Spatiotemporal dynamics of word processing in the human cortex

Understanding language relies on concurrent activation of multiple areas within a distributed neural network. Hemodynamic measures (fMRI and PET) indicate their location, and electromagnetic measures (magnetoencephalography and electroencephalography) reveal the timing of brain activity during language processing. Their combination can show the spatiotemporal characteristics (where and when) of the underlying neural network. Activity to written and spoken words starts in sensory-specific areas and progresses anteriorly via respective ventral ("what") processing streams toward the simultaneously active supramodal regions. The process of understanding a word in its current context peaks about 400 ms after word onset. It is carried out mainly through interactions of the temporal and inferior prefrontal areas on the left during word reading and bilateral temporo-prefrontal areas during speech processing. Neurophysiological evidence suggests that lexical access, semantic associations, and contextual integration may be simultaneous as the brain uses available information in a concurrent manner, with the final goal of rapidly comprehending verbal input. Because the same areas may participate in multiple stages of semantic or syntactic processing, it is crucial to consider both spatial and temporal aspects of their interactions to appreciate how the brain understands words.

Dorsal and ventral streams: a framework for understanding aspects of the functional anatomy of language

Despite intensive work on language-brain relations, and a fairly impressive accumulation of knowledge over the last several decades, there has been little progress in developing large-scale models of the functional anatomy of language that integrate neuropsychological, neuroimaging, and psycholinguistic data. Drawing on relatively recent developments in the cortical organization of vision, and on data from a variety of sources, we propose a new framework for understanding aspects of the functional anatomy of language which moves towards remedying this situation. The framework posits that early cortical stages of speech perception involve auditory fields in the superior temporal gyrus bilaterally (although asymmetrically). This cortical processing system then diverges into two broad processing streams, a ventral stream, which is involved in mapping sound onto meaning, and a dorsal stream, which is involved in mapping sound onto articulatory-based representations. The ventral stream projects ventro-laterally toward inferior posterior temporal cortex (posterior middle temporal gyrus) which serves as an interface between sound-based representations of speech in the superior temporal gyrus (again bilaterally) and widely distributed conceptual representations. The dorsal stream projects dorso-posteriorly involving a region in the posterior Sylvian fissure at the parietal-temporal boundary (area Spt), and ultimately projecting to frontal regions. This network provides a mechanism for the development and maintenance of "parity" between auditory and motor representations of speech. Although the proposed dorsal stream represents a very tight connection between processes involved in speech perception and speech production, it does not appear to be a critical component of the speech perception process under normal (ecologically natural) listening conditions, that is, when speech input is mapped onto a conceptual representation. We also propose some degree of bi-directionality in both the dorsal and ventral pathways. We discuss some recent empirical tests of this framework that utilize a range of methods. We also show how damage to different components of this framework can account for the major symptom clusters of the fluent aphasias, and discuss some recent evidence concerning how sentence-level processing might be integrated into the framework.

A category-specific advantage for numbers in verbal short-term memory: evidence from semantic dementia

This study explored possible reasons for the striking difference between digit span and word span in patients with semantic dementia. Immediate serial recall (ISR) of number and non-number words was examined in four patients. For every case, the recall of single-digit numbers was normal whereas the recall of non-number words was impaired relative to controls. This difference extended to multi-digit numbers, and remained even when frequency, imageability, word length, set size and size of semantic category were matched for the numbers and words. The advantage for number words also applied to the patients' reading performance. Previous studies have suggested that semantic memory plays a critical role in verbal short-term memory (STM) and reading: patients with semantic dementia show superior recall and reading of words that are still relatively well known compared to previously known but now semantically degraded words. Additional assessments suggested that this semantic locus was the basis of the patients' category-specific advantage for numbers. Comprehension was considerably better for number than non-number words. Number knowledge may be relatively preserved in semantic dementia because the cortical atrophy underlying the condition typically spares the areas of the parietal lobes thought to be crucial in numerical cognition but involves the inferolateral temporal-lobes known to support general conceptual knowledge.

The anatomy of semantic knowledge: medial vs. lateral temporal lobe

Semantic knowledge (e.g., long-established knowledge about objects, facts, and word meanings) is known to be severely impaired by damage to the anterolateral temporal lobe. For example, patients with semantic dementia have prominent atrophy in anterolateral temporal cortex and also have significant damage within the medial aspect of the temporal lobe. However, there is uncertainty about the contribution of medial temporal lobe damage, including perirhinal cortex damage, to impaired semantic knowledge. Drawing largely on published material from multiple sources, we compared the performance of severely amnesic patients with large medial temporal lobe lesions and patients with semantic dementia on nine tests of semantic knowledge and two tests of new learning ability. On the tests of semantic knowledge, the amnesic patients performed markedly better than the patients with semantic dementia. By contrast, on the tests of new learning, the patients with semantic dementia performed markedly better than the amnesic patients. We conclude that medial temporal lobe damage impairs the formation of declarative memory, and that semantic knowledge is impaired to the extent that damage extends laterally in the temporal lobe. Reports that the extent of atrophy in perirhinal cortex correlated with the severity of impaired semantic knowledge may be understood by supposing that the extent of damage in many temporal lobe areas is intercorrelated in this progressive disease, and that the extent of atrophy in perirhinal cortex is a proxy for the overall severity of dementia.

Dissociation of numbers and objects in corticobasal degeneration and semantic dementia

BACKGROUND

Semantic memory is thought to consist of category-specific representations of knowledge that may be selectively compromised in patients with neurodegenerative diseases, but this has been difficult to demonstrate reliably across object categories.

METHODS

The authors evaluated performance on several simple measures requiring number representations (including addition and magnitude judgments of single digits), and on a task that requires object representations (an object naming task) in patients with corticobasal degeneration (CBD; n = 13) and semantic dementia (SD; n = 15). They also examined regional cortical atrophy using voxel-based morphometric analyses of high resolution structural MRI in subgroups of five CBD patients and three SD patients.

RESULTS

CBD patients were consistently more impaired on simple addition and magnitude judgment tasks requiring number representations compared to object representations. Impaired performance with numbers in CBD was associated with cortical atrophy in right parietal cortex. By comparison, SD patients demonstrated a greater impairment on a naming task requiring object representations relative to their performance on measures involving number representations. This was associated with left anterior temporal cortical atrophy.

CONCLUSION

The cognitive and neuroanatomic dissociations between CBD and SD are consistent with the hypothesis that number and object representations constitute distinct domains in semantic memory, and these domains appear to be associated with distinct neural substrates.

Cognition and anatomy in three variants of primary progressive aphasia

We performed a comprehensive cognitive, neuroimaging, and genetic study of 31 patients with primary progressive aphasia (PPA), a decline in language functions that remains isolated for at least 2 years. Detailed speech and language evaluation was used to identify three different clinical variants: nonfluent progressive aphasia (NFPA; n = 11), semantic dementia (SD; n = 10), and a third variant termed logopenic progressive aphasia (LPA; n = 10). Voxel-based morphometry (VBM) on MRIs showed that, when all 31 PPA patients were analyzed together, the left perisylvian region and the anterior temporal lobes were atrophied. However, when each clinical variant was considered separately, distinctive patterns emerged: (1) NFPA, characterized by apraxia of speech and deficits in processing complex syntax, was associated with left inferior frontal and insular atrophy; (2) SD, characterized by fluent speech and semantic memory deficits, was associated with anterior temporal damage; and (3) LPA, characterized by slow speech and impaired syntactic comprehension and naming, showed atrophy in the left posterior temporal cortex and inferior parietal lobule. Apolipoprotein E epsilon4 haplotype frequency was 20% in NFPA, 0% in SD, and 67% in LPA. Cognitive, genetic, and anatomical features indicate that different PPA clinical variants may correspond to different underlying pathological processes.

Dissociating person-specific from general semantic knowledge: roles of the left and right temporal lobes

The cognitive architecture and neural underpinnings of different semantic domains remains highly controversial. We report two patients with focal temporal lobe atrophy who presented with contrasting and theoretically informative dissociations of person-specific versus general semantic knowledge. Subject J.P. showed severely impaired person-specific semantics, with relative preservation of knowledge about objects and animals, while subject M.A. exhibited the opposite pattern of performance (good knowledge of people in the context of impoverished general semantics). Voxel-based morphometric analysis of MR images in the two cases established predominantly right temporal atrophy associated with J.P.'s deficit for person knowledge and predominantly left temporal atrophy in M.A. who was impaired in general conceptual knowledge.

Deformation tensor morphometry of semantic dementia with quantitative validation

High-resolution structural MRI scans of 20 subjects diagnosed with semantic dementia were compared against scans of 20 cognitively normal control subjects using whole brain deformation tensor morphometry to study spatially consistent differences in local anatomical size. A fine lattice free-form volume registration algorithm was used to estimate a continuous mapping from a reference MRI to each individual subject MRI. The Jacobian of these transformations at each voxel were used to quantitatively map relative anatomical size in each individual brain. Intensity consistent filtering was applied to the determinant of these Jacobians. A careful validation using manually traced gyral anatomy was carried out and used to select an optimal deformation tensor filter scale at which to examine the anatomical size maps. General linear modeling at each voxel was used to decompose the influence of age and head size from the primary diagnosis. Maps of the T statistic of the diagnosis across the 40 subjects highlighted significant (P < 0.01 Bonferroni corrected) focal tissue contraction effects related to dementia diagnosis in the left temporal pole extending into the hippocampus, occipitotemporal gyrus and parahippocampal gyrus. Some evidence of greater focal contraction in gray over white matter was also apparent. Contraction effects were also seen, but with reduced significance in the right temporal anatomy, focused toward the temporal pole and hippocampal regions. Additional lower significance findings (P < 0.05 permutation corrected) were detected in the left superior frontal gyrus, left orbital gyrus and left parietal lobe.

Distinctive neuropsychological patterns in frontotemporal dementia, semantic dementia, and Alzheimer disease

OBJECTIVE

To assess the ability of a brief neuropsychological bedside screening battery to discriminate between Alzheimer disease, frontotemporal dementia, and semantic dementia.

METHODS

Subjects were 21 patients with frontotemporal dementia, 14 patients with semantic dementia, and 30 patients with Alzheimer disease comparable in terms of Mini Mental Status Examination score, age, and education. Frontotemporal dementia and semantic dementia diagnoses were made clinically using the consensus criteria of Neary et al. 1 Subjects were administered a brief neuropsychological screening assessing episodic memory, working memory, executive function, naming, spatial ability, abstract reasoning, and calculations.

RESULTS

Both the Alzheimer disease and semantic dementia groups were significantly impaired relative to the frontotemporal dementia group on verbal memory, whereas only the Alzheimer disease group was impaired on visual memory. Frontotemporal dementia patients performed significantly worse on backward digit span and made significantly more executive errors than Alzheimer disease and semantic dementia patients. Semantic dementia patients were more impaired than Alzheimer disease and frontotemporal dementia patients on confrontation naming. Discriminant function analyses identified the 5 most discriminating variables that correctly classified 89.2% of cases.

CONCLUSIONS

Frontotemporal dementia, semantic dementia, and Alzheimer disease are associated with distinct neuropsychological profiles that classify these dementia syndromes with considerable success. The neuropsychological profiles highlight the distinctiveness between the 3 syndromes, are consistent with the known loci of neuropathology in these conditions, and can potentially serve as an adjunct to the current clinical criteria.

Language processing: functional organization and neuroanatomical basis

Earlier formulations of the relation of language and the brain provided oversimplified accounts of the nature of language disorders, classifying patients into syndromes characterized by the disruption of sensory or motor word representations or by the disruption of syntax or semantics. More recent neuropsychological findings, drawn mainly from case studies, provide evidence regarding the various levels of representations and processes involved in single-word and sentence processing. Lesion data and neuroimaging findings are converging to some extent in providing localization of these components of language processing, particularly at the single-word level. Much work remains to be done in developing precise theoretical accounts of sentence processing that can accommodate the observed patterns of breakdown. Such theoretical developments may provide a means of accommodating the seemingly contradictory findings regarding the neural organization of sentence processing.

Humor modulates the mesolimbic reward centers

Humor plays an essential role in many facets of human life including psychological, social, and somatic functioning. Recently, neuroimaging has been applied to this critical human attribute, shedding light on the affective, cognitive, and motor networks involved in humor processing. To date, however, researchers have failed to demonstrate the subcortical correlates of the most fundamental feature of humor-reward. In an effort to elucidate the neurobiological substrate that subserves the reward components of humor, we undertook a high-field (3 Tesla) event-related functional MRI study. Here we demonstrate that humor modulates activity in several cortical regions, and we present new evidence that humor engages a network of subcortical regions including the nucleus accumbens, a key component of the mesolimbic dopaminergic reward system. Further, the degree of humor intensity was positively correlated with BOLD signal intensity in these regions. Together, these findings offer new insight into the neural basis of salutary aspects of humor.

Evaluation of voxel-based morphometry for focal lesion detection in individuals

Voxel-based morphometry (VBM) is an automated statistical technique used to detect regional differences in tissue density and tissue amount based on spatially standardized structural magnetic resonance (MR) images. Developed initially to discern differences between groups of subjects, VBM is now being used to characterize structural abnormalities in individual brains. While VBM performance has been qualitatively assessed for this purpose, to date no quantitative validation study has been performed. This study evaluated several commonly used variants of VBM for detecting structural differences at the individual level by assessing their performance in MR images of 10 subjects with stable focal brain lesions. Results were quantitatively compared to expert tracings of the lesions, the current gold standard for lesion detection and delineation. Additionally, analyses using two sets of simulated lesion data were performed to examine the relative impact of the underlying processing steps on VBM results. Performance metrics revealed that (1) for this application, VBM had low sensitivity; (2) detection sensitivity was altered by model parameterization; (3) underperformance was due to the adverse influence of lesions on the preprocessing steps and to insufficient statistical power; and (4) VBM could not satisfactorily delineate the spatial extent of lesions, even in simulations that avoided preprocessing artifacts. In its current form, VBM is not a suitable stand-alone technique for detecting or spatially characterizing focal lesions.

Cognitive and Behavioral Profile in a Case of Right Anterior Temporal Lobe Neurodegeneration

Semantic dementia (SD) is a clinical variant of frontotemporal lobar degeneration (FTLD) characterized by progressive deterioration of semantic memory with relative sparing of other cognitive functions. It is associated with mainly left anterior temporal atrophy, and is also referred to as "left-temporal lobe variant" of FTLD. Recently, patients with mainly right-sided atrophy, or "right-temporal lobe variant"(RTLV), have been described. While some authors have reported that the initial and most significant deficit in these right-sided cases is a difficulty in recognizing famous people, others have observed that major behavioral abnormalities are the presenting symptoms. Here we report a detailed neuropsychological, language, behavioral and neuroimaging assessment of JT, a case of right temporal lobe variant of FTLD. JT showed early and prominent behavioral changes accompanied by a severe impairment in recognizing foods by their look, flavor or name. Later she also developed a difficulty in recognizing familiar people and objects. Standardized caregiver questionnaires of JT's pre- and post-morbid personality and interpersonal functioning showed that she went from being a flexible, dominant, extraverted, person to showing rigid, submissive and introverted behaviors. Her levels of neuroticism significantly increased, while her scores on agreeableness and cognitive and emotional empathy dropped. Voxel-based morphometry (VBM) showed most significant atrophy in the right amygdala/anterior hippocampal complex and collateral sulcus, extending to the right insula. We discuss the atypical cognitive and behavioral features of this case of RTLV of FTLD and stress the importance of behavioral changes and atypical semantic deficits for early diagnosis.

Deterioration of naming nouns versus verbs in primary progressive aphasia

Disproportionate impairment of naming nouns versus verbs and the opposite pattern have been reported in cases of focal brain damage or degenerative disease, indicating that processing of nouns and verbs may rely on different brain regions. However, it has not been clear whether it is the spoken word forms or the meanings (or both) of nouns and verbs that depend on separate neural regions. We tested oral and written naming of nouns and verbs, matched in difficulty, in patients with nonfluent primary progressive aphasia (nonfluent PPA; n = 15), fluent primary progressive aphasia (fluent PPA; n = 7), and amyotrophic lateral sclerosis with frontotemporal dementia (ALS-FTD; n = 6). Patients with nonfluent PPA and ALS-FTD, both individually and as groups, were significantly more impaired on verb naming than on noun naming and significantly more impaired on oral naming than written naming. Patients with fluent PPA showed the opposite pattern for both word class and modality, significantly more impaired naming of nouns versus verbs and significantly more impaired written versus oral naming. Results indicate that separate regions of the brain are essential for access to oral and written word forms of verbs and nouns, and that these neural regions can be differentially damaged in separate forms of PPA.

Structure and Deterioration of Semantic Memory: A Neuropsychological and Computational Investigation

Wernicke (1900, as cited in G. H. Eggert, 1977) suggested that semantic knowledge arises from the interaction of perceptual representations of objects and words. The authors present a parallel distributed processing implementation of this theory, in which semantic representations emerge from mechanisms that acquire the mappings between visual representations of objects and their verbal descriptions. To test the theory, they trained the model to associate names, verbal descriptions, and visual representations of objects. When its inputs and outputs are constructed to capture aspects of structure apparent in attribute-norming experiments, the model provides an intuitive account of semantic task performance. The authors then used the model to understand the structure of impaired performance in patients with selective and progressive impairments of conceptual knowledge. Data from 4 well-known semantic tasks revealed consistent patterns that find a ready explanation in the model. The relationship between the model and related theories of semantic representation is discussed.

Alzheimer’s disease and frontotemporal dementia exhibit distinct atrophy-behavior correlates

RATIONALE AND OBJECTIVES

The purpose of this study was to test the hypothesis that distinct patterns of gray matter atrophy are responsible for unique interruptions of the naming process in Alzheimer's disease (AD) and frontotemporal dementia (FTD).

MATERIALS AND METHODS

Voxel-based morphometry (VBM) was performed to characterize at the voxel level the neuroanatomic changes that occur in AD and FTD based on high-resolution T1-weighted three-dimensional (3D) spoiled-gradient echo images of patients (AD, n = 12; FTD, n = 29) and healthy control subjects (n = 12). The cortical atrophy measurements were correlated with performance on behavioral measures of naming and related processes to identify brain regions that may contribute to this language function.

RESULTS

Both AD and FTD have significant naming difficulty, and this difficulty in naming correlates with a measure of lexical retrieval in both patient groups as well. However, only FTD patients showed a correlation with semantic memory. Areas of cortical atrophy common to AD and FTD were found in the anterior temporal, posterolateral temporal, and dorsolateral prefrontal regions of the left hemisphere. Correlation with naming in both AD and FTD was seen in the left anterior temporal cortex, suggesting that this area may play a role in the lexical retrieval component of naming. We also observed several unique areas of cortical atrophy in temporal and frontal cortices of these patients. Right anterior temporal and left posterolateral temporal regions of atrophy correlated with naming difficulty in FTD, suggesting that these areas may contribute to the semantic memory component of naming. Cortical areas correlating with naming that are not atrophic may represent regions that play an optional role in naming.

CONCLUSION

VBM provides an important first step in analyzing brain-behavior relations in vivo in patients with neurodegenerative diseases. More refined analyses of brain morphology via high-dimensional normalization methods that are capable of modeling local as well as global variability in neuroanatomical structure promise to be even more informative.

Medial temporal lobe activation during semantic language processing: fMRI findings in healthy left- and right-handers

Medial temporal lobe (MTL) areas are well known to serve episodic memory functions; their contribution to semantic memory has been occasionally noticed but not studied in detail. In the present fMRI study, 35 right-handed and 35 left-handed healthy subjects performed a semantic decision paradigm during which subjects heard spoken concrete nouns designating objects and had to decide on whether these objects were available in the supermarket and cost lest then a certain amount of money. The control paradigm consisted of sequences of low and high tones where subjects had to decide whether a sequence contained two high tones. The resulting contrast activation of semantic decision versus tone decision involved neocortical temporal, parietal, and prefrontal areas. Additional significant, bilateral activations in the MTL, the hippocampal formation, and adjacent areas were found. The exact incidence and location of activation was studied in a single-subject analysis for all 70 subjects. At the chosen threshold of P<0.001, 94% of subjects showed activations in the MTL and inferior temporal lobe (ITL). Activations were found along the longitudinal axis of the MTL, including the hippocampal formation and the parahippocampal gyrus. In the ITL, parts of the fusiform and lingual gyri were activated. Activations were similar in right- and left-handers. We conclude from this study that the MTL and parts of the ITL can be added to the areas activated by semantic verbal memory processing.

Category-specific medial temporal lobe activation and the consolidation of semantic memory: evidence from fMRI

Semantic memory consolidation was studied by comparing medial temporal lobe (MTL) fMRI activation to ANIMAL, IMPLEMENT and ABSTRACT nouns in healthy seniors to that of young adults. Relative to healthy seniors, young adults were predicted to show greater MTL activation for IMPLEMENTS, but not ANIMALS, because the ANIMALS category consists of highly intercorrelated and overlapping features that should require less MTL-mediated binding than IMPLEMENTS over a shorter period of time during concept consolidation. ABSTRACT meanings are context-dependent and do not consist of fixed feature sets. Thus it was predicted that ABSTRACT words would not involve age-related feature binding mediated by the MTL. These predictions were confirmed by the results. Our observations are consistent with the hypothesis that the structure of a category influences the consolidation of knowledge in semantic memory.

Neural basis for sentence comprehension deficits in frontotemporal dementia

Many patients with frontotemporal dementia (FTD) have impaired sentence comprehension. However, the pattern of comprehension difficulty appears to vary depending on the clinical subgroup. The purpose of this study was to elucidate the neural basis for these deficits in FTD. We studied patients with two different presentations: Three patients with Progressive Non-Fluent Ahasia (PNFA), and five non-aphasic patients with a dysexecutive and social impairment (EXEC). The FTD patient subgroups were compared to a cohort of 11 healthy seniors with intact sentence comprehension. We monitored regional cerebral activity with blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) while subjects read sentences featuring both a grammatically complex object-relative center-embedded clause and a long linkage between the head noun phrase (NP) and the gap where the NP is interpreted in the center-embedded clause. Subjects decided whether the agent of the action is a male or a female. Healthy seniors activated both ventral portions of inferior frontal cortex (vIFC) and dorsal portions of IFC (dIFC) in the left hemisphere, often associated with grammatical and working memory components of these sentences, respectively. PNFA patients differed from healthy controls since they have reduced activation of left vIFC, while EXEC patients have less recruitment of left dIFC. We conclude that FTD subgroups have distinct patterns of sentence comprehension difficulty in part because of selective interruptions of a large-scale neural network for sentence processing.

Temporal lobe regions engaged during normal speech comprehension

Processing of speech is obligatory. Thus, during normal speech comprehension, the listener is aware of the overall meaning of the speaker's utterance without the need to direct attention to individual linguistic and paralinguistic (intonational, prosodic, etc.) features contained within the speech signal. However, most functional neuroimaging studies of speech perception have used metalinguistic tasks that required the subjects to attend to specific features of the stimuli. Such tasks have demanded a forced-choice decision and a motor response from the subjects, which will engage frontal systems and may include unpredictable top-down modulation of the signals observed in one or more of the temporal lobe neural systems engaged during speech perception. This study contrasted the implicit comprehension of simple narrative speech with listening to reversed versions of the narratives: the latter are as acoustically complex as speech but are unintelligible in terms of both linguistic and paralinguistic information. The result demonstrated that normal comprehension, free of task demands that do not form part of everyday discourse, engages regions distributed between the two temporal lobes, more widely on the left. In particular, comprehension is dependent on anterolateral and ventral left temporal regions, as suggested by observations on patients with semantic dementia, as well as posterior regions described in studies on aphasic stroke patients. The only frontal contribution was confined to the ventrolateral left prefrontal cortex, compatible with observations that comprehension of simple speech is preserved in patients with left posterior frontal infarction.

Spatiotemporal maps of past-tense verb inflection

Does the brain inflect verbs by applying rules, by associative retrieval of the inflected form, or both? We used whole-head magnetoencephalography to spatiotemporally map the brain response underlying verb past-tense inflection. Placing either regular or irregular verbs into the past tense sequentially modulates the bilateral visual, left inferotemporal, posterior superior temporal (Wernicke's area), left inferior prefrontal (Broca's area), and right prefrontal cortices. Although irregular and regular verb inflection evokes similar cortical response patterns, differences in specific frontotemporal regions are observed. At approximately 340 ms, irregular verbs evoke greater response modulation in left occipitotemporal cortex. This modulation occurs when widespread areas are simultaneously active, suggesting that it reflects associative activation necessary for generation of past-tense forms. Subsequently, regular verbs show increased response at approximately 470 ms within left inferior prefrontal regions associated with rule-based inflection. Increased right dorsolateral prefrontal response at approximately 570 ms may represent directed/effortful retrieval of irregular past-tense forms. Thus, the brain inflects verbs by dynamically modulating different functional divisions of an integrated language system.

Changes in cerebral morphology consequent to peripheral autonomic denervation

Pure autonomic failure (PAF) is characterized by an acquired, selective, peripheral denervation of the autonomic nervous system. Patients with PAF fail to generate bodily states of arousal via the autonomic nervous system in response to physical or cognitive effort. We used voxel-based morphometry to test the hypothesis that changes in the morphology of brain regions involved in autonomic control would arise as a consequence to the longstanding absence of peripheral autonomic responses in PAF patients. Optimized voxel-based morphometry of structural magnetic resonance scans was used to test for regional differences in grey and white matter in 15 PAF patients and matched controls. There were no group differences observed in global measures of grey matter, white matter, or cerebrospinal fluid (CSF). We identified morphometric differences reflecting regional decreases in grey matter volume and concentration in anterior cingulate and insular cortices in PAF patients relative to controls. Morphometric differences in brainstem and subcortical regions did not reach statistical significance. Our findings suggest that peripheral autonomic denervation is associated with grey matter loss in cortical regions encompassing areas that we have previously shown are functionally involved in generation and representation of bodily states of autonomic arousal. The nature of these changes cannot be determined from morphometric analysis alone, but we suggest that they reflect experience-dependent change consequent upon loss of afferent input to brain regions involved in representation of autonomic states.

Semantic knowledge in patient H.M. and other patients with bilateral medial and lateral temporal lobe lesions

We investigated the effects of damage to the medial temporal lobe (MTL) and anterolateral temporal cortex on semantic knowledge. We studied eight male controls, two patients with lesions limited to the hippocampal formation, three postencephalitic patients with extensive MTL lesions and variable damage to the lateral temporal cortex, and patient H.M. (whose lesion is limited mostly to the MTL, but who also has minimal damage to the anterolateral cortex). On 13 tests of semantic memory, patients with lesions limited to the hippocampal formation performed similarly to controls. Postencephalitic patients were mildly to moderately impaired on most tests. Patient H.M.'s performance was impaired on only a few tests and was less severely impaired overall than the three postencephalitic patients. A ranking of test scores showed a direct relationship between impairment and the extent of damage to lateral temporal cortex. These findings, and related findings from other studies, point to the importance of anterolateral temporal cortex for semantic knowledge. Patient H.M. performed uniquely in certain respects. For example, when providing definitions of objects, he made many grammatical errors. In contrast, the other patients with large MTL lesions made no more errors than those made by controls. Considering that H.M.'s lesion, both medially and laterally, is less extensive than the lesions in these other patients, it appears unlikely that his shortcomings in language production are related to his temporal lobe lesion.

A voxel-based morphometry study of temporal lobe gray matter reductions in Alzheimer's disease

Several MRI studies have reported reductions in temporal lobe volumes in Alzheimer's disease (AD). Measures have been usually obtained with regions-of-interest (ROI) drawn manually on selected medial and lateral portions of the temporal lobes, with variable choices of anatomical borders across different studies. We used the fully automated voxel-based morphometry (VBM) approach to investigate gray matter abnormalities over the entire extension of the temporal lobe in 14 AD patients (MMSE 14-25) and 14 healthy controls. Foci of significantly reduced gray matter volume in AD patients were detected in both medial and lateral temporal regions, most significantly in the right and left posterior parahippocampal gyri and the left posterior inferior temporal gyrus/fusiform gyrus (P<0.05, corrected for multiple comparisons). At a more flexible statistical threshold (P<0.001, uncorrected for multiple comparisons), circumscribed foci of significant gray matter reduction were also detected in the right amygdala/enthorinal cortex, the anterior and posterior borders of the superior temporal gyrus bilaterally, and the anterior portion of the left middle temporal gyrus. These VBM results confirm previous findings of temporal lobe atrophic changes in AD, and suggest that these abnormalities may be confined to specific sites within that lobe, rather than showing a widespread distribution.

The neuroanatomical and functional organization of speech perception

A striking property of speech perception is its resilience in the face of acoustic variability (among speech sounds produced by different speakers at different times, for example). The robustness of speech perception might, in part, result from multiple, complementary representations of the input, which operate in both acoustic-phonetic feature-based and articulatory-gestural domains. Recent studies of the anatomical and functional organization of the non-human primate auditory cortical system point to multiple, parallel, hierarchically organized processing pathways that involve the temporal, parietal and frontal cortices. Functional neuroimaging evidence indicates that a similar organization might underlie speech perception in humans. These parallel, hierarchical processing 'streams', both within and across hemispheres, might operate on distinguishable, complementary types of representations and subserve complementary types of processing. Two long-opposing views of speech perception have posited a basis either in acoustic feature processing or in gestural motor processing; the view put forward here might help reconcile these positions.

No change in the structure of the brain in migraine: a voxel-based morphometric study

Migraine is a common, disabling form of primary neurovascular headache. For most of the twentieth century it was regarded as a vascular headache whose primary pathophysiology lay in the cranial vasculature. Functional brain imaging using positron emission tomography has demonstrated activation of the rostral brain stem in acute migraine. Voxel-based morphometry is a new fully automated whole brain technique that is sensitive to subtle macroscopic and mesoscopic structural differences between groups of subjects. In this study 11 patients suffering from migraine with aura (10 females, one male: 23-52 years, mean 31); 11 controls (10 females, one male: 23-52, mean 31); 17 patients with migraine without aura (16 females, one male: 24-57, mean 34); 17 controls (16 females, one male: 24-57, mean 34) were imaged with high resolution volumetric magnetic resonance imaging. There was no significant difference in global grey or white matter volumes between either patients with migraine and controls, or patients with aura and without aura. This study did not show any global or regional macroscopic structural difference between patients with migraine and controls, with migraine sufferers taken as homogenous groups. If structural changes are to be found, other methods of phenotyping migraine, such as by genotype or perhaps treatment response, may be required to resolve completely whether there is some subtle structural change in the brain of patients with migraine.

Advanced brain imaging procedures and human memory disorder

The impact of advanced brain imaging procedures in the field of human memory disorder is reviewed, with particular emphasis on current and potential applications that may impact upon the diagnosis and management of memory-disordered patients. While both advanced structural, resting physiological and functional physiological brain imaging procedures have been applied to conditions where memory disorder is a major feature, the specific implications of research findings for diagnosis and treatment in routine clinical practice remain tentative and promising, but not yet substantive enough to inform clinical decisions to a significant degree. In terms of diagnostic applications, several promising areas include dementia, epilepsy, and transient amnesic states. In the case of applications in treatment settings, advanced brain imaging procedures may help to monitor neural correlates of spontaneous recovery or progression of memory function, and may also help in the planning and monitoring of therapeutic intervention.

Primary progressive aphasia: PPA and the language network

Primary Progressive Aphasia (PPA) is a behaviorally focal dementia syndrome with deterioration of language functions but relative preservation of other cognitive domains for at least the first two years of disease. In this study, PPA patients with impaired word finding but intact comprehension of conversational speech and their matched control subjects were examined using voxel-based morphometry (VBM) and functional magnetic resonance imaging (fMRI). fMRI compared signal changes during phonological and semantic language tasks with those during a control task (matching letters). PPA patients showed longer reaction times and reduced accuracy versus controls on the language tasks, but no performance differences on the control task. VBM demonstrated reduced gray matter in left superior temporal and inferior parietal regions in the PPA group. However, these patients showed a normal pattern of activation within the classical language regions. In addition, PPA patients showed activations, not seen in normals, in fusiform gyrus, precentral gyrus, and intra-parietal sulcus. These activations were found to correlate negatively with measures of naming and task performance. The additional activations in PPA may therefore represent a compensatory spread of language-related neural activity or a failure to suppress activity in areas normally inhibited during language tasks.

Patterns of cerebral atrophy in dementia with Lewy bodies using voxel-based morphometry

Previous cross-sectional MRI studies based on region-of-interest analyses have shown that increased cerebral atrophy is a feature of both Dementia with Lewy bodies (DLB) and Alzheimer's disease (AD). Relative preservation of the hippocampus and temporal lobe structures in DLB compared to AD has been reported in region-of-interest-based studies. Recently, image processing techniques such as voxel-based morphometry (VBM) have been developed to provide an unbiased, visually informative, and comprehensive means of studying patterns of cerebral atrophy. We report the first study to use the voxel-based approach to assess patterns of cerebral atrophy in DLB compared to control subjects and AD. Regional gray matter volume loss was observed bilaterally in the temporal and frontal lobes and insular cortex of patients with DLB compared to control subjects. Comparison of dementia groups showed preservation of the medial temporal lobe, hippocampus, and amygdala in DLB relative to AD. Significant gray matter loss was also observed in the thalamus of AD patients compared to DLB.

When action turns into words. Activation of motor-based knowledge during categorization of manipulable objects

Functional imaging studies have demonstrated that processing of man-made objects activate the left ventral premotor cortex, which is known to be concerned with motor function. This has led to the suggestion that the comprehension of man-made objects may rely on motor-based knowledge of object utilization (action knowledge). Here we show that the left ventral premotor cortex is activated during categorization of "both" fruit/vegetables and articles of clothing, relative to animals and nonmanipulable man-made objects. This observation suggests that action knowledge may not be important for the processing of man-made objects per se, but rather for the processing of manipulable objects in general, whether natural or man-made. These findings both support psycholinguistic theories suggesting that certain lexical categories may evolve from, and the act of categorization rely upon, motor-based knowledge of action equivalency, and have important implications for theories of category specificity. Thus, the finding that the processing of vegetables/fruit and articles of clothing give rise to similar activation is difficult to account for should knowledge representations in the brain be truly categorically organized. Instead, the data are compatible with the suggestion that categories differ in the weight they put on different types of knowledge.

Mapping gray matter loss with voxel-based morphometry in mild cognitive impairment

We used voxel-based morphometry to comprehensively and objectively map gray matter loss in 22 patients with amnestic mild cognitive impairment, defined as progressive isolated episodic memory decline. Compared with 22 age-matched healthy controls, patients had highly significant gray matter loss predominantly affecting the hippocampal region and cingulate gyri (posterior and subcallosal part of the anterior), and extending into the temporal neocortex. Compared with 16 age-matched patients with mild Alzheimer's disease, gray matter density was significantly preserved in mild cognitive impairment in the posterior association cortex. This pattern of gray matter loss in mild cognitive impairment agrees with but considerably expands upon previous region-of-interest based MRI studies, and is highly consistent with the course of neurofibrillary tangles across aging and Alzheimer's disease.

Relearning of verbal labels in semantic dementia

Semantic dementia is a degenerative disorder of temporal neocortex characterised by loss of word and object concepts. There is limited evidence that temporary relearning of lost vocabulary may be possible, attributed to sparing of hippocampal structures. However, learning is variable across patients and factors underlying learning success are poorly understood. The study investigated relearning of object names in two severely anomic semantic dementia patients. Following memory models that assume that hippocampal memories require some neocortical representation to underpin them it was predicted that relearning would be influenced by patients' residual semantic information about stimuli. Experiment 1 confirmed that residual knowledge influenced learning success. On the assumption that neocortical knowledge encompasses concepts of space and time, as well as words and objects, it was predicted that learning would be affected by the availability of contextual (temporo-spatial) information. Experiment 2 demonstrated effective learning of object names, attributed to the patient's use of temporal order and spatial position knowledge. Retention of object names over months was linked to the patient's capacity for autobiographical experiential (temporo-spatial contextual) association. The findings indicate that relearning of lost vocabulary is possible in semantic dementia, indicating a role of the medial temporal lobes in the acquisition of semantic information. Effective learning does not imply reinstatement of lost concepts, but, it is argued, does involve some reacquisition of meaning. The findings challenge the traditional semantic-episodic memory dichotomy and are consistent with a "levels of meaning" account of semantic memory.

Degeneracy and cognitive anatomy

Cognitive models indicate that there are multiple ways of completing the same task. This implicit degeneracy cannot be revealed by functional imaging studies of normal subjects if more than one of the sufficient neural systems is activated. Nor can it be detected by neuropsychological studies of patients because their performance might not be impaired when only one degenerate system is damaged. We propose that degenerate sets of sufficient neural systems can only be identified by an iterative approach that integrates the lesion-deficit model and functional imaging studies of normal and neurologically damaged subjects.

Progressive aphasic syndromes: clinical and theoretical advances

PURPOSE OF REVIEW

Knowledge of the neural basis for language and related aspects of cognition has been advanced through detailed studies of patients with primary progressive aphasia. This brief review highlights some recent work.

RECENT FINDINGS

The impairment of semantic knowledge in patients with semantic dementia appears to influence performance in a wide variety of linguistic and cognitive domains, including morphological agreements such as the irregular past tense. Computational studies modeling the deficits of these patients have advanced interpretations of the impairments in semantic dementia. Imaging analyses have confirmed the presence of temporal atrophy cross-sectionally and longitudinally in these patients. In patients with semantic dementia, it appears that both the left temporal and right temporal regions contribute in different proportions to naming and comprehension, although the nature of the process underlying the consolidation of knowledge in semantic memory continues to be actively debated. In patients with progressive non-fluent aphasia, recent work has emphasized an impairment with verbs. Functional neuroimaging work with progressive non-fluent aphasics, compared directly to non-aphasic patients with frontotemporal dementia, has demonstrated a dissociation for grammatical and working memory aspects of sentence processing within the left frontal cortex.

SUMMARY

These findings will improve diagnostic accuracy, prognostic ability, and therapeutic potential in patients with progressive aphasia.

Voxel-based morphometric comparison of hippocampal and extrahippocampal abnormalities in patients with left and right hippocampal atrophy

We used voxel-based morphometry (VBM), an automatic whole-brain MR image analysis technique, to investigate gray matter abnormalities in patients with temporal lobe epilepsy (TLE), in whom hippocampal atrophy (HA) was demonstrated by application of the Cavalieri method of modern design stereology. VBM results (P < 0.05, corrected) indicated preferential gray matter concentration (GMC) reduction in anterior hippocampus in patients with left HA and posterior hippocampus in patients with right HA. GMC reduction was also found in right dorsal prefrontal cortex in left and right HA patients. Prefrontal atrophy may be due to epileptiform excitotoxic discharges from the reciprocally connected pathological hippocampus, and may be the underlying biological cause for executive dysfunction in patients with TLE. GMC excess in ipsilateral parahippocampal, cerebellar, and pericallosal regions was common to both left and right HA groups relative to controls, and is hypothesized to reflect diminished gray-white matter demarcation, underlying white matter atrophy, or structural displacement due to cerebrospinal fluid expansion. However, bilateral temporal lobe GMC excess was observed in left HA patients, while ipsilateral temporal lobe GMC excess was observed in right HA patients. This work demonstrates methodological consistency between automated VBM and manual stereological analysis of the hippocampus in group comparisons, indicates widespread extrahippocampal gray matter abnormalities in unilateral HA, and suggests that there may be inherent differences in the effect of TLE on temporal lobe structures depending on the side of HA.

Relating semantic and episodic memory systems

Episodic and semantic memory are two forms of declarative memory which appear to function in distinct yet interdependent ways. Here we provide direct evidence for a functional relationship between these two memory systems by showing that left lateral temporal lobe regions involved in semantic memory play an important role in accurate episodic memory retrieval.

Verbal memory in left temporal lobe epilepsy: evidence for task-related localization

We explored the hypothesis that components of verbal memory are subserved by separate temporal lobe structures in patients with temporal lobe structures in patients with temporal lobe epilepsy [correction]. Uptake of 18F-fluorodeoxyglucose (FDG) measured by positron emission tomography, hippocampal volume, and memory for arbitrarily and semantically related verbal paired associates were examined in 27 patients with left temporary lobe epilepsy. Scores from memory tests performed outside the scanner were regressed against normalized resting FDG uptake at each voxel. Significant regression was seen in the left perirhinal cortex (Talaraich coordinates x, y, z: -29, 10, -34; p < 0.05) for arbitrarily related word pairs. For semantically related paired associates, significant regression was present in the left inferior temporal gyrus (x, y, z: -48, -18, -24; p < 0.05). In subsequent analyses, mean FDG uptake within a spherical region of interest centered on the perirhinal peak predicted performance on both tasks. Mean FDG uptake in a region of interest centered on the inferior temporal peak made an additional, independent contribution to memory for semantically related pairs. Hippocampal volumes did not explain any additional variance in memory scores. Our findings indicate that heterogeneity in the left temporal lobe structures mediating verbal memory function, and support the view that the perirhinal cortex is an important mnemonic substrate.

Memory consolidation and the hippocampus: further evidence from studies of autobiographical memory in semantic dementia and frontal variant frontotemporal dementia

Studies of autobiographical memory in semantic dementia have found relative preservation of memories for recent rather than remote events. As semantic dementia is associated with progressive atrophy to temporal neocortex, with early asymmetric sparing of the hippocampus, this neuropsychological pattern suggests that the hippocampal complex plays a role in the acquisition and retrieval of recent memories, but is not necessary for the recall of older episodic events. In an alternative view of memory consolidation, however, the hippocampus plays a role in the retrieval of all autobiographical memories, regardless of the age of the memory [Curr. Opin. Neurobiol. 7(1997)217]. This 'multiple trace theory' predicts that patients with semantic dementia should show no effects of time in their autobiographical recall. In this article, we ask whether it is possible to reconcile the data from semantic dementia with the multiple trace theory by investigating whether the time-dependent pattern of autobiographical retrieval seen in the disease is due to (i) patients showing this effect being exceptional in their presentation; and/or (ii) patients with semantic dementia exhibiting impaired strategic retrieval from concomitant frontal damage. A series of experiments in patients with semantic dementia, the frontal variant of frontotemporal dementia and Alzheimer's disease clearly demonstrates that neither of these two factors can explain the documented effect of time seen in semantic dementia. Nonetheless, we discuss how damage to semantic knowledge could result in an autobiographical memory deficit and suggest that data from semantic dementia may be consistent with both views of hippocampal involvement in long-term memory.

Patterns of brain atrophy in frontotemporal dementia and semantic dementia

OBJECTIVE

To identify and compare the patterns of cerebral atrophy associated with two clinical variants of frontotemporal lobar degeneration (FTLD): frontotemporal dementia (FTD) and semantic dementia (SemD).

METHODS

Twenty patients with FTLD were classified as having FTD (N = 8) or SemD (N = 12) based on current clinical criteria. Both groups showed a similar spectrum of behavioral abnormalities, as indicated by the neuropsychiatric inventory. T1-weighted MRI was obtained for each patient and 20 control subjects. The regions of focal gray matter tissue loss associated with both FTD and SemD, as well as those differing between the two groups were examined using voxel-based morphometry.

RESULTS

Regions of significant atrophy seen in both groups were located in the ventromedial frontal cortex, the posterior orbital frontal regions bilaterally, the insula bilaterally, and the left anterior cingulate cortex. The FTD, but not the SemD, group showed atrophy in the right dorsolateral frontal cortex and the left premotor cortex. The SemD, but not the FTD, group showed tissue loss in the anterior temporal cortex and the amygdala/anterior hippocampal region bilaterally.

CONCLUSIONS

Although FTD and SemD are associated with different overall patterns of brain atrophy, regions of gray matter tissue loss in the orbital frontal, insular, and anterior cingulate regions are present in both groups. The authors suggest that pathology in the areas of atrophy associated with both FTD and SemD may underlie some the behavioral symptoms seen in the two disorders.

Object naming and semantic knowledge in temporal lobe epilepsy

Object-naming impairment is common among temporal lobe epilepsy (TLE) patients, but other aspects of semantic memory have received limited attention in this population. This study examined object-naming ability and depth of semantic knowledge in healthy controls (n = 29) and patients with early onset TLE (n = 21). After administration of the Boston Naming Test (BNT), the authors asked participants to provide detailed definitions of 6 BNT objects. The TLE group demonstrated a significant deficit relative to controls in both object-naming ability and semantic knowledge for the target objects, even after controlling for IQ. In a multiple regression analysis that included other neuropsychological test scores as independent variables, the semantic knowledge score was the only significant predictor of patients' object-naming performance. Thus, at the group level, early onset TLE patients have a semantic knowledge deficit that contributes to dysnomia.

Episodic memory: insights from semantic dementia

Semantic dementia, also known as the temporal lobe variant of fronto-temporal dementia, results in a progressive yet relatively pure loss of semantic knowledge about words, objects and people, and is associated with asymmetric, focal atrophy of the antero-lateral temporal lobes. Semantic dementia provides a unique opportunity to study the organization of long-term memory particularly since initial observations suggested sparing of episodic memory. Recent studies reveal, however, a more complex but theoretically revealing pattern. On tests of autobiographical memory, patients with semantic dementia show a 'reverse step function' with sparing of recall of events from the most recent 2 to 5 years but impairment on more distant life periods. Anterograde recognition memory for visual materials is extremely well preserved, except in the most deteriorated cases, although performance is heavily reliant upon perceptual information about the studied stimuli, particularly for items that are no longer known by the subjects. On tests of verbal anterograde memory such as word learning, performance is typically poor even for words which are 'known' to the patients. A source discrimination experiment, designed to evaluate familiarity and recollection-based anterograde memory processes, found that patients with semantic dementia showed good item detection, although recollection of source was sometimes impaired. Semantic knowledge about studied items and measures of item detection and source discrimination were largely independent. The implications of these findings for models of long-term memory are discussed. The results support the concept that episodic memory, or at least the recall of temporally specific autobiographical experiences, draws upon a number of separable memory processes, some of which can function independently of semantic knowledge.

A role for left temporal pole in the retrieval of words for unique entities

Both lesion and functional imaging studies have implicated sectors of high-order association cortices of the left temporal lobe in the retrieval of words for objects belonging to varied conceptual categories. In particular, the cortices located in the left temporal pole have been associated with naming unique persons from faces. Because this neuroanatomical-behavioral association might be related to either the specificity of the task (retrieving a name at unique level) or to the possible preferential processing of faces by anterior temporal cortices, we performed a PET imaging experiment to test the hypothesis that the effect is related to the specificity of the word retrieval task. Normal subjects were asked to name at unique level entities from two conceptual categories: famous landmarks and famous faces. In support of the hypothesis, naming entities in both categories was associated with increases in activity in the left temporal pole. No main effect of category (faces vs. landmarks/buildings) or interaction of task and category was found in the left temporal pole. Retrieving names for unique persons and for names for unique landmarks activate the same brain region. These findings are consistent with the notion that activity in the left temporal pole is linked to the level of specificity of word retrieval rather than the conceptual class to which the stimulus belongs.

Cerebral asymmetry and the effects of sex and handedness on brain structure: a voxel-based morphometric analysis of 465 normal adult human brains

We used voxel-based morphometry (VBM) to examine human brain asymmetry and the effects of sex and handedness on brain structure in 465 normal adults. We observed significant asymmetry of cerebral grey and white matter in the occipital, frontal, and temporal lobes (petalia), including Heschl's gyrus, planum temporale (PT) and the hippocampal formation. Males demonstrated increased leftward asymmetry within Heschl's gyrus and PT compared to females. There was no significant interaction between asymmetry and handedness and no main effect of handedness. There was a significant main effect of sex on brain morphology, even after accounting for the larger global volumes of grey and white matter in males. Females had increased grey matter volume adjacent to the depths of both central sulci and the left superior temporal sulcus, in right Heschl's gyrus and PT, in right inferior frontal and frontomarginal gyri and in the cingulate gyrus. Females had significantly increased grey matter concentration extensively and relatively symmetrically in the cortical mantle, parahippocampal gyri, and in the banks of the cingulate and calcarine sulci. Males had increased grey matter volume bilaterally in the mesial temporal lobes, entorhinal and perirhinal cortex, and in the anterior lobes of the cerebellum, but no regions of increased grey matter concentration.

Large, colorful, or noisy? Attribute- and modality-specific activations during retrieval of perceptual attribute knowledge

Position emission tomography was used to investigate whether retrieval of perceptual knowledge from long-term memory activates unique cortical regions associated with the modality and/or attribute type retrieved. Knowledge about the typical color, size, and sound of common objects and animals was probed, in response to written words naming the objects. Relative to a nonsemantic control task, all the attribute judgments activated similar left temporal and frontal regions. Visual (color, size) knowledge selectively activated the right posterior inferior temporal (PIT) cortex, whereas sound judgments elicited selective activation in the left posterior superior temporal gyrus and the adjacent parietal cortex. All of the attribute judgments activated a left PIT region, but color retrieval generated more activation in this area. Size judgments activated the right medial parietal cortex. These results indicate that the retrieval of perceptual semantic information activates not only a general semantic network, but also cortical areas specialized for the modality and attribute type of the knowledge retrieved.