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Home Page > Mainstream 1 > Mainstream 5

Mainstream 5

Mainstream 5
Summaries and reviews of papers and books related to mainstream consciousness theory

i of the vortex

Llinas, R.

New York School of Medicine

MIT Press ISBN 0-262-12233-2

Llinas argues that much of the activity of the brain is intrinsic or self-generated, with sensory input seen as coming in on top of, and modulating, existing brain activity. The brain is suggested to be a closed system, with sensory input being more about specifying cognitive states than actual information. In this system, sensory cues are incorporated into on-going cognitive states. The more conventional image has been of a passive brain only performing when it receives sensory input.

Electrical oscillation

The author’s major emphasis is on the electrical oscillations in the brain. He describes how the electrical potential across cell membranes is subject to an intrinsic small oscillation. He likens this to gentle ripples on a pond. On occasion, much larger fluctuations arise. These are known as action potentials, and form the basis of communication between neurons. The intrinsic oscillation of a neuron can influence its responsiveness to incoming signals. Llinas regards electrical oscillation as the glue that allows the brain to organise itself. He stresses that simultaneity of neuronal activity is pervasive in the brain, and that this derives from neuronal oscillation. It is suggested that neurons that display rhythmic oscillation may entrain each. Neurons which oscillate in phase (with the peaks and troughs at the same time) can support simultaneity of operation. Llinas uses the example of cicadas that chirp in rhythmic unison because they have an internal clock, which is an intrinsic oscillator. Fluctuations within this rhythm constitute information that is available to individuals or cells remote from one another. Oscillation in phase so as to make scattered elements work together in amplified fashion is known as resonance. A group of neurons that resonate with each other may also resonate with another group that are in an area of the brain remote from them. However, not all neurons resonate at all times. Neurons are able to switch in and out of the oscillatory mode and this allows resonance to occur transiently among different group of neurons. Cells receiving new sensory information may start to resonate with other groups of neurons. Llinas’s own work has uncovered the existence of intrinsic neuronal oscillations and the ionic currents that generate them⁽¹⁾. This is related to membrane conductance, and it has been shown that neurons are capable of generating action potentials without the presence of external input⁽²⁾.

Llinas argues that the brain is a closed rather than an open system. By an open system, he means one that is a reflex that merely receives, processes and outputs data, rather than contributing anything of its own⁽³⁾. The closed-system hypothesis that Llinas favours argues for a mainly self-activating system generating its own intrinsic images. Sensory input only gains significance as a result of the pre-existing state of the brain.

Llinas considers that our understanding of the external world arises from the juxtapositioning of internally generated images with the sensory properties of the external world. The internal images occur through intrinsic properties of the brain. Llinas develops this idea as an explanation of the process of dreaming. Dreams derive from intrinsic activity of neurons. In REM sleep, the brain is not receptive to sensory signals, but only to its intrinsic activity⁽⁴⁾. Studies by Llinas himself⁽⁵⁾ show that 40 Hz activity is present both in the awake condition and in REM sleep, but is greatly reduced during deep sleep, which is characterised by delta waves. In the waking state, auditory signals produced a change in the 40 Hz oscillation, but in REM sleep, there is no change in the oscillation. The significant thing is that in REM sleep the brain is so adjusted as to carry on with its intrinsic or internal activity, and to ignore sensory input.

Interneurons

Interneurons are particularly important in Llinas’s view of the brain. An interneuron is defined as any neuron that does not communicate with the outside world, but only exchanges information with other neurons. The interneurons serve to distribute sensory input to various components of the brain. The interneurons are thus in a position to influence a large number of other neurons and in effect ‘steer with multiple reins’. A particularly sensory input may stimulate a relatively small number of cells, which may activate another small number of interneurons, which then go on to have widespread effects. Interneurons are found throughout the central nervous system and particularly in the thalamocortical areas. Llinas’s emphasis on the intrinsic or internal activity of the brain also leads to him being opposed to the tabula rasa view of the brain as a blank slate at birth ready to be entirely determined by subsequent conditioning. In support of his view, he quotes studies⁽⁶⁾ in which newborn monkeys respond differently to lines of different orientation, although they had never previously seen lines.

Gap Junctions and Gamma Synchrony

Llinas emphasises the importance of gap junctions in the operation of the brain. In addition to the synaptic connections between cells, which involve a neurotransmitter crossing a 20 wide nanometre gap, the gap junctions offer a quicker and more direct connection. Unlike the synapses that require a chemical signal, the gap junctions allow ions to move from one cell to the next, and this constitutes a form of signal transmission that is quicker than the synaptic kind. Moreover, cells that receive such an ion based signal may be activated to fire an action potential. This can result in rapid and synchronous firing of interconnected cells. This allows a group of neurons to fire synchronously, as a result of which other more distant groups may resonate with them in a synchronous signalling pattern. This rapid electric coupling produces simultaneity between many neurons or in Llinas’s words creates the ‘roar of the masses’ rather than a ‘voice in the wilderness.’ This is effectively Llinas’s solution to the binding problem. The synchronous activity of group of neurons at locations remote from one another in the brain combines information from disparate sources and modalities. Llinas suggests that the effect of resonance is to bind together the spatial related processing of different groups of neurons at the same time. He implies rather than says that this accounts for the ‘now’ or ‘present moment’ sensation that is so much a part of consciousness, but which conflicts with special relativity. Llinas points to the example of the electric shocks delivered by electric eels. The motor neurons involved in delivering the shock have axons of varying length depending on their distance from the point from which the shock is administered, arranged so that the charges are delivered simultaneously, without which they would be of little effect. In a similar way, activity in the central and peripheral parts of the human retina is almost synchronous when it subsequently arrives at the thalamus.

These ideas are supported by a series of studies^(7-9) showing widespread synchronicity in the cortex. Synchronous activity in a single column in the visual cortex is observed when particular light bars are presented, This involves gamma (around 40 Hz) oscillation, and this can resonate between different cortical columns, and between regions of the cortex separated by as much as 7mm. The studies show that there are spatially separated ensembles of neurons bound together by an oscillation of around 40 Hz. Studies also indicate that 40 Hz coherent neuronal activity is generated during cognitive tasks. Some proposals suggest that this reflects the resonant properties of the thalamocortical system^(10-14). This coherent activity is a candidate for the unitary binding of sensory perception and thus the unity of consciousness. However, Llinas envisages this as a machine that is constantly ‘humming’, not something that is only switched on when it receives sensory inputs. The 40 Hz oscillation has a high degree of spatial organisation, and its synchronous oscillation may also be capable of producing the temporal conjunction of separate resonating groups of neurons.

Further evidence for the intrinsic or internal quality of much brain activity is seen in the fact that the input that the thalamus receives from the cortex is much larger than its input from the sensory systems. Neurons with intrinsic oscillation situated in the thalamocortical areas are suggested to allow the brain to self-generate oscillatory states that shape the impact of sensory stimuli. The hub position of the thalamus in the brain allows the thalamic nuclei to link to all parts of the cortex. In particular, the association areas of the cortex, which are very large in the human brain, receive input from both the association nuclei of the thalamus and from the sensory cortex. Llinas sees the thalamus as synchronously relating the sensory input from the external world with internally generated ideas and memories.

Consciousness

Llinas’s book repeatedly emphasises the workings of the brain in relation to in relation to the mechanical activity of muscles, their supporting motor neurons and the relevant parts of the cortex. This is not to deny the importance of these areas, but it is arguable that this emphasis facilitates an ultimately unhelpful approach to the problem of consciousness. From the point of view of students of quantum consciousness this is a pity, because Llinas penetrates further than most neuroscientists into the question of gamma synchronisation and gap junctions, bringing him close to the views of Hameroff.

This is an important book because of its argument for the self-generating nature of much of the activity in the brain, and for the role of the gamma synchrony relative to this self-generating nature. Llinas’s insight that the gamma synchronisation across spatially remote groups of neurons may correlate with the ‘now’ sensation of consciousness has explanatory value. Unfortunately, he does not go the further step of producing a mechanism that explained why this correlated activity produces subjective experience rather than unconscious information processing. His description of brain function in respect of the gamma synchrony and the gap junctions bears an extraordinary resemblance to their role the Penrose/Hameroff Orch-OR model of consciousness. This might have sparked an interesting chapter on the possibility of quantum features. However, Llinas doubts that this scenario is worth pursuing seriously. Why does he think this? In the course of a rather wordy paragraph, he comes round to the conclusion that electrical patterns are the same as qualia. This is simply stated as a belief with no plausible reason given, and the matter is simply left at that.

We have to be specific about what Llinas appears to be trying to say here. He is not claiming that electrical oscillations are connected or correlated with qualia in some way but that they are the same thing as qualia. But this gives rise to the problem as to why it is only the electrical patterns in the brain, and only some of those that are conscious and have subjective experience. Elsewhere, he has given a good description of how the electric potential across the cell membrane works in principle in the same way as a battery, but Llinas does not presumably think that batteries are conscious. While brain activity is no doubt related to conscious, there is a requirement to show why these functions produce the property of consciousness or qualia not found elsewhere in the physical universe. Llinas could have attempted an explanation based on complexity or information processing. As far as he will go in this direction is to argue that the process of muscular movement is somewhat similar to the process of producing sensations in the brain. He is probably right in saying that the physical mechanisms are similar, but this does not really bring us any closer to why one class of these mechanisms is especially involved with subjective sensory experience. We have to be specific about what Llinas appears to be trying to say here. He is not claiming that electrical oscillations are connected or correlated with qualia in some way but that they are the same thing as qualia. But this gives rise to the problem as to why it is only the electrical patterns in the brain, and only some of those that are conscious and have subjective experience. Elsewhere, he has given a good description of how the electric potential across the cell membrane works in principle in the same way as a battery, but Llinas does not presumably think that batteries are conscious. While brain activity is no doubt related to conscious, there is a requirement to show why these functions produce the property of consciousness or qualia not found elsewhere in the physical universe. Llinas could have attempted an explanation based on complexity or information processing. As far as he will go in this direction is to argue that the process of muscular movement is somewhat similar to the process of producing sensations in the brain. He is probably right in saying that the physical mechanisms are similar, but this does not really bring us any closer to why one class of these mechanisms is especially involved with subjective sensory experience.