How age of acquisition and lexical frequency differently affect early visual word processing

It has been consistently shown that words with high lexical frequencies are recognized more quickly and accurately than words with lower frequencies in the same language. Word frequency has therefore played a central role in the development of lexical processing models. There is increasing behavioral evidence that the age at which words are learned is a variable with as much significance as lexical frequency. Nevertheless, while neuroimaging and electrophysiological studies have provided remarkable evidence of the influence of word frequency on brain activity, little is known about how words acquired earlier or later in life are represented in the brain. The aim of the present study was to investigate whether the processing of early- vs. late-acquired words affects the timing and topographical distribution of ERP components differently than the processing of high- vs. low-frequency words. Participants were asked to perform an orthographic decision task (detection of a target letter) while EEG measurements were taken from 128 scalp sites. The LORETA source localization technique was applied to identify the intracranial generators of the surface potentials. Reaction times showed an effect of both word age of acquisition (AoA) and lexical frequency as well as an interaction between the two psycholinguistic variables. The electrophysiological data revealed a neuro-functional dissociation between AoA and frequency effects in early visual word processing. AoA modulated the amplitude of left occipito-temporal selection negativity, suggesting an effect of word AoA on early orthographic and lexical access and revealing the crucial role of AoA in determining how words are neurally represented in the ventral pathway. Lexical frequency modulated the amplitude of left anterior negativity, providing evidence for the involvement of the left inferior frontal cortex in the processing of low-frequency words.