Chapter 8. Personal Identity. Section 3 of 4

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Chapter 8
Personal Identity

continued, Section 3 of 4

Third Criterion: Subjectivity

Subjectivity is the quicksilvered criterion, the most elusive of the three Great Criteria of personal identity.

The philosopher Albert Shalom, writing in the mid-80s, could grant corporeal bases for memory and continuity — but not for subjectivity. In his view subjectivity is fundamentally different:

There seems... to be no alternative but to regard the continuity of body or, according to more recent writings, the biographical or autobiographical continuity of experiences, as the only basis on which one can refer to an individual's "identity"....

But when we are dealing with subjectivities, in particular that of the human entity, the situation is not quite so simple. It is through our own awareness of ourselves that this simple model is most clearly seen as inadequate. ...[T]he constant ability of an "I" to be aware of "itself" throughout all bodily changes and throughout all the changes of subjective experience, is a problem which simply cannot be dealt with in terms of that framework.[17]

[The] central difficulty... of all theories which try to derive an adequate theory of the person from the body/mind framework [is that] there is no explanation of how neural activity can give rise to subjective processes at all. It follows that all such theories can be no more than variations on the inexplicable emergence of subjectivity from an initial fertilized ovum considered as a purely physicochemical entity.[18]

That was a mid-80s perspective. For a mid-90s perspective, we can turn to another accomplished philosopher, David Papineau. Papineau makes a novel comparison. He likens the twentieth-century study of consciousness — subjectivity in its fullness — to the nineteenth-century study of life:

It may seem to some readers that a physicalist theory of consciousness will come close to denying the existence of consciousness. But that would be a mistake. It doesn't deny consciousness, just a certain conception of consciousness.

It denies that consciousness is some kind of extra inner light, some further non-physical property which exists over and above any physicalistically specifiable property. But this is quite consistent with holding that consciousness is a real property which distinguishes some kinds of systems from others. This combination of views requires only that we accept that consciousness is identical with some property which is specifiable in a physicalistically acceptable way.

An analogy may be helpful here. In the nineteenth century there was a heated theoretical debate about the essence of life. The participants had a satisfactory enough working notion of life: they agreed about which kinds of behaviour and physical organization are characteristic of life, and in consequence were clear enough about where in practice the line should be drawn. Everything from humans to microbes is alive, while planets and pebbles are dead. (Perhaps there were some borderline cases; but the penumbra of vagueness was not wide.)

Still, despite this wide degree of agreement on the nature of life, nineteenth-century thinkers took there to be a further question. Why are these systems alive? What mysterious power animates them? And why is this power present in certain cases, such as trees and oysters, and not in others, like volcanoes and clouds?

These questions have disappeared from active debate. Biology textbooks sometimes begin with a few perfunctory paragraphs about the distinguishing characteristics of their subject matter. But the nature of life is no longer a topic of serious theoretical controversy. Everybody now agrees that the difference between living and non-living systems is simply having a certain kind of physical organization (roughly, we would now say, the kind of physical organization which fosters survival and reproduction.)

The explanation for this nineteenth-century debate, and its subsequent disappearance, was that it was premised on the notion that living systems were animated by the presence of a special substance, a vital spirit, or elan vital, which was postulated to account for those features of living systems, such as generation and development, which were thought to be beyond physical explanation. And of course, if you do believe in such a vital spirit, then you will want to know about its nature, and why it arises in certain circumstances and not others.

However, nobody nowadays believes in vital spirits any more, not least because it is now generally accepted that the characteristic features of living systems can in principle all be accounted for in physical terms. In consequence, it no longer makes sense to puzzle about why living systems are alive. To be alive is just to be a physical system of a certain general kind. There isn't any extra property present in living systems, over and above their physical features, which distinguishes them from non-living systems. So we have stopped asking questions which presuppose such an extra property.

I recommend that we do the same with consciousness....

...We should reject the theory that consciousness involves an extra inner light in addition to facts of physical organization. But we can reject this theory without rejecting consciousness. Even if consciousness is just a kind of abstract physical organization, the difference between being conscious and not being conscious can still be perfectly real.[19]

Papineau predicts positive results from physicalist studies of consciousness. Two very recent (1997) studies are especially relevant to this essay, in that they point the way towards a corporeal subjectivity criterion. The studies isolate subjective mechanisms within their functional models of conscious brain systems. This functional approach may well address Shalom's concern, and vindicate Papineau's faith in wrinkled clay. We'll review the studies below.

Bernard Baars has developed a "Global Workspace" model of consciousness, as described in a joint article by James Newman, Bernard Baars and Sung-Bae Cho, entitled, "A Neural Global Workspace Model for Conscious Attention."[20] The researchers characterize attention as a "global integration and dissemination system... which controls the allocation of the processing resources of the central nervous system."[21]

Their system model places great importance on the thalamus.[22] The thalamus is a deep-seated limbic structure. Like the hippocampus, it also intercepts transmissions along the brain's sensory pathways. But the thalamus is not an organ of memory like the hippocampus — instead, the thalamus is known to be a regulator of "attentional states," or states of alertness.[23]

Fig. 8.2
Central location of thalamus

The thalamus has connections to and from many regions of the neocortex.[24]

For Newman, Baars and Cho, the thalamus performs attentional functions as part of the Global Workspace model. The Global Workspace is a competitive neural net system wherein independent neural output units compete with each other to see which unit can produce the largest response to each environmental stimulus input. The output unit with the largest response wins the competition. It proceeds to categorize the stimulus, classifying it as a distinct feature of the external environment.

In the thalamus, this competition is thought to "gate" attention. Gating blocks off unimportant sensations before they can reach the neocortex; freeing the cortex to act upon just those features of its environment which are novel, or else important to survival.[25]

Fig. 8.3
Thalamocortical connections

Some long-distance connections between the thalamus and the neocortex[26]

The thalamus alerts the neocortex to these important environmental features. In Figure 8.3 the thalamus is the dark ovoid at the center of the inset image. The larger, main image displays thalamic fiber bundles — they run from the thalamus to other brain regions (which reciprocate with fibers of their own). Note the thickness of the fiber bundles. Like high-voltage power lines, these fibers transmit thalamic alerts with swift and abundant force.[27]

Fig. 8.4
"Wagon wheel" diagram

A "wagon wheel" diagram of central nervous system attention and conscious perception. "Th" is the thalamus; "g_c" is a closed attention gate; and "g_o" is an open attention gate.

In this diagram a visual stimulus has opened an attention gate to the primary visual cortex (V1).[28]

In Figure 8.4 a "wagon wheel" diagram of the Global Workspace places the thalamus at visual center. Newman, Baars and Cho speak of it as the "hub" of their model. Thick fiber "spokes" anchor it to all points on the rim of cortex. In Baars' model the thalamus maintains gated attentional connections to all regions of the central nervous system.

The Global Workspace model outlines a neural mechanism of attention. And this is highly relevant to the subjectivity criterion; because when the mind "pays attention" to some object, it maintains a distinction between itself and that object. The distinction between self and object can also be stated as the distinction between self and not-self, which is perhaps a more general definition of subjectivity. Through this "verbal overlay" we can see more clearly the model's relevance to the subjectivity criterion.
Newman, Baars and Cho summarize the most important properties of their model:

[W]e have introduced a collection of neuroscience and [neural net] models for attention and binding, resource allocation, and second-order gating, which share important features and parallels with a neural Global Workspace System for conscious attention.... While the [neural net] models we have presented only implement partial aspects of the [Global Workspace] system, and even our Wagon Wheel model largely neglects the influences of memory and affective systems upon the stream of consciousness, the outlines of a general framework for understanding conscious processes should be discernable.... This is certainly great progress, given the virtual terra incognita consciousness has been for most of the history of science.[29]

John G. Taylor builds upon this Global Workspace model. In "Neural Networks for Consciousness"[30] Taylor presents a complementary neural net model. His model distinguishes discrete stages in the emergence of consciousness.

Taylor begins his exposition by listing several features of consciousness which any detailed model (neural net or other) must mimic. Taylor categorizes these low-level features of consciousness, as in Table 8.1. Here, "spatial features" place the location of mental activity; "temporal features" delimit the timing of mental activity; and "emergent features" specify the ways in which mental activity emerges as conscious awareness, or "phenomenal experience."

Table 8.1
Criterial features of phenomenal experience (PE)[31]

Feature Types

Features

Spatial features

Localized representations in
    localized modules

Modules well-coupled together

Modules well-coupled to
    higher-level modules

Temporal features

Temporal continuation of activity

Time required to achieve PE
    activation

No gap between different PE
    activations

Emergent features

One-way creation of PE activation

PE created at highest level
    of hierarchy

Rapid emergence once begun

No ability to probe lower levels
    supporting PE but below PE
    creation level

These features of conscious life have been extracted from a mountain of psychological studies. Taylor is careful to work within the real-world limitations which they impose upon the modeler.

To model the emergence of consciousness at its lowest level, Taylor begins with a two-stage neural net model of cortical processing. The two stages correspond with the two lowest levels of mental activity: the first stage is pre-conscious, an "unattended condition"; the stage above it is "passive awareness" (awareness without attention). The third and highest stage, the "attended condition," Taylor models separately.[32]
The images below map Taylor's three stages to three distinct regions of cortical activity. Figure 8.5 is the predictive theory. Figure 8.6, the experimental result.

Fig. 8.5
Levels of activation in modules of the three-stage model, and their corresponding levels of awareness.[33]

Fig. 8.6
Images of brain activity (dark areas) corresponding to the three levels of conscious activation.

"A" shows the unattended condition. "B" shows passive awareness, minus the unattended condition. "C" shows the attended condition, minus passive awareness and the unattended condition.[34]

The result agrees with prediction: each stage of awareness activates additional cortical regions. To see better why this is the case, we can highlight a broad correspondence between Taylor's three stages of awareness and three sectors of Baars' "wagon wheel" attentional model. Figure 8.7 illustrates with color-coded sectors. Note that passive awareness (yellow) charges out of the thalamus' attention gates. When a gate opens, additional cortical regions are activated.

Fig. 8.7
Correspondence between three stages of awareness and three sectors of the "wagon wheel" attentional model.[35]

This broader correspondence, which is as much anatomical as theoretical, bolsters Taylor's argument. In Taylor's model, attentional systems gate thoughts between these discrete stages of awareness.[36] As it happens, Taylor's attentional systems are similar to the thalamic system theorized by Newman, Baars and Cho (as above). There is in fact some overlap between the proposed systems, in and around the thalamus.[37]
Taylor provides clinical results in support of his model, along with details of the model's continuum neural field theory.[38] That mathematical theory defines recursive "bubbles" of persistent cortical activity: bubbles set in motion by the thalamus. These recursive bubbles separate stage-two thoughts (passive awareness) from stage-one thoughts (the unattended condition).[39]

For Taylor, this second-stage recursive separation is the principal requirement of awareness:

In the model, the lower first stage involves modules tranforming inputs into various codes. It is only at the second level that further modules are activated in a manner which brings about the emergence of phenomenal awareness.[40]

Awareness, even passive awareness, entails subjectivity — it's the conscious subject, after all, who is aware. This ubiquitous correlation indicates that the transition from the unattended condition up to passive awareness marks the emergence of subjectivity. (It's a popular idea: many philosophers are now willing to entertain just this sort of deduction.)[41]

The deduction can be stated more dramatically. We can say that Taylor's third stage of consciousness — the "attended condition" — is not necessary for subjectivity. I should clarify this more dramatic statement:

The attended condition engages the brain's frontal lobes for sustained concentration, and for planning and reasoning tasks which pertain to the object of attended concern. Such effort is a hallmark of the human experience, but adult humans sustain the attended condition only intermittently. Moreover, this level of consciousness is entirely absent in young children. Taylor explains:

[T]he late onset of prefrontal myelination [axon sheathing] would prevent much use being made of such frontal sites in the first few years of life, a period in which there is extensive coding of words and objects in posterior sites.[42]

Taylor goes on to cite studies of adults who have maintained conscious life despite damage to the frontal lobes.[43] These case studies provide additional support to the idea that third-stage thoughts can be distinguished from, and are largely independent of, basic subjectivity.

The implication of Taylor's work is that subjectivity emerges within the second stage of mental activity — not the third. Passive awareness would appear to be sufficient.

Newman, Baars and Cho reach a similar conclusion. They deduce that the frontal lobes perform an "executive function" over the limbic system.[44] The researchers take pains to point out the fact that this executive function is not indispensable to consciousness — thalamic competition is judged the more fundamental manifestation of conscious life:

Note that an executive system is not an essential requirement for consciousness. That this is not the case is illustrated by the literature on extensive damage to the frontal lobes of the brain.... In terms of the [Global Workspace] model we have presented, it is not executive attentional processes, but the selective binding of coalitions of active cortical modules via a thalamocortical competition which is the sine qua non for the generation of a coherent stream of conscious representations.[45]

And finally, a picture worth a thousand words: Figure 8.8 shows the thalamus at work.

Fig. 8.8
Thalamus imaged while in the process of stimulating the cortex to attention.[46]

This extraordinary set of images comes from an experiment in which five adults were monitored as they focused attention upon a visual target. These images are a record of activity within each subject's thalamus (blue rectangle).

Subjects attended to the target under three different mental conditions: high, medium and low arousal. Performance during the state of low arousal (morning sleep deprivation) ignited the greatest activity in the thalamus. This activity maps to color pixels, which predominate in the "low arousal" images at left.

The investigators speculate that this record of heightened activity captures the thalamus as it is "working harder" to force a drowsy cortex up into an attentive state.[47] The thalamus, it seems, is here compelling the cortex to "wake up and pay attention."

Remarkable images — and photographic evidence in support of theories which implicate the thalamus in subjective awareness.

So thalamus is king. Taken all in all, the thalamic attentional system would appear to be of utmost importance to the emergence of subjectivity.

Third Conclusion: The subjectivity criterion of personal identity has a corporeal basis.

next Section 4 of 4

Chapter 8, Section 3 Endnotes

[17] Shalom 449.

[18] Shalom 366.

[19] David Papineau, Philosophical Naturalism (Oxford: Blackwell Publishers, 1993) 121-23.

[20] James Newman, Bernard J. Baars, and Sung-Bae Cho, "A Neural Global Workspace Model for Conscious Attention," Neural Networks 10:7 (1997): 1195-1206. The article is reproduced within the next section.

[21] Newman, Baars, and Cho 1195.

[22] See especially Newman, Baars, and Cho 1197-99, Section 2.

[23] See Marvalee H. Wake, ed., Hyman's Comparative Vertebrate Anatomy, 3rd ed. (Chicago: University of Chicago Press, 1992) 681. Quoting:

"The fifth division of the dorsal thalamus is composed of the intralaminar nuclei. The neurons of this group are scattered along the margins of the other thalamic nuclei and constitute an extensive arousal system to the telencephalon. These neurons receive projections from all of the ascending sensory pathways..., and they project to the outer layer of the isocortex.

Such neurons appear to form an alerting or biasing mechanism for the isocortex. They probably do not convey information on where or what is happening in an animal's world, but rather convey that something new or different is happening that 'needs' to be analyzed."

[24] Francis Crick, The Astonishing Hypothesis (New York: Charles Scribner's Sons, 1994) 84.

[25] For a good introduction to competitive neural networks, see McLeod, Plunkett, and Rolls 127-38, Chapter 6.

[26] Hendelman 199.

[27] Thick axons are fastest. Thalamocortical fiber bundles are thick enough to be visible to the unaided eye: the lobotomy procedure severs them. For neuro-computational details of their alerting function, see Anthony Zador, "Thalamocortical Synapses: Sparse but Stentorian," Neuron 23:2 (1999): 198-200.

[28] Newman, Baars, and Cho 1199. Quoting the original caption:

"'Wagon wheel' model of CNS systems contributing to global attention and conscious perception. A1, primary auditory area; BG, basal ganglia; g_c, 'closed' nRt gate; g_o, 'open' nRt gate; MRF, midbrain reticular formation; nRt, nucleus reticularis thalami; PFC, prefrontal cortex; S1, primary somatosensory area; Th, ventral thalamus; V1, primary visual cortex (from Newman et al., 1997)."

[29] Newman, Baars, and Cho 1205.

[30] John G. Taylor, "Neural Networks for Consciousness," Neural Networks 10:7 (1997): 1207-25. The article is reproduced within the next section.

[31] John G. Taylor, "Neural Networks for Consciousness" 1209.

[32] John G. Taylor, "Neural Networks for Consciousness" 1221-22.

[33] John G. Taylor, "Neural Networks for Consciousness" 1212.

[34] John G. Taylor, The Race for Consciousness (Cambridge, Massachusetts: A Bradford Book, MIT Press, 1999) 270. Quoting the original caption:

"Surface-projected schematic representation of mean activated areas of cortex (a) during inattention to an auditorily presented stream of syllables due to attention to a visual signal presented to both eyes. (b) The additional cortical area brought on stream when subjects passively listen to the syllables (no visual distractor). (c) Additional cortical area when attention is focused on the syllables."

[35] The correspondence is this author's interpretation. Note especially that the juxtaposed images do not represent attentional events of the same sensory modality. The wagon wheel drawing illustrates a visual stimulus, whereas the brain images record an auditory stimulus. For the researchers' original captions, refer to notes 28 and 34 of this section.

[36] John G. Taylor, "Neural Networks for Consciousness" 1209-11, 1221-22.

[37] Taylor's model of the "thalamus-NRT-cortex complex" is cited by Newman et al. See Newman, Baars, and Cho 1198.

[38] John G. Taylor, "Neural Networks for Consciousness" 1211-21.

[39] In light of prior arguments, it is perhaps unsurprising to find that Taylor's hypothesized neural net architecture is massively recursive. See especially John G. Taylor, "Neural Networks for Consciousness" 1210, Figure 4; also reproduced within the next section. Taylor provides a supplementary exposition on recursion in the nucleus reticularis thalami (the layer of neurons surrounding the thalamus proper) in John G. Taylor, The Race for Consciousness 149-56.

[40] John G. Taylor, "Neural Networks for Consciousness" 1207. (Quoted from the abstract.)

[41] For example, David Chalmers, a prominent and popular philosopher of mind, has recently (1996) made a strong case for something very similar: namely, the correlation between consciousness and awareness. See David J. Chalmers, The Conscious Mind (New York: Oxford University Press, 1996) 213-46; Chapter 6. Quoting from page 243:

"It is very plausible that some kind of awareness is necessary for consciousness. Certainly all the instances of consciousness that I know about are accompanied by awareness. There seems to be little reason to believe in any instances of consciousness without the accompanying functional processes. If there are any, we have no evidence for them, not even indirect evidence, and we could not in principle. It therefore is reasonable to suppose on the grounds of parsimony that wherever there is consciousness, there is awareness."

[42] John G. Taylor, "Neural Networks for Consciousness" 1221.

[43] John G. Taylor, "Neural Networks for Consciousness" 1221.

[44] Newman, Baars, and Cho 1201.

[45] Newman, Baars, and Cho 1202.

[46] C. M. Portas, G. Rees, A. M. Howseman, O. Josephs, R. Turner, and C. D. Frith, "A Specific Role for the Thalamus in Mediating the Interaction of Attention and Arousal in Humans," The Journal of Neuroscience 18:21 (1998): 8982.

[47] Portas et al. 8987.