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

 Reentry and Recursion

 Reentrant activity leading to recursion is a fundamental feature of thalamocortical activity, and indeed nearly all neural activity.  Reentrant activity is not simple feedback but functions in a network as recursive multiple pathways, which update cyclically on a time scale of tens to hundreds of milliseconds, rapidly converging to the dynamic core’s synaptically connected neuronal network mediating an instantaneous thought. This cyclical neural activity generates gamma (~40-Hz) oscillations in the normal waking state. The converged state of the dynamic core can change as individual thoughts change on an approximate 100 ms basis.  This allows a few cycles of the recursive reentrant activity with the local synaptic loops at the gamma (~40 Hz) rate to converge to the dynamic core.

Comprehension is always accomplished by a "holistic" attempt to integrate the information from all sources that has arrived in the brain up until that point. When further information (from any semantic source) arrives that can disambiguate an earlier piece of information, the model is adjusted accordingly. (Hurley, Dennett, Adams; Inside Jokes, 99)


Reentry and recursion and the thalamocortical system

Core consciousness is created in pulses, each pulse triggered by an object we interact with or that we recall. Each new object triggers the process of changing the proto-self. Proto-self modified by the first object becomes the inaugural proto-self for the new object. Continuity of consciousness is based on the steady generation of consciousness pulses, which correspond to the endless processing of myriad objects, whose interaction, actual or recalled, constantly modifies the proto-self. (Damasio; Feeling of What Happens, 176)

Strong reciprocity exists in thalamocortical connections. (Shepherd; Synaptic Organization of the Brain, 295)

Experimental observations have concluded that sensory information is processed in discrete time segments as low as 12 ms. (Llinás & Paré; Brain Modulated by Senses, 12)

Whereas the 12.5 ms time for the quantum of cognition has been determined psychophysically, another very distinct measurement of the phase shift of 40-Hz oscillatory activity over the human cortex has a 12.5-ms duration as well. (Llinás & Paré; Brain Modulated by Senses, 12)

In alert subjects, continuous 40-Hz oscillations can be recorded over large areas of the surface of the head.  These oscillations are not in phase, but exhibit a 12- to 13-ms phase shift between the rostral and caudal parts of the brain. (Llinás & Paré; Brain Modulated by Senses, 7)

The thalamocortical system, by its hublike organization, allows radial communication of the thalamic nuclei with all aspects of the cortex. These cortical regions include the sensory, motor, and associational areas. These areas subserve a feedforward/feedback, reverberating flow of information. (Llinás; I of the Vortex, 126)

Thalamic input from the cortex is far larger than from the peripheral sensory systems. This suggests that thalamocortical iterative activity is a main mechanism of brain function. (Llinás; I of the Vortex, 124)

Connectivity between the thalamus and the cortex is bidirectional. Layer 6 pyramidal cells project back to the area of the thalamus where their specific input arises, and layer 5 cells project to the nonspecific thalamus. The number of corticothalamic fibers is about one order of magnitude larger than the number of thalamocortical axons. (Llinás; Perception as Oneiric-like, 113)

A large part of the thalamocortical connectivity is organized in what is presently known as reentrant activity (Edelman) or previously viewed as reverberating activity. Only a minor dart of thalamocortical connectivity is devoted to the reception and transfer of sensory input. The number of cortical fibers projecting to the specific thalamic nuclei is much larger than the number of fibers conveying the sensory information to the thalamus. (Llinás; Perception as Oneiric-like, 114)

Reciprocal connections between hippocampus and neocortex, long-term storage of memories. (LeDoux; Synaptic Self, 104)

Widespread cortical and subcortical activity constitutes an image

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)

Layer 5's orderly connections to subcortical structures (e.g., from visual cortex to the pulvinar and the superior colliculus, structures implicated in controlling attention and eye movements) that are reciprocally connected in turn in a topographic manner to multiple visual areas. (Ullman; Sequence Seeking Counterstreams, 265)

Pulvinar, a subcortical nucleus of the thalamus, makes reciprocal connections with all of these visual processing cortical areas. (Van Essen; Dynamic Routing Strategies, 285)



    Return to — Reentry and Recursion

    Link to — Consciousness Subject Outline

    Further discussion — Covington Theory of Consciousness