Cortex and Mind: UnifyingCognitionby Joaquín M. FusterOxford University Press: 2002. 314 pp.$49.95, £47.95

Kevan Martin

“Human cognition may like the wingedhorse take at times its flights toward the starsand forget Earth.” This was Charles Sherring-ton’s own flight of fancy as he pondered theancient question of the relationship betweenthe working of the brain and the working ofthe mind in his Gifford lectures of 1937–38.How close are we now to an answer? JoaquínFuster provides a progress report in Cortexand Mind, in which he considers severalaspects of cognition: perception, memory,intelligence, language and attention. What isstriking from his account is how much wenow rely on data from experimental animals,particularly primates, to understand thephysical basis of human cognition — moreeven than Sherrington, who was perhaps thegreatest of all behavioural physiologists.

Those familiar with Fuster’s previous bookMemory in the Cerebral Cortex (MIT Press,1994) will know that he discovered in the pre-frontal cortex of monkeys a group of neuronsthat sustain their activity between a sensorycue and a delayed motor response. The exis-tence of such ‘working memory’ cells had longbeen predicted, and soon afterwards HiroakiNiki found a second class, which acceleratetheir firing over the delay period.

These two classes are intermixed in mostof the prefrontal cortex and are also found in parietal cortex. It is not yet clear whethersuch sustained activity arises through rever-beration in the recurrent cortical circuits, asproposed by Lorente de Nó and currently thetheorist’s favourite, or through mnemoniccapabilities at the level of single neurons, assuggested by in vitro experiments by AlexeiEgorov and colleagues, and no definitiveanswer is expected soon. But the question isnevertheless important: how do our cognitiveprocesses map onto the brain’s circuits?

For Fuster, this question is decided by his view that the brain is a neural network,currently best represented by connectionistmodels involving parallel distributed pro-cessing. The essential currency of cognitionfor Fuster is the ‘cognit’, a word he coins here to describe an item of knowledge that isstored in a network. Each node of the corticalneural network stores yet more elementaryrepresentations that make up a cognit. Notall cognits are equal. Symbols, for example,are presented in high-ranking cognits, themselves produced by the convergence of

cognits from lower levels. In other words, theessential transactions underlying cognitioninvolve the activation of different networks,arranged in hierarchies, that represent thebuilding blocks of cognition. So, unlike Sher-rington with his ‘million-fold democracy’ ofneurons, Fuster proposes an isomorphismbetween the processes and the structures ofmind and of the cerebral cortex.

This is a nice idea, but how well does it fit the facts? Fuster’s chosen model of the parallel-distributed-processing network hasbeen widely used, especially by cognitive psychologists. Such artificial neural networkshave multiple layers, learn associatively,provide a complete output from incompleteinput data, and show ‘graceful’ degradationin the face of partial damage. These are allpresented as being satisfyingly brain-like.

But in truth, the performance of artificialneural networks is fragile in real-world envi-ronments. They seem ill-suited to represent-ing Fuster’s cognits, because the nodes ofthese networks usually represent functionswith continuous values and their outputscombine with those of all other nodes tocompute the output of the network. Thenodes do not represent discrete bits that cansimply be composed into well-defined higher-order constructs. Indeed, artificial neuronalnetworks can approximate any arbitrary non-linear function without resorting to exten-sive hierarchies, yet hierarchies seem to be afundamental organizing principle in brains.Artificial neural networks also scale notori-ously badly, so most successful simulationshave usually used networks with substantiallyfewer than 1,000 ‘neurons’, in contrast to the100,000 neurons contained in each cubic

millimetre of neocortex. How we learn effec-tively and stably with such large networks is amystery. Most models propose simple feed-forward networks, which do not capture theessential recurrence and local processing ofthe neocortical circuits, nor do they capturethe temporal aspect of processing, which is afeature of perception and action.

The capacity of the brain to excite itself is a fundamental necessity for cognition.And one of the key capacities that workingmemory provides is the possibility to delay a choice of action until the various optionshave been weighed up. This ‘executive’model of prefrontal cortex has been popu-lar since Phineas Gage survived a horrific mining accident in 1848 to become the firstclinical exemplar of the consequences ofdamage to the prefrontal cortex. But as we‘ponder’, ‘deliberate’ or ‘examine’ (all wordsderiving from Latin roots meaning ‘toweigh’), can we find an analogous ‘weighing’in the activity of neurons in our prefrontalcortex, or are the brain computationsinvolved in these cognitive processes carriedout by symbolic manipulations reminiscentof our use of numbers and letters? Exactlyhow do neurons make up one’s mind?

Horace Barlow once proposed that therate of neuronal activity reflects the proba-bility of a certain stimulus being present.Decision-making requires that these proba-bilities be weighed, and one intriguing recentsuggestion, by Michael N. Shadlen, is thatthis occurs in a second stage of processing in which sensory responses are accumulatedfrom pools of neurons over time to form asingle quantity, for example the logarithm ofthe likelihood ratio. The neurons that best

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Making your mind upNeurons with sustained activity could help us to understand cognition.

Growth factor: neurons develop rapidly in the human cortex between 3 and 24 months of age.

© 2003 Nature Publishing Group

fulfil this role are none other than those dis-covered by Fuster and Niki. If their persistentactivity in the absence of a sensory cue isindeed the step of calculating a single decision variable based on information from several sources, then neurophysiolo-gists have actually watched neurons makingup the monkey’s mind. What determines themoment of decision is not yet known, butjust as ‘decide’ once meant to cut off, or bringto an end, so these neurons do indeed stoptheir activity when the decision is made.

There is a strong argument that we havemade such great progress in understandingthe neural basis of cognition only becauseneurons, and the networks that they form,compute in an analogue style. We can get an idea of the underlying computations by measuring the activity of single neurons, orthe strength of the functional magnetic reso-nance imaging signal. It seems fantastic, butFuster’s progress report dares us to believethat the patterns woven by Sherrington’s“enchanted loom”, the cerebral cortex, arenow well on the way to being understood. ■

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

Suffocated or shot?When Life Nearly Died: TheGreatest Mass Extinction of All Timeby Michael BentonThames and Hudson: 2003. 336pp. £16.95, $29.95

Peter J. Bowler

Whatever hit the Earth at the end of the Permian period certainly struck hard, killing90% of living species. Compared with this,the extinction at the end of the Cretaceousperiod was comparatively minor, with only a 50% death rate. Yet the latter event is muchbetter known, because among that 50% were the last of the dinosaurs. Partly for this reason, Michael Benton uses the event at theend of the Cretaceous as an introduction tohis account of the Permian extinction — hewants us to realize how limited it was in com-parison with what he intends to describe.

But there is a deeper reason for linking thetwo episodes: Benton wants to show us howthe catastrophist perspective has re-emergedin modern geology and palaeontology. Heargues that the theory of catastrophic massextinctions was widely accepted in the earlynineteenth century, but was then drivenunderground by the gradualist perspective of Charles Lyell’s uniformitarian geology and Darwin’s theory of evolution. Only in the1970s was catastrophism revived, throughthe claim that the dinosaurs were wiped outwhen an asteroid hit the Earth. Benton shows

us how in the 1990s the evidence began toemerge that the species replacements mark-ing the Permian–Triassic transition were alsosudden, and hence were probably caused bysome environmental trauma. He is describ-ing both a geologically sudden event and arapid transformation in our ideas about theEarth’s past.

As a result, the book is partly historical in nature. It describes how the British geolo-gist R. I. Murchison (himself a catastrophist)defined the Permian rocks of Russia in about1840, and how Lyell and Darwin challengedthe idea of mass extinctions by arguing thatapparently sudden transitions in the fossilrecord were the result of gaps in the evidence,which created illusory jumps between onesystem of rocks and the next.

The triumph of darwinism ensured thatcatastrophist explanations were marginal-ized until they were revived by the asteroid-impact theory for the end of the Cretaceous.Even then, many palaeontologists resisted,arguing that the dinosaurs were declininganyway, so the impact only finished a job that had already been started by gradual envi-ronmental changes. At the time, knowledgeof the Permian–Triassic transition was so limited that gradualism still seemed plausiblehere, too. Benton provides a graphic accountof how more recent evidence has piled up,including his own experiences fossil huntingin Russia, making a catastrophic explanationinescapable.

There is one important twist in the story,however: Benton finds little support for thepossibility that the Permian extinction wascaused by an extraterrestrial agent. Wild theories about periodic bombardments byasteroids have not stood the test of time: thePermian event was probably triggered bymassive volcanism, which injected poisonousgases into the atmosphere, both directly andby triggering the release of methane fromdeep-sea hydrates. Some geologists thinkthat volcanism also played a role at the end of the Cretaceous. Significantly, Benton concludes by considering the implications of the latest, man-made mass extinction, askingwhat light the earlier events can throw on thepotential for survival of modern species.

The historical aspect of Benton’s bookraises some intriguing questions. Many earlycatastrophists postulated the involvement ofextraterrestrial agents — a comet was some-times invoked as the cause of Noah’s flood.

But such ideas went out of fashion inthe mid-nineteenth century, and latercatastrophists, including Murchison,

favoured explanations based on thesupposedly more intense geological acti-

vity in the young Earth. The asteroid-impact theory of dinosaur extinctions

seems to parallel some of the earliest speculations, but Benton has redressed thebalance by favouring internal causes.

My one criticism of his account is that he accepts too readily the assumption thatLyell and Darwin marginalized all supportfor discontinuity in the Earth’s history.There were few outright catastrophists leftby around 1900, but many still believed that the history of life had been punctuated byenvironmental transitions far more rapidthan anything observed in the recent past.

The real triumph of gradualism camewith the modern darwinian synthesis of the mid-twentieth century, and even then itwas confined to the English-speaking world.Benton notes that British and US palaeontol-ogists of the 1950s ignored the catastroph-ism of Otto Schindewolf. But we need to recognize that German palaeontologistssuch as Schindewolf were continuing a long-standing tradition that had proved farmore robust than our modern, Darwin-centred histories acknowledge. The fact that modern catastrophists do not see a link back to that tradition tells us about the effectiveness of the neo-lyellian interlude ofthe mid-twentieth century. ■

Peter J. Bowler is in the Department of SocialAnthropology, Queen’s University Belfast, Belfast BT7 1NN, UK.

Hooke, life and thinkerLondon’s Leonardo: The Life andWork of Robert Hookeby Jim Bennett, Michael Cooper, Michael Hunter & Lisa JardineOxford University Press: 2003. 240 pp. £20, $35

David R. Oldroyd

Some devotees of Robert Hooke haveregarded him as Britain’s greatest scientificgenius of the seventeenth century, the rangeof his interests and achievements being hard to conceive. He is a fruitful subject forhistorical enquiry as he left behind him a large archival trail, and, with his polymathic interests, he has attracted much attention. A good general overview, Robert Hookeby Margaret ’Espinasse (Heinemann), waspublished in 1956. Since then, studies ofHooke have expanded greatly to the pointwhere we have a detailed knowledge of theman, although not all within the pages of a

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Exit stage right — even though Lystrosaurussurvived the extinction at the end of the Permian.

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