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ABSTRACT
In this brief paper, I propose that consciousness is a fundamentally motive phenomenon which integrates and coordinates activity throughout the neuraxis at the broadest level, and that systemic coordination is the product of circulation of a wavetrain through a circuit which encompasses diencephalic and telencephalic structures. This neural wavetrain is the substance of consciousness, and innumerable other wavetrains are the substance of the subconscious. The transformation and interaction of these wavetrains comprises the tapestry of mind. Throughout, I specifically and directly address issues of neurophysiology, psychology, and complex systems behavior.
1. Introduction
Theories of consciousness are far older than recorded history. Most of them comprise a metaphysical framework without a concomitant accounting of the manner and nature of its actualization. The few that attempt such an accounting fall into at least one of the following three categories: those that mystically and indefensibly invoke exotic (and speculative) phenomena of theoretical physics, those that are at odds with extant neuroanatomical, neurodynamic, or myeloarchitectural evidence, and those that simply fail to account for the subjectively unified and centralized, and objectively single-threaded and versatile, nature of consciousness.
The theory I propose below is an attempt to account for the subjective and objective aspects of consciousness, at the level of physiology, without running afoul of the evidence research has uncovered.
2. Summary proposal
Consciousness is a multidimensional wavetrain. A brain is a medium in which wavetrains, consciousness being the foremost such wavetrain, cyclically propagate, are transformed, and transform and initiate other wavetrains. The medium alters the wavetrain, and the wavetrain alters the medium. The innumerable subconscious processes of the mind, processes which at opportune moments enter consciousness, are wavetrains circulating and evolving along physically disjoint pathways.
The means by which such a subconscious wavetrain is made to enter consciousness is by a breaking down of the barrier between the physical substrate of the appropriate subconscious wavetrain, and that of consciousness. In an interruption (an asynchronous realization, a reminder, the deferred recall of a memory, etc.) the state of the subconscious wavetrain is such that the physical substrate is modified to amalgamate the subconscious and conscious wavetrains. In an intentioned (single-threaded, willed, I-want-this) access or incorporation, the state of the conscious wavetrain catalyzes the topological modification.
The myriad disjoint wavetrains of the subconscious join among themselves to form larger, semantically richer wavetrains. Eventually this amalgam of subconscious wavetrains may reach a level of richness and coherency which crosses a threshhold and causes incorporation of the wavetrain into consciousness (the primary wavetrain). Alternatively, the amalgam may reach a dead end and simply decay.
Subconscious wavetrains routinely eavesdrop on the conscious wavetrain, without upsetting it, in order to enrich themselves and increase their relevance. The noise indigenous to the neural substrate also serves to seed subconscious wavetrains. The critical path circuits (particularly the conscious circuit) probably use extensive forward error correction strategies to conceal the inherent noise. The critical point here is that the brain's circuits are divided into two categories: those that deliberately damp out chaotic effects, and those that rely on them. It follows that hard, i.e. phylogenetic, circuits fall into the former category, and that the vast array of soft circuits are formed and dissolved by a process which leverages off the attractor effects of nonlinearity cultivated in an architecture founded on the unifying principal of circulating wavetrains.
An example product of the subconscious amalgamation mechanism is the flash of insight, in which (at an opportune stage) consciousness is interrupted. Hence, the amalgamation process is the primary mechanism of creativity. Here, eavesdrop-based and noise-based seeding both play crucial roles.
A recurrent and common mode of operation is for the primary wavetrain to amalgamate with particular peripheral circuits, discover that the desired information is unavailable, and dissociate from those circuits having left them in a state which will evolve in such a way that they will seek the desired information and interrupt consciousness when it becomes available.
Separable circuits dissociate when the portions of the wavetrain conveyed by each circuit become incoherent relative to eachother. A dominant component of the wavetrain, such as that of the core circuit of consciousness, is usually able to deliberately desynchronize and "spin off" wavetrains associated with amalgamated (but dissociable) circuits, reinstating their physical disjointness. Sometimes such an amalgamated circuit carries a wavetrain that is particularly pesky, and wages a battle of wills with the conscious wavetrain. More often, these internal battles are with organs that are operating by a non-circulatory mechanism (more on this below).
3. Myeloarchitectural hypotheses
3.1 The myeloarchitecture of consciousness
Three circuits are overlaid to construct the circuit that is necessary and sufficient for consciousness. Because consciousness is essentially a predictive, reactive, and motive phenomenon, there is an evident tendency for the organismic substrate to lean toward nuclei implicated in higher motor functioning.
3.1.1 Principal conscious circuit
- the mediodorsal nucleus of the thalamus
- the intralaminar nuclei of the thalamus:
- centromedian
- parafascicular
- central lateral
- paracentral
- corpus striatum, in particular the head and body of the caudate nucleus, and portions of the putamen
- prefrontal cortex
- back to (1)
3.1.2 Thalamocortical adjunct conscious circuit
- the mediodorsal nucleus of the thalamus
- prefrontal cortex
- back to (1)
3.1.3 Telencephalic adjunct conscious circuit
- corpus striatum, in particular the head and body of the caudate nucleus, and portions of the putamen
- prefrontal cortex
- back to (1)
3.1.4 Integration of conscious circuits
These circuits are not actually separate or in any way disjoint, because intranuclear and intraorganic linkages bind them together at all times. Dissociation or interruption of any of these circuits yields results which range from extreme psychosis to catatonic morbidity (interruption of the principal conscious circuit destroys recognizable consciousness). Thus, in functioning humans, this is a single complex circuit, which I call the primary circuit. The wavetrain which courses through this circuit is consciousness.
For comparison, birds have only a circuit which interconnects the thalamus and the corpus striatum; this circuit is impossible with human neurophysiology because the corpus striatum has no efferent projections to the thalamus.
It is striking that the human cerebellum is substantially larger than the primate cerebellum, relative to the rest of the brain. This is indicative of a further reassignment to the cerebellum of responsibility for externally directed motor scripting and coordination functions, permitting a greater proportion of the corpus striatum to concern itself with associative functionalities of intentionality, and therefore, refinement of consciousness.
3.2 On laterosymmetricity
All the circuits detailed in this paper are laterosymmetrically replicated. Four primary translateral links integrate the two hemispheres. The nucleus reuniens links the two thalami in the environs of the paracentral and mediodorsal nuclei. The commissure and body of the fornix link the hippocampal formations with contralateral septal nuclei, thalami, and mammillary bodies. The anterior commissure links much of the temporal lobe with contralateral temporal regions. Finally, the corpus callosum is a very large bundle of some 200 million mostly slow-conducting fibers which link the essential entirety of the two cortical hemispheres. Many of these fibers interconnect homologous regions, and callosal fibers which interconnect homologous regions are mostly inhibitory. There are no fibers which directly connect contralateral corpora striata; any interaction between them is through at least one intervening nucleus and in general more than one.
Given this brief background on interhemispheric commissuration, I leave it largely as an exercise for both myself and the reader to determine what the alien hand syndrome, and functional competence following cerebral hemispheric lobectomy, manifest. The compounding of cortical and striatal decoupling is surely instrumental here; the inability of the nucleus reuniens and interhemispheric links through brainstem nuclei to provide interhemispheric coherency underscores the concentration of high-resolution plastic circuits in telencephalic structures. This suggests that the thalamus, as a node in the critical circuits of higher thought, serves largely as a collection of non-plastic, simple, fixed transformation, association, and relay nuclei - a solid backboard of sorts. The relative non-plasticity of brainstem nuclei has never been a point of serious contention.
3.3 The primary memory coordination circuit
3.3.1 Overview
- the lateral dorsal nucleus and anterior nuclear group of the thalamus
- cingulate cortex
- subiculum
- back to (1)
This circuit, and memory in general, are not in the conscious signal path. At multiple loci, the primary conscious and primary memory circuits junction, and the wavetrains they conduct assume postures of varying relationship. The most important such junctions are probably between the cingulate cortex and orbitofrontal cortex, and between the hippocampal formation and orbitofrontal. The thalamic nuclei implicated in the primary memory circuit are adjacent to the mediodorsal nucleus, and intrathalamic fibers probably allow for a further relation of the conscious and memory wavetrains at the thalamic level.
3.3.2 Memory and isocortex
Four contiguous cortical regions, the temporal (minus primary and paraprimary auditory areas), the insular, the posterior and lateral parietal, and the lateral visual, provide the basic semantic backdrop for consciousness and for the primary memory circuit, and gateway to domain-specific regions of cortex. These extremely plastic regions are intimately and reciprocally interconnected with eachother, with the frontal lobe, with the cingulate cortex, and with the corpus striatum. Without the circuits these regions form, consciousness is essentially meaningless, and long-term memory is non-existent.
Quite apart from the activities of the memory coordination circuit, the memory constellations embodied by these regions are activated implicitly when circumstances, brought about either by external stimulus and therefore signalled by primary and paraprimary cortical regions, or resulting from the incidental evolution of a wavetrain circulating internally (usually a combination of the two), activate a segment of the memory constellation.
Nonetheless, these highly associative non-frontal regions are not in the critical path of consciousness itself. The continuity that is a salient aspect of consciousness is embodied by the conscious circuits that involve the frontal lobe.
3.3.3 Elaborations on the mechanisms of subconscious information management
A circuit involving the pulvinar of the thalamus may facilitate subconscious associational activation of memory constellations in reaction to sensory input. I am struck by the pulvinar's extensive projections to modal integrative (intradomain association) areas of sensory cortex, and its innervation by the superior colliculus, itself a polymodally integrative brainstem sensory nucleus with a cortical arrangement. These projections may comprise a system which primes the cortex so that it will be more readily able to provide information on a demand or interrupt basis to the conscious wavetrain upon amalgamation.
Because the pulvinar's connections with visual and temporal cortex are broad, reciprocal, and topographically regular, the pulvinar may form a node in the circuit which implements the "rendering" capability of the mind - the ability to dream. It is striking that these rendering activities are mutually exclusive with the environmentally-oriented priming described in the previous paragraph - the daydreamer is in his own world. As has long been recognized, somatotopic nociception does not form an element in dreams, and if the pulvinar comprises the selectively dissociable switch which accounts for daydreaming, it follows that nociception will not be dissociable, since the pulvinar neither sends nor receives somasthetic information. Dreams are an audiovisual phenomenon.
The claustrum, a broad, thin lens of neurons situated amidst the white matter of the cerebral hemispheres, between the insular cortex and putamen, probably implements a subconscious sensory FIFO buffer. The claustrum is segmented into regions dedicated to the auditory, visual, and somasthetic domains, and each of these regions has a regular topographic arrangement reciprocally relating corresponding regions in sensory cortex. It is cytoarchitecturally thalamic. My proposal is that fibers from sensory cortex feed a constant, essentially unprocessed stream of sensory data to the claustrum. The claustrum acts as a backboard much as the thalamus does in the conscious circuit. On an interrupt basis, therefore, raw data about very recent sensory input can be replayed. Once the raw data is desired, the claustrocortical circuit can be made to reverberate the same raw data for a substantially longer time, perhaps in the tens of seconds, to the exclusion of new sensory data buffering. This mechanism implements the auditory loop. Conceivably, the reciprocal circuits of the pulvinar and those of the claustrum are used to implement something akin to register renaming or double buffering (both of these terms are borrowed from computer architecture).
3.4 Emotional circuits
3.4.1 The LeDoux circuit
- mediodorsal nucleus of the thalamus
- orbitofrontal cortex
- basolateral nuclei of the amygdala
- back to (1)
3.4.2 The Papez circuit
- anterior nuclear group of the thalamus
- cingulate cortex
- hippocampal formation
- hypothalamus, particularly the mammillary nuclei
- back to 1)
3.4.2 Discussion
Like the primary memory circuit, these circuits are not in the conscious signal path, but also like the primary memory circuit, they junction at multiple loci, and it is unusual for the wavetrains either of them conduct to attain an extreme degree of dissociation from the conscious wavetrain.
The total number of major endogenous emotional circuits is large, perhaps in the hundreds. The caudate nucleus forms a crucial piece of some of these circuits (it is projected to broadly by the amygdala).
3.5 Thalamic organization
At this point, it is well to observe that the thalamus consists of two functionally separable and anatomically contiguous nuclear supergroups. The medial dorsal anterior (MDAT) nuclei, composed of the mediodorsal nucleus, intralaminar group, lateral group, anterior group, and midline nuclei, comprise the integrative and telencephalically-oriented portion of the thalamus. The lateral ventral posterior (LVPT) supergroup is composed of the ventral mass, geniculate bodies, (anomolous) pulvinar, posterior complex, and the (anomolous) reticular nucleus. The MDAT supergroup is not intimately related with any rhombencephalic structures or pathways. Except for the reticular nucleus, all the nuclei of the LVPT supergroup are intimately related to rhombencephalic structures and are linked only with primary and paraprimary regions and organs. The reticular nucleus reciprocates with most, or all, thalamic nuclei, and receives topographically organized projections from much of the cortex, though it has no extrathalamic efferent projections. Its role is unique, and by no means fully understood. The pulvinar, as explained above, has intimate links with both associative cortex and rhombencephalic structures, and can be considered a transitional nucleus functionally midway between the role of the MDAT and that of the LVPT. It is also the only globular thalamic nucleus which substantially straddles the mediodorsal-lateroventral boundary.
4. Speculation on the generic nature of circuits
4.1 Circuit dynamics
There are many circuits that run both directions; without further examination of the particulars of projection fields and intranuclear linkages, the implications of this bidirectionality are unclear. Perhaps wavetrains circulating in both directions along homologous circuits result in something akin to a standing wave, with associated crests and troughs. The standing wave, when present, is inferred by neurons whose dendrites receive input from neurons implementing circulation and from those implementing countercirculation. Such an arrangement is not precluded by known basic cellular neurophysiology. It is perhaps key that the principal conscious circuit is strictly unidirectional - the corpus striatum does not project to the intralaminar nuclei of the thalamus.
An interesting possibility is that circuits (in particular, though hardly restricted to, the primary circuit), may bifurcate and recombine in a single circulation. By this process, one portion of the wavetrain may exhibit processing, perhaps analogous to a group delay, and subsequent recombination convolves the two back together. I am particularly thinking of a bifurcation at the thalamus, at which one portion of the signal passes directly to the cortex, and the other passes through the corpus striatum before reaching the cortex.
Circulation which is tightly localized, usually referred to as reverberation, is probably the mechanism by which modules (of the neurophysiological variety) internally maintain working state - myeloarchitectural plasticity cannot play a role in the maintenance of working state on the time scale of seconds. It is by this mechanism that waylayed or otherwise latent information can serve to modulate the transformational function of the module when and if the module is made to form a portion of a circuit. Once it forms part of a circuit, the information is shuttled on internuclear and interregional trajectories, and by virtue of myeloarchitectural plasticity, is rendered permanent to a degree proportionate to the subjective significance of the information. Here, subjective significance is to be taken to be synonymous with resulting in wavetrains which are efficient at catalyzing changes in myeloarchitecture. This significance will often be dramatically modulated by the emotive circuits, since they junction repeatedly with the primary memory coordination circuit.
The modules and intramodular circuits just described, and Donald Hebb's cell assemblies and local reverberations, are one and the same. This organization is predominant in cortex, and may be similarly predominant in the corpus striatum (based on sketchy cytoarchitectural evidence). This type of modularization is probably not relevant to the physiology of the thalamus.
4.2 Circuit variety
The full variety of all the thousands of endogenous circuits, which at a given time may or may not have a wavetrain flittering through and evolving, implement those basic aspects of mind (tools and influences) that can be called upon by the conscious wavetrain when it has a job for them, or whose wavetrains join with the conscious wavetrain, thereby interrupting it, to provide input considered important or advantageous (though perhaps not by the conscious wavetrain).
Subconscious abstract symbolic cogitation involves circuits many of which do not even descend below the cortex. Circuits involving the hypothalamus, amygdala, and associative cortices, are responsible for a variety of nagging emotional phenomena.
Beyond the thousands of endogenous circuits, innumerable circuits can be learned by dint of the plasticity of the cerebral cortex and of many higher brain nuclei. These circuits are very individual, and embody the full richness and variety of the human tapestry of mind. Many segments of these learned circuits are portions of broader circuit segments. I propose that it is through the delineation of innumerable fine-grained circuits that the endogenous course-grained circuits come to be arranged in a semantically powerful manner.
5. Feed-forward phenomena, two examples
There are many influences on mind by structures outside the primary conscious circuit, which use a power limited in domain, but essentially sovereign therein, to bring about dramatic changes in the substrate of the conscious circuits. The sensation of falling in love - on the face of it similar to the examples from the last paragraph - is not the same, and is a perfect example of such a power. Falling in love is a phenomenon selected for by darwinian evolution which is implemented by the amygdala, and mediated primarily by persistent, long term "pressure" on the caudate nucleus. The caudate nucleus is a crucial component of the circuits that define the individual's high-level and motivational worldview (values, that sort of thing). When the amygdala decides you are going to be in love with someone, it precipitates a dramatic meltdown of this worldview using its broad, rich projections to the caudate nucleus. This is not really a circuit, this is an "out-of-the-blue" modification, and this is exactly how we experience falling in love. Of course, the amygdala precipitates changes in many other parts of the brain, and other changes in the caudate (in particular, construction of new values to replace the melted ones) to implement the complete constellation of separable phenomena collectively referred to as "falling in love."
There are many lower brain nuclei which exert broad influences on higher brain structures in various ways, often involving particular aspects of brain chemistry (particular neurotransmitters). These nuclei have huge, pivotal effects. A good example is a group of them (the raphe, part of the brain stem reticular formation) that are, with the locus ceruleus, responsible for the systemic coordination of sleep. The raphe at issue deal primarily with telencephalic serotonergic innervation, and it is striking that LSD, a serotonin antagonist, yields effects that amount to the breaking into consciousness of the dream state.
These brainstem nuclei do not form nodes in any (actually or potentially) conscious circuits - they exert an external, course-grain tonic effect. However, through such techniques as meditation (meditation is a willed training and simplification of the conscious wavetrain, with the salient characteristic that the wavetrain does not modify the medium, and the medium does not modify the wavetrain), many if not all of them can be coerced into a desired state (within parameters not typically including catatonia or mortality). Systemic innervation with noradrenalin, serotonin, and dopamine, is by brainstem nuclei which are reciprocally related to diencephalic and telencephalic structures; thus course activational tone is modulated both unilaterally by the brainstem, and sympathetically in response to signals from higher brain structures.
6. Future directions
Critical aspects of the neurodynamics underpinning many of my proposals are plainly unjustified in this paper, though some justification does exist in other literature. Future research will be directed specifically at determining if, and precisely how, the physiological substrate is arranged so that wavetrains appropriately amalgamate and dissociate. This is, at present, the weakest link in the chain of reasoning detailed in this paper.
Much greater detail is needed in the delineation of projections and projection fields. This will involve laboratory work, and I would be thrilled to help evaluate and define specific avenues of research. My invocation of the term "wavetrain" throughout this paper leads to an impression of something approximate in time and space; such a smearing is not always the case with consciousness, and the particulars of projections, projection fields, and intranuclear linkages, must reveal how these wavetrains attain a tight spatiotemporal focus. Short of such a demonstration, the concept of the circulating wavetrain is incapable of accounting for the empirical phenomena of mind and behavior.
7. Conclusion
The Circulating Wavetrain theory promises to catalyze solution of many outstanding problems in psychology. It will help in the search for coherent explanations for symptomatic constellations of psychiatric and organic syndromes, and will presumably be helpful in deriving treatment strategies.
The theory, even in the somewhat vague and approximate form in which I have described it here, promises a compelling explanation for the effect music (particularly, hypnotic and rhythmically regular music) has on the mind. I feel this is a good litmus test for any theory which purports to account for the subjective conscious experience.
To me, the most exciting aspect of the Circulating Wavetrain theory is simply that it is a first-order and satisfying ontology of consciousness expressed in the lexical and conceptual vocabulary of mathematics, physics, chemistry, and biology. With the high-resolution hybrid EEG/MEG systems we have reason to expect in the not-distant future, it will be possible for this theory to be empirically verified.
8. Acknowledgements and inspirations
My theory is compatible with (in fact, implicitly subsumes) the Hebbian theory of learning, which is itself a satisfying explanation for how learning is implemented at the level of neurophysiological substrate.
Donald Hebb's learning rule and theories of Cell-Assemblies and neural reverberations, Gerald Edelman's Neural Darwinism, and some of the precepts of the PDP school, are precursors of the Circulating Wavetrain theory.
A paper by Daniel Amit entitled "The Hebbian Paradigm Reintegrated: Local Reverberations as Internal Representations" electronically preprinted in 1994 and published in 1995 was a very concrete source of inspiration. http://www.fiz.huji.ac.il/staff/acc/faculty/damita/psfiles/hebb.ps.Z
For a compelling discussion of the integration of some of the most important regions of isocortex, see "Brain Evolution and Neurolinguistic Preconditions" by Wendy K. Wilkins and Jennie Wakefield. http://cogsci.ecs.soton.ac.uk:80/~bbs/Archive/bbs.wilkins.html
Speculative writings by William Calvin, Francis Crick, and others, seem to lead directly to the Circulating Wavetrain theory.
A vast corpus of wrong theories of consciousness has been as instrumental in forming my theory as have the theories, most of which are of lesser scope, which I believe are essentially correct.
In my insistent quest to grapple with the brain on its own terms, Malcolm Carpenter's Core Text of Neuroanatomy has been a constant companion. His text, a joltingly dense book, has been the primary source of my understanding of nuclear modularization, internuclear linkages and their chemistries, and intranuclear cytoarchitecture.
Steve Harnad's Psycoloquy mailing list has been effective in peppering me over the years with useful information and pointers.
Benoit Mandelbrot's "The Fractal Geometry of Nature," James Gleick's "Chaos" and Murray Gell-Man's "The Quark and the Jaguar" are three books oriented toward a more general audience that have set various ideas running in my head as I was introduced to them at intervals over the last ten years.
Innumerable sources, some anonymous, have shown up in my Altavista searches for more specific information on particular topics.
Appendix 1 - Glossary
Multidimensional wavetrain = a signal which is not scalar. A prototype of such a signal is a train, or sequence, of matrices (a train of tensors). However, such a train is quantized in a way quite dissimilar from the quantization of the multidimensional wavetrains which circulate through circuits in the brain. The multidimensional wavetrain is not an exotic concept; examples of multidimensional wavetrains are optical and sonic wavetrains in three-space; motion holography and true sonic holography (a dense 2-D matrix of computer-controlled loudspeakers) both generate artificial multidimensional wavetrains. Population = a contiguous set of cells of like architecture and chemistry. Nucleus = a contiguous agglomeration of one or more distinct populations, usually bounded by non-nuclear matter of some sort. Projection = a set of fibers that convey information from one population to another. A projection may bifurcate at some point along its trajectory and give off "collaterals" so that more than one population is targetted. A projection is characterized by its length, its myelination, its propagational velocity, and its chemistry (these four dimensions are not orthogonal). The fibers that compose most projections involve a synapse at some point along their trajectories. Projection field = a precise pattern of fiber projection; it is particularly characterized by the mapping function by which a trajectory can be derived from a particular source neuron, and by the projection fields with which it is interleaved at the target. Cortex = a laminar arrangement of distinct population shells encasing a non-laminar nuclear body or fiber mass. Module (neurophysiological) = a small region of a nucleus or cortex whose boundary is characterized by a marked falloff in projections to and from adjacent neurons. Modules in cortex interrelate the different types of neurons in the several layers of a given area of vertically registered cortex, layers which are composed of architecturally distinct types of neurons (and therefore comprise multiple populations with a disciplined interrelation). Medulla = phylogenetically oldest, anatomically most basal, and computationally most primitive, portion of the brain. Rhombencephalic = phylogenetically derived from the medulla. Cerebellum = a vastly intricate rhombencephalic organ, comprising roughly half the neurons of the central nervous system. It is responsible for refinement and embellishment of the motor instruction stream, and learning of automatonesque motor scripts that, once instilled, free the conscious portions of the brain from computationally expensive and time-consuming chores. Cerebral cortex = the external surface of the upper brain, really wrinkly and quite gigantic in humans (actually a gigantic nucleus, though never referred to as such). Isocortex = the portion of the cerebral cortex (often called just "cortex") that has a more or less continuous and unexceptional layered arrangement and manner of connection to internal brain structures and other regions of cortex. Heterocortex = "weird" parts of the cortex whose physical arrangement is strange in one or more ways. Isocortex and heterocortex are continuous; they are all part of the same gigantic nucleus. Domain-specific = a population or cortical area dedicated to operation at the level of an external domain (a sense or motor field) - almost always arranged with a straightforward mapping between the key attribute of the domain and the topographical location in the population of the several neurons associated with that specific range in the domain. Examples are auditory cortex and associated subcortical nuclei (cells positioned based on frequency at which they turn on), visual cortex and associated nuclei, somasthetic cortex and nuclei, and somatomotor cortex and nuclei. Olfaction doesn't have this kind of organization, but that's just one of the many very strange and exceptional aspects of olfaction. Primary cortex = domain-specific cortex Associative = a population that is associated with more than one external domain, or is connected to only such populations. By this narrow definition, many regions classically considered to be associative (particularly paraprimary cortex) are seen to be integrative, though not associative. Paraprimary cortex = region of cortex adjacent to a primary region. Paraprimary regions are domain-specific, though there is a definite integrative aspect to their operation. Relay nucleus = a simplistic nucleus that sends a signal on down the line with little if any transformation of the signal. Reticular formation = a nuclear complex with involved intranuclear and intracomplex connections - compare with relay nuclei. Raphe nuclei = a variety of nuclei distributed throughout the central grey column of the brainstem, which form the most important portion of the brainstem reticular formation. Habenular nuclei = a pair of diencephalic nuclei that serve as a nexus through which telencephalic signals are relayed to brain stem nuclei, and thence on to (largely autonomic) general visceral efferents. Thalamus = an agglomeration of many (25) nuclei positioned just above the brain stem. At least one nucleus of the thalamus connects to every little piece of the whole isocortex. The thalamus has domain-specific nuclei devoted to each of the major sensory domains (except olfaction), a set of motor relay nuclei, and a variety of (far more interesting!) associative nuclei which are connected both to the associative regions of the cortex and to other thalamic nuclei. Diencephalic = phylogenetically derived from the thalamus. Hypothalamus = a set of 15 nuclei that deal with metabolic and reproductive drives and the hormonal aspects of emotion. Hunger, thirst, and sex are star attractions here, though there are smaller players. The mammillary bodies are among these 15 nuclei. Amygdala = a set of 9 nuclei that define and assert middle-tier emotional state, specifically fear, aggression, and probably desire and attachment, but definitely at least some other recognizable and named emotions. Some of these nuclei also exert a direct effect on visceral and vascular state ("so scared he peed in his pants"). Septal nuclei = a pair of telencephalic nuclei which serve as major relay points for signals from the amygdala and hippocampus to the hypothalamus, back to the hippocampus, and to more basal nuclei including (through the habenular nuclei) the brain stem. Corpus striatum = a set of very large nuclei (inches long) with chemically distinct populations organized in a patchy matrix. Basal ganglia = a vast nuclear supergroup composed of the globus pallidus (middle-tier motor association nucleus), corpus striatum, amygdala, septal nuclei, ventral striatum (olfactory nuclei), and nucleus basalis (diffuse cortical cholinergic innervation, analogous to the diffuse innervating brainstem nuclei mentioned in the text). Telencephalic = phylogenetically derived from the basal ganglia. Caudate nucleus = a surprisingly long arc-shaped nucleus, forming the superior portion of the corpus striatum. The caudate nucleus is the primary site of initiation of action (site of high-level intention). The caudate is continous with the amygdala at one end and the putamen at the other. Putamen = the inferior (more basal) portion of the corpus striatum. The putamen is somewhat more wrapped up in the middle-level actualities (motor logistics) of intentionality, but still plays an integral role in the high-level initiation of action. Orbitofrontal cortex = the portions of the cortex, including prefrontal and posterofrontal, that deal with hierarchical and temporal (time) modelling, and half of the mechanism of forward and reverse kinematics. This area is highly associative. The frontmost area of orbitofrontal is called prefrontal and is crucial in modelling the world as a continuous, causal place (and making predictions on that basis). A region known as Broca's Area is in the lateral inferior frontal lobe. this area is where the grammatical, parsing, and reverse-parsing (construction of a sequence of actions, either written letters or spoken syllables) aspects of language are implemented (not as such, this hardware is usurped for the purpose of language processing in the left hemisphere because it happens to be suited for it). Non-auditory temporal, and insular, cortex = regions oriented toward declarative memory, for what it's worth. This is probably the closest to a region that implements what is classically called "memory." It is rich in associations, serving as a metarepository by having innumerable little regions distinguished by the particular sites in other cortical regions to which they connect. zap one of these regions with a few millivolts and you can cause conscious experience of a whole, intact memory - artificially declaring a state which modifies the conscious wavetrain to include the circuitry associated with the memory (completely polymodal and distributed across the whole isocortex, all of which is part of the conscious circuit). Wernicke's Area is in this region. This is the area concerned with the semantic aspect of language. Wernicke's Area and Broca's Area are intimately related and connected by a huge bundle of fibers. Between the two of them, they are responsible for the comprehension and production of language. Only one side of the head uses these regions for language; their twins on the opposite side are used for spatial processing and, well, right brain stuff (language is confined to one hemisphere, usually the left) - no one quite knows everything that can be done with this hardware when it's not used up by the needs of language processing, but Albert Einstein is a good example of someone who pushed that envelope. Never let it be said that science is a left-brain activity! Lateral and posterior parietal = region implementing the other half of forward and reverse kinematics. This region is responsible for "body sense." when you figure out the shape and identity of an object by touch alone, this region is primarily responsible for identifying features and producing (and projecting) a signal which can be integrated with other representations, particularly visual and procedural ones. That's why a brick feels like a red construction material. The precise area that a signal like this projects to is the junction of the temporal, visual, and parietal regions, called the POT (parietal-occipital-temporal; visual is confusingly called occipital in most literature) and also called Wernicke's Area (ooh! we're starting to understand things now!). This is the most associative region of the entire sensory cortex - it is fed by regions which are themselves already associative. The POT exists only in humans - in fact, it exists only in "homo" humans; australopithecus afarensis lacked the POT but homo erectus had it - now there's food for thought. Basically, having a POT is a necessary and sufficient indicator for being human. Lateral anterior visual cortex = a highly integrative, and perhaps associative, area responsible for visual modelling. The portion of the cortex devoted to vision alone is quite massive; we have a large biological investment in visual thought and this investment is evidenced by our culture and behavior (and vice-versa). Regions of isocortex not included in this glossary = domain-specific regions for every sense and motor modality under the sun except for olfaction, which is heterocortex (strange!). Cingulate gyrus = region of heterocortex which mediates recruitment of memories. Because of its inclusion in some very important emotional circuits, I propose that the cingulate gyrus is key for the automated recruitment and unintentioned incorporation into consciousness of memories relating to current circumstances - cingulate and the caudate nucleus are probably the key to understanding and alleviating post-traumatic-stress disorder (whose most devastating symptoms are hypothalamic and amygdalar in nature, but quite probably not autonomously caused by these subcortical nuclear complexes). Hippocampal formation = the region of heterocortex which contains short-term memory storage registers, and coordinates the conversion of the contents of these registers into long-term memories embodied elsewhere in isocortex (largely temporal, see above) and perhaps a non-critical roll in the recall of these memories (placing them into short-term memory registers). The subiculum, dentate gyrus, and hippocampus proper, are all part of this formation, though the dentate is often not included for reasons unapparent to this writer.