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WHY IT MUST BE CONSCIOUSNESS - FOR REAL !

Bernard J. Baars
The Wright Institute
Berkeley, California USA

1. Why do we think the thalamus is involved in conscious experience? There are several reasons.

1.1 Bilateral damage to the thalamus abolishes waking consciousness. The critical site of this damage is believed to be a relatively small cluster of neurons, about the size of a pencil eraser on either side of the brain's midline, called the Intra-Laminar Nuclei (ILN) because they are located inside the white layers (laminae) that divide the two thalami into their major groupings of nuclei. The fact that bilateral damage to the ILNs abolishes consciousness is very unusual. There is no other site in the brain that has this property, except the reticular formation in the brain stem. In contrast, huge chunks of cortex can be damaged without abolishing the STATE of consciousness. (Cortical damage does change the CONTENTS of consciousness, of course).

1.2 All this seems to imply that the ILNs are somehow are necessary to help maintain the state of waking consciousness. I would suggest that the CONTENTS of consciousness are supported elsewhere, primarily in cortex.

2.1 As Newman points out, there is another thalamic nucleus called the reticular nucleus (nucleus reticularis thalami, or nRt), which appears to "gate" sensory channels to the cortex. Thus the process of selecting an inflow of visual, auditory, or body-sensory information to cortex can be controlled by the nRt. Physically, the nRt looks like an eggshell covering the front and sides of each egg-shaped thalamus. Neuranatomists, notably Arnold Scheibeland Marian Diamond, have argued that the nRt involves a body-centered map. The implication therefore is that nRt is involved in attentional selection of sensory input from one or another side of the body. If a cat hears a twig breaking on the left side of its head it can presumably gate all the information from that side of body-space to the cortex, where the input can be analyzed in detail. In humans we can comfortably believe that that information becomes conscious.

2.2. In my theoretical terms (global workspace theory), conscious information is distributed widely in cortex and elsewhere, to facilitate recruitment of unconscious knowledge sources that can work together to identify the input, and to construct a nonroutine action in response to it. (Baars, 1988; Baars, 1996).

3. A number of neuroscientists have proposed that there are looping activation cycles going between the sensory nuclei of each thalamus and corresponding cortical sense projections areas. (e.g. Edelman) Each thalamus maps directly into corresponding sensory and motor cortex, and in the case of vision, it is believed that there are many times more neurons projecting "down" from the visual cortex to the visual nucleus of the thalamus (LGN) as there are going "upward." Such loops are known to be able to amplify one input at the expense of others. Recent single-neuron recording of visually conscious input seems to show that conscious visual input is highly amplified in visual cortex, while simultaneous unconscious input is difficult to find at all. (Logothetis). Thus the thalamic visual nucleus, the LGN, is not just a relay for visual input on its way to cortex, but may actually amplify the input by way of an accelerating looping mechanism, bouncing information between the LGN and early visual cortex in a rising cycle of activity until one flow of input becomes dominant in cortex. (There are many such looping possibilities WITHIN visual cortex as well, and one can imagine a number of such interlocking loops building on each other, to bring an event to consciousness, at least for a few hundred milliseconds).

4.These are the arguments FOR the involvement of thalamus in consciousness. What arguments go against this hypothesis?

4.1 Thalamus does not have the exquisite specificity of content that we experience in looking at a visual scene or listening to a symphony; not to mention the kind of detail we can access when discussing an abstract topic like consciousness. That kind of specificity only emerges in cortex.

4.2. Some critics have argued that thalamus and brain stem may be merely the electrical power cord for the TV set of conscious experience. In some versions of this argument the role of subcortical mechanisms seems almost trivial. However, my reading of the literature is that the "looping hypothesis" of section 3 above is currently much more in favor. Thalamocortical looping suggests that the thalamus is stimulus specific, at least in terms o fhelping to select and amplify certain potentially conscious contents, and perhaps deselecting and minimizing others.

5. But what is the role of the ILNs, the only (two) tiny thalamic structures whose removal will abolish waking consciousness? They do not seem to be nearly big enough to support the details of visual conscious experience. Joseph Bogen has suggested that they may support certain primitive conscious experiences, such as nonspecific pain. Rodolfo Llinas and others point to the ILNs as sources of a pacing rhythm, a 40 Hz oscillation that may serve to "bind" cortical activity in many different locations into one coherent conscious experience.

6. One question I have about this is whether temporal binding is enough, or whether we need spatial binding as well, perhaps via the retinotopic map of V1. (All such sensory maps presumably work in registration with other maps that are known to exist in other parts of the cortex (i.e. right parietal object-centered map), and perhaps the nRT body-centered map.) I believe that parts of this puzzle are coming together very nicely.

References

Baars, B.J. (1988) A cognitive theory of consciousness. NY: Cambridge University Press.

Baars, B.J. (in press, 1996) In the theater of consciousness: The workplace of the mind. Oxford University Press.

Baars, B.J. (1993) How does a serial, integrated and very limited stream of consciousness emerge from a nervous system that is mostly unconscious, distributed, parallel and of enormous capacity? In, Ciba Foundation, Experimental and theoretical studies of consciousness. Chichester, UK: Wiley, 282-290.

Baars, B.J. (1995) Tutorial commentary: Surprisingly small subcortical structures are needed for the STATE of waking consciousness, while cortical projection areas seem to provide perceptual CONTENTS of consciousness. Commentary on J. E. Bogen, On the neurophysiology of consciousness: Part II. Constraining the semantic problem. Consciousness & Cognition, 4 (2).

Bogen, J. E. (1995) On the neurophysiology of consciousness: I. An Overview. Consciousness & Cognition, 1 (4), 52-62.

Newman, J. (1995) Review: Thalamic contributions to attention and consciousness. Consciousness & Cognition, 4 (2).

Scheibel, A.B. (1980) Anatomical and physiological substrates of arousal: A view from the bridge. In J.A. Hobson & M.A.B. Brazier (eds.) The reticular formation revisited. (pp. 55-66). NY: Raven Press.

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