The Emergence of Hemispheric Organization: The case of faces and words

• In Event: How Top-down Influences Shape Visual Development from Infancy to Childhood

Abstract

Despite the similarity in structure, the hemispheres of the human brain have somewhat different functions. A traditional view of hemispheric organization asserts that there are independent and largely lateralized domain-specific visual regions in ventral occipitotemporal, specialized, if not dedicated, and perhaps innate, for the recognition of distinct classes of objects such as words and faces. In this talk, I will offer an alternative account of the organization of the hemispheres, with a specific focus on the development and plasticity of this arrangement.
I will first present an account of interactive and graded organization of both within- and between-hemisphere organization. The crux of the account is that organization emerges from a distance-dependent wiring constraint. The claim is that, prior to the acquisition of literacy, there is no a priori pressure for hemispheric organization, and both hemispheres initially subserve the development of face recognition—indeed, a lesion to either hemisphere in children affects face perception (de Schonen, Mancini, Camps, Maes, & Laurent, 2005). At around ages 5 or 6 years, the onset of reading acquisition triggers a competition between face and word representations, both of which require and, therefore, compete for high acuity region in extrastriate cortex. Word recognition emerges in the posterior ventral occipital cortex of the left hemisphere (LH) because this posterior region is more proximal to left-lateralized language areas (in the modal, right-handed population). In this way, the acquisition of reading obeys the distance-dependent constraint which reduces metabolic demands and potential errors. As the LH is optimized for word recognition, by virtue of competitive and collaborative processes, the RH becomes increasingly optimized for face recognition. As shown in the figure below, across children and young adolescents, the better the reader, the greater the lateralization of faces to the RH. ERP data collected from children also support this view, with significantly greater amplitude of the N170 component for words in the LH over RH and no hemispheric difference in the signal for faces.
This pattern of organization, however, is malleable and findings from children who have had posterior cortical regions resected show normal BOLD activation in fMRI studies of face and word recognition (one individual with RH posterior resection, and normal face and word activation in blue and orange, respectively; (see figure on right below).
Together, these findings support a dynamic interactive process by which hemispheric organization emerges and unfolds with experience rather than being prespecified from birth.

Author

• Marlene Behrmann, Carnegie Mellon University
  Presenting Author