Update TRENDS in Cognitive Sciences Vol.10 No.9 397

Coarse coding can also support the assessment of itemfamiliarity, one of the main memory functions ascribed toPRh [8]. In a recent connectionist model of recognitionmemory [9], the extrahippocampal component that includesPRh supplies a familiarity signal based on extensivelyoverlapping distributions, and yet supports fine discrimi-nations between similar objects in forced-choice recognitionmemory tasks successfully. A model of PRh that incorpo-rates coarse coding thus would not only offer a promisingaccount of its perceptual functions, but would also be in linewith computational accounts of its memory functions.

AcknowledgmentsWe thank Dr Melvyn Goodale for his comments on an earlier draft of thismanuscript. We are also grateful to Edward O’Neil and Benjamin Bowlesfor their contributions to many helpful discussions on this topic. A.C. issupported by a fellowship from the CIHR Group for Action andPerception. S.K. is funded by a CIHR operating grant.

References1 Buckley, M.J. and Gaffan, D. (2006) Perirhinal cortical contributions to

object perception. Trends Cogn. Sci. 10, 100–107

Corresponding author: Marcus, G.F. (gary.marcus@nyu.edu)Available online 8 August 2006.

www.sciencedirect.com

2 Hampton, R.R. (2005) Monkey perirhinal cortex is critical for visualmemory, but not for visual perception: reexamination of the behaviouralevidence from monkeys. Q. J. Exp. Psychol. B 58, 283–299

3 Bussey, T.J. et al. (2005) The perceptual-mnemonic/feature conjunctionmodel of perirhinal cortex function. Q. J. Exp. Psychol. B 58, 269–282

4 Bussey, T.J. and Saksida, L.M. (2002) The organization of visual objectrepresentations: a connectionist model of effects of lesions in perirhinalcortex. Eur. J. Neurosci. 15, 355–364

5 Pomerantz, J.R. and Kubovy, M. (1986) Theoretical approaches toperceptual organization. In Handbook of Perception and HumanPerformance (Boff, K.R. et al., eds), pp. 36.1–36.46, Wiley

6 Holscher, C. et al. (2003) Perirhinal cortex neuronal activity related tolong-term familiarity memory in the macaque. Eur. J. Neurosci. 18,2037–2046

7 Rolls, E.T. (2000) Functions of the primate temporal lobe cortical visualareas in invariant visual object and face recognition. Neuron 27, 205–218

8 Brown, M.W. and Aggleton, J.P. (2001) Recognition memory: what arethe roles of the perirhinal cortex and hippocampus? Nat. Rev. Neurosci.2, 51–61

9 Norman, K.A. and O’Reilly, R.C. (2003) Modeling hippocampal andneocortical contributions to recognition memory: a complementary-learning–systems approach. Psychol. Rev. 110, 611–646

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doi:10.1016/j.tics.2006.07.004

Genes and domain specificity

Gary F. Marcus and Hugh Rabagliati

New York University, 6 Washington Place, New York, NY 10012, USA

Kovas and Plomin’s recent article on ‘generalist genes’ [1]purports to pose a strong challenge to the possibility of

Genes are in essence instructions for fabricating biologi-cal structure. In the construction of a house, one finds both

modular neural or cognitive structure. Pointing to the factthat ‘there is substantial genetic overlap between suchbroad areas of cognition as language, reading, mathe-matics and general cognitive ability’, Kovas and Plomin[1] argue that ‘genetic input into brain structure andfunction is general not modular’.

The observation of genetic overlap in itself is not new.Our group made it recently in a more nativist account [2],while Karmiloff-Smith made it previously in the context ofa less nativist proposal [3]. Given the extensive literatureon a general factor (g) of intelligence, and the well-knowncorrelation between verbal and nonverbal IQ, the overlapobservation itself is unsurprising.

Kovas and Plomin’s anti-modularity interpretation,however, goes far beyond the scope of these data. Asresearchers in other areas of biology recognize [4], mod-ularity certainly does not require that all genes or evenmost of the genes involved in a given process be domain-specific; only that some genes (or even some portionsthereof) be differentially expressed. Modularity can arisefrom the actions of a handful of ‘upstream’ regulatorygenes, even if many or all downstream genes are broadlyshared across domains.

some repeated motifs and some specializations for particu-lar rooms. Every room has doors, electrical wiring, insula-tion andwalls built upon a frame ofwooden studs.However,the washroom and kitchen vary in the particulars of howthey use plumbing array fixtures, and only a garage islikely to be equipped with electric doors (using a novelcombination of electrical wiring and ‘doorness’). Con-structing a home requires both domain-general anddomain-specific techniques. The specialization of a givenroomprincipally derives from theways inwhich high-leveldirectives guide the precise implementation of low-leveldomain-general techniques.

When it comes toneural function, the real question ishow‘generalist genes’ fit into the larger picture. Continuing theanalogy, onemightaskwhetherdifferent ‘rooms’ of thebrainare all built according to exactly the same plan, or whetherthey differ in important ways, while depending on commoninfrastructure.KovasandPlomin [1]presumethat thesheerpreponderance of domain-general genes implies a singlecommon blueprint for the mind, but it is possible that thegeneralist genes are responsible only for infrastructure (e.g.the construction of receptors, neurotransmitters, dendriticspines, synaptic vesicles and axonal filaments), with asmaller number of specialist genes supervising in a waythat still yields a substantial amount of modular structure.Kovas and Plomin [1] offer no data to indicate otherwise.

398 Update TRENDS in Cognitive Sciences Vol.10 No.9

References1 Kovas, Y. and Plomin, R. (2006) Generalist genes: implications for the

cognitive sciences. Trends Cogn. Sci. 10, 198–2032 Marcus, G.F. (2004) The birth of the mind: How a tiny number of genes

creates the complexities of human thought. Basic Books3 Karmiloff-Smith, A. (1998) Development itself is the key to

understanding developmental disorders. Trends Cogn. Sci. 2, 389–398

Corresponding author: Kovas, Y. (y.kovas@iop.kcl.ac.uk)Available online 8 August 2006.

www.sciencedirect.com

4 Schlosser, G. and Wagner, G.P., (eds) (2004) Modularity in developmentand evolution. University of Chicago Press

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doi:10.1016/j.tics.2006.07.003

Letters Response

Response to Marcus and Rabagliati ‘Genes anddomain specificity’

Yulia Kovas and Robert Plomin

Institute of Psychiatry, King’s College London, SGDP Centre Box P050, London SE5 8AF, UK

Ourhypothesis of ‘generalist genes’ – that is, the samegenesthat affectmost cognitive abilities and disabilities – is based

Fortunately, this is an empirical issue about DNA poly-morphisms that does not require resorting to metaphors

on the consistent finding of high genetic correlations amongthem. In this issue, Marcus and Rabagliati [1] accept thisfinding but argue for the possible importance of specialistgenes that conform to a modular view of the brain.

Finding high genetic correlations means that genesmust be generalists at the psychometric level at whichthese traits have been assessed. Therefore, a genetic poly-morphism that is associated with individual differences ina particular cognitive ability will also be associated withother abilities. The question is how these generalist geneswork in the brain. Does a genetic polymorphism affect justone brain structure or function, which then affects manycognitive processes, as suggested by a modular view ofbrain structure and function (mechanism 1 in [2])? Thismodel assumes that brain structures and functions are notgenetically correlated – genetic correlations arise only atthe level of cognition. Another possibility, which we thinkis more probable, is that the origin of the general effect of agenetic polymorphism is in the brain because the poly-morphism affects many brain structures and functions(mechanisms 2 and 3 in [2]). Of course, some polymorph-ismsmight have general effects via mechanism 1 and otherpolymorphismsmight have general effects viamechanisms2 and 3, as Marcus and Rabagliati suggest.

such as house-building. We did not say that the case formechanism 3 was proven, which is what Marcus andRabagliati imply with their partial quote. The full quotefrom our article is: ‘In our opinion, these two key geneticconcepts of pleiotropy and polygenicity suggest that thegenetic input into brain structure and function is generalnot modular’. Pleiotropy (in which a gene affects manytraits) is a general rule of genetics. Polygenicity (in whichmany genes affect a trait) is becoming another rule ofgenetics for complex traits and common disorders. As wepoint out, polygenicity greatlymultiplies andmagnifies thepleiotropic effects of generalist genes. A more empiricalreason for suggesting that the origin of generalist genes isin the brain is that gene-expression maps of the braingenerally indicate widespread expression of cognition-related genes throughout the brain.

References1 Marcus, G.F. and Rabagliati, H. (2006) Genes and domain specificity.

Trends Cogn. Sci. 10, 397–3982 Kovas, Y. and Plomin, R. (2006) Generalist genes: implications for the

cognitive sciences. Trends Cogn. Sci. 10, 198–203

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doi:10.1016/j.tics.2006.07.002