- The main idea in this series of essays is that subjective awareness (more precisely, what I call C) depends upon the intralaminar nuclei of each thalamus (hereafter, ILN). This implies that the internal structure and external relations of ILN make subjective awareness possible. An array of material relevant to this proposal was briefly reviewed in Part I (Bogen, 1995). This Part II considers in more detail some semantic aspects and a bit of philosophic background as these pertain to propositions O, 1, and 2 of Part I. Part II should be read in conjunction with Part I. o 1995 Academic Press. Inc.
- O. A PHYSIOLOGY OF CONSCIOUSNESS NEEDS SEMANTIC SPECIFICATION
- Investigations of consciousness can be carried out at many levels, in various ways. What appeals to me, as a consequence no doubt of my upbringing,1 is an anatomicophysiologic approach. As mentioned in Part I (Bogen, 1995), this view, while explicitly mechanistic, is not necessarily materialistic, a point elaborated in Proposition 14 of this Part II.
- I believe that elucidation of consciousness at a physiological level is both attainable and desirable. Consideration of this goal for some years suggests to me that a principal difficulty arises from the multiplicity and heterogeneity of "consciousness" as discussed by different authors, sometimes differing from time to time or even page to page in just one author. As Natsoulas (1983) put it, "No individual familiar with the history of psychology will be surprised that, at the present time, conceptual confusions and difficulties in mutual comprehension attend scientific discussions of consciousness, even where the participants are relatively sophisticated students of the topic." An essential step, therefore, is to sharpen or constrain our description of the phenomenon, a step to some extent separable from showing the phenomenon's physiology. These two steps are not entirely separable because increased physiological knowledge can suggest (and has suggested) the constraints to be adopted.
- The program suggested here is to take what often appears to be a rather diffuse, intuitive notion (i.e., "consciousness") and narrow it down, by stripping off various layers of implication and connotation, to what I believe is at the core (which
1 Extensive autobiographical information here would be inappropriate; suffice to say that as early as age 5 I was fascinated by the rabbit experiments my mother was doing to earn her Ph.D. in biochemistry.
- I call C) of this notion. In other words, C is intended to be a highly constrained concept suitable for neurologizing and, at the same time, reasonably representative of what is indispensable about "consciousness."
- We cannot now ken the entirety of consciousness and should not bite off more than we can chew. What we need first is a bite (from the juiciest, tasty middle) which is small enough to be thoughtfully chewed and swallowed.
- 1. C MUST INCLUDE SUBJECTIVITY
- Your subjective experience is different from mine, ours from a monkey's or a cat's, and all of these from a bat's. But do they not all have something crucial in common? Thomas Nagel's oft-cited essay (1974) asserted the unreasonableness of attempting an objective (i.e., scientific) explanation of the subjective. He wrote, "With consciousness it [the mind-body problem] seems hopeless." It may have seemed hopeless to him, but it does not seem hopeless to others, including me. This is a question, I suppose, of what one judges might be a productive line of inquiry.
- Nagel went on to say, "Conscious experience is a widespread phenomenon. It occurs at many levels of animal life." Here we are in agreement. That is, I believe that subjective awareness is possessed by many animals, although the things of which they are aware may differ widely. These essays treat only briefly the variety in content of consciousness which is dependent upon species differences and variation in individual experiences, as well as alterations by drugs, hypnosis, etc. These are all satellite diversions which eventually (but not this early on) deserve exploration.
- As this essay continues, it will become progressively more clear what I mean (and do not mean) by C. For the moment, C means "subjective awareness." I will try, as the characterization of C proceeds, to take note of the comment by Donald MacKay (1969, p. 83)
- ... in the early stages particularly, the technical equivalent of a common term should conform as far as possible with common usage. The technician's effort to sharpen the concept should at least in principle allow the technical equivalent to be substituted for the term, without violation of basic sense or grammar, in as many contexts as possible.
- One way in which I diverge from MacKay's advice is to use the symbol C, rather than asking the reader to keep constantly in mind that the word "consciousness" is being used in a narrow, less familiar way.
- 2. C NEED NOT BE FORMALLY DEFINED TO BE USEFULLY CONSIDERED
- You are likely conscious (hence, endowed with C) while reading this and you probably believe that I was conscious when I wrote it. Although there are troublesome exceptions (Allport, 1988), we commonly think we know consciousness when we see it. As an example, consider the importance of consciousness for a neurosurgical decision whether or not to operate upon an acutely brain-sick patient, and when. In spite of CAT scans, PET scans, magnetic resonance imaging and angiography, quantitative EEG, Doppler sonography, and other "hi tech" methods now clinically available, it is still the case that in the neurosurgeon's multifactorial equation, the level of consciousness (rather, its rate of change) is the major variable. If the patient's level of consciousness is decreasing, aggressive intervention is needed, and the more rapid the decrease, the more urgent the need.2
- Our ability to label levels of consciousness is largely the result of ostention. Ostention means that I convey what I mean by ''x'' by saying "x'' when pointing to n objects, and "not x" when pointing to m others. A problem with ostention is that it does leave considerable room for misunderstanding. Ostention contributes both to our having some common understanding and to our uncertainty about (and disagreement with) what others mean. But in spite of some inevitable ambiguity, ostention works, not only for people but also for artifacts. Quite often, the training of artificial networks involves the presentation of a large number of examples from which categories are eventually abstracted. Indeed, the success of both people and artificial networks suggests that this approach can at times be better than beginning with a formal definition.
- By a "formal definition," I mean a statement of the conditions which are both necessary and sufficient. Such a definition usually needs rather than precedes an adequate theory (Edelman, 1989, p. 19). As Weiskrantz (1986, p. 166) put it, "Definitions are more helpful after one considers the body of background knowledge to a concept than before." P. S. Churchland (1988, p. 284) pointed out,
- The idea that if only we could get the words correctly defined then we would understand the phenomenon is seductive but misguided. The words will come to have a more precise meaning as they are more deeply embedded within the framework of an empirical theory ... [and] some philosophers have called the "define-the-words-first" strategy the heartbreak of premature definition. [The italicized passage she attributes to Dennett.]
- As we come to understand C better, we will progressively approach a formal definition. Meanwhile, we must content ourselves with a characterization which, while insufficient for a formal definition, is sharper than that given solely by ostention .
- It was pointed out to me by Professor Asa Kasher that the process I am advocating here was discussed in detail by Rudolf Carnap (1950) who called it "explication." He wrote,
- By the procedure of explication we mean the transformation of an inexact, prescientific concept [the datum] into a new exact concept ... since the datum is inexact, the problem itself is not stated in exact terms; and yet we are asked to give an exact solution. This is one of the puzzling peculiarities of explication. It follows that, if a solution for a problem of explication is proposed, we cannot decide in an exact way whether it is right or wrong. Strictly speaking, the question whether the solution is right or wrong makes no good sense because there is no clear-cut answer. The question should rather be whether the proposed solution is satisfactory, whether it is more satisfactory than another one, and the like.
- Carnap noted that Arne Naess introduced a related concept: precisation. In paraphrase, the concept of Naess seems to me as follows: the formulation "C" is more precise than "consciousness" if there are properties of "consciousness" which are not properties of "C," but there are no properties of "C" which are not also properties of "consciousness." What I pursue in this essay might better be called "precisation" than "explication" since C cannot yet be made "exact" in Carnap's sense, although C will be more sharply characterized than the amalgamate datum (consciousness) with which we begin.
2 Widely accepted among neurosurgeons for the labeling of lower levels of consciousness (because of its interobserver reliability) is the Glasgow Coma Scale (Teasdale & Jennett, 1974).
- 3. C INVOLVES SOME BACKGROUND PRESUPPOSITIONS
- A program of precisation is inevitably affected by preconceptions. I will try to make explicit a few of mine. Already mentioned is my bias for constraining C in the light of and with an emphasis on neuroanatomy. Three others will be discussed next.
3.1 C Can Have (although It Need Not Have) a Causal Role in Our Behavior
- Some suppose that consciousness (including C) is an epiphenomenon, like the sounds of a beating heart which can inform us about the state of the heart although having no influence on the heart's functions. When we reject this view (of C as epiphenomenon) it may be, at least in part, for personal reasons. It seems to challenge our selfhood or in some such way makes us feel uncomfortable. But there are less self-serving rebuttals; in his monograph on consciousness, Baars (1988) devotes Chapter 10 to this question. One reason which many find compelling is the appearance and persistence of C under selective pressure during mammalian (and other?) evolution. Flanagan (1991, p. 39) quotes William James, arguing that it is, "... inconceivable that consciousness has nothing to do with a business which it so faithfully attends." Of course, it is conceivable (else why say it isn't?); "highly improbable" is how we might now describe the idea that consciousness is only epiphenomenal.
- When someone honestly says, "I am thinking it over," this need not be simply a report of what is happening. Rather, the person's C of what is happening can itself affect what happens. More specifically, when the neuronal circuits generating C are active concurrently with the activity of circuits involved in some decision, the decision is often (certainly not always) affected one way or another by the concurrence.3 As mentioned in Part I, Section 2, being "endowed with C" includes that a pattern of activity so endowed has an increased likelihood of influencing other neuronal activity. This might well cause some change in ongoing behavior. At times this may be as simple a matter as inhibiting further action while detailed processing occurs elsewhere. This is elaborated in Section 16.
3.2. We Are Looking for Mc, Not C
- C is provided by some cerebral mechanism, Mc. It is this mechanism which we hope to locate and ultimately analyze. Pointing to C may turn out to be like pointing to the wind. We can point to the effects of the wind, and we can often give a good account of what causes the wind, the causes often being quite distant (in miles) from the effects. An objective here is to constrain C sufficiently that Mc will be identifiable within and as part of cerebral anatomy.4 The effects of C, when it is generated by Mc, will depend on the nature of each of the many things distant from but communicating with Mc. Which of these effects might be included in "consciousness" is important, but is not the immediate problem.
3 The variety of psychoneural correspondence assumed here is briefly discussed in Footnote 11.
3.3. C Is an Emergent Property of Mc
- The term "emergent" is used variously (Wimsatt, 1976, 1985). What I mean here is a property of a set of elements, which is neither explicable nor predictable solely on the basis of the properties of those elements together with some simple aggregative operation such as addition, multiplication, or integration (in the math sense).5 Understanding of an emergent property (generally not present in what we call "collections") requires specification of the particular way in which the elements are configured and the particular way they communicate elsewhere.
- Knowledge of the configurational properties can sometimes give us predictability (even constructability) in the absence of a complete description of the parts making up the whole. A common example is a wheel; no matter (within limits) what it is made of, it rolls. How the emergent properties of the wheel are manifested also depends upon how it is connected; sometimes wheels turn without rolling.
3.3.1. Internal configuration. With respect to the importance of internal arrangements, examples abound in organic chemistry. A particularly notorious example is thalidomide. The left-handed form of thalidomide is a fairly safe, mild sedative whereas the right-handed form (when taken in early pregnancy) causes severe congenital defects such as absent or deformed limbs (Atkins, 1987). Simple emergent properties may require only minimal specification of internal arrangements. It was shown by Hopfield (1982) that certain emergent properties could arise within a system whose only specification is that it be composed of a large number of nonlinear, richly interconnected components. For example, associative memory is likely a "natural--almost spontaneous-property of neuron ensembles" (Tank & Hopfield, 1987).
- On the other hand, when specific problems are to be solved, efficient computation is favored by the introduction of specific configurational properties (such as introducing an appropriate pattern or "syntax" of inhibitory relations) and by "forward engineering," the provision of a restricted set of hypothetical solutions (Hopfield & Tank, 1986).6
- To reiterate, C should not be expected to result from the simple piling up of decision elements, neuronal or otherwise. What we will need in order to explain Mc is a specification of its internal neuronal interactions, possibly diffuse as well as synaptic (Fuxe & Agnati, 1991). And we need a specification of how Mc is connected to elsewhere in a brain.
4 As Charles Yingling put it in recent conversation, "We will have a better chance of finding the how and why, if we can first decide on the what and where."
5 The property of "critical mass" might be an exception, probably not the only one.
6 The words in quotes are also in quotes in Hopfield and Tank (1986).
3.3.2. External connections. The stationary but rotating wheel exemplifies the importance of external connections. More complicated examples exist in the functions of various cerebrocortical areas. It has been found that the areas in temporal lobe which ordinarily have an auditory function can be converted to visual function by manipulations in very early development (the manipulations consist in removing the source of auditory input combined with removing the usual targets for neurons carrying visual information) (Sur, Garraghty, and Poe, 1988). The variations in types and distribution of neurons in various brain areas are to considerable extent dependent upon where they receive and send information. Cells which have longer axons have, in general, larger cell bodies. Understanding this point will help when we come to the question of identifying which thalamic cell groups belong to the intralaminar nuclei (ILN).
- That we can to some extent ascertain the function of some brain part by knowing how it is connected is an essential aspect of our search for Mc. That is, I have suggested we first locate Mc in order to facilitate our subsequent efforts to analyze its internal workings. As pointed out in Part I, the external connections of ILN are particularly weighty evidence that ILN provide the best candidate for Mc.
- 4. CONSCIOUSNESS INCLUDES BOTH A PROPERTY (C) OF VARIABLE INTENSITY AND A GREAT VARIETY OF CONTENTS
- This proposition is a postulate rather than something for which I provide any evidence.
- I am concerned that we be able to consider the intensity of C and its neural substrate (Mc) without, at the same time, worrying about the specific contents of consciousness. I shall refer at times to the content (and shall suggest that this is commonly derived from thalamocortical interaction). However, it is essential to make this distinction and to focus on the neural substrate for intensity, rather than the content which is typically transient and often idiosyncratic. As noted in Part I, this important distinction has already been made by Baars (1993).
- The same distinction has been made by others. Grossman (1980) wrote: "We can also introspectively discriminate between the contents of consciousness ... and the quality of being conscious." And there is a resemblance here to the distinction made by G. E. Moors between "the sensing, which alone is distinctively mental, from the sense datum sensed" (Peters & Mace, 1967). Landesman (1967), quotes Moore as follows,
- The sensation of blue differs from that of green. But it is plain that if both are sensations they also have some point in common.... I will call this common element "consciousness".... We have then in every sensation two distinct terms, (I) "consciousness," in respect of which all sensations are alike, and (2) something else [the object], in respect of which one sensation differs from another. (Philosophical Studies, London, 1922).7
7 Possibly relevant also is Edelman's (1992) distinction between "primitive" and "higher" consciousness.
- 5. AVAILABILITY TO VERBAL OUTPUT (AVO) NEED NOT ACCOMPANY C
- When dealing with human subjects, availability to verbal output is commonly (and for the most part properly) taken as evidence for subjective states. If a subject denies feeling a pressure on the skin, we ordinarily assume it was not "felt," i.e., had no accompanying subjectivity, even if we see correlated voltage changes (evoked potentials) when recording from the subject's head. If someone says of a wound suffered in midst of battle, "I didn't feel it at the time," we believe. The reasons we believe include that person's behavior at the time, and/or because we ourselves have on occasion sustained a wound without feeling it.
- But C does not depend upon availability to verbal output, nor need it involve any linguistic competence. We need only ascribe subjective states (e.g., pain, thirst) to monkeys to establish this point. Whether or not the monkey's subjective states can encompass "higher thoughts" is not essential to possessing the property C. Language and/or "higher thoughts" are varieties of content. The phenomenon of automatic utterance shows us that availability to verbal output (AVO) need not be accompanied by C. On the other hand, what we ordinarily take as signs of C (certain nonverbal behaviors) can be produced by neural activity which does not have AVO. These facts may help us by stimulating some questions, such as: what are the connections between C and AVO? One hint seems to come from the AVO of much right hemisphere information when the corpus callosum is intact, rather than severed. This AVO depends on communication over long distances via collections (tracts) of nerve fibers. One might suppose that AVO of information already in the left hemisphere (of a right hander) is often similarly communicated, i.e., by long tracts. If so, various kinds of cerebral processing could lose AVO by interruption of nerve fiber tracts, i.e., by disconnection (Geschwind, 1965). This sort of disunification may tell us something about AVO, but it does not explain C or Mc.
- 6. C IS IDENTICAL WITH ONLY PART (SUBJECTIVITY) OF WHAT IS COMMONLY MEANT BY "SELF"
- The word "self" seems to be used even more variously than the word "consciousness." My Webster's Collegiate Dictionary has about 350 usages spread over three and a half pages.
- As a possible example of a relatively narrow usage, Dennett (1991) refers to the "self as the center of narrative gravity." This is a felicitous phrase; but linguistic ability (good or bad) is not essential to the C we share with so many other species. At another extreme, Galin (1992) proposed that "self" be defined as ". .. the overall organization that makes a person a unity." This may not be the most inclusive usage possible, but it is surely a contender, there being such a multitude of hormonal and neural unifying mechanisms. We need not define "self" nor attempt a catalogue of what it might contain, in order to point to an aspect which is typically intended (often implicitly) when the word "self" is used. Flanagan (1991, p. 352) quotes William James as saying, "Whatever I may be thinking of, I am always at the same time more or less aware of myself, of my personal existence." Flanagan goes on to observe,
- This low level sense of "me-ness," of "something happening here" does seem to underlie all conscious experience. All conscious experiences are, in addition to being experienced, experienced as attached to the subject of these very experiences.
- There may be exceptions (extreme absorption or certain meditative states). But Flanagan describes what I intend to be a characteristic of C. That is, "attachment [of an experience] to the subject of the experience" is part of what I have described earlier as "endowing with C some particular content." If we use "Pi" to mean a particular pattern of neuronal activity then "endowing with C" includes "giving subjectivity to P;." [As mentioned in Part I, Section 2, "endowing with C" has a second aspect: it includes an additional opportunity for Pi to influence activity elsewhere, especially impending action.]
- 7. C DOES NOT REQUIRE THE SELF-NONSELF DISTINCTION
- A particularly prominent aspect of consciousness which I intend not to be a property of C, but rather a particular content, is the self-nonself distinction (S/NS-D) .
- First, the S/NS-D is commonly made without C. The S/NS-D is routinely made by our immune systems. And it is made by creatures (e.g., amoebae) without any central nervous system. The S/NS-D does not require C.
- Does C require the S/NS-D? Considering reports of an "oceanic" state, a feeling of "oneness with the world," the answer is immediately "no." This is because the "oceanic" feeling is an example of a subjective state without the S/NS-D.
- More compelling perhaps is the existence of pathologic states in which the S/NS-D is grossly erroneous but C persists with the same dynamic range and most of the usual potential contents. First, there are brain lesions in which a subject's limbs (more often the left limbs) are not considered part of the subject's self (Bisiach, Vallar, Perani, Papagno, & Berti, 1986; McGlynn & Schacter, 1989; Bisiach & Geminiani, 1991). And there are states without demonstrable brain lesion, as with phantom limbs, in which things nonexistent for others are felt as part of the self(cf. Melzack, 1989). In addition to the foregoing, there are reports of people "identifying" their "selves" with external objects, or other persons, or even all persons, or maybe even all living things. These observations exemplify how idiosyncratic (highly personal) the S/NS-D can sometimes be. Hence, we should not consider the S/NS-D as an integral part of C. Rather, it is one more example of content which must be brought into consciousness by the "appropriate interaction" between Mc and the neural activity representing the S/NS-D.
- Certain disorders of "selfhood" are associated with lesions of parietal cortex (Critchley, 1953; Hecaen & Albert, 1978; Benton & Sivan, 1993; Stein, 1992). For example, when someone has an occipital lesion resulting in hemianopia, unawareness of the visual loss typically means that the lesion extends into parietal cortex (Griisser & Landis, 1991). It is likely that parietal cortex plays a special role in the S/NS-D (Damasio, 1994). If so, awareness of the S/NS-D would require the "appropriate interaction" between parietal cortex and Mc.
- It may help if we consider a specific example whose physiology is somewhat understood; we adopt temporarily the view that "self" means everything in one's body. Let us compare the report, "I feel thirsty" with a second report, "I smell something burning." Most of the time, the first statement ("I feel thirsty") means "I am aware of a brain state ascribable to some goings-on in my body proper." (By "body proper'' I mean the body exclusive of brain.) The goings-on are usually either an increase in blood osmolality (detected in the hypothalamus) or a significant decrease in intravascular volume (detected in the right atrium and great veins). Less often, the subjective state (thirst) reflects a brain state attributable to dryness of the tongue, or to elevated body temperature, or any combination of the above. (It is worth noting, in passing, that discrimination among these causes is usually not crucial to an appropriate response to thirst, although it might be helpful.) Whatever the cause, thirst typically reflects an aspect of one's self.
- The second verbal report, "I smell something burning," also reflects a particular brain state. This may, on quite rare occasions, arise spontaneously in brain, as in the variety of epilepsy called "uncinate fits." Also rarely, it may reflect some condition in the nose (part of the body proper). But most of the time, by far, it reflects a brain state best ascribable to conditions outside the body. When it does not, we tend to consider it pathologic, i.e., suggestive of some disease process. Whatever the loci of the neural activity making the S/NS-D, making this decision is not a function of Mc. The neural activity making the S/NS-D need not be endowed with C; indeed, much of the time it probably is not.
- 8. C DOES NOT NECESSARILY INVOLVE "SELF-AWARENESS"
- One of the properties commonly ascribed to consciousness, especially in higher primates, is "awareness of self." Does this mean one's own body image, body boundaries, state of mind, state of health? Does it include hopes, intentions, desires, or recollections of long-past experiences? All of these, and more, are contents of consciousness. They may be endowed with C from moment to moment. But none of them is necessary for C. Note that if self-awareness begins with an awareness of the S/NS-D, then proposition 7 entails this proposition 8.
- 9. THE CONTENTS ENDOWED WITH C ARE VERY RESTRICTED AT ANY ONE TIME
- In common use nowadays is the metaphor of a "spotlight" or "searchlight" of consciousness, a small circle of bright light, surrounded by a halo of lesser illumination (the penumbra). The contents brightly illuminated, as well as the penumbra, are at any one time a very small fraction of what is potentially available to C. When the spotlight was first pictured (so far as I am aware) by Richard Jung (1954), he called the penumbra a "fringe" and said it was "hazy"; in the discussion of Crick and Koch (1990), the penumbra is considered hazy and treated in part as the result of an inhibitory surround.
- According to Julesz (1991), "the single searchlight metaphor originated with Helmholz (1896)." William James (1890/1925), according to Julesz, "opened up an entirely new field of inquiry called divided attention." Calling the penumbra a "fringe" is different from James' views on what he called the "fringe" as has recently been pointed out by Mangan (1993) and by Galin (1993, 1994). Galin (1994) noted that James' fringe is not the dim or fuzzy fringe of the spotlight metaphor. "James' fringe represents a separate class of information than the nucleus, not just the same kind of information at a lower resolution'' (Galin, 1994, p. 10). Also, "metaphoric language often carries unwanted connotations which can be misleading" (p. 9). Defects of the spotlight metaphor include the image of a sharp, closed boundary between what is illuminated and what is not. A bit better in this respect is the description by Wigner (1967),
- There always seems to be some single sensation or thought at the center of my attention, but there are other sensations which cast shadows on the center, as if they were just outside my field of vision." (p. 190)
- Whether or to what extent the spotlight metaphor is helpful, the main point is that what is "endowed with C" at any one time is picayune compared to what is potentially available. One importance of this fact (relatively little content at any one time) is that it allows us to consider structures which are small (relative to a cerebral hemisphere) as candidates for Mc.
- 10. THE NEURAL SUBSTRATE FOR MC MUST HAVE IMMEDIATE ACCESS TO MOTOR SYSTEMS
- This anatomic requirement is ascribed to Mc as a consequence of 3.1 above.8 The "crucial core of consciousness" not only includes subjective awareness of various contents (sensations, feelings, etc. which are often described as "sensory") but must also have a means to influence action. Mc must be so connected as to take account of the subjective experience that one is responsible for an action. This was considered in Part I where we took note of the large efference from ILN to the striatum, an important reason for attributing Mc to ILN.
- 11. C IS NECESSARILY OF THE PRESENT, BUT NOT NECESSARILY OF THE FUTURE
- Anticipation (or expectation) is not necessary for the subjective awareness of ongoing activity. Examples include: holding one's breath, feeling pain or thirst, or feeling sad. Each of these may be accentuated by or even mainly arise from anticipation. But the neural activity subserving C is distinguishable from the neural activity responsible for the specific content of C, including the neural activity associated with anticipation. I appreciate that this is a severe constraint. However, experience, both my own and that reported by others, with bifrontal lobectomies and lobotomies has led me to conclude that anticipation (indeed, any concern for future consequences) is only one of many specific contents to which C can attach.
8 I am indebted to Francis Crick for repeatedly stressing this point.
- 12. HUMAN C DOES NOT REQUIRE PREFRONTAL CORTEX
- When neuropsychologists and behavioral neurologists describe "frontal lobe symptoms" or "frontal lobe functions," they have in mind a large expanse of cortex more precisely termed "prefrontal" (Levin, Eisenberg, & Benton, 1991; Damasio & Anderson, 1993). They are not using the term "frontal lobe" as it is used in anatomy texts where "frontal lobe" includes everything anterior to the central (Rolandic) sulcus.
- Prefrontal cortex has come to be precisely defined as the large expanse of each frontal lobe which is reciprocally connected with the mediodorsal nucleus (MD) of the thalamus (Divac, 1988; Fuster, 1989).
- The reciprocal connections between MD and prefrontal cortex make up the white matter in the medial inferior aspect of each frontal lobe. These tracts can be easily severed by inserting a spatula through two burr holes in the top of the skull. This procedure (bimedial leukotomy) has occasionally been used to treat excruciating pain caused by head or neck cancer; it is remarkably reliable. Postoperatively the patient's conversation, food preferences, memories, and awareness of current events remain intact. And both verbal and bodily signs of distress are absent. When specifically asked if they have "pain," they typically say that they do, and in the same location as before. But they express no concern about this, and rarely if ever request pain medication. The main drawback of the procedure is that the patients also evidence minimal concern for the future consequences of their behavior which on some notorious occasions has included urinating or defecating in public. The connections between MD and prefrontal cortex are valuable, but required for C they are not.
- I bring up the nonnecessity for C of prefrontal cortex for two reasons: as a further illustration of how neurologic experience has contributed to my proposed precisation of C and to illustrate how otherwise knowledgeable people have been misled because they lack personal experience with humans who have had massive bifrontal damage. For example, a famous astrophysicist with whom I had dinner, a man also known for his ability to explain difficult material to the general public, opined: "Consciousness depends on the frontal lobes." "How so?" I asked. "Because (1) they are for the anticipation of consequences [probably true] and (2) anticipation of consequences is a defining characteristic of consciousness." (This second claim is the source of his error--if his circle of acquaintances were wider he might have met many persons whom he would consider conscious but who minimally consider consequences.) I then asked, "What would you say if someone had severe frontal damage and was still conscious?" "Is that so?" he replied, and changed the subject.
- Humans from whom the prefrontal cortex has been removed bilaterally appear to be both subjectively aware and volitional, whatever the extent to which they are neglectful, shortsighted, unconcerned, apathetic, perseverative, restless, impulsive, or even explosive. A half-century ago it was shown that normal IQ9 could be present after bifrontal lobectomy "despite the loss of somewhere around 15 percent by weight of the total mass of the cerebrum" (Hebb, 1959; see also Eslinger & Damasio, 1985. Benson, 1994, provides other examples.) In one study, both IQ and good psychiatric recovery were actually found to correlate positively with the amount of frontal lobe damage from lobotomies done 25 years or so before (Stuss & Benson, 1986, p. 10).
9 I am aware of the misleading nature of "IQ" as a measure of cognitive ability as was expertly reviewed by Lezak (1988). However, I find it hard to believe that someone does well on so-called IQ tests without possessing C.
- 13. C IS NOT "TIME BINDING"
- One of the most reliable signs of a bilateral prefrontal lobectomy in monkeys is their inability to do delayed-alternation tasks (Jacobsen & Nissen, 1937; Mishkin, 1957; Iversen & Mishkin, 1970; Pribram, Plotkin, Anderson, & Leong, 1977; Markowitsch, Pritzel, Kessler, Guldin, & Freeman, 1980; Fuster, 1989; Sawaguchi & Goldman-Rakic, 1991).
- In this task, the monkey must remember, during a brief delay, which of two otherwise identical containers held the reward (e.g., a banana chip) on the immediately preceding trial. To obtain a reward on the current trial, the monkey must choose the other container.
- Similar impairment of delayed responses was found following frontal lobectomy in humans (Prisko, 1963).10 Milner and Corsi (Milner, 1971) went on to use "recency" tasks to show in these patients a disturbance in the temporal ordering of events (Milner, Corsi, & Leonard, 1991; McAndrews & Milner, 1991). Luria's (1966) frontal lobe patients had "memory disorders" most evident when two tasks requiring recollection were given sequentially; the items from one task interfered with those from the other. Hecaen and Albert (1978) pointed out that most of Luria's findings were based on cases of large frontal tumors which likely affected other brain parts as well as prefrontal cortex. This methodologic problem qualifies ascription of the timing deficit solely to damage of either prefrontal cortex or MD, but it does not affect the important conclusion that patients unable to properly order recent events can be conscious. Moreover, deficits in temporal organization have been found in patients whose frontal lesions were not caused by tumors (Petrides & Milner, 1982; Shimamura, Janowski, & Squire, 1990).
- 14. C REQUIRES FUNCTIONING BRAIN, BUT THIS MECHANISTIC APPROACH IS ONTOLOGICALLY NEUTRAL
- I am publicly agnostic as to whether Self exists without body, whether Cosmic Mind exists, and the like. What I am inclined to believe (it is called "cerebralism") is that to the degree that something nonmaterial has observable effects, it is only through functioning brain.11 But this does not entail materialism since it is also consistent with some forms of dualism.
10 The prefrontally damaged patients of Verin, Partiot, Pillon, Malapani, Agid, and Dubois (1993) chose alternately from the start, unlike both normal and more posteriorly damaged patients who alternated only after a dozen or so trials had made clear what the rule was. When the rule then was changed to nonalternation, the two control groups soon recognized the new rule, whereas the prefrontal patients perseverated, i.e., kept alternating even though unrewarded (see also Levin, Culhane, Fletcher, Mendelsohn, Lilly, Harward, Chapman, Bruce, Bertolino-Kusnerik, & Eisenberg, 1994).
- Ontologic dualism was the creed of many past creators of neuroscience. Fritsch and Hitzig (1870) were prominent pioneers in cortical localization. Summing up some experimental results, they said,
- ... one might express himself thus: there was some motor connection between the soul and the muscle, while the connection from muscle to soul was somewhere interrupted. 12
- Hughlings Jackson was forthrightly dualistic and went so far as to say,
- We cannot understand how any conceivable arrangement of any sort of matter can give mental states of any kind ... I do not trouble myself about the mode of connection between mind and matter. (Taylor, 1931, Vol. I, p. 52).
- At one time, Jackson went so far as to say, "Psychical states are never states of the organism." (Taylor, 1931, Vol. II, footnote on p, 95).
- Sir Charles Sherrington closed the introduction to his Integrative Action (1947,p. xxiv) as follows,
- Of these two views Cajal tells how he was for a time a zealous disciple of the former, and noticed that to his practical life adherence neither to the one nor to the other seemed to make any difference whatever. ... that our being should consist of two fundamental elements offers I suppose no greater improbability than that it should rest on one only."
- Sir Francis Walshe was one of the more profound thinkers (as well as a shaker and a mover) of 20th century clinical neurology, When criticizing Karl Lashley's metaphysics he said,
- Yet, when not on this subject, no one could be a more penetrating student of the nature of cerebral organization than he was. This, surely is the proper business of the physiologist, and it is not facilitated by attaching to it, like a sinker, an irrelevant materialist ideology (Walshe, 1965, p. 208).
11 This is to say: a small subset of cerebral processes maps onto (not just "into") mentation. [The word "onto" means that the target of the mapping is exhausted by the mapping, so that nothing is left over. As Haugeland (1980) put it, "The central intuition is that 'fixing' the physical fixes everything, or that nothing [mental] could have been otherwise without something physical having been otherwise."] And an even smaller subset of cerebral processes has the potential to be endowed with C. No one now knows how the set of brain states specifically maps onto mental states, but it is surely not one-to-one for each individual brain state (Sperry, 1952, p. 309). A variety of distinguishable brain states likely correspond to a small number of mental states, in a many-to-few mapping. Within the network described by Hopfield (see Section 3.3.1), "The flow is not entirely deterministic, and the system responds to an ambiguous starting state by a statistical choice between the memory states it most resembles" (p. 2557). This is one example of a probabilistic many-to-few mapping; such a brain-onto-mind mapping imposes some limitation on the reverse, a mind-to-brain reduction. To speak of a "brain/mind relation" is to suppose that we obtain information from our own mentation which needs to be correlated with information about brain states. That is, for the foreseeable future, our knowledge of what goes on inside a human head will come from two sources, introspection and observation. This is epistemologic dualism but is ontologically neutral. That is, cerebralism is compatible with either ontologic dualism or materialism. Moreover, for the foreseeable future, cerebralism is much less likely to be eliminative of folk psychology than to be revisionary (cf. P. S. Churchland, 1995, her footnote 13; see also Horgan & Woodward, 1985).
12 This quotation is from the translation by Gerhardt von Bonin (1960, p. 96). In the translation by Wilkins (1965, p. 27) the word "psyche" is used instead of "soul"; this is a pivotal, conflationary change as discussed in Footnote 13.
- Walshe might have added, if he had been so inclined, that a humane clinical neurology can be practiced without attaching to it a dualistic metaphysics.
- The achievements of the foregoing historical figures (and many others) show that a dualistic metaphysics need not impede scientific success. The best minds of the Western world have long argued on both sides the problem of monism versus dualism, especially after, in the words of L. J. Rather (1965, p. 5), "Descartes' rediscovery of and emphasis on the primacy of awareness." Rather's 16-page discussion is a gem; among other things, he points out that, "the problem is not likely to be resolved within the set of presuppositions that helped generate it."
- Is it unfruitful that so many engage in this unending controversy? (Rather slyly called it "intellectual tauromachy"). For some it may reflect fear (or fear of a fear) of a postvital void. In any case, there is no need here to either affirm or deny ontologic dualism; cerebralism can work with either.13
- 15. C NEEDS ONLY FUNCTIONING BRAIN
- C is minimally if at all dependent upon continued communication of brain with the body proper (although some output channel is needed to detect it--something resembling EEG or PET might serve). When characterizing C we need not to confuse it with more elaborate notions of either "mind" or "consciousness." Foster Kennedy (1939) claimed of mind,
- It affects and is affected by the whole organism; and consciousness is found, not as the resident of parochial places, but as the emergent distillate of the total structure.
- Consider next the 1948 view of that eminent anatomist, Paul Yakovlev, when he wrote that mind is in,
- ... every heartbeat, every twitch of muscle, every movement and posture ... [it] is an integral part of the total behavior which evolves and proceeds as a unity in time (p. 315).
- My sometimes facetious physical chemistry professor once said (in 1951), "I know that I think with my big toe because when my big toe hurts I don't think as well." One might as well say, "I think with my rocks, because when I lose a diamond earring it interferes terribly with my thinking."
13 In view of the "near completeness" of physics, how something without spatial extent (e.g., the Soul) could affect brain is, and has long been, a problem: the "nexus problem." But this is not the same as the "mapping problem," i.e., how brain produces mind. Some superlative scientists [e.g., Sperry (1980), Edelman (1992), Crick (1994)1 have explicitly denied ontologic dualism, thus denying the existence of any nexus problem. Others [e.g., Popper & Eccles (1981), MacKay (1980), Eccles (1989)1 being dualists, have explicitly explored the nexus problem. Unclear is how either attitude, whatever other virtues they possess, contributes to the mapping problem. Many people have mistakenly conflated these two problems.
- Let's get serious. When an individual has had a high spinal injury, such as a C, (first cervical segment) transection, it is indeed serious, as in the case I attended of an athletic father of four who fell from a backyard trapeze onto his head. In such cases, movement is lost below the face. To be kept alive, such individuals need artificial respiration. Also lost are somatic sensory inputs from below the throat and ears. Yet, such individuals are convincingly conscious in their responses to questions and, although what they say reflects their distressing circumstance, their capacity for awareness seems no less than before.
- A case of medullary infarction described by Plum and Posner (1985, p. 29) indicates that transections even higher than C,, i.e., near the level of cranial nerve VIII (the auditory nerve), are compatible with, as they put it, "the behavioral appearance of consciousness."
- Intermittently during those final days, she had brief periods of unresponsiveness, but then awakened and signaled quickly and appropriately to questions demanding a yes or no answer and opened or closed her eyes and moved them laterally when commanded to do so. There was no other voluntary movement.
- A cat with an even higher transection of the brain stem (at the level of cranial nerve V) although deaf, is able to respond to olfactory and visual input with the EEG characteristics and tracking eye movements we recognize as concomitants of awareness (Battini, Moruzzi, Palestini, Rossi, & Zanchetti, 1959).
- Alema, Perria, Rosadini, Rossi, and Zattoni (1966) injected barbiturate into the vertebral circulation of 19 humans, producing loss of pupillary light reflex, corneal reflex, and both facial and eye movements for 3 to 4 min. However, loss of button presses to acoustic or visual signals and verbal responses to questions were only occasionally affected and in those cases for no more than 10 s. There was never slowing of the EEG. The authors concluded,
- In man the most important subcortical structures ultimately responsible for maintenance of the level of consciousness are located rostral to the brain stem, perhaps in the diencephalon.
- Complementing the above cases with thalami intact are cases with severe, bilateral thalamic damage but relatively intact brainstem and cerebral cortex (Kinney, Korein, Panigrahy, Dikkes, & Goode, 1994).
- 16. THE CONTRIBUTION OF C TO BEHAVIORAL UNIFICATION SHOULD NOT BE OVERESTIMATED
- By behavioral unification, I mean what is sometimes called "organismal unity," i.e., parts of the organism function to further the survival, and success otherwise, of the whole organism. Well-known examples can be seen in plants where the unification is mainly hormonal (Thimann, 1977; Jones, 1994). Hormonal integrations are important in mammals also, but they generally proceed at a much slower pace than the unifications effected by the central nervous system (CNS).
- An essential aspect of "unification" is a selection or choice among alternative responses to a specific stimulus; this depends as much upon the ongoing state at the time of stimulation as it does upon the nature of the stimulus. In the CNS, the "ongoing state" is largely a function of the neuropil, the feltwork of interwoven nerve cell processes, and locally connected interneurons. Neuropil is prominent throughout the neuraxis from top (cortex) to bottom (tip of the spinal cord). Hence, alternative responses are available even at the spinal reflex level.
- As a particularly simple example of neuropil function we can consider the withdrawal reflex. A sharp stimulus to the toe of a spinal frog typically results in one of two movements: flexion of the leg if the tip of a toe is stimulated when the leg is extended, but extension of the leg if it is already flexed. The "switching mechanism," as Magnus (1924) called it, as well as the circuits for coordinated motion of one leg (while suppressing the other), are present in the spinal cord of a mammal, as well as a frog.
- In Magnus' words (describing hind limb behavior in a paraplegic dog),
- If, therefore, one and the same sensory stimulus brings about at one time flexion and at another time extension, a specific switching process must take place each time in the spinal cord centers (p. 32).
- However, if you are aware that you are about to be stimulated, a pattern of descending cerebrofugal inhibition can often prevent any observable response.
- As more levels of CNS are added above,14 the alternatives available are altered and, in general, expanded. This is particularly evident with enlargement of the cerebrum, including an increase in the cortical connections with Mc. However, the absolute expansion of alternatives influenceable by C remains a relatively small part of the overall expansion.
- As Danto (1985) put it:
- Choosings between alternative courses of action, in the preponderance of motor acts we perform, occur as the outcome of deliberations of which we are barely conscious, if at all. ... Happily, we are so wired that deliberation may occur without the mediation of consciousness .. ."
- The same point was made by Purves and Lichtman (1985),
- Consciousness, after all, is only a small corner of our neural universe. Man's view tends to be biased because our minds have no direct access to the myriad neural mechanisms that allow us to function successfully in daily life ... (p. 355).
- It is essential to understand that most bodily adaptations (including postural adjustments, sensorimotor coordination, phoneme generation during speech, as examples, as well as most autonomic regulation) not only can proceed independently of C but, more often than not, are unavailable to C. The pupillary constriction to light is one of the most familiar. An interesting partial exception is the regulation of body temperature. When we are cold enough to shiver, usually we feel cold and we feel the shivering. In this case we are directly aware 15 of aspects of an ongoing bodily adjustment; of both its cause (lower body temperature) and its result (rapid muscle contractions), However, we probably do not have direct awareness of the brainstem neural activity producing the muscle activity. Awareness of shivering contributes little (if at all) to the ongoing adaptive processes. This seems to me to be a circumstance in which C can appear to be merely epiphenomenal.
14 "Adding above" to the neuraxis can be understood both in a phylogenetic (evolutionary) sense and in terms of how much CNS is left attached to the spinal cord when the neuraxis is truncated. For example, a midbrain animal (all nerve tissue removed above the midbrain) has a larger repertoire of behavior than an animal decerebrated at a lower level such as by a cut through the midbrain (the so-called intercollicular decerebration). The reverse ("subtracting from above") occurs with cerebral lesions. The effects of cerebral lesions depend upon lesion site, but they typically have the effect of simplifying the subject's behavior (i.e., reducing availability of potential alternatives).
- Breathing is different. When people say, "I was shivering," the meaning of "I" is quite different from its meaning in "I was holding my breath,"16 Both the control of and the sensory feedback from respiratory movement are ordinarily without C. But both can easily be endowed with C, (Again, this probably does not include awareness of the neural activity in the respiratory centers in the brain stem.) It is difficult (although seemingly obtainable by practice) to be aware of one's own breathing without affecting (by cerebrofugal output) the ongoing neural activity in the brainstem. In breathing, as opposed to shivering, epiphenomenality seems to require a lot of practice!
- An epiphenomenalist might say that the report, "I am holding my breath," reflects an awareness of something happening in which the awareness has no more causal effect than it does in "I am shivering." But I believe there is a significant difference. The difference is in the neurophysiology; it goes something like this:
- The verbal report, "I am shivering," reflects an awareness of proprioceptive information ascending from the spinal cord, and conceivably but improbably some collateral information from the brain stem neurons which are generating the motor output which produces (via spinal cord) the shivering. The verbal report (written at the time) "I am holding my breath," reflects an awareness not only of ascending information from spinal cord (and possibly brain stem) but in addition signals a thalamic (ILN) contribution to the descending (cerebrofugal) control of brainstem respiratory mechanisms.
- One can also be aware of holding one's breath, but without doing it deliberately (when frightened, for example). That could be epiphenomenal.
15 By "directly aware," I mean that feeling cold and feeling shivery are qualia (as some philosophers call them). As Daniel Dennett (1988) noted, the words qualia (plural) and quate (singular) are not transparently clear. Even pain seems sometimes to be an elaborated rather than pure percept, although some types of pain (e.g., trigeminal neuralgia) are probably as pure as contents of "direct awareness" can get. One of Dennett's arguments against the existence of qualia is that what was thought to be a single quale or "raw feel" (e.g., the sensation elicited by a specific musical chord) can be heard, after musical training, as distinct tones, simultaneously present; i.e., qualia are often divisible. My reaction is, so what? What we are directly aware of no doubt depends on our education--it does not mean we were unaware of the chord before the education. This is the argument made by Paul Churchland (1989) on this point, if I understand him correctly.
- 16 Ryle (1949), for example, recognizes in a lengthy discussion (pp. 183-198) what he calls "the systematic elusiveness of 'I'" and the "elasticities in the uses of' 'I' and 'Me'. .. ." He distinguishes among usages of "logically different types." The distinctions I suggest here depend upon differences in neuronal referents. In particular, there is a difference between awareness (endowment with C) of ascending (sensory) information versus awareness of descending (motor) information which is more accessible to ILN efference.
- Holding one's breath need not involve C at all, as reflected in the statement, "Well maybe I was holding my breath but I didn't do it deliberately and I was unaware of it at the time." The foregoing different reports about breathing reflect differences in neuronal activity; one of our goals is to describe those neuronal differences.
- OVERALL CONCLUSION
- A physiologic understanding of consciousness is desirable and will be facilitated, at this stage, by adopting severe constraints on what aspects are to be explained. The constraints employed should reflect our current anatomic and neurologic knowledge.
- The aspects here considered essential require a mechanism (Mc) for the generation of subjectivity which, by virtue of its anatomical connections, will be able to exert a causal influence on behavior. Subsequent essays in this series will explore in detail the evidence that Mc is mainly subserved by the thalamic intralaminar nuclei.
- Thanks for generous advice, not all of it followed, to M. Adelson, B. Baars, P. S. Churchland, R. Douglas, J. Fuster, D. Galin, D. Hilbert, A. Kasher, N. Kinsbourne, M. O'Dell, A. Scheibel, C. Yingling, and E. Zaidel.