Differences in functional magnetic resonance imaging activation by category in a visual confrontation naming task

Knowledge of language function and underlying neural networks gained from focal seizures and epilepsy surgery

The effects of epilepsy and its treatments have contributed significantly to language models. The setting of epilepsy surgery, which allows for careful pre- and postsurgical evaluation of patients with cognitive testing and neuroimaging, has produced a wealth of language findings. Moreover, a new wave of surgical interventions, including stereotactic laser ablation and radio frequency ablation, have contributed new insights and corrections to language models as they can make extremely precise, focal lesions. This review covers the common language deficits observed in focal dyscognitive seizure syndromes. It also addresses the effects of surgical interventions on language, and highlights insights gained from unique epilepsy assessment methods (e.g., cortical stimulation mapping, Wada evaluation). Emergent findings are covered including a lack of involvement of the hippocampus in confrontation word retrieval, possible roles for key white matter tracts in language, and the often-overlooked basal temporal language area. The relationship between language and semantic memory networks is also explored, with brief consideration given to the prevailing models of semantic processing, including the amodal Hub and distributed, multi-modal processing models.

Laterality of anterior temporal lobe repetitive transcranial magnetic stimulation determines the degree of disruption in picture naming

The bilateral anterior temporal lobes play a key role in semantic representation. This is clearly demonstrated by the performance of patients with semantic dementia, a disorder characterised by a progressive and selective decline in semantic memory over all modalities as a result of anterior temporal atrophy. Although all patients exhibit a progressive decline in both single-word production and comprehension, those with greater atrophy to the left anterior temporal lobe show a stronger decline in word production than comprehension. This asymmetry has been attributed to the greater connectivity of the left anterior temporal lobe with left-lateralised speech production mechanisms. Virtual lesioning of the left ATL using offline repetitive transcranial magnetic stimulation (rTMS) has been shown to disrupt picture naming, but, the impact of right ATL rTMS is yet to be explored. We tested the prediction that disruption of picture naming in normal participants by rTMS should be greater for the left than the right ATL. We found a significant increase in picture naming latencies specifically for stimulation of the left ATL only. Neither left nor right ATL TMS slowed performance in a number naming control task. These results support the hypothesis that although both temporal lobes are part of a widespread semantic network in the human brain, the left anterior temporal lobe possesses a stronger connection to left-lateralised speech production areas than the right temporal lobe.

A unified model of human semantic knowledge and its disorders

How is knowledge about the meanings of words and objects represented in the human brain? Current theories embrace two radically different proposals: either distinct cortical systems have evolved to represent different kinds of things, or knowledge for all kinds is encoded within a single domain-general network. Neither view explains the full scope of relevant evidence from neuroimaging and neuropsychology. Here we propose that graded category-specificity emerges in some components of the semantic network through joint effects of learning and network connectivity. We test the proposal by measuring connectivity amongst cortical regions implicated in semantic representation, then simulating healthy and disordered semantic processing in a deep neural network whose architecture mirrors this structure. The resulting neuro-computational model explains the full complement of neuroimaging and patient evidence adduced in support of both domain-specific and domain-general approaches, reconciling long-standing disputes about the nature and origins of this uniquely human cognitive faculty.

EEG source connectivity analysis: from dense array recordings to brain networks

The recent past years have seen a noticeable increase of interest for electroencephalography (EEG) to analyze functional connectivity through brain sources reconstructed from scalp signals. Although considerable advances have been done both on the recording and analysis of EEG signals, a number of methodological questions are still open regarding the optimal way to process the data in order to identify brain networks. In this paper, we analyze the impact of three factors that intervene in this processing: i) the number of scalp electrodes, ii) the combination between the algorithm used to solve the EEG inverse problem and the algorithm used to measure the functional connectivity and iii) the frequency bands retained to estimate the functional connectivity among neocortical sources. Using High-Resolution (hr) EEG recordings in healthy volunteers, we evaluated these factors on evoked responses during picture recognition and naming task. The main reason for selection this task is that a solid literature background is available about involved brain networks (ground truth). From this a priori information, we propose a performance criterion based on the number of connections identified in the regions of interest (ROI) that belong to potentially activated networks. Our results show that the three studied factors have a dramatic impact on the final result (the identified network in the source space) as strong discrepancies were evidenced depending on the methods used. They also suggest that the combination of weighted Minimum Norm Estimator (wMNE) and the Phase Synchronization (PS) methods applied on High-Resolution EEG in beta/gamma bands provides the best performance in term of topological distance between the identified network and the expected network in the above-mentioned cognitive task.

Grey and white matter correlates of picture naming: evidence from a voxel-based lesion analysis of the Boston Naming Test

A number of recent studies utilizing both functional neuroimaging and lesion analysis techniques in neurologic patients have produced conflicting results with respect to the neural correlates of picture naming. Picture naming involves a number of cognitive processes, from visual perception/recognition to lexical-semantic retrieval to articulation. This middle process, the ability to retrieve a name associated with an object, has been attributed in some cases to posterior portions of the left lateral temporal lobe and in other cases, to anterior temporal cortex. In the current study, we used voxel-based lesion symptom mapping (VLSM) to identify neural correlates of picture naming in a large sample of well-characterized left hemisphere (LH) patients suffering from a range of naming deficits. We tested patients on the Boston Naming Test (BNT), a clinical, standardized measure of picture naming that is widely used in both clinical and research settings. We found that overall performance on the BNT was associated with a network of LH regions that included significant portions of the left anterior to posterior middle temporal gyrus (MTG) and superior temporal gyrus (STG) and underlying white matter, and extended into left inferior parietal cortex. However, when we added covariates to this analysis that controlled for deficits in visual recognition and motor speech in order to isolate brain regions specific to lexical-semantic retrieval, the significant regions that remained were confined almost exclusively to the left mid-posterior MTG and underlying white matter. These findings support the notion that a large network in left peri-Sylvian cortex supports picture naming, but that the left mid-posterior MTG and underlying white matter play a critical role in the core ability to retrieve a name associated with an object or picture.

Difficulty processing temporary syntactic ambiguities in Lewy body spectrum disorder

While grammatical aspects of language are preserved, executive deficits are prominent in Lewy body spectrum disorder (LBSD), including Parkinson's disease (PD), Parkinson's dementia (PDD) and dementia with Lewy bodies (DLB). We examined executive control during sentence processing in LBSD by assessing temporary structural ambiguities. Using an on-line word detection procedure, patients heard sentences with a syntactic structure that has high-compatibility or low-compatibility with the main verb's statistically preferred syntactic structure, and half of the sentences were lengthened strategically between the onset of the ambiguity and its resolution. We found selectively slowed processing of lengthened ambiguous sentences in the PDD/DLB subgroup. This correlated with impairments on measures of executive control. Regression analyses related the working memory deficit during ambiguous sentence processing to significant cortical thinning in frontal and parietal regions. These findings emphasize the role of prefrontal disease in the executive limitations that interfere with processing ambiguous sentences in LBSD.

MRI diffusion tensor tracking of a new amygdalo-fusiform and hippocampo-fusiform pathway system in humans

PURPOSE

To use MRI diffusion-tensor tracking (DTT) to test for the presence of unknown neuronal fiber pathways interconnecting the mid-fusiform cortex and anteromedial temporal lobe in humans. Such pathways are hypothesized to exist because these regions coactivate in functional MRI (fMRI) studies of emotion-valued faces and words, suggesting a functional link that could be mediated by neuronal connections.

MATERIALS AND METHODS

A total of 15 normal human subjects were studied using unbiased DTT approaches designed for probing unknown pathways, including whole-brain seeding and large pathway-selection volumes. Several quality-control steps verified the results.

RESULTS

Parallel amygdalo-fusiform and hippocampo-fusiform pathways were found in all subjects. The pathways begin/end at the mid-fusiform gyrus above the lateral occipitotemporal sulcus bilaterally. The superior pathway ends/begins at the superolateral amygdala. The inferior pathway crosses medially and ends/begins at the hippocampal head. The pathways are left-lateralized, with consistently larger cross-sectional area, higher anisotropy, and lower minimum eigenvalue (D-min) on the left, where D-min assesses intrinsic cross-fiber diffusivity independent of curvature.

CONCLUSION

A previously-undescribed pathway system interconnecting the mid-fusiform region with the amygdala/hippocampus has been revealed. This pathway system may be important for recognition, memory consolidation, and emotional modulation of face, object, and lexical information, which may be disrupted in conditions such as Alzheimer's disease.

Who is who: areas of the brain associated with recognizing and naming famous faces

OBJECT

It has been hypothesized that specific brain regions involved in face naming may exist in the brain. To spare these areas and to gain a better understanding of their organization, the authors studied patients who underwent surgery by using direct electrical stimulation mapping for brain tumors, and they compared an object-naming task to a famous face-naming task.

METHODS

Fifty-six patients with brain tumors (39 and 17 in the left and right hemispheres, respectively) and with no significant preoperative overall language deficit were prospectively studied over a 2-year period. Four patients who had a partially selective famous face anomia and 2 with prosopagnosia were not included in the final analysis.

RESULTS

Face-naming interferences were exclusively localized in small cortical areas (< 1 cm2). Among 35 patients whose dominant left hemisphere was studied, 26 face-naming specific areas (that is, sites of interference in face naming only and not in object naming) were found. These face naming-specific sites were significantly detected in 2 regions: in the left frontal areas of the superior, middle, and inferior frontal gyri (p < 0.001) and in the anterior part of the superior and middle temporal gyri (p < 0.01). Variable patterns of interference were observed (speech arrest, anomia, phonemic, or semantic paraphasia) probably related to the different stages in famous face processing. Only 4 famous face-naming interferences were found in the right hemisphere.

CONCLUSIONS

Relative anatomical segregation of naming categories within language areas was detected. This study showed that famous face naming was preferentially processed in the left frontal and anterior temporal gyri. The authors think it is necessary to adapt naming tasks in neurosurgical patients to the brain region studied.

ERP assessment of functional status in the temporal lobe: Examining spatiotemporal correlates of object recognition

We have previously used functional magnetic resonance imaging to assess specific anterolateral temporal regions involved in object recognition, providing a novel technique for evaluating functional status in temporal lobe epilepsy (i.e., a site directed approach). However, a method that combines high temporal resolution with spatial mapping is needed to isolate the specific processes associated with these regions (i.e., a process specific approach). In the current study, we evaluated the cognitive processes associated with object recognition and their relationship to the temporal lobe using high-density event-related potentials (ERPs). N300 and late positive component (LPC) responses were examined using a word-picture matching task, with three factors: congruity (match/mismatch), level of abstraction (basic/superordinate), and object category (natural/artificial). The N300 and LPC were analyzed in terms of their temporal characteristics as well as their spatial characteristics through source analysis. The results showed that the N300 primarily indexed processing of congruity and level of abstraction, whereas the LPC primarily indexed processing of object category. In agreement with previous functional imaging studies, N300 and LPC source analysis results confirmed temporal lobe involvement in object recognition. Importantly, LPC object category differences were detected in the anterior temporal lobe. Individual subject analyses revealed that these anterior temporal lobe differences were reliable--with greater activity for natural objects in 84% of subjects. The findings are discussed in terms of clinical applications that use spatiotemporal ERP differences to evaluate functional status of the temporal lobes.

A review of functional imaging studies on category specificity

A review of 20 functional imaging studies that compared visual processing of natural objects and artifacts in normal subjects is presented. The studies included fulfilled three criteria: (i) they used pictures as stimuli, (ii) they were based on direct contrasts between categories, and (iii) they reported findings in Talairach space. Not a single area is consistently activated for a given category across all studies. In contrast, 11 out of 29 regions are reported activated by both artifacts and natural objects. It is argued that the inconsistency is unlikely to reflect differences between studies in task requirements alone. Rather, the most likely causes of the inconsistency are: (i) adoption of liberal statistical thresholds that may yield false-positive activations, (ii) limited sensitivity due to few observations, and (iii) failure to match categories on confounding variables such as familiarity and visual complexity. Of the most consistent activations found, none appear to be selective for natural objects or artifacts. The findings reviewed are compatible with theories of category specificity that assume a widely distributed conceptual system not organized by category.

A site directed fMRI approach for evaluating functional status in the anterolateral temporal lobes

Functional magnetic resonance imaging (fMRI) is increasingly being used for neurosurgical planning. One potential application relates to identifying eloquent cortex in regions immediately adjacent to epileptogenic foci in temporal lobe epilepsy (TLE). While medial temporal structures, such as the hippocampus and amygdala, are typically removed during surgery, it is often difficult to determine whether nearby cortical regions in the anterolateral temporal lobe should be spared. An essential first step is to identify a method of activating these regions in healthy individuals. The purpose of this study was to develop a site directed fMRI approach for evaluating functional status in the anterolateral temporal lobes. A picture-word matching task, with object category and level of abstraction factors, was used to characterize temporal lobe activation. Whole brain analysis at the group level confirmed the involvement of the temporal poles as well as adjacent superior, middle and inferior temporal gyri within a larger object recognition network. A region-of-interest analysis on the anterolateral temporal lobe demonstrated that activation varied across conditions and regions for individuals. Importantly, it was possible to detect activation in one or more conditions and/or regions for all individuals--demonstrating that it is possible to evaluate functional status. The findings provide the foundation for a novel fMRI approach in neurosurgical planning for TLE.

Task-related laterality effects in the lateral occipital complex

Using functional imaging, we investigated the effects of two different tasks on activation in the lateral occipital complex (LOC). Alternating blocks of intact and scrambled objects were presented. In one task, subjects responded when an object repeated (matching task). In a second task subjects silently named objects (naming task). Identical objects (tools, animals and letters) were presented for both tasks. A relative measure of the number of voxels activated in LOC in left and right hemispheres was calculated for each task across a range of thresholds. Also the effects of task demands on category specific areas in LOC were examined. The object matching task resulted in proportionally more activity in the right hemisphere. The object naming task resulted in proportionally more activity in the left hemisphere, most prominently in the anterior portion of LOC. Effectively, changing the task changed the lateralization of activation to intact objects in LOC. In contrast, changing the task did not change the lateralization of category-specific activations. The results suggest that there are task-related top-down influences on the activation of neural populations in LOC as a whole, but the lateralization of category-specific regions in LOC is independent of task demands and may reflect bottom-up processing.

How necessary are the stripes of a tiger? Diagnostic and characteristic features in an fMRI study of word meaning

This study contrasted two approaches to word meaning: the statistically determined role of high-contribution features like striped in the meaning of complex nouns like "tiger" typically used in studies of semantic memory, and the contribution of diagnostic features like parent's brother that play a critical role in the meaning of nominal kinds like "uncle." fMRI monitored regional brain activity while participants read complex noun descriptions consisting of statistically high-contribution and low-contribution features; and nominal kind descriptions consisting of diagnostic and characteristic features. We found different patterns of activation depending on the type of noun and the type of feature contributing to the noun. Complex nouns recruited significantly greater bilateral superior temporal and left prefrontal activation compared to nominal kind nouns, while nominal kind nouns activated bilateral medial parietal and right inferior parietal regions more than complex nouns. Moreover, features making a statistically high contribution to complex noun meaning activated right inferior frontal cortex relative to low-contribution features, while diagnostic features of nominal kinds activated left dorsolateral prefrontal and right parietal regions more than characteristic features. These findings are consistent with the hypothesis that at least two different neural mechanisms appear to support word meaning: one driven by a statistically determined approach to feature knowledge, and the other sensitive to the qualitatively critical role that a specific diagnostic feature plays in word meaning.

Category-specific effects in semantic memory: category-task interactions suggested by fMRI

Much work has investigated the neural representation of specific categories of knowledge, but relatively scant attention has been paid in the cognitive neuroscience literature to the semantic processes that contribute to semantic memory. In this study, we monitored regional cortical activity with fMRI while healthy young adults evaluated visually displayed NATURAL KIND, ARTIFACT, and ABSTRACT nouns with two standard tasks: Typicality judgments and Pleasantness judgments. We observed a significant interaction effect between the category of knowledge and the type of judgment used to evaluate members of these semantic categories. Typicality judgments recruited greater temporal-occipital activation relative to Pleasantness judgments of the same category, and this was seen for comparisons of all three semantic categories. However, when contrasted with Typicality judgments, Pleasantness judgments activated a different anatomic distribution for each semantic category. These findings are consistent with a dynamic approach to semantic memory that includes at least two components: semantic knowledge and semantic processes that interpret this knowledge in several ways depending on the particular semantic challenge.

Meta-analyses of object naming: effect of baseline

The neural systems sustaining object naming were examined using the activation likelihood estimation (ALE) meta-analysis approach on the results of 16 previously published studies. The activation task in each study required subjects to name pictures of objects or animals, but the baseline tasks varied. Separate meta-analyses were carried out on studies that used: (1) high-level baselines to control for speech processing and visual input; and (2) low-level baselines that did not control for speech or complex visual processing. The results of the two meta-analyses were then compared directly, revealing a double dissociation in the activation pattern for studies using high and low baselines. To interpret the differential activations, we report two new functional imaging experiments. The aim of the first was to characterize activation differences associated with visual stimuli that are typically used in baseline conditions (complex visual features, simple structures, or fixation). The aim of the second was to classify object-naming regions in terms of whether they were engaged preferentially by semantic or phonological processes. The results reveal a remarkably precise correspondence between the areas identified by the meta-analyses as affected differentially by baseline and the areas that are affected differentially by non-object structure, semantics or phonology. As expected, high-level baselines reduced object-naming activation in areas associated with the processing of complex visual features and speech production. In addition, high-level baselines increased sensitivity to activation in areas associated with semantic processing, visual-speech integration and response selection. For example, activation in the anterior temporal areas that neuropsychological studies have associated with semantic processing was more strongly activated in the context of high-level baselines. These results therefore have implications for understanding the convergence of functional imaging and neuropsychological findings.

Effects of gender on blood flow correlates of naming concrete entities

A cross-cohort PET analysis was performed in 62 normal subjects (31 men and 31 women) to address the issue of whether men and women have different physiologic correlates of naming visually presented concrete entities. The subjects named nonunique concrete entities in one or more conceptual categories and also performed a face orientation decision task. A second analysis was performed in 24 additional subjects to assess whether there were gender effects related to the face orientation decision task and to constrain the interpretation of the first analysis. Male subjects engaged the left inferotemporal region and several other left hemisphere regions more than female subjects did during visual naming. Areas showing more activity in female subjects included the right inferior frontal gyrus and right precentral cortex, regions that were less active in visual naming than in the face orientation decision task. In other words, the male subjects engaged the latter regions less or deactivated them more than female subjects. The results can be interpreted as showing a greater modulation of activity in both hemispheres for men compared to women. Although the gender effects we found are smaller than the task effects, they are not negligible for the purposes of performing and interpreting functional imaging studies.

Categorization of object descriptions in Alzheimer's disease and frontotemporal dementia: limitation in rule-based processing

Studies of semantic memory in probable Alzheimer's disease (AD) have focused on the degradation of semantic knowledge, but other work in AD suggests an impairment in the semantic categorization processes that operate on this knowledge. We examined the categorization of object descriptions, where semantic category membership judgments were based on rule-based or similarity-based categorization processes. We found that AD patients were selectively limited in their semantic categorization under conditions requiring a rule-based approach. However, AD patients did not differ from healthy seniors under conditions based on judgments of overall similarity. We showed that this was not due to nonspecific or overall task-related difficulty associated with the rule condition by asking the subjects to use similarity-based judgments of perceptually degraded versions of the stimuli. The results of this condition did not differ from other similarity-based judgments but did differ from the rule-based condition in AD. Rule-based judgments of semantic category membership correlated with executive measures of inhibitory control and mental search, but not with measures of episodic memory or overall dementia severity, suggesting a contribution of executive resources to rule-based semantic categorization. Moreover, the pattern of limited rule-based categorization in AD closely resembled the performance profile of patients with frontotemporal dementia, further implying that executive resource limitations underlie AD patients' limited rule-based semantic categorization. These findings suggest that semantic memory difficulty in AD is due in part to a deficit in executive processes that are central to rule-based categorization in semantic memory.

A functional magnetic resonance imaging study in patients with benign essential blepharospasm

OBJECTIVE To identify blinking-induced functional magnetic resonance imaging (fMRI) activation patterns in five benign essential blepharospasm (BEB) patients and five age-matched control subjects. METHODS fMRI brain activation maps were obtained during repeated conditions of spontaneous and voluntary blinking in BEB and control groups. Blood oxygen level-dependent intensity images were collected from two separate runs as 16 axial and 16 coronal, 8 mm thick slices using a T2-star weighted gradient echo EPI sequence, coregistered with anatomic images. Spatially normalized and isotropically blurred activation maps for each subject were combined within groups of BEB patients and control subjects to generate maps of the intersubject mean fractional signal change.RESULTS Substantially greater activation during spontaneous and voluntary blinking was seen in BEB patients compared with control subjects in the anterior visual cortex, anterior cingulate cortex, primary motor cortex, central region of the thalamus, and superior cerebellum. In both groups, activations were generally greater for voluntary than for spontaneous blinking. CONCLUSIONS The activations observed might represent a hyperactive cortical circuit linking visual cortex, limbic system, supplementary motor cortex, cerebellum, and supranuclear motor pathways innervating the periorbital muscles.

Functional MRI of language: new approaches to understanding the cortical organization of semantic processing

Until recently, our understanding of how language is organized in the brain depended on analysis of behavioral deficits in patients with fortuitously placed lesions. The availability of functional magnetic resonance imaging (fMRI) for in vivo analysis of the normal brain has revolutionized the study of language. This review discusses three lines of fMRI research into how the semantic system is organized in the adult brain. These are (a) the role of the left inferior frontal lobe in semantic processing and dissociations from other frontal lobe language functions, (b) the organization of categories of objects and concepts in the temporal lobe, and (c) the role of the right hemisphere in comprehending contextual and figurative meaning. Together, these lines of research broaden our understanding of how the brain stores, retrieves, and makes sense of semantic information, and they challenge some commonly held notions of functional modularity in the language system.

Functional neuroimaging studies of category specificity in object recognition: a critical review and meta-analysis

Functional neuroimaging studies in which the cortical organization for semantic knowledge has been addressed have revealed interesting dissociations in the recognition of different object categories, such as faces, natural objects, and manufactured objects. The present paper critically reviews these studies and performs a meta-analysis of stereotactic coordinates to determine whether category membership predicts patterns of brain activation across different studies. This meta-analysis revealed that, in the ventral temporal cortex, recognition of manufactured objects activates more medial aspects of the fusiform gyrus, as compared with natural object or face recognition. Face recognition activates more inferior aspects of the ventral temporal cortex, as compared with manufactured object recognition. The recognition task used--viewing, matching, or naming--also predicted brain activation patterns. Specifically, matching tasks recruit more inferior occipital regions than do either naming or viewing tasks, whereas naming tasks recruit more anterior ventral temporal sites than do either viewing or matching tasks. These findings indicate that the cognitive demands of a particular recognition task are as predictive of cortical activation patterns as is category membership.