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

Stochastic Nature of Neuronal Behavior

Neuronal Populations

Because single neurons have small and uncertain effects on other neurons, the cortical description must be carried out in terms of neuronal populations rather than at the level of individual cells. (Stevens; Cortical Theory, 242)

Layer 4 neurons have a dendritic tree with a diameter of about 0.3 mm. (Stevens; Cortical Theory, 242)

Layer 4 is about 0.3 mm thick, and cortex has a density of about 10⁵ neurons per mm³. (Stevens; Cortical Theory, 242)

All of the neurons in layer 4 that fall within a cylinder having a radius of about 0.3 mm will have overlapping dendritic trees. The number of neurons that overlap is estimated to be approximately 8000. A significant fraction of this population of neurons should represent essentially the same information. (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)

Neurons of the same functional class and in the same cortical layer share nearly the same potential synaptic inputs whenever their cell bodies are separated by several hundred microns or less, and the degree of similarity in their inputs increases as the distance between cell bodies decreases. (Stevens; Cortical Theory, 243)

Neuronal ensembles, not individual neurons. (Merzenich; Neural Representations, 65)

 Stochastic Nature of Neuronal Behavior

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)

Basic unit of the synaptic network selective process is not the individual neurons but the neuronal group. There are perhaps 100 million neuronal groups in the brain, and they range in size from 50 to 10,000 neurons. Neurons are only effective in groups, working toward a single goal, such as discriminating color or producing emotion. By virtue of their size, neuronal groups can compensate for individual cell deaths. (Ratey; User's Guide to Brain, 142)

A theory of cortex must be coarse-grained and treat cortical inputs and outputs as continuous variables that represent the summed behavior of appropriately sized and selected neuron populations. (Stevens; Cortical Theory, 243)

The prominent recurrent nature of lateral intracortical connections and relatively wide spatial distribution of cortical inputs mean that the cortical output at any one location must depend on both the input and output over relatively great expanses of cortex. That is, the output at any one point must be a functional, of both inputs and outputs. (Stevens; Cortical Theory, 243)

The fast dynamics of local recurrent networks may be facilitated by the fact that cortical neurons in the awake state have a low spontaneous rate of firing (a few Hz),    which means that any small additional input    may produce some spikes sooner than otherwise would have occurred,    because some of the neurons may be very close to a threshold of firing. (Rolls; Memory, Attention, and Decision-Making, 26)

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)

 

 

Return to — Neurons and Synapses

Return to — Neural Network

Link to — Consciousness Subject Outline

Further discussion — Covington Theory of Consciousness