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Morphological Language Processing in the Brain

Major levels of linguistic structure

Introduction

What is a Morpheme

Morphemes are the smallest individually meaningful units of a language. They are also the minimum units of the lexicon. One's lexicon is the total stock of morphemes in a language. A morpheme may be a whole word or part of a word . For example, the morpheme "s" is not spoken independently in ordinary English speech. Therefore, "s" is not a word on its own. However, it is meaningful as it can make a word plural, thus making it a morpheme.

Notably, it has been shown that a number of factors seem to affect morpheme processing in the brain. These include frequency and priming effects. The frequency of exposure to morphemes may affect the identification times of complex words. Frequency effects have been found to be strongest for stem frequencies in which the stem morpheme containing the root of the word was manipulated. For example, the morpheme 'DEAL' in the word 'DEALER'. .Furthermore, priming effects have been shown to affect the processing of morphemes. Studies suggest that previous exposure to a morpheme within the experiment results in faster processing and identification for derived words (words containing more then one morpheme). Low frequency primes were shown to yield longer identification times than that of high frequency primes . It is these factors which need to be considered when discussing studies regarding morphological processing in the brain.

Brain Areas Associated with Morphological Processing

Broca's Area

Areabroca

It has been shown that Broca's Area plays a significant role in the processing of morphemes in the brain. Broca's area (shown on the right) has been shown to play a role in hominid speech production and language comprehension. Moreover this brain region has been associated with characteristics of brain damage relating to aphasia - which is a chronic language impairment disorder relating to the modalities of writing, reading, speaking, and listening. In the study by Moro and colleagues, semantic relationships were controlled for by requiring the detection of anomalies in written sentences containing pseudowords. In both the morphological processing conditions, as well as morphosyntactic conditions in which semantic value was applied to morphemes in the sentence, the selective deep component of Broca's area and of a right inferior frontal region was shown to be active via positron emission tomography scanning (PET). Positron emission tomography is a method of brain imaging in which radioactive glucose is injected into the participant's blood stream. This method relies on the assumption that active brain regions will consume the radioactive glucose, thus allowing researchers to detect active brain regions via the radioactive tags.

Moreover, within these systems, the left caudate nucleus and insula were activated; but only during syntactic processing, thus indicating their role in syntactic computation independent from morphological processing. These finding suggest in vivo evidence (within participant) of a neural network for morpheme and syntax processing as well as computation.

Studies examining patients with Aphasia suggest that these areas may also be responsible for grammar processing as well. In the study conducted by Shapiro and colleagues, results suggest that grammar processing is not epiphenomena- in which processing occurs separately and secondary to language processing, but instead is processed as one computation. This was shown in fluent aphasia patients who had difficulty processing nouns relative to verbs in naming and sentence comprehension. Moreover, the patients displayed selective difficulty in producing the plural forms of nouns as well as pseudowords presented as nouns- which required the use of morphemes. However, patients were able to produce the phonologically identical third-person singular forms of the corresponding verb homonym and of the same pseudowords presented as verbs. These results suggest that grammar computation occurs in syntactic streams, and may be mediated by morphemes.

Models of Morphological Processing

As previously mentioned, frequency and priming effects have been shown to affect morpheme processing, and should be taken into consideration when examining studies regarding morpheme processing. Additionally it should be noted that words with internal structure consisting of root words, are processed differently than those with no internal structure, or words which are not derived from root units It is suggested that words with internal structure are encoded by their constituent morphemes and not by their full orthographic cluster.

Probabilistic Models of Morphological Processing

Fermı́n Moscoso del Prado Martı́n, Aleksandar Kostić and R.Harald Baayen propose an artificial neural network model of morphological processing, which involves probabilistic parameters. In this model, a mathematical formula is used to formulate morphological complexities. Residual information is calculated for a word, which is the probability of the word being correct, as well as belonging to an appropriate category when conducting morphological and semantic tasks. This model has proved to perform well in many morphological language tasks. Furthermore other models have not been able to account for such a wide variety of words used by Moscoso and colleagues, including monomorphemic, polymorphemic, and compound words. This model is used for predicting response latencies (times) in visual lexical decision tasks and further provides a parsimonious account of processing (see parsimony)

Morphemes and Prosodic Components in Language Recognition

In this section, Feng's work on morpheme and speech recognition in terms of prosodic characteristics, that is the rhythm, stress and innotation of speech, will be examined. In this model, prosodic morphemes and fixed content morphemes are recognized by two separate processes. Fixed content morphemes refer to those morphemes which have a consistent sound correlated with meaning. That is no matter what suffix the morpheme is attached to, it is pronounced the same. Inversely, prosodic morpheme pronunciation changes and depends on the context or root the morpheme is attached to. The prosodic characteristics of a prosodic morpheme are referred to as it's shape. Thus inherently resulting in fixed content morphemes embodying a different shape.

However, prosodic and fixed content morphemes also have a lot in common. They both characterize the basic meaningful units of language, as well as link phonological content and semantic meaning. Thus a general template theory, in which there is a separate template stored in the mind for each word, often cannot account for prosodic morphemes. Feng proposes a model in which fixed content and prosodic morphemes are processes separately. This is rationalized due to their distinctly different shape and underlying characteristics. Feng's model states that prosodic morphemes are recognized by their shape while fixed content morphemes are recognized by their content. Further research is needed in order to determine where these two constructs of language are processed in the brain as well as a theoretical differences between prosodic and fixed content morphemes.

Work by Raveh has concluded that another factor influencing language recognition, specifically the time of language processing, are priming and frequency effects. Both of which influence prosodic and fixed content morphemes in similar ways, however through distinct pathways.

Morphemes and Word Recognition

In the literature, many previous studies have suggested that orthographic characteristics are processed before any semantic derivatives can be comprehended. Semantic derivatives refer to the semantic value, or meaning associated with a unit of language. However, Feldman and O'Connor suggest that semantic value is significantly more involved in word recognition. Paradigms of word recognition show that morpheme facilitation was the strongest when morpheme primes were semantically transparent, or easy to perceive, in juxtaposition to primes which were not semantically related. These results limit the notion of word processing which states that humans perceive language form first and meaning later, as semantic priming has been shown to significantly influence morphological processing. Thus if morphological processing is influenced, word processing should also theoretically be influenced by semantic value.

Morphological Relevance in Second Langauge Learning

Inflectional morphemes have been shown to be extremely difficult to acquire when learning a second language, especially english. Inflectional morphemes are basic units of language which are used in order to change a verb's tense or a noun's number. It has been found that when learning a second language adults are insensitive to number morphemes. Here, number morphemes are units used to change a noun's number. In the study conducted by Jiang, Chinese ESL speakers (English as a second language) were instructed to read English sentences for comprehension. Their reading times were recored in order to determine if they were sensitive to idiosyncrasies in which disagreement in the sentence does not involve number morphemes. The results show that participants are sensitive to idiosyncrasies but not number morphemes. This insensitivity to number morphemes suggests that their morphological knowledge is not an integrated part of their automatic second language competence. . A comprehension-based model has been proposed to explain this phenomena. This models states that deficits in morpheme comprehension, when learning a second language, results from being unable to effectively retrive this information from memory. Hoever if this model were true, than one should see a gradual decrease in deficiency with increased exposure to a language. However, evidence shows that even highly proficient speakers of a second language have difficulties with number morphemes.

Larsen- Freeman has also noted that inflectional morphemes are unproblematic for children to acquire when learning a first language, however they still remain a substantial obstacle for older individuals learning a second language. Interestingly, children do experience some difficulties with inflectional morphemes, such as over-generalization of rules, and these problems arise in almost all languages. Larsen-Freeman concludes that more research is needed in order to determine is cause surrounding inflectional morphemes and suggests age of acquisition for the language may play a significant role.

Second Language Learning and ERP

EEG recording

In the study conducted by Hane, Mueller and Clahsen, localized brain activity for various morphemes were found to differ in participants speaking German as a second language. Two subsets of morphemes were studied, verb inflections and noun plurals. Event related potential (ERP) results show that in participants who speak German as a second language, greater anterior negativity, followed by an N600 component was found when overgeneralizing inflectional morphemes but attempting to use correct morphemes. Furthermore when attempting to use plural morphemes, participants elicited a N600 component when correcting using 's' morphemes, and showed a N400 component when overgeneralizing. In conclusion, these results show differential processing of various morpheme subtypes, as well as when they are correctly and incorrectly used in participants speaking a second language. It should be noted here, that ERP components refer to extracted, average activity across the scalp corresponding to a stimulus onset. These averages (appearing in a graph) are used to characterize voltage potential on the scalp with a stimulus onset (see Event related potential for more information on this method).

Impairments of Morphological Processing

Morphological Mapping in the Brain of Dyslexic Patients

Fusiform gyrus animation

Dyslexia is characterized by the difficulty in learning to read and spell. Research has shown that dyslexia affects comprehension on levels of phonology, orthography and morphology. Using functional magnetic resonance imaging ( fMRI ), Richards and colleagues have shown that brain activity indicating areas responsible for processing orthography, phonology and morphology are significantly different when comparing dyslexic patients to controls.

Specifically orthography processing, which can be considered processing the "visual word form", has been shown to significantly differ in fMRI studies comparing dyslexic patients and controls. The fusiform gyrus has been suggested to be the area in the brain in which orthography is processed. It has been shown that children with dyslexia differ significantly from those without during orthographic processing tasks that require access to precise spelling patterns. The fusiform gyrus shows less activity on these trials involving dyslexic patients.

Morphological Processing in the Brain of Patients with Down-Syndrome

It has been shown that the language performance of those individuals with down syndrome are fundamentally similar to those patients with specific language impairment when examining the following constituencies; tense, tense inflections, & non-tense morphemes. Specific language impairment is characterized by language deficits which are not accountable by genetic or developmental discrepancies.

Participants consisted of children with specific language impairment (SLI), with down syndrome (DS), and with typical language development (TL). All of which were matched on mean length of utterance (MLU) of spoken sentences. After completing the various morphological tests of tense, tense inflections, & non-tense morphemes, results showed that children with SLI performed significantly more poorly on these task than children with TL. Furthermore, the DS group performed significantly more poorly on tasks of tense inflections and non-tense morpheme composites than the TL group. Although there were no statistical differences found between the SLI and DS groups on any of the previously mentioned morpheme measures, they were not similar in the ways in which they executed regular past morphemes (ed), irregular third person singular morphemes (has, does) and present progressive morphemes (ing). Furthermore, the SLI and DS groups both performed significantly worse on sentence imitation tasks. Thus it has been suggests that similar deficits in large scale morphological processing in the brain may be similarly affecting both groups but not in more minuet computational systems of morphology.

Literature Summary Tabe

Literature Summary for Brain Areas Discussed in the Article

Concept	Brain Area Involved	Specific Aspect of Morphology	Implications
Broca's Area	Right Inferior Frontal Region	All	Evidence suggests a neural network for morpheme and syntax processing and computation
Broca's Area	Left Caudate Nucleus and Insula	All	These areas were active only during syntactic processing. This indicates their role in syntactic computation independent from morphological processing. These finding suggest in vivo evidence of a neural network for morpheme and syntax processing and computation.
Dyslexia	Fusiform Gyrus	Orthography	Using functional magnetic resonance imaging ( fMRI ), Richards and colleagues have shown that brain activity responsible for processing orthography, phonology and morphology are significantly different when comparing dyslexic patients to controls.