Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Oscillation, Synchronization

Neural network activity with its ubiquitous recurrent and reentrant circuitry generates electrical signals that can be detected on the scalp (Electroencephalogram EEG) and by invasive probes into the brain.  The spectral frequencies of these neural signals have a power law characteristic, displaying an inverse linear relationship on a log-log plot.  This kind of relationship is called "pink noise."

Simultaneous processing of vast numbers of independent local circuits leads to gamma oscillations detected in EEG. Reentrant neural activity in more widespread neural circuits leads to lower frequency oscillations in the power spectrum.

The electrical potential recorded outside the skull is replete with oscillatory activity. The frequency of these oscillations ranges from 1 Hz to ~100 Hz. (Koch; Quest for Consciousness, 38)

Understanding how interactions take place between oscillations at different frequencies may be important for unraveling the functional importance of each frequency, and how the respective distinct functions might be interrelated. (Traub; Cortical Oscillations, 66)

In the rhythm of oscillations, no single cell joins in all the activity all the time, but overall, there are sufficient cells to maintain a synchronous activity for long periods of time. (Greenfield; Private Life of Brain, 193)

Oscillations have been documented in the brains of numerous mammalian species, ranging from very slow oscillations with periods of minutes to very fast oscillations with frequencies reaching 600 Hz. (Buzsáki - Rhythms of the Brain, 112)

Rhythmic activity can be generated by two main mechanisms -- intrinsic membrane properties and synaptic circuits. (Shepherd and Koch; Synaptic Circuits, 30)

The main biophysical idea is that rhythmicity is generated by a depolarization process that is autocatalytic (positive feedback), followed by a slower repolarization process (negative feedback). (Wang; Oscillatory and Bursting, 687)

Temporal coherence - timeness is consciousness - perceptual unity based on spacial and temporal conjunction. (Llinás; I of the Vortex, 120)

Simultaneity of neuronal activity arising from intrinsic oscillatory electrical activity, resonance, and coherence are at the root of cognition. (Llinás; I of the Vortex, 12)

We remain agnostic with respect to the relevance of gamma oscillations to conscious perception. (Crick & Koch; Consciousness and Neuroscience, 46)

 

EEG Patterns of Activity

Surface EEG shows typical patterns of activity that can be correlated with various stages of sleep and wakefulness. Normal human EEG shows activity over a range of 1-30 Hz with amplitudes in the range of 20-100 μV. Alpha waves (8-13 Hz) of moderate amplitude are typical of relaxed wakefulness and are most prominent over parietal and occipital sites. Lower-amplitude beta activity (13-30 Hz) is more prominent in frontal areas and over other regions during intense mental activity. Alerting a relaxed person results in the desynchronization of the EEG, with a reduction in alpha activity and an increase in beta. Theta waves (4-7 Hz) and delta waves (0.5-4 Hz) are normal during drowsiness and earliest slow wave sleep. (Kandel; Principles of Neural Science, 916)

 

Asynchronous Local Circuits and the Associative Property of Memory

Local circuits tend to operate asynchronously. They are drawn into coherence by the associative property of memory.

By the associative property of memory, neural assemblies aggregate via the laws of Gestalts into the sparse but widespread neural assemblies of the dynamic core of consciousness.

The enormous associative capabilities of the dynamic core are ideal to link or hierarchically organize a series of preexisting unconscious routines into a particular sequence. (Edelman; Universe of Consciousness, 187)

Recurrent neural networks can generate an asynchronous state characterized by arbitrarily low mean spiking correlations despite substantial amounts of shared input.

The asynchronous state of local circuits provides a highly differentiated neural network with rich information content. The associative property of memory amalgamates these myriad asynchronous local circuits into the highly integrated dynamic core of the neural network.

Orderings of computations is necessary for performance. Ordering does not take place by a strict serial organization. Instead, computations pass information back and forth to coordinate their results. (Posner; Constructing Neuronal Theories of Mind, 198)

Precise connections exist between anatomically distant areas. A particular anatomical area is active whenever its computation is required. Since computations are often contingent on information from another area, information is fed back to reenter the critical areas. (Posner; Constructing Neuronal Theories of Mind, 198)

Working memory is beginning to be accepted as simply one of a range of important systems underlying complex cognition, many of which operate relatively automatically. (Baddeley; Working Memory, 173)

Research study — Asynchronous State in Cortical Circuits

Research study — Gamma Oscillations and Cellular Pathologies

 

Power Spectrum of EEG Has 1/f Distribution

Brain oscillators should be considered as a system of oscillators with an intricate relationship between the various rhythmic components. (Buzsáki - Rhythms of the Brain, 119)

Synaptic activity in the brain can lead to a complex system organized at multiple timescales. (Buzsáki - Rhythms of the Brain, 119)

Slow rhythms involve very large numbers of cells and can be "heard" over a long distance, whereas localized fast oscillations involving only a small fraction of neurons may be conveyed only to a few partners. (Buzsáki - Rhythms of the Brain, 119)

Even for modest firing frequencies such as 20 Hz, the action potential amplitude, measured about 300 µ from the soma, attenuates to less than half of its amplitude at lower frequencies. (Andersen; Hippocampus Book, 146)

Power spectrum of the EEG is a straight line on a log-log plot, the hallmark of scale-free systems (i.e. systems that obey power laws) (Buzsáki - Rhythms of the Brain, 119)

The amplitude of the EEG power spectrum increases as the frequency decreases.  This inverse relationship is expressed as the "one over f" power spectrum (also called "pink" noise). (Buzsáki - Rhythms of the Brain, 119)

Human Listeners prefer sounds with 1/f spectra

Noise with 1/f spectra; it is partially random and partially predictable. (Gazzaniga; Human, 238)

Amplitude and pitch fluctuations of natural sounds such as running water, rain, and wind, often exhibit 1/f spectra. (Gazzaniga; Human, 238)

Human listeners reportedly preferred 1/f-spectra melodies to melodies with faster or slower changes in pitch and loudness. (Gazzaniga; Human, 239)

 

Mean Frequencies of Brain Oscillator Families Are Not Integer Multiples

A critical aspect of brain oscillators is that the mean frequencies of the neighboring oscillatory families are not integer multiples of each other. Adjacent bands cannot simply lockstep because the prerequisite for stable temporal locking is phase synchronization. (Buzsáki - Rhythms of the Brain, 120)

The 2.17 ratio (natural logarithm e) between adjacent oscillators can give rise only to transient or metastable dynamics, a state of perpetual fluctuation between unstable and transient phase synchrony, as long as the individual oscillators can maintain their independence and do not succumb to the duty cycle influence of a strong oscillator. (Buzsáki - Rhythms of the Brain, 120)

Local Circuits Link Hierarchically into a Widespread Network

When brain rhythm is fast, only small groups can follow the beat perfectly, because of the limitations of axon conductance and synaptic delays. (Buzsáki; Rhythms of the Brain, 122)

VFO is coherent over distances of at least 300 µ, so that hundreds of cells (at least) must be participating. (Traub; Cortical Oscillations, 262)

VFO in the hippocampus, neocortex, and other telencephalic structures arises because of relatively rare spontaneous action potentials, percolating from axon to axon across gap junctions, with period determined by the global topological network structure, rather than by intrinsic membrane or synaptic conductances. (Traub; Cortical Oscillations, 262)

Slow oscillations, spanning numerous axon conduction delay periods, allow the recruitment of very large numbers of neurons. (Buzsáki; Rhythms of the Brain, 122)

Even for modest firing frequencies such as 20 Hz, the action potential amplitude, measured about 300 µ from the soma,    attenuates to less than half of its amplitude at lower frequencies. (Andersen; Hippocampus Book, 146)

Locally emerging stable oscillators in the cerebral cortex are constantly being pushed and pulled by the global dynamics. (Buzsáki - Rhythms of the Brain, 120)

Despite the chaotic dynamics of the transient coupling of the oscillators at multiple spatial scales, a unified system with multiple time scales emerges. (Buzsáki - Rhythms of the Brain, 120)

Rhythmic Neuronal Activity during Movement

Neural Population Dynamics -- Quasi-oscillatory neural responses are present when a monkey 'reaches'. Rotations of the population state are a prominent feature of the cortical response during reaching. These population-level rotations are a relatively simple dynamical feature yet explain seemingly complex features of individual-neuron responses,

 

 

Research study — Neuronal Oscillations as a Mechanism of Attentional Selection

Research study — Gamma Oscillations in Hippocampus

Research study — Oscillations and Schizophrenia

Research study — Oscillatory Network Dynamics Mediate Visual Perception

 

 

Binding of specific and nonspecific gamma band activity

 

International Congress Series 1250 (2003) 409-416

Consciousness and the thalamocortical loop

Rodolfo Llinás

Department of Physiology and Neuroscience, New York University Medical School, 550 First Avenue,

New York, NY 10016, USA

A neuronal circuit that may subserve temporal binding is formed by gamma oscillations in neurons in specific thalamic nuclei, which establish cortical resonance through direct activation of pyramidal cells and feed forward inhibition through activation of 40-Hz inhibitory interneurons in layer IV. These oscillations re-enter the thalamus via layer-VI pyramidal-cell axon collaterals, producing thalamic feedback inhibition via the reticular nucleus.

In a second system, the intralaminar nonspecific thalamic nuclei project to cortical layers I and V and to the reticular nucleus. Layer-V pyramidal cells return oscillations to the reticular nucleus and intralaminar nuclei. The cells in this complex have been shown to oscillate at gamma band frequency and to be capable of recursive activation.

It is also apparent from the literature that neither of these two circuits alone can generate cognition. Indeed, as stated above, damage of the nonspecific thalamus produces deep disturbances of consciousness, while damage of specific systems produces loss of the particular modality. Although at this early stage, it must be quite simple in its form, the above finding suggests a hypothesis regarding the overall organization of brain function.

This rests on two tenets: First, the ‘‘specific’’ thalamocortical system is viewed as encoding specific sensory and motor activity by the resonant thalamocortical system specialized to receive such inputs (e.g. the LGN and visual cortex). The specific system is understood to comprise those nuclei, whether sensorimotor or associative, that project mainly, if not exclusively, to layer IV in the cortex. Second, following optimal activation, any such thalamocortical loop would tend to oscillate at gamma band frequency and activity in the ‘‘specific’’ thalamocortical system could be easily ‘‘recognized’’ over the cortex by this oscillatory characteristic.

In this scheme, areas of cortical sites ‘‘peaking’’ at gamma band frequency would represent the different components of the cognitive world that have reached optimal activity at that time. The problem now is the conjunction of such a fractured description into a single cognitive event. We propose that this could come about by the concurrent summation of specific and nonspecific 40-Hz activity along the radial dendritic axis of given cortical elements, that is, by coincidence detection.

 

 

Oscillations

Neural oscillations  unify the brain's disparate components. (Hobson; Consciousness, 72)

Rhythmic activity can be generated by two main mechanisms — intrinsic membrane properties and synaptic circuits. (Shepherd and Koch; Synaptic Circuits, 30)

Many types of neurons in the vertebrate central nervous system possess complex and highly nonlinear ionic conductances that allowed these cells to respond to inputs by oscillating at various frequencies. (Shepherd and Koch; Synaptic Circuits, 30)

Intrinsic membrane properties were first found in pacemaker neurons in central pattern generator circuits controlling breathing, walking, and other highly stereotyped behaviors in invertebrates. (Shepherd and Koch; Synaptic Circuits, 30)

Cells in globus pallidus and in substantia nigra pars reticulata have very high rates of tonic activity. Fire rhythmically at a rate determined by membrane characteristics. (Wilson; Basal Ganglia, 373)

Mutual inhibitory connections  synchronize interneurons, producing oscillations at various frequencies, including theta (5 Hz) and gamma (40 Hz) frequencies. (Johnston; Hippocampus, 423)

Semisynchronous, neuronal oscillations in the 25-55 Hz band could cause neurons to be coordinated, giving rise to short-term memory and thus to awareness. (Koch and Crick; Neuronal Basis, 109)   [Edelman's dynamic core]  [Baddeley - Working Memory] 

Gamma Frequency Oscillations

High frequency oscillations (40-60 Hz) of a large proportion of the neuronal population in a given area appear to be a common occurrence in awake, behaving animals. (Shepherd and Koch; Synaptic Circuits, 30)

40 Hz oscillations are present at many levels in the CNS, olfactory bulb, specific and nonspecific thalamic nuclei, reticular thalamic nucleus, and neocortex. (Llinás & Paré; Brain Modulated by Senses, 7)

Fourier analysis of the spontaneous, broadly filtered rhythmicity (1-200 Hz) demonstrated a large peak of activity at 40-Hz over much of the cortex. (Llinás; Perception as Oneiric-like, 118)

The 40-Hz oscillation is a candidate mechanism to produce temporal conjunction of rhythmic activity over a large ensemble of neurons. (Llinás; I of the Vortex, 124)

Experiments using magnetoencephalography (MEG) has demonstrated that the coherence and oscillations at 40 Hz may be centrally related to cognition. (Llinás & Paré; Brain Modulated by Senses, 7)

Neurons with intrinsic oscillatory capabilities in the complex synaptic network allow the brain to generate dynamic oscillatory states, which can shape the computational events  evoked by sensory stimuli. (Llinás; Perception as Oneiric-like, 114)

Magnetoencephalographic recordings performed in awake humans revealed the presence of continuous and coherent 40-Hz oscillations over the entire cortical mantle. (Llinás; Perception as Oneiric-like, 115)

There is experimental evidence for the existence in the nervous system of temporal signal structure of appropriate nature to encode binding by temporal synchrony. (von der Malsburg; Binding Problem, 137)

Temporal bandwidth of neural signals is severely limited. It is not possible within short periods of time to express complicated multilevel binding structures in terms of signal correlations. (von der Malsburg; Binding Problem, 140)

 

It is unlikely that the processing time in each cortical area is sufficiently long for a stochastic iterative process, or for temporal encoding and synchronization of multiple different populations of neurons. (Rolls & Treves; Neural Networks, 177)

My speculation is that the associative property of memory will allow a correlation between neuronal assemblies, sufficient for integration of widespread neuronal activity into the dynamic core of consciousness.

When stimulus-dependent temporal synchronization has been rigorously tested with information theoretic approaches, it is then found that most of the information available is in the number of spikes, with rather little, less than 5% of the total information, in stimulus-dependent synchronization. (Rolls; Memory, Attention, and Decision-Making, 325)

There is little additional information, to the great deal available in the firing rates, from any stimulus-dependent cross-correlations of synchronization that may be present. (Rolls & Deco; Noisy Brain, 62)

 

Gamma oscillations should be confined to discrete active locations rather than being diffusely present over a wide cortical region. (Buzsáki; Rhythms of the Brain, 245)

Recording sites as close as 3--4 mm from each other in the visual cortex yielded quite different amplitudes of gamma oscillations. (Buzsáki; Rhythms of the Brain, 245)

Induced gamma activity emerges at a variable latency between 150 and 300 ms after stimulus onset, approximately at the time when stimuli acquire meaning. (Buzsáki; Rhythms of the Brain, 244)

Theta Frequency Oscillations

Anterior cingulate cortex (ACC) monitoring activity in the theta frequency band could be seen as a violation of expectation process; i.e., a monitoring process that compares and analyzes the similarities and differences between an expected stimulus or action and a presented or performed stimulus or action. (Posner; Cognitive Neuroscience of Attention, 313)

Hippocampal theta oscillations are related to episodic and semantic memory. (Buzsáki; Rhythms of the Brain, 331)

Phase coding of place cell firing with the respect to the concurrent theta rhythm of the EEG    (O'Keefe) (Andersen; Hippocampus Book, 727)

The theta rhythm is a large-amplitude oscillation of around 6 to 10 Hz of the EEG    and is present whenever the rat is moving its head    through the environment. (Andersen; Hippocampus Book, 727)

 

Research study — Theta Rhythms Coordinate Hippocampal—Prefrontal Interactions

Research study — Interneurons Project Long-Range in Hippocampus and Entorhinal Cortex

 

Rostrocaudal phase shift of 40-Hz signals

Phase comparison of the oscillatory activity recorded from different cortical regions revealed the presence of a 12- to 13-msec phase shift between the rostral and caudal pole of the brain. (Llinás; Perception as Oneiric-like, 115)

Overall speed of the rostrocaudal scan, which averaged approximately 12.5 msec, corresponded quite closely to half a 40-Hz period. This number is the same as that calculated for a quantum of consciousness in psychophysical studies in the auditory system. (Llinás; Perception as Oneiric-like, 120)

Correlated spikes, Stochastic nature of spikes

Correlated spikes from full spike trains may be more meaningful than responses from individual neurons. (Mumford; Neuronal Architectures, 136)

Set of spikes may be much less stochastic, carrying information transmitted between areas, and hopefully correlated much more precisely and predictably with identifiable aspects of an input. (Mumford; Neuronal Architectures, 136)

The reliability of spike transmission increases steeply for approximately 20 to 40 synchronous thalamic inputs in a time window of 5 milliseconds, when the reliability per spike is most energetically efficient. The optimal range of synchronous inputs is influenced by the balance of background excitation and inhibition in the cortex, which can gate the flow of information into the cortex. Ensuring reliable transmission by spike synchrony in small populations of neurons may be a general principle of cortical function.  (Spike Synchrony in Small Populations of Neurons)

 

Resonance and dissonance of firing

Resonance manifests itself by potentiation of firing, dissonance by its extinction. (Changeux; Neuronal Man, 139)

Rhythmic EEG

A pronounced rhythm dominates the EEG throughout the nonprimate mammalian limbic system during exploratory activity and learning. Theta rhythm range 5-12 Hz. (Eichenbaum; Olfactory Perception and Memory, 181)

Simultaneity of neuronal activity is the most pervasive mode of operation of the brain.  Neuronal groups that oscillate in phase, i.e. coherently, support simultaneity of activity. (Llinás; I of the Vortex, 10, 42, 48, 56, 57, 63)  (Crick 22)  (Damasio 95)  (Neil’s 227)

Scherzo of Schubert's Piano Quartet No.8 requires repetitive hand movements at approximately 8 Hz, which approaches the upper limit for finger movements by professional pianists. (Llinás; I of the Vortex, 30)

The spinal cord is capable of sustaining a rhythmic movement -- like a decapitated chicken -- but it cannot organize and generate a directed movement. (Llinás; I of the Vortex, 44)

Intrinsic neuronal oscillation - sensory input is necessary for the modulation of ongoing movement. (Llinás; I of the Vortex, 42)

25-100 Hz, gamma; possible relevance to consciousness. (Zeman; Consciousness, 84)

Four rhythms commonly encountered in EEGs. beta 14-25 Hz, alpha 8-13 Hz, theta 4-7 Hz, delta <4 Hz. - (illustration) (Zeman; Consciousness, 85)

Only synchronized activity in substantial numbers of cortical cells could generate currents large enough to be detected over the scalp. Source of these currents is activity in the dendrites of cortical neurons. (Zeman; Consciousness, 86)

Many neurons active, often rhythmic. If neurons are suitably interconnected, intrinsically rhythmic patterns can generate widespread synchronous patterns of EEG. (Zeman; Consciousness, 86)

Neurons whose spontaneous rhythms contribute to the EEG lie in two regions: (1)cerebral cortex, (2) straitened confines of the thalamus. (Zeman; Consciousness, 86)

Thalamus, the gateway to the cortex, is especially well placed to synchronize rhythms throughout the brain. (Zeman; Consciousness, 86)

Alpha (Koch 38)

40 Hz (Koch 39)

 

Thalamocortical circuits oscillate in NREM sleep

When the activation level of the brainstem falls, even a little, the thalamocortical circuits begin to oscillate. This kind of synchrony contributes to the global loss of consciousness that occurs in NREM sleep. (Hobson; Consciousness, 70)

Oscillations of the thalamocortical circuits that occur at sleep onset are robust and so highly synchronous that they cause the characteristic EEG pattern of slow wave sleep. (Hobson; Consciousness, 71)

In drowsiness or non-REM sleep, the thalamic relay cells go into an oscillatory mode in which they alternate between short, high-frequency bursts and extended periods of hyperpolarization, repeating at a frequency of 7-14 Hz. (Mumford; Thalamus, 981)

Embryos generate continuous bouts of muscle tremor, not unlike small epileptic fits. (Llinás; I of the Vortex, 63)

 

Synchronization

The term ‘synchronization’ should be interpreted broadly to mean ‘coherence of neuronal assemblies’ at the group level (lower frequencies),    not necessarily synchronized individual neuronal firing (higher frequencies).

Each mental act is characterized by simultaneous neuronal activity in different brain areas. (Poppel; Time Perception, 987)

 

Research Study — Working Memory Fronto-Parietal Synchronization — Lateral prefrontal and posterior parietal cortical areas exhibit widespread, task-dependent, and content-specific synchronization of activity  during working memory tasks in humans and monkeys.

 

Consciousness is a noncontinuous event determined by simultaneity of activity in the thalamocortical system. (Llinás; I of the Vortex, 124)

Intralaminar nuclei of the thalamus send diffuse projections to most areas of the cerebral cortex and help to synchronize its overall level of activity. (Edelman; Universe of Consciousness, 107)

Perhaps music creates a resonance in the brain between neurons firing in synchrony with a sound wave and a natural oscillation in the emotion circuits? (Pinker; How the Mind Works, 538)

Neurons several millimeters apart in the same or different stages of the visual system, and even across the two cerebral hemispheres, have been shown to come together in time transiently by gamma frequency synchronization. (Buzsáki; Rhythms of the Brain, 240)

Synchronized Oscillations in Consciousness

If synchronized oscillations are required for consciousness, intrinsic rhythmicity of neuronal discharge allows for the rhythmic pacing of brain activity. (Zeman; Consciousness, 294)

Synchronous firing of neurons involved in a common activity is often rhythmic, in the gamma band, 25-100 Hz. (Zeman; Consciousness, 294)

Ubiquitous, bidirectional connections between related brain regions facilitates synchronization. (Zeman; Consciousness, 294)

Rhythmic, synchronous firing of intricately-connected neuronal feedforward and feedback networks generate the gamma-band electrical and magnetic signals measured externally near the brain. Intricately-connected neuronal  feedforward and feedback networks, with reentry and recursion, result in coherent firing and the generation of gamma-band electrical and magnetic signals.

Research study — Synchronized Oscillations and Inhibitory Interneurons — Selectively modulating multiple distinct circuit elements in neocortex,   gamma-frequency modulation of excitatory input was found to enhance signal transmission by reducing circuit noise and amplifying circuit signals.

Synchronized Activity in Consciousness differs greatly from Synchronization in Epilepsy

The synchronization that is hypothesized for the function of binding in consciousness differs greatly in detail from the synchronization that is associated with epilepsy. Synchronization associated with epilepsy is fairly widespread over the brain, is relatively low frequency about 10 Hz and typically lasts many seconds or minutes. In contrast the synchronization that is hypothesized for the function of binding in consciousness is associated with multiple small neural assemblies (perhaps ~8000 neurons within a volume of ~0.1 mm³) that lock into synchronization across widespread cortical and subcortical areas after only a few cycles of oscillation around 40 Hz and remain synchronized less than perhaps 0.3 sec before being desynchronized by some active process.

Only synchronized activity in substantial numbers of cortical cells could generate currents large enough to be detected over the scalp. Source of these currents is activity in the dendrites of cortical neurons. Many neurons active, often rhythmic. If neurons are suitably interconnected, intrinsically rhythmic patterns can generate widespread synchronous patterns of EEG. Neurons whose spontaneous rhythms contribute to the EEG lie in two regions: (1) cerebral cortex, (2) straitened confines of the thalamus. Thalamus, the gateway to the cortex, is especially well placed to synchronize rhythms throughout the brain. (Zeman; Consciousness, 86)

Firing pattern of a population of pyramidal cells in Layer 5 can initiate synchronized rhythms and project them on neurons in all layers. (Ullman; Sequence Seeking Counterstreams, 265)

Feature Binding through Oscillatory Synchrony

Representation of the various attributes of the visual world by distributed neuronal assemblies can be bound together harmoniously in the time domain through oscillatory synchrony. (Buzsáki; Rhythms of the Brain, 232)

Features processed in separate parts of the cortex by different sets of neurons are bound into a complex representation in a matter of 200 ms or so. (Buzsáki; Rhythms of the Brain, 232)

 

Research study — Synchronization of Neuronal Activity by Interneurons

Research study — Spike Synchronization in Motor Cortical Function

Research study — Synchronized Brain Interactions for Memory and Decision-Making

 

Gap junctions

Gap junction channels, direct flow of current from cell to cell, rapid and synchronous firing of interconnected cells. (Llinás; I of the Vortex, 90)

Gap junctions, synchronizing hippocampal GABA cells. (LeDoux; Synaptic Self, 61)

The overall stochastic nature of neuronal behavior suggests that the physiologically meaningful signal from cortex should be the average firing rates of a. population of perhaps 100 to 1000 neurons near a particular cortical site. (Stevens; Cortical Theory, 243)

Brain has 10¹¹ neurons. With 1000 neurons per assembly, there are ~10⁸ assemblies. Notice that the combinatorics of ~10⁸ assemblies is beyond comprehension, however, relationships among the assemblies will vastly constrain the possibilities.

Although the 1000 neurons in an assembly may be highly correlated and often operate in unison, there will likely be linkages with other assemblies in which the correlations within the assembly will be less pronounced. In effect, the assemblies with have variability, especially with the plasticity that is associated with memory and learning.

The dynamic core of consciousness in a waking state of reverie might involve perhaps 10% or less of the ~10⁸ assemblies. In a highly challenged state of alert attention, the dynamic core might involve over 50% of the assemblies.

Shape of the dendritic tree may affect spike output pattern

Theoretical work has suggested that the shape of the dendritic tree is a major factor in controlling the pattern of spike output from neurons, (Douglas; Neocortex, 465)

Consciousness is a noncontinuous event determined by simultaneity of activity in the thalamocortical system. (Llinás; I of the Vortex, 124)

Coherence between different brain areas, higher frequency range about 25 to 70 Hz, links together the separate areas. (Calvin; Neil's Brain, 227)

Traditionally, the cerebellum is considered to be concerned with the coordination and synchrony of motion. (Edelman; Universe of Consciousness, 45)

Four rhythms commonly encountered in EEGs. beta 14-25 Hz, alpha 8-13 Hz, theta 4-7 Hz, delta <4 Hz. - (illustration) (Zeman; Consciousness, 85)

Original hypothesis was that the phase-locked firing of a set of neurons at 40 Hz was the neural correlate of visual awareness. (Koch and Crick; Neuronal Basis, 101)

Firing pattern of a population of pyramidal cells in Layer 5 can initiate synchronized rhythms and project them on neurons in all layers. (Ullman; Sequence Seeking Counterstreams, 265)

Distributed groups of neurons, synchronous activity. (Singer; Neuronal Synchronization, 109)

Cortical neural signals have a very pronounced stochastic structure, and nervous tissue is evidently designed to create and preserve it. An assumption of statistical independence of signals therefore must be faulty, and strong signal correlations must be present in the brain. (von der Malsburg; Binding Problem, 139)

 

SQUID measurement of 40-Hz oscillation in magnetic fields

The 40-Hz oscillation is a candidate mechanism to produce temporal conjunction of rhythmic activity over a large ensemble of neurons. (Llinás; I of the Vortex, 124)

Coherence measurements of MEG signals over the hold extent of the cerebral hemispheres indicates that significant coupling in the gamma frequency band is present in the waking brain as well is during REM sleep. (Buzsáki; Rhythms of the Brain, 243)

Rudolfo Llinás detected magnetic field oscillations associated with brain activity. A superconducting quantum interference device (SQUID) was used in the quiet environment of a shielded room to detect the tiny magnetic oscillations produced by brain activity. In 1990 he recorded a rapid oscillation in the gamma frequency band in waking subjects, which was also present during REM but absent from slow wave sleep. The Rudolfo Llinás results hint at a role for fast oscillations, around 40 Hz, in the genesis of consciousness. (Zeman; Consciousness, 90)

Limited controlled synchronization of rapid neuronal discharge might play a role in perception, memory and movement. (Zeman; Consciousness, 293)

Reentry and Synchronization

Reentry is the central organizing principle that governs the spatiotemporal coordination among multiple selectional networks of the brain. (Edelman; Wider than the Sky, 41)

Consequence of this dynamic process is the widespread synchronization of the activity of widely distributed neuronal groups. Binds their functionally segregated activities into coherent output. (Edelman; Wider than the Sky, 41)

Reentry solves the binding problem. Through reentry, for example, the color, orientation and movement of a visual object can be integrated. (Edelman; Wider than the Sky, 41)

The idea that an iterative algorithm is carried out in the thalamocortical loop has received experimental confirmation in observed oscillations. (Mumford; Thalamus, 982)

Tendency to Oscillate controlled by Inhibition

Models of cortical networks reveal the need to regulate the tendency of recurrent networks to oscillate. (Sejnowski; Thalamocortical Oscillations, 980)

The excitability of neurons can be controlled by inhibition. (Sejnowski; Thalamocortical Oscillations, 980)

Inhibition is an efficient mechanism for synchronizing large populations of pyramidal neurons because of voltage-dependent mechanisms in their somas and the strategic location of inhibitory boutons on the initial segments of axons, where action potentials are initiated. (Sejnowski; Thalamocortical Oscillations, 980)

 

Hierarchical visual processing

Small networks of cells combine to represent objects and people. Place and frequency coding relate to single cells. Time or phase coding relates to the activity of groups of cells. Neurons that represent the disparate features of a single object -- which may be widely spread across the brain -- are associated by firing at the same moment. Synchronous firing of neurons involved in a common activity is often rhythmic, in the gamma band, 25-100 Hz. If synchronized oscillations are required for consciousness, intrinsic rhythmicity of neuronal discharge allows for the rhythmic pacing of brain activity; ubiquitous, bidirectional connections between related brain regions facilitates synchronization. (Zeman; Consciousness, 294).

Synchronized activity across brain regions at around 40 Hz, a signature of wakefulness, provides a mechanism by which the contents of consciousness can be bound into a unified whole. 40 Hz oscillation, signature of perceptual awareness and a candidate for the mechanism of binding, may prove to be the most convincing physiological correlate of consciousness. (Zeman; Consciousness, 301, 324).

Suprachiasmatic nucleus; intrinsically rhythmic, cycle close to 24 hours. Pacemaker for the body's circadian rhythms of activity. (Zeman; Consciousness, 103)

Neurons are coincidence detectors: large numbers of other cells connect to them. Synchronous firing of networks of cells is probably a feature of brain regions controlling movement as well as of regions involved in sensation. (Zeman; Consciousness, 295)

 

Temporal coherence - timeness is consciousness - perceptual unity based on spacial and temporal conjunction. (Llinás; I of the Vortex, 120)

 

Coherence between different brain areas, higher frequency range about 25 to 70 Hz, links together the separate areas. (Calvin; Neil's Brain, 227)

Bursts of Firing

Potentiation requires a high-frequency train of stimuli for its induction and persists for up to a few minutes after the end of the train. (Andersen; Hippocampus Book, 211)

Because principal neurons    frequently discharge in bursts of action potentials,    the degree of postsynaptic facilitation or depression  during such bursts may contain much of the information  transmitted through the network. (Andersen; Hippocampus Book, 211)

 

Quantum of Cognition

A quantum of cognition (not to be confused with ‘quantum’ in quantum physics; they are completely unrelated) is measured to be a well-defined 12-15 millisecond time epoch. (Llinás; I of the Vortex, 206)

The perceptual capability of the central nervous system is such that for two sensory stimuli to be perceived as two distinguishable sensory events, there must be a minimum of 12.5 milliseconds separating these events. (Llinás; I of the Vortex, 206)

A quantum of cognition requires the patterned activation of millions or even hundreds of millions of cells. (Llinás; I of the Vortex, 206)

Electricity is the only medium fast enough and far-reaching enough to support the rapid and widespread ensemble activity underlying sensory experience within perceptual time frames. (Llinás; I of the Vortex, 207)

 

 

 

Link to — Consciousness Subject Outline

Further discussion — Covington Theory of Consciousness