Gerald M.Edelmann A UNIVERSE OF CONSCIOUSNESS Basic 2000

pg 138 ...This analysis shows that high complexity originates from continuing interactions between the brain and an external environment of much greater potential complexity. Simulations with simple linear systems indicate that systems with random connectivity have low complexity values. However, if the connectivity of these systems is allowed to change through a selection procedure in such a way as to increase their match to the statistical regularities of an external environment, their complexity increases considerably. Moreover, everything else being equal, the more complex the environment, the larger the complexity of the systems that achieve high values of matching. It is thus the adaptation of the brain's reentrant circuits to the demands posed by a rich environment, based on principles of natural, developmental, and neural selection that leads to a high complexity, as reflected by increased values of matching and degeneracy. And it is only after such a level of complexity has been achieved that an adult brain, even when relatively isolated as in dreaming, can generate integrated neural processes of sufficient complexity to sustain conscious experience. The question can now be posed: Can we use the concepts and measures developed here to specify under which conditions populations of neurons contribute to conscious experience? Determining Where the Knot Is Tied: The Dynamic Core Hypothesis In this chapter, we first reviev observations indicating that despite their wide distribution, only a subset of the neuronal groups in our brain comtributes directly to conscious experience at any given time. What, then, if anything, is special about these neuronal groups, and how should they be identified both in theory and experiment? The dynamic core hypothesis is our answer to this question. This hypothesis states that the activity of a group of neurons can contribute directly to conscious experience if it is part of a functional cluster, characterized by strong mutual interactions among a set of neuronal groups over a period of hundreds of milliseconds. To sustain conscious experience, it is essential that this functional cluster be highly differentiated, as indicated by high values of complexity. Such a cluster, which we call the "dynamic core" because of its ever-changing composition yet ongoing integration, is generated largely, althongh not exclusively, within the thalamo-cortical system. The dynamic core hypothesis leads to sperific predictions concerning the neural basis of conscious experience. Unlike hypotbeses that merely invoke a correlation between conscious experience and this or that neural structure or group of neurons, the dynamic core hypothesis accounts instead for the general properties of conscious experience by linking these properties to the specific neural processes that can give rise to them. ----------------------------------------------------------------- The evidence examined so far suggests that to support conscious experience, the activity of distributed populations of neurons must be integrated through strong, rapid neural interactions. We have also shown that neural processes underlying conscious experience must be sufficiently differentiated, as indicated by the loss of consciousness when neural activity is globally tomogenous or hypersynchronous, as is the case during slow-wave sleep and generalized epilepsy. Finally, we have noted that every conscious task seems to require the activation or deactivation of many regions of the brain. These regions typically include portions of the thalamocortical system, although they are not necessarily limited to it. The issue we consider now is whether the neural processes underlying consciaus experience extend to most of the brain in some nonspecific fashion or are restricted to a particular subset of neurons therefore posing the question of what is special about this subset. HOW MUCH OF THE BRAIN DOES A THOUGHT REQUIRE? After reviewing the scant physiological literature of his age, William James concluded that there was as yet no evidence for restricting the neural correlates of consciousness to anything less than the entire brain. Since the days of James, however, scientists have discovered that only a certain portion of the neural activity in the brain contributes to consciousness directly - as assessed by experiments with stimulation and lesions - or is directly correlated with aspects of conscious experience - as assessed by studies recording neural activity. On the basis of studies of lesions and stimulation, we are confident that, for example, the activity of certain brain regions, such as the cerebral cortex and thalamus, is more important than the activity of other regions. Moreover, there is reason to believe that a substantial proportion of neural activity, even in the cerebral cortex, does not correlate with what a person is aware of. This lack of correlation is indicated by recent studies of binocular rivalry, for instance. As we discussed previously, if two incongruent images, such as a vertical and a horizontal grating, are presented simultaneously to each eye, a person perceives only one image at a time, with an alternation every few seconds in perceptual dominance - the image the person is conscious of. Studies recording neural activity in monkeys have revealed that a large proportion of neurons in the primary visual cortex and other early stages of the visual system continue to fire to their preferred stimulus even when the stimulus is not being consciously perceived. In higher visual areas, however, most of the neurons fire in response to the percept. In the MEG study of rivalry in humans described in chapter 5, we found that even when the subject was not conscious of a stimulus, steady-state responses at the frequency at which a stimulus was flickered could be recorded in many regions of the brain, including the frontal cortex. However, the responses of only a subset of brein regions were actually correlated with the conscious perception of a stimulus. Recording studies have also demonstrated that the activity of many neurons in sensory and motor pathways can be correlated with rapidly varying details of a sensory input or a motor output but do not seem to map to conscious experience. For example, patterns of neural activity in the retina and other early visual structures are in constant flux and correspond more or less faithfnlly to spatial and temporal details of the rapidly changing visual input. However, a conscious visual scene is considerably more stable, and it deals with properties of objects that are invariant under changes in position or illumination, properties that are easily recognized and manipulated. For example, when we see a hummingbird fluttering its wings, we can recognize it and grasp it, whether it is against the sunny sky or in a canopy of trees, whether it is distant or nearby, and whether it is turned toward or away from us. Moreover, much evidence indicates that during each visual fixation, we extract the meaning or gist of a scene, rather than its innumerable and rapidly varying local details. We certainly could not describe the precise position of the wings during the bird's flight. In fact, we are surprisingly blind to or unconscious of considerable changes in a visual scene as long as its meaning or gist is not affected. In reading a text, for instance, we usually do not take account of the typeface unless it is exotic or when we have its recognition as a specific aim. These more invariant aspects of a scene are the ones that appear to be really important and informative about it and can usefully control behavior and planning. Thus, rapidly changing patterns of activity.in the retina and other early visual structures seem to contribute to conscious visual perception by affecting the responses of higher areas indirectdy, rather than directly. The neurological evidence is in agreement with these observations. In adults, lesions of the retina produce blindness, but they do not eliminate the possiLility of conscious visual experience, as evidenced by the persistence of visual imagery, visual memories, and visual dreams. On the other hand, lesions of certain visual cortical areas eliminate all visual aspects of perception, imagery, and dreaming.> Another indication that a significant proportion of neural activity goes on without directly contributing to conscious experience comes from considering cognitive tasks that are highly prac~iced, as we did in chapter 5. Much of our adult cognitive life is the prodoct of highly automated routines that make it possible to tallc, listen, read, write, and so on fast and effortlessly. Neural processes devoted to carrying out such routines do not contribute directly to conscious experience, although they are essential in determining its content. For example, when we want to express a certain idea, such routines guarantee that in general the appropriate words will come to mind without any explicit conscious effort. As we mentioned, some evidence indicates that neural circuits that carry out such highly practiced neural routines may become functionally insulated. As we discuss in chapter 14, these circuits are not integrated with more distributed neural processes except at the input and output stages. Similarly, neural events that are too fleeting or too weak to participate in sustained, distributed interactions are also unlikely to contribute to conscious experience. Neural activity that is suffficient for triggering a particular behavioral response but insofficient in strength or duration to affect a distributed neural process may be responsible for many examples of perception without awareness. Experiments involving the direct or indirect stimulation of cortical areas also soggest the existence of significant limits to the spread of fast, distributed interactions in the brain. Many brain regions can be briefly stimulated or lesioned without producing direct or immediate functional effects on other regions, despite the presence of anatomical pathways linking them. Likewise, the lesioning or stimulation of these regions does not have direct consequences on conscious experience. These observations suggest that transient changes in the activity of such regions are functionally insulated from those of other parts of the brain, at least for short p Finally, modeling studies indicate that although the sheer anatomical connectivity of the brain may hint that everything can interact with everything else, several factors ensure that the emergence of fast, effective interactions is not a global phenomenan. The organization of the anatomical connectivity of certain brain systems, such as the thalamocortical system, is much more effective in generating coherent dynamic states than that of other regions, such as the cerebellum." Such studies also suggest that despite the continuity of anatomical connectivity in the cortex, nonlinear interactions among neuronal groups, that are due, for instance, to the activation of socalled voltage-dependent connections, may transiently increase the strength of the interactions among a subset of groups, leading to the formation of distinct functional boundaries. Moreover, these modeling studies also suggest that although all elements of the brsin are likely to be functionally interactive over a sufficiently long time scale, only certain interactions are fast enough and strong enough to lead to the formation of a functional cluster within a few hundred milliseconds (see chapter 10). THE DYNAMIC CORE HYPOTHESIS Taken together, these observations support the conclusion that at any given time, only a subset of the neuronal groups in the human brain - although not a small subset - contributes directly to conscious experience. This conclusion, in turn, rsises a question that epitomizes the entire issue of the neural basis of consciousness - a question that is as simple to formulate as it is difficult to answer. What, if anything, is special about these neuronal groups, and how should they be identified? Laying the ground for an adequate answer to this question has been our main goal for a good part of this book. As we have argued, assuming that certain local properties of neurons may sooner or later hold the key to the mystery of consciousness is entirely unsatisfactory. How could having a specific location in the brain, firing in a particular mode or at a particular frequency, being connected to certain other neurons, or expressing a particular biochemical compound or gene endow a neuron with the remarkable property of giving rise to conscious experience? The logical and philosophical problems of hypostatization associated with such assumptions are all too obvious, as both philosophers and scientists have noted many times. Consciousness is neither a thing nor a simple property. Instead, our approach has been to focos on the fundamental properties of conscious experience - such properties as integration and differentiation - and explain them in terms of neural processes. The previous discussions amply indicate that if integration and differentiation are indeed fundamental features of consciousness, they can be explained only by a distributed neural process, rather than by specific local properties of neurons. Can we formulate a hypothesis that explicitly states what, if anytlung, is special about the subsets of neuronal groups that sustain conscious experience and how they can be identified? We believe that we are now in a position to do so, and indeed to do so concisely. The hypothesis states: 1. A group of neurons can contribute directly to conscious experience only if it is part of a distributed functional cluster that, through reentrant interactions in tbe thalamocortical system, achieves high integration in hundreds o" to emphasize both its integration and its constantly changing composition. A dynamic core is therefore a process, not a thing or a place, and it is defined in terms of neural interactions, rather than in terms of specific neural location, connectivity, or activity. Although a dynamic core will have a spatial extension, it is, in general, spatially distributed, as well as changing in composition, and thus cannot be localized to a single place in the brain. Furthermore, even if a functional cluster with such properties is identified, we predict that it will be associated with conscious experience only if the reentrant interactions within it are sufficiently differentiated, as indicated by its complexity. While we envision that a functional cluster of sufficiently high complexity can be generated through reentrant interactions among neuronal groups distributed particularly within the thalamocortical system and possibly within other brain regions, such a cluster is neither coextensive with the entire brain nor restricted to any special subset of neurons. Thus, the term dynamic core deliberately does not refer to a unique, invariant set of areas of the brain (whether prefrontal, extrastriate, or striate cortex), and the core may change in composition over time. Since our hypothesis high-lights the role of the functional interactions among distributed groups of neurons, rather than their local properties, it considers that the same group of neurons may sometimes be part of the dynamic core and underlie conscious experience, but at other times may not be part of it and thus be involved in unconscious processes.' Furthennore, since participation in the dynamic core depends on the rapidly shifting functional connectivity among groups of neurons, rather than on anatomical proximity, the com the core can transcend traditional anatomical boundariesdS Finally, as suggested by imaging studies, the exact composition of the core related to particular conscious states is expected to vary significantly from person to person. |

HOME      BOE     SAL     TEXTE