Overworking the Hippocampus

Commentary on Jeffrey A. Gray, for Behavioral and Brain Sciences, vol. 18, no. 4, 1995, pp. 677-78.

Daniel C. Dennett
Center for Cognitive Studies
Tufts University
Medford MA 02155
email ddennett@diamond.tufts.edu

Overworking the Hippocampus

Gray mistakenly thinks I have rejected the sort of theoretical enterprise he is undertaking, because, according to him, I think that "more data" is all that is needed to resolve all the issues. Not at all. My stalking horse was the bizarre (often pathetic) claim that no amount of empirical, "third-person point-of-view" science (data plus theory) could ever reduce the residue of mystery about consciousness to zero. This "New Mysterianism" (Flanagan, 1991) is one that he should want to combat as vigorously as I have done.

I am all in favor of devising and investigating models of the sort Gray attempts, and for the reasons he gives. Gray's model, in fact, provides an excellent opportunity to clarify the major theoretical claims Kinsbourne and I have made (Dennett, 1991, Dennett and Kinsbourne, 1992), because it exhibits, in crisp detail, the intersection of two central misunderstandings. First, Gray has been seduced by one of the pet themes of the Mysterians: consciousness is all-or-nothing (Searle, 1992, p.83) and (hence) cannot be composed of lots of non-conscious (or quasi-conscious) elements distributed around in a large system. This leads Gray to overlook the possibility that a sufficiently powerful combination of merely "syntactic" (and unconscious) phenomena, like those he illustrates by DNA protein-synthesis, might account for the semantic competence that is, surely, our best hallmark of consciousness. Second, Gray is still an adherent of the "wasp-waist" (Kinsbourne, 1993) or "bottleneck" (Dennett, 1991) vision of consciousness as a property that gets conferred on the contents of various cerebral vehicles when they arrive somewhere special. (The idea is that arrival at this inner observer is straightforwardly analogous to such macroscopic events as the arrival of sound waves at the ears of whole observers.) Putting the two bad ideas together inspires Gray to try to promote his model of a subicular comparator system, which in its own right has many excellent features, into a job it could not possibly handle: the seat of consciousness.

Let us suppose that Gray has made the case for the existence, location, and general physico-chemical nature of a comparator system, straddling various perceptual (and other) pathways, and let us suppose moreover that he is right about its function of providing a "familiarity signal" (Dennett, 1979, p. 101)Endnote 1 when it matches current activity with activity in a previous "moment". Now the hippocampal mechanism that accomplishes this postulated comparison task must operate on some "syntactic" principle: the brute physical pattern of activity in the perceptual stream will trigger the alarm or it won't, depending on whatever features of those local patterns the comparator is sensitive to. Such a syntactic comparison will have been designed (by an evolutionary process, as Gray says) to track or mirror whatever semantic regularities the brain must most reliably respond to. The most interesting--if tacit--claim made by Gray's model, I think, is that this comparator-task is so important that this is a place in the brain to look for major convergence of the syntax and semantics of the whole system. The claim, in effect, is this: just as the protein-specifying semantics of DNA codons is the key to unlocking the mysteries of inheritance, so the perceptual-feature-specifying semantics of syntactic patterns detectable in the hippocampal comparator is a key to unlocking the mysteries of intentionality!

Now if Gray is wrong about this--if the syntactic patterns to which his hippocampal comparator is sensitive are relatively crude in what they track--then he has at best discovered a bit-player in the drama of consciousness. But even if he were right that the comparator is a major locus of semantic competence, this would still give it only a contributory role in the whole task of content-discernment (and appreciation) that is the work of consciousness (agreeing with Gray that if consciousness did no work, as some mysterians suppose, then it could not have evolved).

The truly hopeless Cartesian alternative, of course, would be to postulate that the resources of an entire mind or comprehending intelligence are somehow brought to bear in this local region, rendering it capable of directly recognizing (appreciating, discriminating) the purely semantic similarities and differences in the stream. Gray seems to lapse into this view when he supposes "a second set of events--conscious events--as occurring besides, and in some way as yet unknown linked to, the neural events that constitute the subicular comparison process." [ms p.24) He does not address the crucial question directly. He supposes that "multi-modal and highly elaborated perceptual descriptions" [p.16] together with information on other relevant topics (e.g. "information concerning motivational and reinforcing events" [ms p20]) are "circulated" through the subicular area, but of course unless the subicular area can understand all the information available in its "input," its homuncular role ("seen this, done that") must be limited to syntactic comparisons. But Gray contrasts the power of the subicular comparator to the power of other cognitive machinery, claiming for it that "semantically interpreted content [emphasis in original] of representations plays a causal role in the unfolding of events." [p.24] At that point, by concentrating all the power of mentality into his subicular subsystem, he forces it to be composed of wonder tissue, but this is a fatal misstep from which he could gracefully retreat.

"Merely syntactic" comparator operations are not nothing--they are, indeed, the very elements out of which any non-miraculous theory of consciousness or intelligence must be composed--but it is precisely this limited role that must prevent the subicular area (or any other neuro-anatomically restricted region) from being "the place where it all comes together" for consciousness. There can be bottlenecks--and we may suppose for the sake of argument that Gray has located a functionally important one--but not appreciating bottlenecks. And since the dimly-envisaged capacity to appreciate the contents is well-nigh definitional of consciousness, one could never motivate the claim to have discovered the seat of consciousness (as opposed to the seat of some more minor functional component of mentality) without showing how arrival at that proposed seat led to or accomplished that appreciation.

Gray sees that this is a problem, and adjusts his hypothesis several times to make it at least not obvious that he couldn't solve it, but the adjustments have the effect of conceding, not rebutting, the main lines of our objections. It is not just the obvious problem that is pointed out by Libet in his communication with Gray: the privileged location of the activity necessary and sufficient for consciousness implies that "destruction of the subicular area" should obtund conscious experience, a prediction that Gray himself describes as "apparently false" [ms p.17]--see also the commentary by Kinsbourne. It is the larger theoretical problem of trying to localize in space and time phenomena that by their very nature involve global competences that could not (except in ill-envisaged miracles) be the responsibility of a single module.

Might the hippocampal comparator then be the gateway into consciousness, the porter's lodge if not the Combination Room? (Gray speaks of the comparator's "feedback" as "contributing in a more nuanced manner to the description of the perceived world that finally enters [emphasis added] consciousness" [p.19].) It might be a gateway, one among many, but what matters in any case for consciousness of some "message" is not the order or time of arrival at any gateway, but the way the message is eventually dealt with by subsequent processes, in whatever order.

[1300 words]


Dennett, D., 1979, "On the Absence of Phenomenology," in D. Gustafson and B. Tapscott, eds, Body, Mind and Method: Essays in Honor of Virgil C. Aldrich, Dordrecht: D. Reidel, pp93-114.

Dennett, D., 1991, [as in target article]

Dennett and Kinsbourne, 1992, [as in target article]

Flanagan, O., 1991, The Science of the Mind (2nd Edn), Cambridge, MA: MIT Press.

Kinsbourne, M. 1993, "Integrated cortical field model of consciousness," in J. Marsh, ed., Experimental and Theoretical Studies of Consciousness, Ciba Foundation Symposium 174, New York: John Wiley & Sons, pp. 43-50.

Searle, J., 1992, The Rediscovery of the Mind, Cambridge, MA: MIT Press.


1. In this article I postulated just such a system as part of a purely speculative--and neuroanatomically uninformed--account of how déjà vu might be caused; many are the grounds for looking for not just one but many comparator systems.