Gerald M.Edelmann
A UNIVERSE OF CONSCIOUSNESS
Basic 2000
Perception into Memory:
The Remembered Present
pg. 102To uncover the neural mechanisms of consciousness, it is useful to keep in mind the distinction between primary consciousness and higher-order consciousness. Primary consciousness is seen in animals with certain brain structures similar to ours. These animals appear to be able to construct a mental scene but have limited semantic or symbolic capabilities and no true language.
Higher-order conseiousness (which flourishes in humans and presupposes the coexistence of primary consciousness) is accompanied by a sense of self and the ability in the waking state explicitly to construct past and future scenes. It requires, at the minimum, a semantic capability and, in its most developed form, a linguistic capability.
______________________________________________________In this chapter, we present a model that accounts for the appearance of primary consciousness in the course of evolution that is consistent with a selectionist view of the brain. We briefly consider the neural requirements to be incorporated in such a model.
The first requirement is perceptual categorization, the ability to carve up the world of signals into categories adaptive for a given animal species.
The second requirement is the development of concepts. We propose that concepts arise from the mapping by the brain itself of the activity of the brain's own areas and regions.
Two more requirements for conscious experience are the appearance of a categorical mem-ory responsive to value and the activity of reentry, which is the fundamental inte-grative mechanism in higher brains. We propose that primary consciousness emerged in evolution when, through the appearance of new circuits mediating reen-try, posterior areas of the brain that are involved in perceptual categorization were dynamically linked to anterior areas that are responsible for a value-based memory.
With such means in place, an animal would be able to build a remembered pre-sent—a scene tbat adaptively links immediate or imagined contingencies to that animal's previous history of value-driven behavior.
_____________________________________________________The greatest challenge to modern neuroscience is to provide an adequate analysis of the brain mechanisms that give rise to consciousness. In this chapter, we consider a proposal for the neural mechanisms of consciousness that is based on the selectional brain theory. To understand the neural processes that underlie consciousness, one must first understand a number of other brain processes at a variety of organizational levels. These processes include
perceptual categorization;
concepts;
value;
memory;and, at the neural level, special dynamic processes of cortico-thalamic organization.
Without such an understanding, it is no wonder that the apparently simultaneous and complex experiences of various sensations, moods, scenes, situatedness, thoughts, feelings, and emotions, all occurring in parallel or in serial causal sequences, can appear hopelessly disconnected in their complexity from any brain-based mechanisms proffered to explain them.
And since, unlike the procedures of physics, the phenomenal experience itself is part of the object of scrutiny, it is no wonder that some students of the subject have pointed out that conscious experience and its purported underlying brain mechanisms are printed in non-interchangeable currencies. We believe, on the contrary, that the currencies can be related, but that the exchange demands some prerequisite understanding.
PREREQUISITES FOR A MODEL OF PRIMARY CONSCIOUSNESS
In analyzing consciousness, we deliberately avoid addressing too many difficult problems at once or being distracted by its rich phenomenology. In line with this restraint, we emphasize the useful distinction between primary consciousness and higher-order consciousness.
Primary consciousness—the ability to generate a mental scene in which a large amount of diverse infor-mation is integrated for the purpose of directing present or immediate behavior—occurs in animals with brain structures similar to ours. Such ani-mals appear able to construct a mental scene but, unlike us, have limited semantic or symbolic capabilities and no true language.
Higher-order consciousness is built on the foundations provided by primary consciousness and is accompanied by a sense of self and the ability in the waking state explicitly to construct and connect past and future scenes. In its most devel-oped form, it requires a semantic capability and a linguistic capability. By necessity, only individuals who are endowed with higher-order conscious-ness can report conscious states and speak about consciousness; they are conscious of being conscious.
In what follows, we concern ourselves mainly with primary consciousness but bring in higher-order consciousness when it affords experimental insights. In the last part of the book, we deal in detail with some of the more intriguing aspects of higher-order consciousness, including thought, language, the notion of the self, and self-reference.
Before we consider a mechanistic model for the appearance of primary consciousness during the course of evolution, we must rapidly review certain essential neural processes. What we need to consider are the structures and mechanisms that must be described to account for the consciousness that we ascribe to dogs and to ourselves when, in certain subjective states, we are least in bondage to language. Here we must face a number of complex processes and their interactions, all of which we have touched on before. There are four.
The first is a property shared by all animals—perceptual categorization, the abil-ity to carve up the world of signals into categories useful for a given species in an environment that follows physical laws but itself contains no such cate-gories. Along with the control of movement, perceptual categorization is the most fundamental process of the vertebrate nervous system.
We have described how it occurs in higher vertebrates as a result of reentrant signal-ing among the various areas of the brain that are present within global map-pings. It occurs, usually simultaneously, in a number of modalities (inclnding sight, hearing, joint sense, or kinesthesia) and in a variety of submodalities (within the visual modality, for example, color, orientation, and motion).
The next process required for understanding primary consciousness is the development of concepts. By concept, we do not mean a sentence or propo-sition that is subject to the tests of the philosopher's or logician's truth table. Instead, we mean the ability to combine different perceptual categorizations related to a scene or an object and to construct a "universal" reflecting the abstraction of some common feature across a variety of such percepts. For example, different faces have many different details, but the brain somehow manages to recognize that they all have similar general features. It has been proposed that concepts arise as a result of the mapping by the brain itself of the activity of the brain's different areas and regions. By these means, various common features of responses to different signals can be abstracted—for example, general features of a given form of object motion may be abstractly obtained when the brain, say of a cat, can register a particular state (described here verbally for explanatory purposes) as "cerebellum and basal ganglia active in pattern a, neuronal groups in premotor and motor regions engaged in pattern [, and visnal submodalities x, y, and z simultaneously interactive." Higher-order maps register these activities and yield an output corresponding to the notion that an object is moving forward in reference to the cat's body. Forward motion is a concept. Of course, no words are involved. No simple combination of the maps that are reentrantly interac-tive to yield perceptual categorizations can lead to this abstractive capability. What is required is higher-order mapping by the brain itself of the cate-gories of brain activity in these various regions.
Two more processes, related respectively to memory and value, must be understood before we describe a mechanism for primary consciousness. As we have seen, according to the theory of neuronal group selection (TNGS), memory is the capacity specifically to repeat or suppress a mental or physical act. That capacity arises from combinations of synaptic alterations in reen-trant circuits.
Furthermore, because a selectional nervous system is not preprogrammed, it requires value constraints to develop categorical responses that are adaptive. The diffuse ascending value systems of the brain are known to be richly connected to the concept-forming regions of the brain, notably the frontal and temporal cortex, but also to the so-called limbic sys-tem, a set of brain regions located on the medial (internal) side of the brain that form a circle around the brainstem. These regions affect the dynamics of individual memories, which, in turn, are established or not, depending on positive or negative value responses. The rich psychological literature on learning suggests that
value, emotional responses, and salience provide strong constraints on the establishment of a conceptual, category-based memory.
The death of President John F. Kennedy, for example, carried rich emotional freight, and many people report that they remember exactly what they were doing and where they were when they first heard of it. The synap-tic alterations that combine to develop various individual memories, collec-tively constituting a «value-category memory," are essential to a model of primary consciousness.
THE CRITICAL ROLE OF REENTRY
We have one final process to consider before we turn to a description of the mechanisms of consciousness. This process, reentry, is the third main tenet of the TNGS. As we previously discussed, reentry is a process of ongoing parallel and recursive signaling between separate brain maps along massively parallel anatomical connections, most of which are reciprocal.
It alters and is altered by the activity of the target areas it interconnects. Reentry is not only the most important integrative mechanism in higher brains, but is conceptually the most challenging of the principles proposed in the TNGS. It is critical to a variety of processes, ranging from perceptual categorization and motor coordination to consciousness itself. In chapter 4, we gave the example of a string quartet in which the players were linked by myriad fine threads that gave ongoing signals shared among the otherwise independent instrumentalists and thus coordinated their individual performances. In our brains, the "threads" are actually parallel, reciprocal fibers connecting separate maps; the neural firings among these fibers go from one map to another and then come back or reenter in a constant dynamic interchange. This interchange synchronizes and coordinates the functions of the various maps.
Reentry plays the central role in our consciousness model, for it is reen-try that assures the integration that is essential to the creation of a scene in primary consciousness.
Integration can best be illustrated by considering exactly how functionally segregated maps in the cerebral cortex may operate coherently together even though there is no superordinate map or logically determined program. The organization of the cerebral cortex is such that even within a single modality, for example, vision, there is a multitude of specialized or functionally segregated maps devoted to different submodalities—color, movement, and form. Despite this diversity, we are aware of a coherent perceptual scene. When we see such a scene, we are not aware of colors, movements, and forms separately and independently, but bind the color with the shape and the movement into recognizable objects.
Our ability to act coherently in the presence of diverse, often conflicting, sensory stimuli requires a process of neural interaction across many levels of organization without any superordinate map to guide the process. This is the so-called binding problem: How can a set of diverse and functionally segregated maps cohere without a higher-order controller?
Within a single area, linking must occur among various neuronal groups in the same feature domain or submodality. Examples are perceptual groupings within a map in sensing color or in another map sensing movement. At a higher level, binding must take place among different distributed maps, each of which is functionally segregated or specialized. Binding, for example, assures the integration of the neuronal responses to a particular object con-tour with its color, position, and direction of movement.
Since there is no superordinate map that coordinates the binding of the participating maps, the question arises: How does binding actually take place? A set of models and computer simulations has shown that binding can occur as a result of reentry across brain maps that establishes short-term temporal correlations and synchrony among the activities of widely spaced neuronal groups in different maps. As a result, neurons in these groups fire at the same time. Thus, reentry correlates a large number of dynamic circuits in space and time. The selection of those circuits that are temporally correlated under constraints of value leads to a coherent output. This binding principle, made possible by reentry, is repeated across many levels of brain organization and plays a central role in mechanisms leading to consciousness.
PRIMARY CONSCIOUSNESS:
THE REMEMBERED PRESENTWith a grasp of the mechanisms of reentry and the notions of perceptual categorization, concept formation, and value-category memory in hand, we can now formulate a model of how primary consciousness arose in the course of evolution.
The model assumes that during evolution, the cortical systems leading to perceptual categorization were already in place before primary consciousness appeared. With the further development of sec-ondary cortical areas and the various cortical appendages, such as the basal ganglia, conceptual memory systems emerged.
At a point in evolutionary time corresponding roughly to the transitions between reptiles and birds and reptiles and mammals, a critical new anatomical connectivity appeared.
Massively reentrant connectivity arose between the multimodal cortical areas carrying out perceptual categorization and the areas responsible for value-category memory.
This evolutionarily derived reentrant connectivity is implemented by several grand systems of corticocortical fibers linking one part of the cortex to the rest and bv a large number of reciprocal connections between the cortex and the thalamus (see figure 4.4A).
The thalamocortical circuits mediating these reentrant interactions originate in the major subdi-visions of the thalamus: structures known as the specific thalamic nuclei, the reticular nucleus, and the intralaminar nuclei. The specific nuclei of the thalamus are the ones that are reentrantly connected with the cerebral cor-tex; they do not communicate directly with each other, but the reticular nucleus has inhibitory connections with those nuclei and can act to select or gate various combinations of their activity. The intralaminar nuclei send diffuse projections to most areas of the cerebral cortex and help to synchronize ist overall level of activity.
All these thalamocortical structures and their reciprokal connections acting together via reentry lead to the creation of a conscious scene.
PRIMARY CONSCIOUSNESS
FIGURE 9.1 MECHANISMS OF PRIMARY CONSCIOUSNESS. Signals related to volue and categorized signals from the outside world are correlated and lead to mem-ory in conceptual areas. This memory, which is capable of conceptual categorization, is linked by reentrant paths (the heavy lines) to current perceptual categorization of world signals. This reentrant linkage results in primary consciousness. When it occurs through many modalities {sight, touch, and so for~h), primary consciousness is of a Nscene" made up of obiects and events, some of which are not causally connecied to each other. An animal with primary consciousness con nonetheless connect these obiects and events through memory via its previous value-laden experience.The dynamic reentrant interactions that occur via the connections between memory systems and systems for perceptual categorization take place within periods ranging from hundreds of milliseconds to seconds—the "specious present" of Wlliam James. Strongly interaccing neuronal groups that change and are differentiated from each other can be integrated by these means. These groups are distributed in the thalamocortical system in ways that we described in chapters 5 and 6.
What emerges from their inter-actions is an ability to construct a scene. The ongoing parallel input of sig-nals from many different senson' modalities in a moving animal results in reentrant correlations among complexes of perceptual categories that are related to objects and events.
Their salience is governed in that particular animal by the activity of its value systems. This activity is influenced, in turn, by memories conditioned by that animal's history of reward and punishment acquired during its past behavior.
The abiliy of an animal to connect events and signals in the world, whether they are causally related or merely con-temporaneous, and, then, through reentry with its value-category memory system, to construct a scene that is related to its own learned history is the basis for the emergence of primary consciousness.
The short-term memory that is fundamental to primary consciousness reflects previous categorical and conceptual experiences. The interaction of the memory system with current perception occurs over periods of fractions of a second in a kind of bootstrapping: What is new perceptually can be incorporated in short order into memory that arose from previous categorizations.
The ability to construct a conscious scene is the ability to construct, within fractions of seconds, a remembered present.
Consider an animal in a jungle, who senses a shift in the wind and a change in jungle sounds at the beginning of twilight. Such an animal may flee, even though no obvious danger exists. The changes in wind and sound have occurred independently before, but the last time they occurred together, a jaguar appeared; a con-nection, though not provably causal, exists in the memory of that conscious individual.
An animal without such a system could still behave and respond to partic-ular stimuli and, within certain environments, even survive. But it could not link events or signals into a complex scene, constructing relationships based on its own unique history of value-dependent responses. It could not imagine scenes and would often fail to evade certain complex dangers.
It is the emer-gence of this ability that leads to consciousness and underlies the evolution-ary selective advantage of consciousness. With such a process in place, an animal would be able, at least in the remembered present, to plan and link contingencies constructively and adaptively in terms of its own previous history of value-driven behavior.
Unlike its preconscious evolutionary ancestor, it would have greater selectivity in choosing its responses to a complex envi-ronment. As we have emphasized, if there is one central structural principle that underlies the appearance of consciousness, it is the emergence during evolution of new anatomically based reentrant systems. Under the con-straint of values, these systems serve to relate new forms of memory to per-ceptual and conceptual activities in the brain.
Following the appearance of primary consciousness and after the evolu-tionary emergence of higher-order consciousness with language in humans, one of the most important global values related to survival was the continu-iy and coherence of the self.
Reentry, continually operating in a degenerate system to allow recategorical memory, provides a fundamental mechanism by which this continuity can be achieved despite ongoing internal and exter-nal change. The paradox that concerned James—how momentary conscious states could be reconciled with various previous states, creating a stable sense of present unity or personal identiy—is dissolved when one under-stands the dynamic nature of the functional integration imposed by reentry between perceptual and memorial systems.
Of course, such a self appears in humans who are capable of higher-order consciousness, which emerged later in evolution when a new set of reentrant connections were established between language centers and conceptual centers. We consider the implications of this second transcendent evolutionary event involving reentry in the last part of this book.
The model we have proposed here is, of course, bare bones. Nevertheless, it provides a firm neuroanatomical and neurophysiological foundation for understanding the neural mechanisms underlying conscious-ness. It also provides a unitary framework that allows us to pursue the goal we stated at the beginning of this book: to formulate a specific hypothesis about the kinds of neural processes that can acconnt for the fundamental integrative and informative properties of conscious experience.
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