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

Modularity of the Brain

The modularity of the brain can be considered in terms of anatomical modularity and functional modularity.  The anatomical modularity, although seemingly jumbled and complex, has been established as a result of natural selection in many ancient environments.  Functional modularity and the anatomical modularity are highly interdependent.

Anatomical modularity, which has been established by evolution from reptiles, through mammals and then through primates and humans, includes: (1) brainstem for basic life processes, (2) limbic system for rapid emotional response to environmental stimuli, and (3) cortex to provide refinement to behavior.

Six major brain divisions: medulla, pons, cerebellum, midbrain, diencephalon, and cerebral hemispheres or telencephalon. Each of these divisions is found in both hemispheres. (Kandel; Principles of Neural Science, 319)

The human cortex is comprised of four major lobes that provide functionality for sensory perception and behavioral action. The insula is considered to be the fifth and smallest lobe of the brain. A large part of the human brain is devoted to visual processing in the occipital lobe. The parietal and temporal lobes are heavily involved in interpreting and processing the sensory information.  The frontal lobe is highly involved in the mechanisms of action and decisions for action.

The brain has two hemispheres with the functionality lateralized, most notably having language functions specialized in the left hemisphere for most people.

Vast majority of the cortex is given over to sensory processing -- only the frontal lobes are dedicated to non-sensory tasks. (Carter; Mapping the Mind, 115)

Declarative memory functions are concentrated in the hippocampus and associated structures.

The composition of the limbic system, originally suggested in mid-20th-century, has continually been reinterpreted to refine its composition and extent as new experimental data have become available.  My discussion of the limbic system will include the orbitofrontal cortex, the anterior cingulate gyrus, and perhaps some functions of the hippocampus.  These structures will be included along with the usual structures of the limbic system such as the amygdala.

Structural and functional modularity includes: (1) neuromodulatory neurons in the brainstem and midbrain, and (2) basic homeostasis systems of the brainstem, hypothalamus, and functions of the sleep/wake cycle.

Most importantly, the thalamocortical system provides the mechanism of the dynamic core of consciousness and the means by which the thalamus can constrain the sensory input to the cortex and focus neural processing for attention.

 

Link to — Functional Modularity of the Cortex

 

Functional modularity of the neocortex is more directly associated with consciousness. The dynamic core consists of a subset of neuronal activity that is momentarily united/integrated in circulating neural spikes that mediate a conscious thought. Although some neural assemblies may never participate in the dynamic core, most assemblies will frequently participate in the dynamic core. The configuration of circulating spikes of the dynamic core changes every few milliseconds as thoughts change. Typically, the dynamic core will consist of a sparsely distributed subset of neural activity in many modular areas, with many millions of neurons and billions of synapses involved. The neuronal and synaptic composition of the dynamic core and the modular areas involved will change from moment to moment as thoughts change. The varying patterns of fMRI images reflect these changes in neuron and synapse activity.

Brain region map is generally consistent among people, yet can vary. (Ratey; User's Guide to Brain, 269)

Modularity of the brain can be considered in terms of Fuster’s Perception-Action Cycle.

Evolutionary development from (1) brain stem, (2) limbic system, and (3) cortex.

Massive expansion of the surface area of the cerebral cortex in humans accommodates many more columns and thus provide greater computational power. (Kandel; Principles of Neural Science, 331)

Architecture of the Cerebral Cortex  (Koch; Quest for Consciousness, 117)

Many functionally distinct cortical regions, over 30 in the visual system. (Stevens; Cortical Theory, 239)

Brain Modularity and Consciousness

Consciousness is mediated by widespread neural activity in many of the modular areas of the brain. Consciousness is an emergent property of the cortical activity that mediates a particular momentary thought, and since thoughts are fleeting from moment to moment, the subset of neural activity for thoughts changes on a momentary basis. The dynamic core of consciousness neural activity is focused in the thalamocortical system (thalamus and cortex reentrant circuits) but also includes working memory (frontal cortex with its subdivisions), emotional functionality of the limbic system, basal ganglia, cerebellum and other subcortical areas controlling movement, and the neuromodulatory systems of the brain stem.

According to Gazzaniga's model of consciousness, there maybe hundreds, if not thousands, of modules contributing to our conscious experience, each contributing specialized bits of information. (Gazzaniga; Left Hemisphere/Right Hemisphere, 266)

 

Modularity provides specialized areas for rapid parallel processing

 

Link to — Dual Routes of Responses

 

Thalamocortical system provides connectivity for the parallel processing in the modular areas.

Working memory is a high-level association area.

Thalamocortical system binds working memory with (1)  specialized sensory modular areas in parietal cortex and with (2) specialized motor modular areas in frontal cortex.

Thalamus provides selective functionality for attention.  Executive function of working memory controls the selective functionality of the thalamus.  The thalamus receives feedback from all cortical processing areas.

Thalamocortical System

Cortex, thalamus

Multiple ways to categorize the modularity of the brain

There are multiple ways to categorize the modularity of the brain. I will probably use several of these ways, employing a category that seems most appropriate in a particular instance.

Gross physical demarcation

Rhombencephalon (hindbrain), mesencephalon (midbrain), proencephalon (forebrain)   (Llinás; Workings of the Brain; 23)

Five Divisions of the Brain -- Telencephalon, Diencephalon, Mesencephalon, Metencephalon, Myencephalon. (Pinel; Anatomy of Human Brain, 74)

Cortical, subcortical

 

 

Brain is modular, but some functions are widely distributed

Some functions such as vision, hearing, sensory function, motor function, and language are generally localized to modular areas. Notable exceptions are consciousness and long-term memory, which are widely distributed.

 

 

Cortical Modularity

The neocortex is involved in higher functions such as sensory perception, generation of motor commands, spatial reasoning, conscious thought and language.

Posterior sector of the cortical hemispheres is dedicated to sensation, perception, and perceptual memory. (Fuster; Prefrontal Cortex, 380)

Frontal sector of the cortical hemispheres is dedicated to action and executive memory. (Fuster; Prefrontal Cortex, 380)

 

Cortical Lobes

Posterior cortex is structured to compute judgments of particular properties of a stimulus object or scene by segregating the input information and directing it to appropriate sets of structures or modules that perform specialized types of computation. (LaBerge; Attentional Processing, 101)

Thirty-two or so areas of the visual cortex. (LaBerge; Attentional Processing, 101)

Processing some property of an object, such as its location, shape, color, depth, and direction of movement. (LaBerge; Attentional Processing, 101)

Four lobes: frontal, parietal, occipital, temporal (Hobson; Consciousness, 58)

Cerebral cortex is divided into four major lobes: frontal, parietal, temporal, occipital. (Kandel; Principles of Neural Science, 325)

 

Frontal Lobes

 

Temporal Lobe

Temporal lobe has distinct regions that carry out auditory, visual, and memory functions. (Kandel; Principles of Neural Science, 325)

Two major parts of the inferotemporal cortex are: (1) the posterior part, which contains a subarea TEO that is specialized for fine discrimination of forms, and (2) an anterior part of the temporal lobe (sometimes labeled TE), which contains the mnemonic properties necessary for identification of an object. (LaBerge; Attentional Processing, 109)

 

Cingulate Cortex

Cingulate cortex surrounds the dorsal surface of the corpus callosum. (Kandel; Principles of Neural Science, 325)

 

Insular Cortex

Insular cortex forms the medial limit of a lateral sulcus. (Kandel; Principles of Neural Science, 325)

Lateral sulcus or Sylvian fissure separates the temporal lobe from the frontal and parietal lobes. (Kandel; Principles of Neural Science, 325)

Within Brodmann's  classic cortical map, the insula is considered to be the fifth and smallest lobe of the brain, comprising Brodmann's areas 13--16. The insula lies within the convergence of the frontal, temporal, and parietal lobes, and can be seen only when these lobes are retracted. (Miller; Human Frontal Lobes, 62)

 

Asymetry of the cerebral hemispheres - Language

The localization of functions in the brain began with the identification of a cortical speech center by Broca and was followed by the discovery of point-to-point somatotopic maps in the motor and sensory cortices (Penfield and Rasmussen 1950), and in the thalamus (Mountcastle and Hennemann 1949, 1952). (Llinás; Perception as Oneiric-like, 115)

Left and right frontal lobes are differentiated.  Left frontal lobe is more specialized for language-related functions.  Right frontal region is dominant in social cognition and emotions. (Miller; Human Frontal Lobes, 7)

Left hemisphere is specialized for language, speech, and intelligent behavior, while the right hemisphere is specialized for such tasks as recognizing faces, focusing attention, and making perceptual distinctions. (Gazzaniga; Human, 291)

Language depends largely on left-hemisphere structures in more than 95% of people, including many left-handers. (Damasio; Descartes’ Error, 66)

The last stage of the brain's development creates the specialization of the hemispheres. (Ratey; User's Guide to Brain, 200)

Frontal Lobe

Prefrontal cortex can be parcellated into orbital, dorsolateral, and cingulate regions. (Miller; Human Frontal Lobes, 7)

Five distinctive frontal subcortical systems: (1) supplementary motor area, (2) frontal eye fields, (3) dorsolateral prefrontal, (4) orbitofrontal, (5) anterior cingulate cortex. (Miller; Human Frontal Lobes, 7)

Cingulate cortex is particularly important for initiation of behavior. (Miller; Human Frontal Lobes, 9)

Medial frontal cortex comprises the supplementary motor area and the anterior cingulate cortex. (Miller; Human Frontal Lobes, 15)

 

Research study — Prefrontal Cortex Functionally Compartmentalized

Research study — Modularity of Categorization in Parietal Cortex

 

Cortical Columns

About 148,000 neurons beneath a square millimeter of cortical surface; organized into minicolumns of about 100 neurons each, which are sometimes organized into macrocolumns of about 300 minicolumns. (Calvin; Neil's Brain, 92)

 

Layers of Cortex

Layered functionality of cortex from layer 1 through layer 6 in the neocortex and layer 1 through 3 in the paleocortex of hippocampus.

Typical cerebral cortex has a surface area of 2200 cm² and a thickness between 1.5 and 4.5 mm in humans. (Science, “From the Connectome to the Synaptome,” 26 Nov 2010, vol. 330 no. 6008, p.1200)

Although all neocortex areas have six layers, the relative number of cells in each layer and the size of the layer are quite variable and specialized for the specific function. Visual cortex, a primary sensory area, has many cells in the layer IV the input layer, whereas the motor cortex has very large neurons in layer V, the output layer. (Sanes; Development of the Nervous System, 52)

Connections to the layers of cortex are shown in a Cortical Layers Diagram.

Layer IV is typically the input layer. (Calvin; Neil’s Brain, 90)

Layers V and VI are typically output layers. (Calvin; Neil’s Brain, 90)

Layers I - III provide interconnections, sideways connections. (Calvin; Neil’s Brain, 90)

Thalamic fibers stop in well-defined layers, particularly layer IV and part of layer III. Layer IV can be considered the main gateway to the cortex. (Changeux; Neuronal Man, 56)

Pyramidal cells that send axons to the thalamus are located in the deepest layer of the cortex (layer VI) or in the lower part of Layer V. (Changeux; Neuronal Man, 57)

Pyramidal cells sending axons to nonthalamic subcortical centers are situated in layer V. (Changeux; Neuronal Man, 57)

Axons that project back to the cortex come from layers II and III. (Changeux; Neuronal Man, 57)

Lower-layer hypothesis states that the neural correlates of visual awareness occur mainly in the lower layers 5 and 6 of the cortex. The input layer as well as neurons in the upper layers 2 and 3 are assumed to be mainly concerned with unconscious processing. (Koch and Crick; Neuronal Basis, 101)

 

Brodmann’s Areas

Brodmann Areas defined in 1909 on the basis of differences in thin cortical sections. (Changeux; Neuronal Man, 20)

Brodmann Areas, 52 areas, early 20th century, based on neuron size and relative layer thickness. (Calvin; Neil’s Brain, 93)

Brodmann's Areas of the Human Neocortex.  (Koch; Quest for Consciousness, 118)

Brodmann's areas - (diagram) (Damasio; Descartes’ Error, 27)

 

Link to — Brodmann areas diagram

Korbinian Brodmann (1868-1918)

 

Cortical Modularity

(paraphrase of Eichenbaum, Cognitive Neuroscience of Memory, p. 176ff)

Korbinian Brodmann in 1909 provided the first map of the cere­bral cortex in which cortical areas were subdivided based on cell types and their laminar organization. Several subsequent mappings of the human and animal cortex followed, and there was (and still is) disagreement about the number and types of areas, and about their evolutionary origins. From the outset, it was expected that the micro-anatomical differences among these areas would provide the substrate for functional differences. Brodmann’s original parcellation has been extensively supplemented by further anatomical techniques, histochemistry, plus stimula­tion and recording studies. This further work through the decades has provided substantially greater resolution of the anatomical divisions and, perhaps more important, given us a greater understanding of the functional distinctions among these areas in a num­ber of species. Moreover, functional neuroimaging, using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), has provided strong confirmation of the functional specialization of different cortical regions. These techniques have permitted detailed mapping of functionally distinct cortical areas as they were engaged in highly specific types of information processing.

A few general principles emerge from these cortex modularity studies. First, the cortex can roughly be categorized into posterior fields that are involved in perceptual processing, and ante­rior areas that are involved in motor processing. Second, in the posterior cortex, most of the fields are divided by sensory modality. Third, the fields in both the anterior and posterior cortex involve processing hierarchies. In the anterior cortex, there is the primary motor area just in front of the central sulcus, where the muscles of the body are mapped in a topographic organization, with adjacent areas of cortex representing muscle groups in adjacent areas of the body. The primary motor cortex is the origin of a progression of projections to higher-order processing areas that are involved in the sequencing and organization of response output, and, more generally, in the planning, executing, and withholding of goal-directed behaviors.

In the posterior cortex there are distinct primary areas for each sen­sory modality. Each of these is characterized by cells that respond to stimulation within a small circumscribed spatial region of the sensory field, known as a receptive field, and respond preferentially to other specific trig­ger features of the stimulus. The receptive fields and other trigger features are organized in a topographic map of the sensory field and of other rel­evant sensory dimensions, such that adjacent neurons represent contiguous parts of the field and closely related dimensions of the trigger features. For each sensory modality these primary areas are the origins of a hier­archy of specialized processing regions leading to ever more complex perceptual areas. Eventually, some of these streams of sensory processing are combined in multimodal cortical areas, which in turn project to the supramodal processing areas in frontal, temporal, and parietal cortices.

(end of paraphrase)

 

 

Limbic System Modularity

The term "limbic" system derives from the ringlike arrangement of allocortical structures, including the amygdala, hippocampus, entorhinal cortex, and hypothalamus, that provide a relative distinct border separating the brain stem from the new cortex. (Buzsáki; Rhythms of the Brain, 281)

Brodmann area 24 in the cingulate cortex is at the crossroads of pathways linking the limbic system with the frontal lobe. (Fuster; Prefrontal Cortex, 199)

Emotions are among the very oldest of our brain properties. (Llinás; I of the Vortex, 156)

Hypothalamus, brain stem and the limbic system intervene in body regulation and in all neural processes on which mind phenomena are based (perception, learning, recall, emotion, feeling, reasoning, creativity). (Damasio; Descartes' Error, 123)

No generally accepted criteria for stipulating which areas of the brain belong to the limbic system. (LeDoux; Synaptic Self, 211)

Amygdala

Amygdala is the part of the limbic system most specifically involved with emotional experience. (Kandel; Principles of Neural Science, 988)

Entorhinal Cortex

The entorhinal cortex (EC) is the main interface between the hippocampus and neocortex. The EC is located in the medial temporal lobe and functions as a hub in a widespread network for memory and navigation. The EC-hippocampus system plays an important role in autobiographical/declarative/episodic memories and in particular spatial memories including memory formation, memory consolid,ation and memory optimization in sleep.

Hippocampus

Hippocampuses, which are evolutionarily ancient structures deep inside the temporal lobes, are involved in the process whereby short-term memory gets transferred to long-term memory, and also spatial memory. (Gazzaniga; Human, 20)

It is convenient to think of the hippocampus having a binding function that binds together the storage sites that were established independently in several cortical regions, so that the storage sites are strongly connected with one another. (Squire & Kandel; Memory, 110)

Hypothalamus

The hypothalamus has many small subregions whose functions are to regulate hunger, thirst, temperature, sexual behavior, and similar body operations. (Crick; Astonishing Hypothesis, 88)

Hypothalamus, brain stem and the limbic system intervene in body regulation and in all neural processes on which mind phenomena are based (perception, learning, recall, emotion, feeling, reasoning, creativity). (Damasio; Descartes' Error, 123)

Orbitofrontal Cortex

Orbitofrontal cortex (OFC) and the insular; two major components of the paralimbic belt. (Miller; Human Frontal Lobes, 59)

 

Subcortical Modularity

Functional modularity in the brainstem that regulates life processes. Pituitary, hypothalamus, thyroid. Modularity that implements and regulates movement: FAPs, cerebellum, basal ganglia.

Subcortical modularity supports the neocortex, but is not directly a part of the neural correlate of consciousness.

Basal Ganglia

Neurons in the basal ganglia regulate movement and contribute to certain forms of cognition such as the learning of skills. (Kandel; Principles of Neural Science, 331)

Basal ganglia receive inputs from all parts of the cerebral cortex but send their output only to the frontal lobe through the thalamus.  (Kandel; Principles of Neural Science, 331) 

Striatum is the major recipient of inputs to the basal ganglia from the cerebral cortex, thalamus, and brain stem. (Kandel; Principles of Neural Science, 855)

Cerebellum

All vertebrate animals have a cerebellum with highly preserved phylogenetic homology, and it continues to serve identical functions. (Buzsáki; Rhythms of the Brain, 363)

Cerebellum is concerned with the regulation of highly skilled movements, especially the planning and execution of complex spatial and temporal sequences of movement, including speech. (Purves; Neuroscience, 435)

Cerebellum might be involved in musical emotion. (Levitin; Your Brain on Music, 178)

The cerebellum is a system for adaptive feedforward motor control. (Rolls & Treves; Neural Networks, 189)

Reticular Formation

The reticular formation is best viewed as a heterogeneous collection of distinct neuronal clusters in the brainstem tegmentum that either modulate the excitability of distant neurons in the forebrain and spinal cord, or coordinate the firing patterns of more local lower motor neuronal pools engaged in reflexive or stereotypical somatic motor and visceral motor behavior. (Purves; Neuroscience, 399)

Neuromodulatory Systems

Major neuromodulatory systems of the brain. (1) noradrenergic, (2) adrenergic, (3) dopaminergic, (4) serotonergic, (5) cholinergic, (6) histaminergic. (Kandel; Principles of Neural Science, 890)

Neuromodulatory neurons: (1) relatively few and relatively small, (2) highly localized to a few brain stem nuclei, (3) they are pacemaker cells; rhythmical and spontaneous, (4) fire at relatively low rates, metronome-like, (5) project their fine, multiply branching processes all over the brain and spinal cord. (Hobson; Dreaming, 63)

 

 

Link to — Consciousness Subject Outline

Further discussion — Covington Theory of Consciousness