Scientific Understanding of Consciousness
Cortical Networks for Vision and Language
Science 3 December 2010: Vol. 330 no. 6009 pp. 1359-1364
Stanislas Dehaene 1,2,3,4, Felipe Pegado 1,2,3, Lucia W. Braga 5, Paulo Ventura 6,
Gilberto Nunes Filho 5, Antoinette Jobert 1,2,3, Ghislaine Dehaene-Lambertz 1,2,3,
Régine Kolinsky 7,8, José Morais 7 and Laurent Cohen 9,10,11
1INSERM, Cognitive Neuroimaging Unit, Gif sur Yvette 91191, France.
2Commissariat à l’Energie Atomique, Direction des Sciences du Vivant, I2BM, Neurospin center, Gif sur Yvette 91191, France.
3University Paris-Sud 11, 91405 Orsay, France.
4Collège de France, 11 Place Marcelin Berthelot, 75005 Paris, France.
5SARAH Network–International Center for Neurosciences and Rehabilitation, QL 13, Lago Norte, 71.535-005 Brasilia, DF Brazil.
6Faculty of Psychology, University of Lisbon, 1600-214 Lisbon, Portugal.
7Faculty of Psychology, Université Libre de Bruxelles (ULB), 1050 Brussels, Belgium.
8Fonds de la Recherche Scientifique (FNRS), 1000 Brussels, Belgium.
9Université Pierre et Marie Curie-Paris 6, Faculté de Médecine Pitié-Salpêtrière, 75013 Paris, France.
10Assistance Publique–Hôpitaux de Paris, Groupe hospitalier Pitié-Salpêtrière, Department of Neurology, 75651 Paris, France.
11INSERM, Centre de Recherches de l’Institut du cerveau et de la moelle épinière, UMRS 975, Paris, France.
Does literacy improve brain function? Does it also entail losses? Using functional magnetic resonance imaging, we measured brain responses to spoken and written language, visual faces, houses, tools, and checkers in adults of variable literacy (10 were illiterate, 22 became literate as adults, and 31 were literate in childhood). As literacy enhanced the left fusiform activation evoked by writing, it induced a small competition with faces at this location, but also broadly enhanced visual responses in fusiform and occipital cortex, extending to area V1. Literacy also enhanced phonological activation to speech in the planum temporale and afforded a top-down activation of orthography from spoken inputs. Most changes occurred even when literacy was acquired in adulthood, emphasizing that both childhood and adult education can profoundly refine cortical organization.
Practically all adult neuroimaging experiments are performed in highly educated college students. The observed brain architecture therefore reflects the influence of culture and education over and above spontaneous brain development. Indeed, the acquisition of reading, a major event in children’s lives, is now recognized as capable of changing both brain anatomy and brain activation. In the auditory modality, literacy leads to phonemic awareness, the ability to manipulate the smallest units of spoken language [i.e., phonemes], and alters online speech processing. At the visual level, developmental neuroimaging studies in normal and dyslexic children show that, with reading acquisition, a specific brain site in left occipito-temporal cortex, which has been termed “visual word form area” (VWFA), starts to respond to orthographic stimuli in the learned script.
These observations leave many important questions unanswered. First, does literacy primarily lead to cooperative or to competitive effects on cortical processing? Two theoretical positions can be contrasted. The first view, derived from animal studies of environmental enrichment and sensory plasticity, emphasizes that perceptual learning entails beneficial modifications of cortical maps, including sharpened receptive fields and neuronal tuning curves correlated with behavioral improvements. Without denying these positive effects, the second view emphasizes that reading is a cultural invention too recent to involve dedicated genetic or developmental mechanisms. Thus, during education, reading processes must invade and “recycle” cortical space devoted to evolutionary older functions, opening the possibility that these functions suffer as reading expertise sets in. Much like expertise for nonface stimuli induces a reduction in face responses, reading, which recruits an identical cortical site in all cultures, might entail a reorganization of nearby responses to faces, houses, and objects. We thus sought to understand which of these stimuli are processed in the VWFA area before reading and how their cortical representation, which gets refined during the school years, is affected by literacy.
A second issue is that, at present, most functional imaging studies of illiteracy only contrasted schooled versus unschooled adults. Because these studies did not include “ex-illiterate” adults who did not attend school but learned to read during adulthood, they confounded the effects of schooling and literacy. The only important exception focused solely on how brain anatomy is changed by literacy. In this study, we separated the functional effects of schooling and literacy by comparing illiterates, ex-illiterates, and adults schooled in childhood.
We scanned a total of 63 Portuguese and Brazilian participants. Our sample included 32 unschooled adults (10 illiterates and 22 ex-illiterates with variable reading skills), and 31 schooled and literate adults. The latter group included 11 literate subjects matched to the illiterates in socioeconomic status. Reading skills were verified through behavioral tasks of letter identification, word and pseudo-word reading (with or without speed pressure), and sentence reading.
Literacy, whether acquired in childhood or through adult classes, enhances brain responses in at least three distinct ways. First, it boosts the organization of visual cortices, particularly by inducing an enhanced response to the known script in left occipito-temporal cortex and by augmenting early visual responses in occipital cortex, in a partially retinotopic manner. Second, literacy allows practically the entire left-hemispheric spoken language network to be activated by written sentences. Thus reading, a late cultural invention, approaches the efficiency of the human species’ most evolved communication channel, namely speech. Third, literacy refines spoken language processing by enhancing a phonological region, the planum temporale, and by making an orthographic code available in a top-down manner. These largely positive changes should not hide that literacy, like other forms of expertise, also leads to cortical competition effects. A significantly reduced activation was found for checkerboards and faces. The intriguing possibility that our face perception abilities suffer in proportion to our reading skills will be explored in future research.
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