Understanding lexical and orthographic processing of Persian : the role of age of acquisition

Abstract

The primary aim of this thesis is to understand the mechanism of word processing in an Indo-European language (i.e. Persian) with a borrowed Semitic script (i.e. Arabic) using a psycholinguistic approach. This research reports fresh psycholinguistic norms for the psycholinguistic properties of object (picture) names and written word stimuli for Persian for the first time and investigates the effect of these properties on lexical retrieval and more specifically oral reading for both normal and impaired Persian speakers. The main hypothesis studied is whether there is an independent effect of the age of acquisition (AoA) of a word on lexical retrieval in Persian and the locus of this effect on adult lexical processing using behavioural, event related potential (ERP) and neuro-linguistic experiments. The predictions tested are that (a) words learnt earlier in life have a processing advantage i.e. faster processing time and fewer errors in lexical retrieval (b) AoA leaves a neural signature on lexical processing and (c) and that AoA preserves lexical retrieval in Persian speakers who have brain damage.

The results of the study broadly confirmed these predictions. The key finding is that although AoA has an effect on lexical retrieval in Persian (Z =9.25, p < .00001), the effect of this variable is more pronounced when the mapping between input and output is more arbitrary such as for oral reading of written words with relatively opaque spellings (Z =-2.00, p < .05). Normal and aphasic speakers of Persian both read opaque words less efficiently than transparent words (Z =3.93, p < .0001), especially when these words are acquired later in life (Z =2.48, p < .05). Crucially, the evidence from patients with aphasia revealed that oral reading in Persian requires both semantic and nonsemantic routes due to varying degrees of orthography to phonology transparency in Persian.

The theoretical implications of the thesis are (1) AoA has an independent and durable effect on lexical retrieval during both spoken and written word processing tasks for Persian speakers (2) AoA clusters with psycholinguistic properties that have a semantic quality rather than simply reflecting token frequency of exposure and (3) a semantic locus for the effect of AoA was confirmed using ERPs suggesting a neural signature of the AoA effect located in the (semantic) N400 F(1,22) = 9.35, p < .01 and LPC components F(1,22) = 21.18, p <.0001. The primary conclusion is that AoA has an effect on spoken and written word processing in Persian a prototypical example of an Indo-European language with a Semitic script.