​FUSIFORM GYRUS, READING AND FACE RECOGNITION

The acquisition of reading is a process that profoundly changes the brain because, unlike language, which is an innate faculty, reading must be learned. In the " neuronal recycling " hypothesis, according to which the brain uses pre-existing neural circuits capable, thanks to cerebral plasticity, of modifying their functions, the human being is able to learn to transform signs on a page into a meaning. This learned capacity relies on the decoding of visual forms (graphemes) in an area of the visual cortex that is very stable from one individual to another, called the Visual Word Form Area (VWFA). This specialized area is located on the ventral side of the brain, in the fusiform gyrus of the left hemisphere (see also Minimal computational models accounting for emergence of a specific region underlying orthographic coding (cea.fr).

The area specialized in face recognition, called the Fusiform Face Area (FFA), is also located in the fusiform gyrus. This region is, in most people, more prominent in the right hemisphere and is adjacent to the word area. It develops slowly until adolescence, i.e. at the same time as reading progresses.

PROBLEMATICS

Neuroscientists are still struggling to understand how such selective activation of neighboring regions of the fusiform gyrus is constructed during development, and in particular, how the acquisition of reading, an acquired cultural ability, competes, or not, with the ancient cortical circuitry involved in the visual recognition of faces and objects.

THE "BIASED CONNECTIVITY" HYPOTHESIS

The aim of the present study was to refine the understanding of the spatial organization of ventral visual areas in the early stages of learning to read. To do so, the researchers measured functional MRI responses to written words, faces, houses and checkerboards in three groups of 20 French children: 6-year-old pre-readers, 6-year-old beginning readers* and 9-year-old advanced readers. The development of responses to words and faces differs greatly between groups of children: specific responses to written words are essentially absent in pre-readers, and appear rapidly with reading acquisition, regardless of age. Responses to faces, which are not very visible in pre-readers, develop slowly, as a result of age and not reading. These two specializations for words and faces compete to occupy initially unspecialized regions from two "cores" distinct by their functional connectivity, one to language circuits (VWFA) and the other to emotional circuits (FFA). It is probably the weak specialization of these regions in the young child that allows reading to be installed rapidly at this age, whereas this learning is more difficult in adulthood. Thus, the acquisition of reading does not compete directly with the face recognition system, but hinders their slow evolution in the left hemisphere, thus accentuating the pre-existing bias towards the right hemisphere (figure).

This study shows that the increasing specialization of areas of human visual cortex for words and faces occurs on two very different time scales and varies as a function of age, education, and constraints imposed by connectivity with distant brain regions. In both cases, these developments are made possible by the persistent plasticity of fusiform regions.

Contact : Ghislaine Dehaene-Lambertz ghislaine.dehaene@cea.fr ; gdehaene@gmail.com

*A major difficulty with previous studies is that age tends to be systematically confounded with reading performance, making it difficult to distinguish between changes related to cultural causes and those related to brain maturation. Because the academic year and the calendar year do not coincide, pre-readers in kindergarten and beginning readers in first grade had roughly the same age (6 years), but very different reading abilities.