INTRODUCTION Perceiving words is a task that entails both bottom-up and top-down processes. Sensory representations, cognitive mechanisms and linguistic knowledge all play an important role in word processing. Post-perceptual decision in word processing requires higher levels of linguistic response (e.g. count the number of words that are names of vegetables), an example of what is termed semantic categorization. Semantic categorization refers to a basic understanding and use of concepts in semantic memory (Kutas & Iragui, 1998). The distributions of a category s characteristic properties are viewed as better exemplars of the category (Schaeffer & Wallace, 1969). Complex neural mechanisms are maintained for semantic storage and retrieval which processes the distinctive features of words, in addition to learned associations between specific words and other semantic entries in the storage system. Semantic categorization mainly depends on different processes. The frequency of language use, linguistic aspects of words, characteristic properties etc. all play a role in semantic categorization (Smith, 1995). Concreteness, word frequency and typicality effects have an effect of semantic categorization. Concreteness effect was explained by different theories, Paivio (1971, 1986) theory which explains that there are two structurally and functionally different representational systems, an imagery system specific for the depiction and processing of non-verbal stimuli and a verbal system specific for the handling of linguistic stimuli. Concrete words have an advantage over abstract words due to both the imagery and verbal system. Context availability theory (Bransford & McCarrell, 1974) states that contextual information is more for concrete words compared to abstract words. Frequency of usage of words has an advantage in processing frequent and infrequent words (Polich & Donchin, 1988). Typical words of 1
category were processed quicker compared to atypical category. Processing of better exemplars of category was faster than poorer exemplars of category in both children and adults (Jerger & Damian, 2005). Semantic categorization with one language symbol alone is influenced by numerous factors as mentioned above, the same in a bi/multilingual would be extremely complex. Bilingualism is use of two or more languages in daily communication (Grosjean, 2010). The language representation in bilinguals has been explained by different models and theories. Green (1986) proposed the single network model which states that there is a single language area in the brain which is responsible for two languages in bilinguals. On the other hand the procedural model (Ullman, 2001) which has explained that different areas of activation in the brain for two languages. Another model by Dijkstra and Van Heuven (2002) described the bilingual language processing. It mainly focuses on language proficiency of first language (L1) and second language (L2), which suggests that due to the lesser proficiency levels of L2 in bilinguals there is a delay in processing and activation of brain for L2 compared to L1. The delicate nature of brain is vulnerable to brain damage and other problems like stroke. Due to cerebral damage, some aspects of the language processing may be preserved but not others (Goodglass, Klein, Carey, & Jones, 1966). Aphasia refers to a language disorder where the disturbance of any or all of the skills in relation to spoken and written language may be impaired due to damage to the particular neuro anatomical areas. Aphasia is mainly divided into two types i.e. fluent and non-fluent. Broca s aphasia is the most common type of non-fluent aphasia (Goodglass, Kaplan, & Baressi, 2001). 2
Deficits in semantic categorization have been reported in persons with aphasia (Schuell, & Jenkins, 1961). In literature, two types of tests have been used to assess the language processing (semantic processing) these are behavioural measurements and electrophysiological measurements. Milberg and Blumstein (1981) studied semantic judgment task in persons with aphasia and results revealed that major deficits were seen in semantic judgment tasks in persons with aphasia compared to neuro-typical participants. Persons with aphasia require more time for categorization of words i.e. frequent words and infrequent words (Gerratt & Jones, 1987). Language processing was assessed through event related potentials (ERP) to check the brain activity and higher level activation. Enormous research has been done using event related potentials for language processing on neuro-typicals (Canseco-Gonzalez, 2000). Negativity around 400 milliseconds is termed as N400 which is responsible for lexical-semantic processesing (Friederici, 1995; Kutas & Hillyard, 1983). In neurotypicals longer N400 latencies were seen for abstract words compared to concrete words (Zhang, Guo, Ding, & Wang, 2006; Tolentino & Tokowicz, 2009). Strong exemplars of category elicited better processing than the weaker exemplars of the category (Mehta, Jerger, Jerger, & Martin, 2009). Need for the study There has been growing number of cross-linguistic studies in persons with Broca s aphasia. There are at least two reasons for the same. First, it has been shown that the manifestation of Broca s aphasia differs across languages, and then in order to comprehend better, the nature of linguistic disorders in persons with aphasia (PWA), cross-linguistic evidence is crucial. Second, the Principles and Parameters theory of Universal Grammar (Chomsky, 1981, 1986, 1995; Chomsky & Lasnik, 1993) gives a 3
productive structure to explain the difference in performance across languages in persons with aphasia. In modern era event related potentials (ERP) techniques have been used with increasing frequency in an attempt to study the possible relationships between brain responses and linguistic processes (Warner, Teyler, & Thompson, 1977). ERP components sensitive to syntactic and semantic factors have been identified in neurotypical individuals. However, not many reports have been documented in the literature on electrophysiological correlates of semantic processing in persons with aphasia, more so, in bilinguals (Brown, Marsh, & Smith, 1973). Researchers have used visual modality to elicit ERP responses in neuro-typical participants and few have tried on persons with aphasia. Similarly very few studies using auditory mode have been reported in neuro-typical participants but not much literature is available in persons with aphasia. Moreover, the documented ERP studies were carried out on mild comprehension deficits in persons with aphasia. Reports from these studies have shown either irregular variations in ERP wave forms or no robust ERP responses. Further most of the documented studies have been carried out on monolingual participants and have used either the behavioural or ERP methods. Hence, it is imperative to investigate semantic categorization using both the behavioural and electrophysiological methods (event-related potentials) in neuro-typical individuals and persons with Broca s aphasia especially using the auditory stimuli. It is also felt that the results of the study will provide more corroborative evidences about the robustness of each method to process the semantic information and in turn its effect on the judgement skills of monolingual and bilingual persons with Broca s aphasia and 4
Aim of the study The aim of the current study was to investigate semantic category judgement using a lexical decision task in a group of persons with Broca s aphasia (monolingual and bilingual) and Objectives of the study The main objectives of the study were 1. To measure the latency and amplitude of N400 event related potentials using semantic judgment task in: a. Monolingual persons with Broca s aphasia b. Bilingual persons with Broca s aphasia, and c. Neuro-typical individuals. 2. To measure the reaction time in semantic judgment task in a. Monolingual persons with Broca s aphasia b. Bilingual persons with Broca s aphasia, and c. Neuro-typical individuals. 3. To compare N400 (latency and amplitude) and reaction time within and between the persons with Broca s aphasia (monolingual and bilingual) and 4. To investigate the correlation between latency and amplitude of N400 and reaction time in persons with Broca s aphasia (monolingual and bilingual), and 5