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consciousness [5]. How do neural processes come to havea qualitative ‘feel’? Why do we have a subjective inner lifeat all? Edelman’s view is that the hard problem is largelya linguistic one. The human brain has evolved to be ableto make very fine discriminations, and according toEdelman, qualia simply are those high-order discrimin-ations; there is no need to talk about anything extra. Hisway of expressing the mind–brain relationship is thatconscious experience (C) is ‘entailed’ by the underlyingneural processing (C 0), not that phenomenology is causedby neural activity but rather it is a simultaneous propertyof that activity. He also notes that because the physicalworld is ‘causally closed’, C itself cannot be causal, onlyphysical events or C 0 can be. We might talk of consciousprocesses causing actions but that is only because C isreliably coupled to the underlying causal C 0 events. The‘entailment’ of C here is somehow not quite the same asbeing ‘supervenient’ or ‘epiphenomenal’ – terms thatothers have used – entailment implies that C could notsimply be removed leaving qualia-free C 0 processes; a non-conscious zombie is a logical impossibility. And Edelman’sreply to those who want his scientific analysis to explainthe actual feeling of qualia is that ‘It suffices to explain thebases of these distinctions – just as it suffices in physics togive an account of matter and energy, not why there issomething rather than nothing.’ (p. 146). Many will feelthat this brushes aside the hard problem and leaves quite

Corresponding author: Vincent Walsh (v.walsh@ucl.ac.uk).Available online 1 October 2004

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a lot to be explained. This ‘physicalist’ position naturallyavoids any form of dualism, which is where some othershave ended up [5]; Edelman is quite clearly keen not tojoin them. But what if they are right; what if conscious-ness is a fundamental property of the universe, differentin nature to physical ‘stuff ’? Edelman states on page 5 thatconsciousness is always embodied and that ‘post-mortemexperience is simply not possible.’ One could argue thatthat is an unscientific statement, as we cannot prove iteither way this side of ‘mortem’. We must wait and see –only then will the problem of consciousness finally besolved for each of us.

References

1 Gray, J. (2004) Consciousness: Creeping up on the Hard Problem,Oxford University Press

2 Koch, C. (2004) The Quest for Consciousness: A NeurobiologicalApproach, Roberts & Co.

3 Morrison, I. (2004) A vision of consciousness (Review of Koch: TheQuest for Consciousness). Trends Cogn. Sci. 8, 444–445

4 Schooler, J.W. (2002) Re-representing consciousness: dissociationsbetween experience and meta-consciousness. Trends Cogn. Sci. 6,339–344

5 Chalmers, D.J. (1996) The Conscious Mind: In Search of a Funda-mental Theory, Oxford University Press

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doi:10.1016/j.tics.2004.09.006

Here we go again.The Deja vu Experience by Alan S. Brown, Psychology Press, 2004. $44.95 (272 pp.) ISBN 1 84169 075 9

Vincent Walsh

Institute of Cognitive Neuroscience and Department of Psychology, University College London, 17 Queen Square,

London WC1N 3AR, UK

When people who are not scientists come to my house fordinner, they often ask how work is going. What they meanis can I tell them anything interesting about things theyassociate with the ‘it must be fascinating’ occupation ofbeing a psychologist (cognitive neuroscientist isn’t yet anoccupation that trips off most people’s tongues). Well ofcourse I can. They are mesmerised by the details ofwhether an interaction between two visual areas has awindow of 10, 50 or 100 milliseconds; they nod vigorouslyin appreciation when I describe the advantages of a figure-of-eight magnetic stimulation coil over a circular one; andif I can just get them to roll up their napkins into a tubeand peer through them so that they really get the point ofvoid versus real colours, I can sense their excitement bythe increased speed of their eating and drinking. Havingdone my bit for the public understanding of science, I canpour myself a drink.

Cognitive neuroscience is a laboratory science and we

are therefore limited in our ability to probe peoples’everyday experiences by the compromises necessary tocontrol variables in the lab. We therefore do apparentlystrange things: we study vision without eye movements,reaching without head movements, emotions throughspeeded responses. it is an easy list to generate. We dothese things for good reason of course, but they are notwhat many undergraduates expect when they sign up tostudy psychology. I’m guessing, but maybe my dinnerguests would be more interested by something that relatesto their own experience – depression, skill at music,absent mindedness, supernatural experiences (at whichpoint I treat them to a demonstration of levitation causedby rage), or deja vu. I have been caught out by deja vumany times. It is a common experience, we do all knowwhat it feels like (or so I thought) and it is worthy of anexplanation. Alan S. Brown has written the ‘everythingyou ever wanted to know but were afraid to ask’ bookof deja vu.

So what do we know about deja vu and what can we dowith it? The answer I take from Brown’s book is that we

Update TRENDS in Cognitive Sciences Vol.8 No.11 November 2004484

know enough to bring it fully into the realms of cognitivescience and explore it using modern techniques, and thatif we do so we may learn not only something about thisphenomenon but also about perception, memory andawareness. Brown is aware of the limits of trying tobring a fleeting and unpredictable experience into the labbut his discussion of perceptual explanations of deja vuare worth consideration. He suggests, for example, thatinattentional blindness paradigms could be adapted tostudy deja vu. Discussions of memory and dual processingare perhaps less convincing, and the lack of any consensusconcerning a link between neurological conditions anddeja vu is disappointing (but not the fault of the author).The difficulty in taking from this book any ideas forresearch lies in the large number of factors that have beeninvestigated only superficially. The result is many vaguepossibilities. Summarizing memory explanations ofdeja vu, for example Brown’s sentences run ‘A variety ofexplanations. have been proposed. The simplest per-spective is. It is also possible. Deja vu could also.Familiar elements could come from. Also may occur.’I think this is Brown’s point – the subject needs a serioussustained treatment to weed out the weaker possibilities.Brown is aware of the difficulties: ‘the deja vu experiencerepresents the clash between two simultaneous andopposing mental evaluations: an objective assessment ofunfamiliarity with a subjective evaluation of familiarity’(p. 2) and in research, according to Brown, ‘there is still anunfortunate tendency.to sidestep common experiences’.Well, common is not always so easy even to describe.Table 2.1 lists over 30 attempts to describe deja vu since1844, and Table 2.2 over 50 definitions. My own internallife is not so grandiose as to have experienced ‘recognitionof the immemorially known’ but I do recognise ‘a weirdfeeling that one has been through all this before, as if timehad slipped a cog and were now repeating itself ’ (I believethis is also called having a grant application rejected). Theserious point is that to study deja vu we need to know whoexperiences it, when and under what conditions. Peoplecan be asked to keep deja vu diaries or can be askedretrospectively about their experiences. Both methodshave obvious weaknesses and if one is seriously thinkingabout research there is daunting number of variables to

Corresponding author: Georg Northoff(georg.northoff@medizin.uni-magdeburg.de).

Available online 1 October 2004

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consider: two thirds of people experience something theycall deja vu; it is more often experienced outdoors, in theafternoon or evening and later in the week. The incidencedecreases with age, is more common in travellers thannon-travellers, in liberals than in conservatives and inthose with moderate rather than fundamental religiousbeliefs. Subject recruitment might require a hardyresearch assistant.

The book also contains a reminder of the law of physicsthat ‘supernature abhors a vacuum’ and any vacuum inscientific psychology will inevitably be filled by para-psychology. You will learn that deja vu experiencescorrelate positively with beliefs in ESP, precognition, theLoch Ness monster, Sasquatch etc. Your experience mayalso be evidence of reincarnation, a collective unconsciousor telepathy. Closer to earth the neurological evidence issparse but tantalising. Hughlings Jackson proposed anassociation between deja vu and aura in epilepsy butsubsequent investigations present a complicated picture.Surgical interventions preceded by brain stimulation haveshown that the experience can be elicited. Penfield, forexample reports a patient who said he had a strangefeeling as though he were ‘in the future listening to thepast’ (p. 85). The experience in epileptic aura does seemto be associated with temporal lobe epilepsy of right-hemisphere origin and deja vu in epileptic aura is moreprotracted and subject to repetition than in normalexperience, but whether what is described as deja vu inthese situations is the same as the experience withoutpathology requires clarification.

In the face of so many descriptions and variables but solittle research into neural mechanisms, Neppe’s warningthat ‘one single explanation for deja vu is probably asuntrue as a single cause for headache’ (p. 113) seemsentirely reasonable. That we should treat deja vu as anexperience to be investigated, I am convinced of bythis book; where one would start is a different matter.One thing is certain, however. My next dinner guestsare in for a treat. ‘Ah deja vu, I’m sure you’ve askedme that before.’

1364-6613/$ - see front matter Q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.tics.2004.09.002

Letters

Why do we need a philosophy of the brain?

Georg Northoff

Laboratory of Neuroimaging and Neurophilosophy, Departments of Psychiatry, Psychotherapy, and Psychosomatics,

Otto-von-Guericke University of Magdeburg, Leipziger Strasse 44, 39120 Magdeburg, Germany

I would like to provide some answers to the questionsraised by Hardcastle [1] in her review of my latest book [2].I did not intend to show how the mind can be reduced to

the brain. Rather than offering a novel solution to themind–brain problem, I question its implicit presuppositionof how our brain can give rise to the concept of mind. Myanswer is that our brain suffers from a knowledge gapbecause it remains unable to perceive itself directly asbrain. I call this ‘autoepistemic limitation’, and as a result,