An ERP measure of AoA and orthographic transparency effects on visual word recognition in Persian

Abstract

INTRODUCTION: Detecting an effect of age of acquisition (AoA) in expert visual word recognition is a controversial issue. One issue is whether AoA has a truly unique effect in word naming and another concern is whether this effect is lexical, semantic or phonological in nature. The arbitrary mapping hypothesis assumes that the AoA effect is dependent on depth of orthographic transparency with robust AoA effects for opaque words but not for transparent words. This is because transparent words can benefit from the predictable orthography-to-phonology mappings that characterize transparent scripts (see Juhaz 2005; Johnston & Barry, 2006 for a review). METHODS: In this study the unique nature of AoA in visual word recognition in Persian (a language with variable orthographic transparency) was investigated using event-related potential (ERP) methods. The degree of orthographic transparency in Persian is determined by vowel transcription in which transparent words have long vowels represented in a letter format, while opaque words have short vowels with no letter/grapheme representation in the expert script. 23 native Persian speakers performed a visual lexical decision task with 120 early-acquired and late-acquired transparent and opaque words whilst controlling for word frequency, imageability, word length and bigram frequency. RESULTS: The ERP analysis revealed greater N170 negativity for early versus late acquired words (PO5-PO8) only in the right hemisphere i.e. PO6 and PO8 electrodes (p < .05). Moreover, there was a significantly greater N400 negativity for late versus early-acquired words across the central and centro-parietal electrodes (p < .01). Critically, we found an interaction between spelling transparency and AoA at a late positive component with significantly higher positivity for late acquired opaque words compared to early acquired opaque words and no difference for the transparent words. CONCLUSIONS: The findings of a greater negativity at the N170 component for early versus late acquired words especially in the right parietal-occipital hemisphere suggests more efficient orthographic processing for words learnt earlier in life. The greater N400 negativity (i.e. greater lexical-semantic retrieval) from the long-term memory store for late acquired words compared to early words is in agreement with semantic accounts of the AoA effect. In fact, finding more efficient orthographic and semantic processing for early versus late words is consistent with a neural plasticity hypothesis indicating that early learning can take advantage of a more flexible neural system and may induce multiple neural representations in the brain. The interaction between AoA and orthographic transparency is consistent with the arbitrary mapping hypothesis and might be due to a need for further phonological or semantic input for late acquired opaque words . Taken together, there is evidence for both accounts of the AoA effect using ERP suggesting that the effect can arise at different levels of processing and add to behavioral findings by showing the time-course of these multi-focal effects.