Morphology, language and the brain: the decompositional substrate for language comprehension

Lateralization of the arcuate fasciculus from childhood to adulthood and its relation to cognitive abilities in children

The arcuate fasciculus is a major white matter tract involved in language processing that has also been repeatedly implicated in intelligence and reasoning tasks. Language in the human brain is lateralized in terms of both function and structure, and while the arcuate fasciculus reflects this asymmetry, its pattern of lateralization is poorly understood in children and adolescents. We used diffusion tensor imaging (DTI) and tractography to examine arcuate fasciculus lateralization in a large (n = 183) group of healthy right-handed volunteers aged 5-30 years; a subset of 68 children aged 5-13 years also underwent cognitive assessments. Fractional anisotropy and number of streamlines of the arcuate fasciculus were both significantly higher in the left hemisphere than the right hemisphere in most subjects, although some subjects (10%) were right lateralized. Age and gender effects on lateralization were not significant. Children receiving cognitive assessments were divided into three groups: a "left-only" group in whom only the left side of the arcuate fasciculus could be tracked, a left-lateralized group, and a right-lateralized group. Scores on the Peabody Picture Vocabulary Test (PPVT) and NEPSY Phonological Processing task differed significantly among groups, with left-only subjects outperforming the right-lateralized group on the PPVT, and the left-lateralized children scoring significantly better than the right-lateralized group on phonological processing. In summary, DTI tractography demonstrates leftward arcuate fasciculus lateralization in children, adolescents, and young adults, and reveals a relationship between structural white matter lateralization and specific cognitive abilities in children.

Gyral and sulcal cortical thinning in adolescents with first episode early-onset psychosis

BACKGROUND

Psychosis is associated with volumetric decreases of cortical structures. Whether these volumetric decreases imply abnormalities in cortical thickness, surface, or cortical folding is not clear. Due to differences in cytoarchitecture, cortical gyri and sulci might be differentially affected by psychosis. Therefore, we examined differences in gyral and sulcal cortical thickness, surface, folding, and volume between a minimally treated male adolescent population with early-onset first-episode psychosis (EOP) and a healthy control group, with surface-based morphometry.

METHODS

Magnetic resonance imaging brain scans were obtained from 49 adolescent EOP patients and 34 healthy control subjects. Subjects were younger than 18 years (age range 12 years-18 years), and EOP patients had a duration of positive symptoms of <6 months.

RESULTS

Early-onset first-episode psychosis was associated with local bilateral cortical thinning and volume deficits in both the gyri and sulci of the superior temporal cortex and the inferior, middle, medial, and superior prefrontal cortex. In the pars triangularis and opercularis cortex of patients, gyral cortical thickness was thinner, whereas sulcal thickness was not. Patients exhibited cortical thinning together with a decreased degree of cortical folding in the right superior frontal cortex.

CONCLUSIONS

Cortical thinning of both gyri and sulci seem to underlie most cortical volume deficits in adolescent patients with EOP. Except for the right superior frontal region, the degree of cortical folding was normal in regions showing decreased cortical thickness, suggesting that the process of cortical thinning in adolescent patients with EOP primarily takes place after the formation of cortical folds.

An Event-Related Potential Study of Cross-modal Morphological and Phonological Priming

The current work investigated whether differences in phonological overlap between the past- and present-tense forms of regular and irregular verbs can account for the graded neurophysiological effects of verb regularity observed in past-tense priming designs. Event-related potentials were recorded from sixteen healthy participants who performed a lexical-decision task in which past-tense primes immediately preceded present-tense targets. To minimize intra-modal phonological priming effects, cross-modal presentation between auditory primes and visual targets was employed, and results were compared to a companion intra-modal auditory study (Justus, Larsen, de Mornay Davies, & Swick, 2008). For both regular and irregular verbs, faster response times and reduced N400 components were observed for present-tense forms when primed by the corresponding past-tense forms. Although behavioral facilitation was observed with a pseudopast phonological control condition, neither this condition nor an orthographic-phonological control produced significant N400 priming effects. Instead, these two types of priming were associated with a post-lexical anterior negativity (PLAN). Results are discussed with regard to dual- and single-system theories of inflectional morphology, as well as intra- and cross-modal prelexical priming.

Where are the human speech and voice regions, and do other animals have anything like them?

Modern lesion and imaging work in humans has been clarifying which brain regions are involved in the processing of speech and language. Concurrently, some of this work has aimed to bridge the gap to the seemingly incompatible evidence for multiple brain-processing pathways that first accumulated in nonhuman primates. For instance, the idea of a posterior temporal-parietal "Wernicke's" territory, which is thought to be instrumental for speech comprehension, conflicts with this region of the brain belonging to a spatial "where" pathway. At the same time a posterior speech-comprehension region ignores the anterior temporal lobe and its "what" pathway for evaluating the complex features of sensory input. Recent language models confirm that the posterior or dorsal stream has an important role in human communication, by a reconceptualization of the "where" into a "how-to" pathway with a connection to the motor system for speech comprehension. Others have tried to directly implicate the "what" pathway for speech comprehension, relying on the growing evidence in humans for anterior-temporal involvement in speech and voice processing. Coming full circle, we find that the recent imaging of vocalization and voice preferring regions in nonhuman primates allows us to make direct links to the human imaging data involving the anterior-temporal regions. The authors describe how comparison of the structure and function of the vocal communication system of humans and other animals is clarifying evolutionary relationships and the extent to which different species can model human brain function.

The processing of English regular inflections: Phonological cues to morphological structure

Previous studies suggest that different neural and functional mechanisms are involved in the analysis of irregular (caught) and regular (filled) past tense forms in English. In particular, the comprehension and production of regular forms is argued to require processes of morpho-phonological assembly and disassembly, analysing these forms into a stem plus an inflectional affix (e.g., {fill}+{-ed}), as opposed to irregular forms, which do not have an overt stem+affix structure and must be analysed as full forms [Marslen-Wilson, W. D., & Tyler, L. K. (1997). Dissociating types of mental computation. Nature, 387, 592-594; Marslen-Wilson, W. D., & Tyler, L. K. (1998). Rules, representations, and the English past tense. Trends in Cognitive Science, 2, 428-435]. On this account, any incoming string that shows the critical diagnostic properties of an inflected form - a final coronal consonant (/t/, /d/, /s/, /z/) that agrees in voicing with the preceding segment as in filled, mild, or nilled - will automatically trigger an attempt at segmentation. We report an auditory speeded judgment experiment which explored the contribution of these critical morpho-phonological properties (labelled as the English inflectional rhyme pattern) to the processing of English regular inflections. The results show that any stimulus that can be interpreted as ending in a regular inflection, whether it is a real inflection (filled-fill), a pseudo-inflection (mild-mile) or a phonologically matched nonword (nilled-nill), is responded to more slowly than an unambiguously monomorphemic stimulus pair (e.g., belt-bell). This morpho-phonological effect was independent of phonological effects of voicing and syllabicity. The findings are interpreted as evidence for a basic morpho-phonological parsing process that applies to all items with the criterial phonological properties.

Symmetric abnormalities in sulcal patterning in schizophrenia

To compare the morphology of the cerebral cortex and its characteristic pattern of gyri and sulci in individuals with and without schizophrenia, T1-weighted magnetic resonance scans were collected, along with clinical and cognitive information, from 33 individuals with schizophrenia and 30 healthy individuals group-matched for age, gender, race and parental socioeconomic status. Sulcal depth was measured across the entire cerebral cortex by reconstructing surfaces of cortical mid-thickness (layer 4) in each hemisphere and registering them to the human PALS cortical atlas. Group differences in sulcal depth were tested using methods for cluster size analysis and interhemispheric symmetry analysis. A significant group difference was found bilaterally in the parietal operculum, where the average sulcal depth was shallower in individuals with schizophrenia. In addition, group differences in sulcal depth showed significant bilateral symmetry across much of the occipital, parietal, and temporal cortices. In individuals with schizophrenia, sulcal depth in the left hemisphere was correlated with the severity of impaired performance on tests of working memory and executive function.

Spatiotemporal signatures of large-scale synfire chains for speech processing as revealed by MEG

We report a new brain signature of memory trace activation in the human brain revealed by magnetoencephalography and distributed source localization. Spatiotemporal patterns of cortical activation can be picked up in the time course of source images underlying magnetic brain responses to speech and noise stimuli, especially the generators of the magnetic mismatch negativity. We found that acoustic signals perceived as speech elicited a well-defined spatiotemporal pattern of sequential activation of superior-temporal and inferior-frontal cortex, whereas the same identical stimuli, when perceived as noise, did not elicit temporally structured activation. Strength of local sources constituting large-scale spatiotemporal patterns reflected additional lexical and syntactic features of speech. Morphological processing of the critical sound as verb inflection led to particularly pronounced early left inferior-frontal activation, whereas the same sound functioning as inflectional affix of a noun activated superior-temporal cortex more strongly. We conclude that precisely timed spatiotemporal patterns involving specific cortical areas may represent a brain code of memory circuit activation. These spatiotemporal patterns are best explained in terms of synfire mechanisms linking neuronal populations in different cortical areas. The large-scale synfire chains appear to reflect the processing of stimuli together with the context-dependent perceptual and cognitive information bound to them.

Early decomposition in visual word recognition: Dissociating morphology, form, and meaning

The role of morphological, semantic, and form-based factors in the early stages of visual word recognition was investigated across different SOAs in a masked priming paradigm, focusing on English derivational morphology. In a first set of experiments, stimulus pairs co-varying in morphological decomposability and in semantic and orthographic relatedness were presented at three SOAs (36, 48, and 72 ms). No effects of orthographic relatedness were found at any SOA. Semantic relatedness did not interact with effects of morphological decomposability, which came through strongly at all SOAs, even for pseudo-suffixed pairs such as archer-arch. Derivational morphological effects in masked priming seem to be primarily driven by morphological decomposability at an early stage of visual word recognition, and are independent of semantic factors. A second experiment reversed the order of prime and target (stem-derived rather than derived-stem), and again found that morphological priming did not interact with semantic relatedness. This points to an early segmentation process that is driven by morphological decomposability and not by the structure or content of central lexical representations.

Longitudinal studies of semantic dementia: the relationship between structural and functional changes over time

The pattern of brain atrophy in semantic dementia and its associated cognitive effects have attracted a considerable body of research, but the nature of core impairments remains disputed. A key issue is whether the disease encompasses one neurocognitive network (semantics) or two (language and semantics). In order to address these conflicting perspectives, we conducted a longitudinal investigation of two semantic dementia patients, in which behavioural performance across a range of measures of language and semantic performance was assessed and interpreted in the context of annually acquired MRI scans. Our results indicated a core semantic impairment in early stages of the disease, associated with atrophy of the inferior, anterior temporal cortex. Linguistic impairments emerged later, and were contingent on atrophy having spread into areas widely believed to subserve core language processes (left posterior perisylvian, inferior frontal and insular cortex). We claim, therefore, that phonological, syntactic and morphological processing deficits in semantic dementia reflect damage to core language areas. Further, we propose that much of the current controversy over the nature of deficits in semantic dementia reflect a tendency in the literature to adopt a static perspective on what is a progressive disease. An approach in which the relationship between progressive neural changes and behavioural change over time is carefully mapped, offers a more constraining data-set from which to draw inferences about the relationship between language, semantics and the brain.

Are regular and irregular verbs dissociated in non-fluent aphasia?

The cognitive mechanisms and neuroanatomical substrates used by the brain to effortlessly generate morphologically complex words (write + ing --> writing) are little understood. The left inferior frontal gyrus (LIFG, including Broca's area) is often implicated as being involved, although its specific role is unclear. Data from brain damaged individuals, particularly those with Broca's aphasia, are often used as evidence to support or refute various theoretical perspectives. Typically, performance on two types of morphologically complex verbs, regulars (walk-walked, slip-slipped) and irregulars (sing-sang, sleep-slept) is contrasted for evidence of single or double dissociations. The question of how Broca's aphasic individuals dissociate in their production of inflectional morphology is important to our understanding of how the brain is organized to compute morphologically complex words. This article is a synthesis of research studies investigating the production of morphologically complex regular and irregular verbs in individuals with Broca's aphasia. The question being asked is if there is a robust and consistent dissociation, and if this dissociation can be tied to lesions of the left frontal lobe. This meta-analysis of 75 patients failed to show a single consistent dissociation pattern and over half the datasets had no significant difference between regulars and irregulars. There was also no relationship of any performance pattern to frontal lobe lesions, highlighting the difficulty of identifying any single neuroanatomical lesion for regular-irregular verb production deficits. The implications for various theoretical and neuroanatomical hypotheses are discussed. The role of neuropsychological dissociations in constraining hypothesis of normal neuroanatomical organization is evaluated.

From cognitive to neural models of working memory

Working memory refers to the temporary retention of information that was just experienced or just retrieved from long-term memory but no longer exists in the external environment. These internal representations are short-lived, but can be stored for longer periods of time through active maintenance or rehearsal strategies, and can be subjected to various operations that manipulate the information in such a way that makes it useful for goal-directed behaviour. Empirical studies of working memory using neuroscientific techniques, such as neuronal recordings in monkeys or functional neuroimaging in humans, have advanced our knowledge of the underlying neural mechanisms of working memory. This rich dataset can be reconciled with behavioural findings derived from investigating the cognitive mechanisms underlying working memory. In this paper, I review the progress that has been made towards this effort by illustrating how investigations of the neural mechanisms underlying working memory can be influenced by cognitive models and, in turn, how cognitive models can be shaped and modified by neuroscientific data. One conclusion that arises from this research is that working memory can be viewed as neither a unitary nor a dedicated system. A network of brain regions, including the prefrontal cortex (PFC), is critical for the active maintenance of internal representations that are necessary for goal-directed behaviour. Thus, working memory is not localized to a single brain region but probably is an emergent property of the functional interactions between the PFC and the rest of the brain.