The relationship between the age of acquisition (AoA) of words and their cerebral hemispheric representation is controversial because the experimental results have been contradictory. While a large number of studies have found that words acquired early in life are also lateralized (as are later words) in the left hemisphere (LH), some evidence (particularly Bowers et al., 2013) suggests that early words have a bilateral (or rightward) representation because the corpus callosum is not fully developed in the early years and the two hemispheres function in parallel. In the present study, we conducted two experiments based on a divided visual hemifield paradigm, testing words that were classified according to an objective criterion of AoA. The results of both experiments were consistent and demonstrated that early words (as well as later words) are processed more quickly and efficiently in the LH, with no AoA x Hemisphere interaction. These results do not support the hypothesis of Bowers et al. (2013) and clearly show that words learned during the early years of life are also processed preferentially in the LH, similar to words learned later. These results are discussed from a theoretical standpoint, considering that all previous experiments on AoA and brain hemispheres have been conducted with adult participants.
