Metaphors of Consciousness in the Brain
Metaphors of Consciousness in the Brain
Feb 10, 2003, 05:47 PM
Joined: Jan 22, 2003
Member No.: 9
METAPHORS OF CONSCIOUSNESS AND ATTENTION IN THE BRAIN
Bernard J. Baars
Scientific metaphors have long provided heuristic tools for approaching novel problems. Today, the neurobiology of consciousness and attention is once again a central concern, presenting formidable conceptual and empirical challenges. It therefore makes sense to try out the heuristic tools that have worked before. Many current ideas fit the broad theme of a theater metaphor. This idea can be worked out in detail, resulting in testable hypotheses with clear relevance.
Metaphors and analogies have a long history in scientific thought. They include the Rutherford planetary analogy for atomic structure, the clockwork metaphor for the solar system, and Harvey's pump metaphor for the heart. (1) Such heuristic metaphors are useful especially when the sciences encounter a topic that has no clear precedent. In the case of consciousness and attention neuroscience appears to confront unprecedented problems once again. A classical metaphor for consciousness has been a "bright spot" cast by a spotlight on the stage of a dark theater, integrating multiple sensory inputs into a single conscious experience, and then disseminating it to a vast unconscious audience. Such a theater stage is called a "global workspace" in cognitive theory. (2) It implies both convergence of input, and divergent dissemination of the integrated content. In this century, features of the theater metaphor have been suggested by neurobiologists from I.P. Pavlov to Francis Crick. Indeed, nearly all current hypotheses about consciousness and selective attention can be viewed as variants of this fundamental idea. (2, 3) If that is true, its pros and cons may be worth exploring.
In 1984 Francis Crick proposed a "searchlight of attention" metaphor for thalamocortical interaction, specified in terms of testable hypotheses at the cellular level. As Crick wrote,
"What do we require of a searchlight? It should be able to sample activity in the cortex and/or the thalamus and decide 'where the action is.' It should then be able to intensify thalamic input to that region of the cortex, probably by making the active thalamic neurons in that region fire mor rapidly than usual. It must then be able to turn off its beam, move to the next place demanding attention, and repeat the process. ... It seems remarkable, to say the least, that the nature of the reticular complex (of the thalamus) and the behavior of the thalamic neurons fit this requirement so neatly. " (p. 368-9) (4)
Crick derived four testable hypotheses from this metaphor. If this was the only use of the searchlight notion, it could be discarded as having done its job. But it has more uses. For example, Crick suggests that "there may be at least two searchlights: One for the first visual area and another for all the rest." One could add thalamocortical searchlights for auditory and somatosensory cortex, perhaps interacting in a mutually inhibitory fashion, so that only one sensory searchlight could be turned on at any time. But we can be aware of more than sensory inflow. Inner speech and visual imagery can compete for access to consciousness. Recent evidence indicates that inner speech involves auditory and speech perception cortex, and that visual projection areas participate in visual imagery. (5) However, humans have conscious access to ideas as well, which may involve prefrontal activation (6). Conscious contents also influence motor output, involving prefrontal, motor, and anterior cingulate cortex. Since all these cortical regions interact with corresponding thalamic nuclei, the searchlight metaphor may generate testable hypotheses about the role of consciousness and attention in all these parts of the brain. (7)
But that is not all. Real searchlights are guided to their targets, suggesting executive control. And real searchlights are useless without an audience to whom the contents in the illuminated spot are disseminated. In the brain the "audience" may consist of unconscious regions of cerebral cortex, hippocampus, basal ganglia, amygdala, etc., that may be activated by conscious contents. The audience for a brain searchlight may also include executive/interpreter systems, such as Gazzaniga's "narrative interpreter" of the left hemisphere. (8) Other executive regions of prefrontal cortex may also receive conscious information. Thus searchlight metaphors do not stand alone. They imply a larger framework, a surrounding "theater."
Cognitive models of memory seem to have exactly this same set of implications: A working memory whose active items are conscious and reportable, controlled by an executive, with an audience of memory systems to receive its contents. (9) "Cognitive architectures" are large-scale simulations that have been developed since the 1950s by A. Newell, H.A. Simon, J.R. Anderson and others. (10) They have been used to model a range of behavioral tasks from chess-playing to language comprehension, memory retrieval, and decision-making. Cognitive architectures resemble theaters, typically taking input into a narrow "stage" of working memory, interacting with a large "audience" of semantic networks, automatic routines, and memory systems. This theoretical tradition has been systematically related to consciousness, at least qualitatively, in a framework called global workspace theory (2). For example, all cognitive architectures treat active elements in working memory as reportable; but reportability is the most widely used operational definition of conscious contents. Elements outside working memory are automatic or in longterm memory, and are therefore unreportable and unconscious. Cognitive architectures seem to reflect the same theater metaphor that is implicit in the searchlight notion.
Theater models are also consistent with proposals for integration of perceptual features, and for "convergence zones" combining various inputs into unified neural representations. Damasio (11) has suggested that consciousness may be associated with cortical convergence zones, but of course theaters exist to allow numerous convergent influences to shape a coherent performance on stage --- which is then distributed divergently to the audience. Schacter (12) notes that conscious or explicit processes involve integration across multiple dissociable subsystems, which is precisely what theaters are good for. The widely discussed "binding function" of consciousness involves yet another compatible feature of the same underlying theme. Gazzaniga (13) has proposed that conscious experiences involve a kind of "publicity organ" in the society of mind, just as a theater allows one to make public selected information. Finally, a vast unconscious audience of specialized neural assemblies and routines is found almost universally in contemporary thinking about the brain (14). In all these proposals, the fundamental function of the theater architecture is to create widespread access --- to make possible novel adaptive interactions between the sensory inflow, motor outflow, and a range of knowledge sources in the brain.
The theater metaphor has encountered criticism from Dennett and Kinsbourne (3), who agree that it is implicit in much of our thinking, but claim that it is "Cartesian" and misleading. A "Cartesian theater" in their view has a "point center" where all sensory input converges, like the pineal gland in Descartes' 17th century view of the brain. However, neither Crick's thalamocortical searchlight nor cognitive architectures propose a single, central point center. Rather, all current proposals involve "binding," "convergence zones," or "working memories" for the integration of conscious input. These ideas are not conceptually problematic. More broadly, Dennett and Kinsbourne maintain that there is no single place in the brain where "it all comes together," as suggested by Damasio, Crick and Koch, and others. However, recent single-cell studies by Sheinberg and Logothetis (15) suggest strong convergence of conscious visual object information in inferotemporal cortex and the superior temporal sulcus in the macaque. Ninety percent of visual neurons in these areas respond differentially to the conscious but not the unconscious visual flow in a binocular rivalry task. Since this area integrates many visual features into object representations, it may indeed be a place where conscious visual information comes together.
Other philosophical critics maintain that consciousness could not possibly play the role attributed to it by theater hypotheses, because computers can simulate such hypotheses without consciousness. But the brain does many things differently from computers, and few scientists would rely on computers in lieu of direct evidence on the neurobiology of consciousness. Still other philosophers claim that some aspects of consciousness, such as subjectivity, may be inherently inexplicable. But that implies a misunderstanding of the scientific enterprise. The goal is a modest increment in knowledge. We simply cannot know today whether we can eventually understand a problem like subjectivity. That may become clearer as more plausible hypotheses are tested. In sum, such philosophical challenges do not invalidate a useful thinking tool.
Indeed, it appears that humans can access a great range of brain functions by way of conscious sensory feedback. No one knows directly which groups of vocal tract muscles they use to say a word, but by way of conscious sensory feedback a wide variety of vocal parameters are controlled. Conscious feedback seems to create spectacular access not only to skeletal muscles, but even to autonomic musculature over the short term. Biofeedback control over single neurons and whole populations of neurons almost anywhere in the brain is well established (21). To establish control over a single spinal motor unit we merely monitor its electrical activity, amplify it, and play it back over headphones; in a half hour subjects have been able to play drumrolls on single motor units, isolated from adjacent units. To gain control over alpha waves in occipital cortex we merely sound a tone when alpha is detected in the EEG, and shortly subjects can learn to increase the amount of alpha at will. Consciousness of sensory feedback appears to be a necessary condition to establish biofeedback control, though the neural activities themselves remain entirely unconscious. It is as if mere consciousness of results creates access to unconscious neuronal systems that are normally inaccessible and autonomous.
Consciousness thus seems to be needed to access at least four great bodies of unconscious knowledge: autobiographical memory, which is believed to require hippocampus; the lexicon of natural language, thought to involve speech perception areas of both hemispheres; automatic routines that control actions, involving motor and prefrontal cortex, basal ganglia, and cerebellum; and, by way of sensory feedback, even the detailed firing of neurons and neuronal populations. Many parts of the brain may therefore be recruited by conscious contents.
Some anatomical structures may function much like the basic elements of a theater. They may integrate, shape, display and disseminate conscious contents, to be received and analyzed by other brain structures, and to receive feedback from them.
1. Convergence zones: The "theater stage."
Sensory projection areas of the posterior cortex may provide one kind of "theater stage," when "lit up" by attentional activation, displaying coherent conscious information to be distributed frontally and subcortically. (18) In the case of visual consciousness, an essential structure is the first cortical projection area V1, whose lesioning leads to blindsight --- visual knowledge without visual consciousness. But higher visual lesions lead to selective impairment of conscious motion, color, or objecthood. Thus we must include V1, V2, V3, V4, MT, and IT for multiple levels of visual content (22). As pointed out above, recent single-cell work by Logothetis and colleagues strongly suggests that fully integrated conscious visual information does not emerge until the anterior pole of the temporal cortex. This makes a great deal of sense, because neurons in these areas respond to whole objects, combining information from previous levels. The sensory projection areas for audition and the body senses may play similar roles. Even abstract conscious contents, such as meaningful ideas, often appear to be mediated by sensory indices like words, images and sensory metaphors (23). Recent fMRI work suggests that the left prefrontal cortex may play a crucial role in semantic access (6). Finally, conscious/ voluntary control involves frontal cortex, including the anterior cingulate, which seems to "light up" in all tasks that require effortful attention (24).
Multiple theater stages. If each sensory area has its own kind of consciousness, plus abstract and voluntary kinds of conscious involvement, how do we cope with not just one, but five or more theater stages, over which the spotlight of attention can play? One hypothesis is that the spotlight of attention can switch from visual to auditory, somatosensory, abstract, or voluntary cortex in multiples of 100 msec steps (2). Such an arrangement would make it possible for several "stages" to operate together. Each one may broadcast widely to the audience of unconscious networks, as soon as the spotlight touches on it. There are other ways to get multiple global workspaces to cooperate and compete, but this is a testable first hypothesis.
Inner speech, imagery, and working memory. Both auditory and visual consciousness can be activated internally as well as externally. Inner speech is a particularly important source of conscious auditory-phonemic events, and visual imagery is useful for spatial problems. They are often taken as the two basic components of cognitive working memory, and they are now known to use corresponding sensory cortex (5, 9, 25). Internally generated somatosensory imagery may reflect emotional and motivational processes, including feelings of pain, pleasure, hope, fear, sadness, etc.
2. Selective attention: "Searchlight" control.
How are conscious contents selected? The thalamus is ideally situated for controlling sensory traffic to cortex, and among thalamic nuclei, the reticular nucleus (nRt) is known to exercise inhibitory modulation over the sensory nuclei. This is indeed an expansion of Crick's 1984 proposal for visual attention (above). nRt operates under dual control of frontal executive cortex and automatic interrupts from areas such as the brain stem, pain systems, and emotional centers like the amygdala and limbic cortex. It is these attentional interrupt systems that presumably allow significant stimuli like one's own name to "break through" into consciousness in a selective listening task, when the name is spoken in the unconscious channel. Interrupt control is quite separate from frontal executive (voluntary) control. Posner (24) suggests that effortful visual attention operates through the anterior cingulate cortex.
3. Receiving regions: The "audience."
What brain regions receive conscious information? We have already listed some possibilities. Consciousness seems to be needed to access at least four great bodies of unconscious knowledge: autobiographical memory, which is believed to require hippocampus; the lexicon of natural language, thought to involve speech perception areas of both hemispheres; automatic routines that control actions, requiring motor and prefrontal cortex, basal ganglia, and cerebellum; and, by way of sensory feedback, even the detailed firing of neurons and neuronal populations. The amygdala is also known to receive information about visual facial expressions. Area 46 of the prefrontal cortex contains another visual map, and neurons in this area are believed to support one kind of working memory (26).
4. Broadcasting of selected contents: "Speaking to the audience."
How is conscious information disseminated? Sensory conscious events from posterior cortex may be broadcast frontally and subcortically. Since there are many spatial maps throughout the brain, the "trade language" of the brain may consist of activated maps, coordinated by temporal oscillations. High fidelity is important to such broadcasting, which implicates the "labeled line" system of the brain (27). Labeled line fibers emerging from posterior sensory cortex include corticocortical axon bundles, the arcuate fasciculi, and the posterior portions of the corpus callosum. A second major system of high-fidelity transmission operates via the thalami, including the mediodorsal nuclei that project to prefrontal cortex.
Labeled line fibers also connect to subcortical structures, including the limbic brain, hippocampi, amygdalae and basal ganglia, all of which are known to have precise spatial maps. Since such connections are typically bidirectional, it seems plausible that labeled line tracts establish activation loops, lasting up to tens of seconds. Significant conscious events can be renewed by inner speech, by visual imagery, or by conscious emotional feeling states, thus re-initializing such activity loops. Storage of such activated information in long term memory may occur via NMDA synapses (28).
5. Unconscious systems that shape conscious events: "Backstage."
How are conscious contents shaped? Behind the stage in a theater are many people who shape and influence the performance without themselves being visible: They include the playwright, makeup artists, the stage director, and the like. There are analogous "contextual" systems in the brain that shape conscious contents while being unconscious. In the visual system sensory contents seem to be produced by the ventral visual pathway, while unconscious contextual systems in the dorsal pathway define a spatial object-centered framework within which the sensory event is defined. There is a major difference between damage to content regions compared to contextual areas. In the case of lesioned content systems like the ventral pathway, the subject can generally notice a missing aspect of normal experience; but for damaged context systems, one no longer knows even what to expect. Without a spatial framework for vision, it is hard to define what may be missing. This may be why parietal neglect is so often accompanied by anosognosia, a massive loss of knowledge about one's body space (29).
6. Narrative observer and executive systems: The "stage director."
How do conscious events influence decision-making and motor control? Gazzaniga (13) describes conditions under which split-brain patients encounter conflict between right and left hemisphere functions. Such patients often use the left hemisphere to talk to themselves, sometimes attempting to force the right hemisphere to obey its commands. When that proves impossible, the left hemisphere may rationalize or reinterpret events. The left-brain "narrative interpreter" receives its own flow of sensory inflow from the right visual field, so that in a real sense it "observes" a conscious flow of visual information. The right hemisphere may have a parallel executive interpreter that observes its own conscious flow from the left visual field. While the right-brain observer does not speak, it may be able to deal better with anomaly via irony, jokes, and other emotional strategies. Each interpretive systems can control its own voluntary motor functions. There is an obvious analogy with a stage director, observing events on stage and ordering changes where needed. One possibility is that full consciousness does not exist without the participation of such self systems, which may be centered in prefrontal cortex.
Many proposals about brain organization and consciousness reflect a single underlying theme, which can be labeled the "theater metaphor." In these views the overall function of consciousness is to provide very widespread access to unconscious brain regions. Such access is needed for global activation, coordination and control. The theater metaphor yields testable hypotheses about perceptual binding, thalamocortical interaction, working memory and selective attention, multimodal convergence zones, aspects of hemispheric specialization, and much more. It seems as if we have many variations on a single theoretical theme that appears time and time again.
Sep 01, 2011, 04:23 AM
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Spontaneous neuronal activity is essential to neural development. Until recently, neurons were believed to be the only excitable cells to display spontaneous activity. However, cultured astrocytes and, more recently, astrocytes in situ are now known to exhibit spontaneous Ca2+ transients. Here we used Ca2+ imaging of astrocytes from transgenic mice for the simultaneous monitoring of [Ca2+]ichanges in large numbers of astrocytes. We found that spontaneous activity is a common property of most brain astrocytes that is lost in response to a lesion. These spontaneous [Ca2+]i oscillations require extracellular and intracellular Ca2+. Moreover, network analysis revealed that most astrocytes formed correlated networks of dozens of these cells, which were synchronous with both astrocytes and neurons. We found that decreasing spontaneous [Ca2+]i transients in neurons by TTX does not alter the number of active astrocytes, although it impairs their synchronous network activity. Conversely, bicuculline-induced epileptic patterns of [Ca2+]itransients in neurons cause an increase in the number of active astrocytes and in their network synchrony. Furthermore, activation of non-NMDA and NMDA ionotropic glutamate receptors is required to correlate astrocytic networks. These results show that spontaneous activity in astrocytes and neurons is patterned into correlated neuronal/astrocytic networks in which neuronal activity regulates the network properties of astrocytes. This network activity may be essential for neural development and synaptic plasticity.
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