Mind Time: The Temporal Factorin Consciousnessby Benjamin LibetHarvard University Press: 2004. 288 pp.$29.95, £19.95, €27.70

Kevan Martin

“I did not know I loved you till I heardmyself telling you so — for one instant Ithought, ‘Good God, what have I said?’ andthen I knew it was the truth.” Bertrand Rus-sell’s report of his late-night conversationwith Ottoline Morrell captures the essentialthesis of Benjamin Libet’s monograph onthe temporal factor in consciousness. Libetreviews his extended set of experiments,which he believes show that Russell wasright: reality is running about half-a-secondahead of our conscious awareness of it. Thebad news is that we never catch up.

Studies of awareness and consciousnessby neurobiologists and psychologists arenow mainstream, and even young scientistscan expect to pursue a well-funded career inconsciousness studies. It was not always so.As Francis Crick records in his own book onconsciousness, The Astonishing Hypothesis(Simon & Schuster, 1994), Libet only daredswitch to the study of consciousness after hegot tenure. It is fortunate for us that he did,and that he has presented us here with whatamounts to a retrospective exhibition of hiswork. The book is something of an atavism,resembling more an early work of his mentorJack Eccles than the glossy volumes that nowappear in the field. But the refreshing result is that we are immediately engaged in anearnest one-to-one tutorial with Libet.

Libet was fortunate to link up with theneurosurgeon Bertram Feinstein, who gavehim the chance to study the neocortex of con-scious patients who were undergoing brainsurgery under local anaesthesia. Libet tookadvantage of the discovery by Wilder Penfieldand Herbert Jasper that conscious sensationsof touch, referred to a particular place on theskin,can be evoked by electrically stimulatingthe surface of the somatosensory cortex.

Libet’s initial discovery was that the trainof stimulation pulses had to last for 200–500milliseconds before the sensation of touchwas perceived consciously. This delay — the‘mind time’ — led him to propose his ‘time-on theory’, which states that conscious sen-sory experience requires that the appropriatebrain activities turn on for at least 500 ms,and that shorter brain activities may lead toan unconscious mental function. This latterhypothesis is needed to explain fast reactiontimes, such as those of champion 100-metresprinters who leave their blocks a mere 130 ms

after the starting pistol fires, suggesting thatthey are only aware that the gun has firedafter they have left their blocks. But can thebrain states of a patient undergoing brainsurgery really be extrapolated to an athletepoised to break a world record?

In the visual system, electrical stimula-tion of the cortex in humans does not pro-duce much more than tiny spots of lightcalled phosphenes, as Giles Brindley discov-ered in the early 1960s. Bill Newsome and his colleagues found that direct electricalstimulation of the visual area called MT inmonkeys influences the motion perceptionof the monkey only if it is combined with areal visual stimulus. The direct electricalstimulation on its own does not evoke a per-ception, or at least one that the monkey canreport, so it has been difficult to use animalmodels to explore such issues.

Single impulses evoked in a sensory nerveby touching the skin can be detected, so onecriticism of Libet’s result is that when hestimulates the brain it evokes a rather coarsespatiotemporal pattern of cortical activa-tion. The brain may simply be doing its bestto make sense, literally, of the artificiallyinduced activity of neurons that are normallyactivated from a patch of skin. This may takelonger than usual, as the neural programsneed to do the interpreting.

Libet’s most controversial claim is that,because subjectively we believe that we are

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immediately aware of the stimulus, theremust be a disjunction between subjectivetiming and neuronal timing. On the basis ofvarious experiments, he suggests that oursense of immediacy is an illusion that arisesbecause we refer the timing of the experienceback to the time of the earliest cortical acti-vity, evoked some 10–30 ms after the skin istouched. The neural mechanisms by whichall this precise time calculation has to be carried out is,of course,unknown.

Those of us for whom a slip of the tongueis a regrettably familiar event might be comforted to read here that, as in the case of sensory events, our conscious intention to act lags 300–500 ms behind the relevantbrain events that actually lead to action.This of course raises the thorny question ofwho is actually doing the intending, as ourconscious selves seem to be lounging in the back seat being chauffeured around by our zombie doppelgänger. The best we cando, it seems, is occasionally lunge for thesteering wheel when our limousine takes awrong turn. This is perhaps not quite asstrange as it sounds,because highly practisedmovements, such as those of skilled musi-cians, are produced at rates that seem fasterthan thought.

In Libet’s work, philosophers have foundgrist for what they do best. Indeed, his experiments, along with Larry Weiskrantz’sdiscovery of blindsight, must rank as one

Time waits for no manDo you know what you think before you see what you say?

Quick off the mark: Michael Johnson can be up and running before he is aware that the gun has fired.

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© 2004 Nature Publishing Group

A global problemGlobal Change and the EarthSystem: A Planet under Pressureby W. Steffen, A. Sanderson, P. D. Tyson,J. Jäger, P. A. Matson, B. Moore III, F. Oldfield,K. Richardson, H. J. Schellnhuber,B. L. Turner II, R. J. Wasson Springer: 2004. 336 pp. $129, £77, €99.95

Hans von Storch

This book “focuses on the profound trans-formations of Earth’s environment that isnow apparent, a transformation owing notto the great forces of nature or to extra-terrestrial sources, but to the number andactivities of people — the phenomenon ofglobal change”.

The authors have adopted a systematicapproach. They begin by describing theEarth system — the whole of the physical andecological environment that determines thestate of the Earth — which they view as oneintegrated system. Next they discuss thedynamics of the Earth system as it was beforehuman influence: the characteristics of tem-poral and spatial variability, the role of bio-logical processes, the connectivity betweenthe different parts of the system, and the roleof nonlinearities, such as the browning ofthe Sahara. The third chapter describes the

Anthropocene, the most recent part of Earthhistory, which has been, and continues to be, massively influenced by humans. Herethe authors identify the drivers of change,characterize these changes, and put theminto perspective in relation to the overallchange of the Earth system.The fourth chap-ter then deals with the responses of the Earthsystem to human activities. The authorscover the entire spectrum of anthropogenicchange, from emissions, changes in land useand land cover, and fires in South-east Asia,to the effect on coral reefs and the overallresponse of the Earth system with respect toclimate and the water cycle, for example.They then examine the consequences ofthese changes for human well-being.The keypoints here relate to anticipation of futurechange, risks to key resources, and dangers to the Earth system as a whole. There is also a rather brief discussion of human percep-tions of global change. In the final chapter,the authors draw various conclusions aboutthe contemporary knowledge base, Earth-system science and the need for a toolkit with which to pursue the overarching goal of global sustainability.

The book is part of a series published for the International Geosphere–Biosphere

Programme (IGBP),one of four internationalglobal environmental-change research pro-grammes sponsored by the InternationalCouncil for Science. The IGBP’s objectivesare “to describe and understand the inter-active physical, chemical and biologicalprocesses that regulate the total Earth Sys-tem,the unique environment that it providesfor life, the changes that are occurring in thissystem, and the manner in which they areinfluenced by human actions”.

Global Change and the Earth System isrichly illustrated with many useful coloureddiagrams,and numerous boxes by additionalauthors provide more detailed informationabout specific issues. However, the wealth ofdetail makes it hard to read, and at times thediagrams are not properly explained.

Is it a good book? I am not entirely con-vinced. It deals with a broader set of topics(including the number of McDonalds res-taurants around the world, and the emissionof heavy metals) than the IntergovernmentalPanel on Climate Change (IPCC) AssessmentReports. But the IPCC reports are written by a broader community of scientists, withmore diverse expertise, greater emphasis on the certainty as well as the uncertainty of knowledge, and with more convincing

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of the major contributions of experimentalpsychology to modern philosophy of mind.Humans and monkeys with extensive damage to their primary visual cortex areclinically blind, but Weiskrantz found thatthey were surprisingly accurate about iden-tifying the position and shape of objectswhen he forced them to guess. Philoso-phers, however, have not always been asappreciative of Libet’s gift as they have of Weiskrantz’s. Paul Churchland, TedHonderich and Daniel Dennett, in particu-lar, have attacked Libet’s methods, resultsand ideas (and,in fairness,also each others’).What so vexes them is Libet’s relegation ofconsciousness to the role of observer andoccasional vetoer of action, which compro-mises both the cartesian view, in which themind controls the machinery of the body,and the materialist view, where mind andbrain events are identical.

Libet has replies for his critics, of course,but in developing his alternative theory, heends up putting the ghost back in themachine in the form of a ‘conscious mentalfield’emanating from the brain.This may begoing too far, but whether or not one agreeswith his thesis or not,one must acknowledgethat his pioneering experimental work hascertainly been stimulating. ■

Kevan Martin is at the Institute ofNeuroinformatics, ETH/University of Zurich,Winterthurerstrasse 190, Zurich 8057, Switzerland.

A world of paint

Visual art has one of thelongest and fullest records ofany human activity. Humanshave been recording imagesof the world around them formore than 30,000 years. InAtlas of World Art (LaurenceKing, £75), art historian JohnOnians has coordinated the work of a group of anthropologists,archaeologists and art historians to compile a temporal and geographic history of art. Pictures of art, like the African masqueradeshown here, take secondplace to detailed mapsshowing the culturalinfluences and location of key sites and artistic centreson each continent in sevendifferent periods, from40,000–5,000 BC to thetwentieth century.

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