really experience pain, although even this comes in two versions of (1.) they don't really experience pain and (2.) they experience it but don't introspect about it, although weirdly this may also not be really experiencing pain. The non-consciousness of animals, which has thankfully become less fashionable in recent years is a necessary position for Ramachandran because his theory of qualia and consciousness is tied to the emergence of self-consciousness, which for Ramachandran only occurs in humans. The non-consciousness at least of mammals, here developed in a rather confused way, has always seemed unlikely, and may possibly be an unwitting inheritance from the Christian dogma that animals do not have souls. The notion appears improbable, given that other mammals have a similar brain architecture and nervous system to humans and their observed behaviour in the face of pain or fear is very similar, which places a heavy burden of evidence on those who claim that for mysterious reasons the response to pain or fear comes from a different source than in humans. For instance, at one point, Ramachandran seems to say that the animal response comes from the spinal cord, so he appears to be saying that the somatosensory cortex is switched off in some mysterious way in non-human mammals. Apart from anything else, like so much of consciousness studies, this discussion is light on actual science. Assertions about the mind, as in this spinal cord claim are thrown around without any reference to the underlying neuroscience. All these assertions about animal non-consciousness are necessary, because for Ramachandran consciousness emerges from the self and only humans have a self. His view of consciousness is that up to a particular point in evolution, at the point where humans emerge, there are just unconscious representations of the external world, but at the human stage comes a meta representation, or a representation of the representations, which is claimed to constitute the self, which in turn constitutes the qualia or subjective experience. The problem with this is that it is yet another version of the all too common proposition in neuroscience that if one video camera is pointed at another or a film is filmed consciousness will emerge. This is really to say that if one copies something one can add something to it without introducing any new physical process. The world would be a very different from what it is if this was true. In this situation, if the first lot of representations are unconscious, a representation of these will simply be a copy, summary or integration of the originals and therefore as true copies equally unconscious, unless a new physical process or property is introduced. This is nowhere suggested in this discussion. An easier approach is to allow animals some qualia, which all the evidence of biology and behaviour suggests that they have, and then accept that the self is part of the contents of consciousness, in that we have the subjective experience (illusory or otherwise) of being a freestanding entity. All that can really be said for Ramachandran's theory is that it one better than the papers where the self is deconstructed into narrative history and the sense of the boundaries and position of the body, and the problem of consciousness is then declared to have been solved. 2.) Fransisco Varela In:- Conservations on Consciousness Susan Blackmore Oxford University Press In terms of consciousness studies, Varela is best known for his enactive or embodies view of the nervous system and cognition. Essentially his argument is that consciousness arises through our embodiment. This needs to be a neural part in the brain but also a 'pheno' part in the body. His theory is beguiling but ultimately unconvincing. He says, and I agree with this part, that we need to account for the intimacy of consciousness. He thinks this requires something different from a normal computer. His answer his that we experience ourselves intimately because we are embodied, rather than as a brain in a vat type of computer. The fact of being embodies brings with it the experience of being embodied. If we touch an object we feel its solidity and inertia, we experience something of the laws of physics governing the external world. This is supposed to combine with neural activity to produce consciousness. As I say, it is initially beguiling, but it actually has a difficult to digest message. It's saying that the information processing in the body (ex-brain) can do something that the brain can't. It's not clear why this should be so. Nothing we know about the body or its nervous system suggest that it has some processing of property that is not available to the brain, the reverse if anything. Admittedly, the computer/brain in a vat attitude of the last century failed to take account of the degree of interactivity between the brain and the body, and particularly between the limbic system and the visceral responses. However, none of this warrants attributing something the brain doesn't have to the body. Varela's example of experiencing the solidity and inertia of an object, and then explaining this in terms of the body reverses the correct logical order. The subjective experience of solidity is the thing that needs to be explained, where as here it is treated as the explanation. The problem is why the brain/body system should have that experience from an object, and why it isn't handled by unconscious processing, in the way that a computer can unconsciously register and respond to an object it is monitoring, for instance by setting off an alarm. The additional artificial intelligence related argument that a computer could become conscious if it was embodied fails if this difficulty cannot be overcome. 3.) Susan Greenfield In:- Conservations on Consciousness – Ed. Susan Blackmore Susan Greenfield is critical of functionalism and particularly of the idea that consciousness is not something in addition to intelligence, vision and other things that the brain provides. She points out that when she taught neuroscience at Oxford, after going through various neuroscience processes, students would still assume that it required consciousness for them to actually see something. But consciousness, they thought, was not included in the syllabus. Greenfield does not think that it's possible to separate consciousness from such functions as vision or emotion. She is also critical of robots that just simulate aspects of human behaviour. She argues that this is not really what consciousness is about. One can be conscious when what is not doing anything. She further remarks that "other people can seem to be utterly brain-dead, like many of the people I know who just sit around", but are nevertheless conscious. She thinks that consciousness should be disassociated from behaviours. 4.) What are the local circuit design features concerned with coordinating rhythms? Miles Whittington, Nancy Kopell, Roger Traub Newcastle University, UK and IBM In:- Dynamic Coordination in the Brain – Eds. Christoph von der Malsburg, William Phillips & Wolf Singer INTRODUCTION: This chapter examines some of the detail of gamma and other rhythms in the brain as studied by recent neuroscience. It emphasises the relationship between cortical rhythm generation and cortical function. In particular, it indicates gap junctions between axons as a driver of brain rhythms including gamma rhythms. In terms of consciousness studies, this is important background given the correlation between the gamma synchrony and consciousness. The cortex gives rise to rhythmic activity over a broad range of frequencies. This involves rhythmic change of the neuronal membrane electrical potential between periods of activity and periods of quiescence. Rhythmic activity can rise as high as 400 Hz when glutamate is involved. Evidence suggests that the majority of rhythm generating properties amongst neurons are in local circuits. Very selective frequency filters for neuronal inputs can determine which local circuit rhythm a neuron can be involved in. This leads to resonance between particular neurons. P. Synaptic inhibition is an important cause of rhythm generation in local networks through the theta to gamma range (up to 80 Hz). Even very low levels of neuronal excitatory activity can cause inhibitory interneurons to fire. The frequency of rhythms is mainly set relative to inhibitory postsynaptic potentials. Theta rhythms also depend on inhibition. Local rhythms above the gamma range are known as 'high gamma' or 'VFO' and may relate to high frequency discharges in interneurons (1. Buzsaki et al, 1992). These higher frequencies may be generated via gap junctions rather than synapses. Gap junctions between axonal compartments can allow rapid transmission of action potentials from one axon to another. These very fast oscillations (VFO) can be nested within slower ones. In persisting gamma rhythms, VFOs can be related to each period and they are seen as a driving force in these rhythms. Networks of such interconnected axons can generate rhythms, and gap junction coupled axonal networks are suggested to generate rhythms in local circuits. Cell assemblies are defined by synchronisation of axon spikes with a near-zero time delay. This is thought to be possible because of inhibitory neuronal activity. With gamma rhythms there is often coordination of activity in a number of separate neuronal populations. In addition to coordination of rhythms at the same frequency, there is also coordination of rhythms at different frequencies in the same or separate brain regions. Gamma rhythms are generated by interaction between principle cells and interneurons. Slower theta rhythms derive from a different set of interneurons inhibiting the dendritic compartments of principle cells. The relationship between the two rhythms is suggested to be handled by the interaction of two types of inhibitory interneuron. Here output from at least one neuron determines both circuits. In some cases, the duration of a locally generated rhythm is an integer multiple of another locally generated rhythm. A ratio of about 1.6 between frequencies expressed at the same time may allow information channels to operate at the same time without interference. The phase relationship between different rhythms also shows a cycle, and reduction of cortical excitation can see a stabilisation of the phase relationship between cortical layers. Rhythms are seen to coordinate firing patterns of neurons. The phase relationship between rhythms at spatially separated brain sites governs the timing of local activity and also future interactions between the two sites. The amount of coherence of rhythms governs the degree of communication between structures. The multiple frequencies in the cortex suggests an overall scheme where different frequency channels are used to process different types of information. So far this has been observed mainly in the association cortex. Gamma rhythms in the visual, parietal and entorhinal cortex are different from one another. Even within the visual field different levels of detail can fall into different frequency channels, with for instance general features related to theta frequencies, but more detailed features related to beta frequencies. Information processed at different frequencies can later be combined at some further frequency. 5.) Coordination in Circuits Mayank Mehta et al In:- Dynamic Coordination in the Brain – Eds. Christoph von der Malsburg, William A. Phillips, & Wolf Singer INTRODUCTION: This chapter examines aspects of neural synchronies, compression of information in relation to neuron oscillations, and the gamma synchronies involvement in communication between neural assemblies in different areas of the brain. All layers of the cortex have a variety of inhibitory interneurons using the GABA neurotransmitter. These interneurons control cortical activity through their connections with excitatory neurons. Inhibitory synapses are often located near the soma (main body) of a neuron in a position to influence excitatory inputs flowing from the dendrites to the soma. Both the excitatory and the inhibitory neurons are connected to each other within and between cortical layers. Excitatory-inhibitory networks (E-I networks) are not confined to the cortex, but are widely dispersed in the brain. When a stimulus arrives, excitatory pyramidal neurons respond and their firing rates may rise as high as 100 Hz. This activation in pyramidal neurons in turn drives the inhibitory neurons to briefly shut down the pyramidal neurons, prior to being synchronously released from inhibition. The inhibitory GABA receptors provide a time constant, and are basic to the 30-100 Hz gamma frequency oscillations that are taken as an indication of activity in the cortex. Synchronised oscillations at a range of frequencies occur in many brain areas during perception, attention, motor planning and sleep. The 4-12 Hz theta oscillations are present in the hippocampus during spatial exploration, and are present in the visual, parietal, hippocampus and prefrontal areas during working memory. The 10-30 Hz beta frequencies are seen in the visual and motor areas. Gamma oscillations are found in visual and prefrontal areas. Lower frequency oscillations can at times be suppressed in favour of the gamma synchrony, but at other times lower frequency oscillations can facilitate the gamma synchrony. Synchrony is important for the transmission of information between areas of the brain. For instance inputs from a small number of neurons in the thalamus to a cortical column are more effective if they are synchronised. A small number of neurons is thus sufficient to transmit information. Speed and flexibility of response is also seen as being improved by synchrony. This economy in transmission is particularly important given the energy intensive nature of axon spiking. Neurons change their firing rate in response to changes in sensory stimuli. The brain has to deal with two influences, its internally generated oscillations and external stimuli. If a neuron is receiving a low level of external stimuli, it will only spike at the peak of an oscillation, but if it is receiving a high level of input, it can spike at any point in the oscillation. The post-synaptic neuron can measure whether the spike occurred at a high or low point of the pre-synaptic oscillation. The activity of pre-synaptic neurons can be thus compressed into an oscillation cycle in downstream neurons and perceived as a single unit. This might also help to bind together different signals so as to determine the relationship between them. Information is seen as being produced by the activity patterns of groups of neurons. A group of coactive neurons could form within a gamma cycle. These are referred to a cell or neural assembly. The membership of the neural assembly is flexible and can change rapidly. Each gamma cycle may contain an assembly that triggers the formation of another assembly in the next cycle. Decisions are suggested to derive from coordinated activity patterns in different neural assembles dispersed across different regions of the brain. Inhibitory connections between cell assemblies could both synchronise gamma activity between assembles and increase overall firing rates. 6.) Neocortical Rhythms Wolf Singer In:- Dynamic Coordination in the Brain Eds. Christoph von der Malsburg, William Phillips, & Wolf Singer INTRODUCTION: Singer's chapter looks to come near to finalising the case for a correlation between the gamma synchrony and consciousness. Conscious stimuli are associated with phase locking of gamma oscillations across spatially distributed regions of the cortex, and also with increases in synchrony without increases in rate of firing. In terms of perception, neurons responding to eyes, noses etc can synchronise to identify a face, but when the facial components are scrambled the synchrony, but not the discharge strength, disappears. This chapter discusses the rhythmic modulation of neuronal activity. During processing in the cortex, the brain increasingly selects for the relationship between objects. This involves interactions between different parts of the cortex. There is a requirement to cope with the ambiguity of the external world. The environment may contain objects with contours that overlap, or are partly hidden, and these conflicting signals have to be resolved in the cortex. Further to this some objects are encoded in different sensory modalities. Evidence suggests that this process involves not only individual neurons but also assemblies of neurons (1. Singer, 1999, 2. Tsunoda et al, 2001). The possible conjunctions in perception are too large to be dealt with by individual neurons, and utilise assemblies of neurons with each neuron relating to particular aspects of the object. There appear to be two stages to this process. There is a signal to indicate that certain features are present. This operates on a 'rate code' basis, where a higher discharge frequency codes for a greater probability of a particular feature being present. However, evidence (3. Gray et al, 1989) shows that neurons in the primary visual cortex synchronise their spiking, and it is proposed that synchronisation of responses could code from relatedness. Synchronised inputs to neurons have a stronger impact than unsynchronised signals. This applies not only to the processing of perception, but also to learning, where precise temporal relations between pre and post-synaptic activity leads to strengthening of synapses. Signal processing and learning thus appear to be related processes. Synchronisation of discharges is often associated with oscillations in populations of neurons. These oscillations are driven by inhibitory neurons through both synapses and gap junctions. (4. Kopell et al, 2000, 5. Whittington et al, 2001) Inhibitory inputs to pyramidal cells favour discharges at depolarising peaks and this allows synchrony in firing. Locally synchronised oscillations can become phase locked with others that are spatially separated with zero delay. Oscillations in different frequencies can exist together, and this combination may deal with composite objects or movement trajectories. Synchronisations may be used at all stages of neural processing from the retina through to the cortex. This appears to serve the function of creating a relationship between spatially distributed responses, for instance signalling a relationship between neuron groupings that may be utilised in future processing, notably perceptual grouping. Synchronisation involves oscillations at up to 90 Hz in both the beta and gamma range. This synchronisation is related to connections linking cortical columns encoding for linked features. The inferior temporal cortex is regarded as the likely site for the production of visual objects, and object-related assemblies are associated with synchronisation. In one study, neurons responding to eyes, noses and faces were shown to synchronise to recognise a face. If the individual components were scrambled into a non-face arrangement then synchrony did not arise. However, the scrambling into non-face did not alter the discharge rate, only the synchrony. Focus of attention on objects also caused increased synchrony in the beta and gamma bands. Here again synchronisation does not necessarily relate to increased discharge rates. Coherent oscillations across distributed areas of the cortex including executive areas are seen as facilitating the routing of activity and the rapid response of different areas of the cortex. Evidence also indicates a close correlation between gamma synchronisation and conscious processing (6. Melloni et al, 2007). Activity related to conscious responses is more synchronised, but not more vigorous. In human subjects conscious processing has been related to phase locked gamma oscillations in widely distributed cortical areas, whereas unconscious processing produced only local gamma activity (6. Melloni et al, 2007). References:- 1.) Singer, W. (1999) - Neuronal synchrony: A versatile code for the definition of relations - Neuron, 24 (1), pp. 49-65 2.) Tsunoda, K. et al (2001) - Complex objects represented in inferotemporal cortex by the recombination of feature columns - Nature Neuroscience, 4 (8): pp. 832-8 3.) Gray et al (1989) - Oscillatory responses exhibit inter-columnar synchronisation which reflects global stimulus properties - Nature, 338: pp. 334-7 4.) Koppel, N. et al (2000) - Gamma and beta rhythms have different synchronisation properties - PNAS, 97, (4): pp. 1867-72 5.) Whittington et al (2001) - Synaptic and non-synaptic mechanisms underlying stimulus induced gamma oscillations - Journal of Neuroscience, 21 (5): pp. 1727-1738 P. 6.) Melloni, L. et al (2007) - Synchronisation of neural activity across cortical areas correlates with conscious perception - Journal of Neuroscience, 27 (11): pp. 2858-2865 7.) Dynamic 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. 8.) Oscillation-supported information processing and transfer at the hippocampus-entorhinal- neocortical interface Gyorky Buzsaki & Kamran Diba In:- Dynamic Coordination in the Brain: From Neurons to Mind – Eds. Christoph von der Malsburg, William Phillips & Wolf Singer Oscillatory coupling is suggested to both package information, and to facilitate an exchange of information. Two network patterns are prominent in the hippocampal system. These are theta oscillation at 4-10 Hz and sharp waves at 140-200 Hz. Theta is associated with preparatory activity and REM sleep. Neocortical to hippocampus information flow takes place during theta oscillation, while hippocampal to neocortex information flow involves sharp waves. The theta oscillation occurs across the whole hippocampal region and derives from a variety of electrical oscillations including membrane voltage oscillations. The theta oscillations can have gamma oscillations nested within them. These are generated by interneurons or the interaction of interneurons and other cells. Neurons that discharge within the same gamma cycle are defined as a neural assembly. In the hippocampus, theta oscillations can be replaced by sharp waves (SPW). These are related to synchronous discharges from pyramidal cells and are associated with synaptic changes. It has been hypothesised that the sharp waves are crucial to transferring memories from the hippocampus to the cortex to become part of long-term memory. This hypothesis is supported by experimental data, and it is also thought that the sharp waves relate to recent waking experience. Oscillations and synchrony are seen as creating conditions for transfer of information between different brain structures. The transfer of information from the hippocampus to the neocortex is during sleep. In the waking state the flow of information based on theta oscillations is in the opposite direction. In this state the neocortex produces gamma bursts that arrive back at the hippocampus at a phase of the theta cycle. The hippocampus generates sharp waves during sleep, at which time much of the neocortex has a very low frequency. The sleep spindles seen in the thalamocortical system are associated with this process of information transfer from the hippocampus to the neocortex. 9.) Coordination in brain systems Moser, E. I. et al In:- Dynamic Coordination in the Brain: From Neurons to Mind – Eds. Christoph von der Malsburg, William Phillips & Wolf Singer Neuroimaging of the human brain indicates coordination of activity between different brain regions. Large scale coordination is shown to span the entire brain. Rhythmic modulation of electrical activity is seen as a possible mechanism to change the couplings amongst neurons. Networks undergoing electrical oscillation facilitate the establishment of synchrony through entrainment and resonance. The precision of synchronisation increases with the frequency of the oscillation. Synchronisation increases the influence of the output of cell assemblies on target neurons. In synchronised cell populations response to strong excitatory inputs will occur earlier than weak inputs on the rising phase of the oscillation, and this acts as a code to indicate the relative strength of signals. Studies of the retina show that this process indicates that relative strength of visual stimuli. Moser's group considered that oscillation-based synchronisation has a role in cognitive processing. Studies have shown that visual attention correlates with increases in coherence between the parietal and the frontal cortex. They also show an increase in coherence between two different frequency bands, 35-55 Hz (gamma) for bottom-up attention and a lower frequency band for top-down attention. Coupling between neuron populations depends on the phase relationship between the different groups. Oscillations in different frequency bands such as theta, beta and gamma can coexist. The point in the phase of an input can code for whether it is processed or suppressed. Brain activity has been shown to be organised in spatiotemporal patterns corresponding to gamma fluctuations. These appear to be related to learnt activity, and may represent an attractor that recruits particular brain networks. The role of neuromodulators is suggested to be one of providing the necessary conditions for oscillations rather than directing the oscillations. This chapter also discusses the question of the zero-phase lag between oscillations in different populations of neurons. This zero-phase lag is ubiquitous in the brain, manifesting over large spatial separations and even between the hemispheres. This is despite significant conduction times between the separated populations. However, it is admitted that the mechanism for the synchronous gamma firing is not well understood. 10.) Neural Coordination and Human Cognition Catherine Tallon-Baudry In:- Dynamic Coordination in the Brain: From Neurons to Mind – MIT Press Brain imaging in recent years has led to the brain being seen in terms of a large number of functional regions, and this in turn creates a need to explain how the activities of these regions are coordinated. This chapter emphasises the distinction between learned routes in the brain and the coordination that is needed to deal with new perceptions and behaviours. Initial waves of feed forward activity are mainly related to unconscious processing. Feedback and recurrent processing between parts of the brain have been related to processing over a longer duration and may be related to conscious activity. Delta waves in the brain are related to sleep, theta to memory, while beta and gamma are associated with attention and the binding together of activity in different areas of the brain. The author refers to "a large and converging body of evidence that grouping features into a coherent percept is accompanied by changes in the gamma range" (1. Jensen et al, 2007, 2. Tallon-Baudry, 2009), although oscillatory synchrony in the alpha range is also found (3. Mima et al, 2001, 4. Freunberger et al, 2008). Brain networks influence oscillatory synchrony from the theta to the gamma range. The laying down and retrieval of memories involves both the theta and gamma oscillatory synchrony. Short-term memory activity involves both beta and gamma oscillations. Local gamma oscillations relate to activity in the visual, medial temporal lobe and frontal areas. Particular cognitive processes can relate to gamma oscillations in different locations. Selective attention can involve gamma oscillations at particular subfrequencies and in particular brain areas. Distinct bands with the gamma range relate to both visual awareness and spatial attention. The frequency content of oscillations in the gamma band has been suggested to code for features such as spatial frequency or the direction of sound. Oscillations could be used to group and separate chunks of data from what went before and came after. This has been related to beta oscillations. There has been a further tentative suggestion that very slow oscillations could be related to the 'psychological present' or the few seconds which form a perceptual unity that does not require the effort of recall. It is further suggested that distinct oscillatory frequencies could be used to accomplish a task requiring more than one function, such as searching for a person in a crowd. This is taken to suggest that activity between different frequency bands can be coordinated in the brain. Experimental evidence suggests that spatially distant areas of the brain oscillate in the same frequency range and with constant phase relationship and that this activity can play a cognitive role (5 & 6. Tallon-Baudry 2001 &4, 7. Melloni et al, 2007, 8. Doesburg et al, 2008). Amplitude as well as phase coupling can occur between frequency bands at S.integrating information and distinguishing between information. References:- 1.) Jensen, O. et al (2007) - Human gamma frequency oscillations associated with attention and memory - Trends in Neuroscience, 30 (7): pp. 317-324 2.) Tallon-Baudry, Catherine (2009) - Gamma band synchrony in visual cognition - Frontier Bioscience, 14, pp. 321-32 3.) Mima, T. et al (2001) - Interhemispheric synchrony correlates with object recognition - Journal of Neuroscience, 21 (11): pp. 3942-3948 4.) Freunberger, R. et al (2008) - Alpha phase coupling reflects object recognition - NeuroImage, 42, (2) pp. 928-935 5.) Tallon-Baudry, Catherine (2001 |
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