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

Hierarchically Arranged Network of Synaptic Connections

Human brain has approximately 100 billion neurons, and each neuron on average connects to about 1000 other neurons. (Gazzaniga; Human, 283)

Brain has about 100 trillion synaptic connections. (Gazzaniga; Human, 283)

Each neuron receives about 10⁴ synapses and communicates with about 10⁴ other neurons. (Stevens; Cortical Theory, 242)

A given axon generally arborizes over a considerable region of cortex with an arbor diameter of perhaps 0.5 mm, and forms about 2000 boutons, each of which makes one or two synapses. (Stevens; Cortical Theory, 242)

Every human brain has billions of neurons that together make trillions of synaptic connections among one another. During wakefulness and during sleep, during thoughtfulness and during boredom -- at any one moment, billions of synapses are active. (LeDoux; Synaptic Self, 49)

Hierarchical Arrangement of Neural Networks Circuits

Brain circuits can be thought of as hierarchically arranged circuits linked together by synaptic connections. (LeDoux; Synaptic Self, 49)

In most brain systems information processing is organized hierarchically. (Kandel; Principles of Neural Science, 324)

Patterns of interconnectivity between visual cortical areas had been classified in a hierarchical organization of ten levels. (LaBerge; Attentional Processing, 105)

A general rule of connectivity between cortical areas is that an area receiving fibers from another area reciprocates the connection. (LaBerge; Attentional Processing, 106)

Synaptic functionality in processing pulse-spike information in spines and dendritic trees

• Neurons communicate information via neurotransmitters in synapses.

• Brain circuits can be thought of as hierarchically arranged circuits linked together by synaptic connections. (LeDoux; Synaptic Self, 49)

• It is estimated that a large feedback loop covering the entire brain takes only five or six synapses. (Andreasen, Creating Brain, 59)

• Every human brain has billions of neurons that together make trillions of synaptic connections among one another. During wakefulness and during sleep, during thoughtfulness and during boredom -- at any one moment, billions of synapses are active. (LeDoux; Synaptic Self, 49)

• The dynamic core that mediates consciousness consists of a subset of active network neurons at a particular moment.

• Three main neuroanatomical motifs in the brain: (1) thalamocortical, (2) polysynaptic loop structure, (3) diffuse ascending projections of the different value systems. (Edelman; Wider than the Sky, 26)

• A neuron’s thousands of dendritic synapses receive pulse-like input from other neurons.

• Whenever a large group of spikes arrives nearly simultaneously, a neuron will generate an action potential, which is transmitted down its axon. This near simultaneous arrival of input spikes creates pulse-like behavior. The following reference provides an informative discussion of synaptic functionality in processing pulse-spike information in spines and dendritic trees. (Koch; Shepherd; Synaptic Organization of the Brain, 1-36)

• 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)

• The behavior of cortex at a particular point would then be described by the firing in a population of neurons. The total firing that represents this population would be determined by a weighted average of the appropriate neurons in the cortical region that surrounded the point, perhaps with weights that are described by a spatial Gaussian. Moving from one cortical location to an adjacent one, the variables describing cortical state would vary continuously with cortical position. (Stevens; Cortical Theory, 243)

• In addition to the preponderance of synaptic connections between neurons, Rudolfo Llinás argues for the electrical connectivity of some neurons via gap junctions, which provides faster network connectivity for synchronization and binding than could be provided via diffusion of neurotransmitters across synapses.

Projection Neurons Linked by Interneurons

Projection neurons have relatively long axons that extend out of the area in which their cell bodies are located. (LeDoux; Synaptic Self, 49)

Interneurons link their short axons to nearby neurons, often projection neurons, and are involved in information processing. (LeDoux; Synaptic Self, 49)

Projection neurons tend to be idle in the absence of inputs. Inhibitory interneurons are often active all the time. (LeDoux; Synaptic Self, 50)

Excitation and inhibition in circuits - (diagram) (LeDoux; Synaptic Self, 52)

Interneuron Connectivity

Scientific understanding of cortical interneurons has changed dramatically in the decade spanning the year 2000. Interneurons are now understood to be a large family of intrinsically different cells with unexpectedly complex circuit wiring. (Buzsáki; Rhythms of the Brain, 69)

Existence of interneuron-specific interneurons provides support for a unique organization of the inhibitory system. (Buzsáki; Rhythms of the Brain, 71)

Long-range interneurons with distant clouds of terminal boutons are separated by myelinated axon collaterals that provide fast conduction speed for temporal synchrony of all terminals. (Buzsáki; Rhythms of the Brain, 70)

Neurons are coincidence detectors and are superbly sensitive to signal correlations.

A correlation pattern can selectively activate receiving circuits of appropriate structure. In the simplest case this is possible if the circuit that receives the pattern is isomorphic to the circuit that created it. (von der Malsburg; Binding Problem, 142)

Whereas in classic neural networks neurons do not pay attention to fine signal structure, in the dynamic link architecture the incoming signals have to be correlated in time. (von der Malsburg; Binding Problem, 142)

A given set of neurons can support a large number of activity patterns that differ only in their fine temporal structure. (von der Malsburg; Binding Problem, 142)

Each neuron can participate in many synfire chains and even several times in the same chain. Synfire chains are proposed as the basic building blocks of a compositional cognitive system. (von der Malsburg; Binding Problem, 142)

Signal correlations are evaluated by synapses by modifying their dynamic weight. The interaction between signal correlations and synaptic dynamics has the form of a positive feedback loop. This feedback loop is the basis for a system of rapid network self-organization. (von der Malsburg; Binding Problem, 142)

Signal correlations act back on the network and modify its structure by rapid synaptic modification. This leads to a run-away situation that comes to a halt when a network is reached in which the signal structure and the connectivity structure are consistent with each other. (von der Malsburg; Binding Problem, 142)