Structural connectivity of the human anterior temporal lobe: A diffusion magnetic resonance imaging study

The impact of bilateral versus unilateral anterior temporal lobe damage on face recognition, person knowledge and semantic memory

The functional importance of the anterior temporal lobes (ATLs) has come to prominence in two active, albeit unconnected literatures-(i) face recognition and (ii) semantic memory. To generate a unified account of the ATLs, we tested the predictions from each literature and examined the effects of bilateral versus unilateral ATL damage on face recognition, person knowledge, and semantic memory. Sixteen people with bilateral ATL atrophy from semantic dementia (SD), 17 people with unilateral ATL resection for temporal lobe epilepsy (TLE; left = 10, right = 7), and 14 controls completed tasks assessing perceptual face matching, person knowledge and general semantic memory. People with SD were impaired across all semantic tasks, including person knowledge. Despite commensurate total ATL damage, unilateral resection generated mild impairments, with minimal differences between left- and right-ATL resection. Face matching performance was largely preserved but slightly reduced in SD and right TLE. All groups displayed the familiarity effect in face matching; however, it was reduced in SD and right TLE and was aligned with the level of item-specific semantic knowledge in all participants. We propose a neurocognitive framework whereby the ATLs underpin a resilient bilateral representation system that supports semantic memory, person knowledge and face recognition.

Overlapping neural correlates underpin theory of mind and semantic cognition: Evidence from a meta-analysis of 344 functional neuroimaging studies

Key unanswered questions for cognitive neuroscience include whether social cognition is underpinned by specialised brain regions and to what extent it simultaneously depends on more domain-general systems. Until we glean a better understanding of the full set of contributions made by various systems, theories of social cognition will remain fundamentally limited. In the present study, we evaluate a recent proposal that semantic cognition plays a crucial role in supporting social cognition. While previous brain-based investigations have focused on dissociating these two systems, our primary aim was to assess the degree to which the neural correlates are overlapping, particularly within two key regions, the anterior temporal lobe (ATL) and the temporoparietal junction (TPJ). We focus on activation associated with theory of mind (ToM) and adopt a meta-analytic activation likelihood approach to synthesise a large set of functional neuroimaging studies and compare their results with studies of semantic cognition. As a key consideration, we sought to account for methodological differences across the two sets of studies, including the fact that ToM studies tend to use nonverbal stimuli while the semantics literature is dominated by language-based tasks. Overall, we observed consistent overlap between the two sets of brain regions, especially in the ATL and TPJ. This supports the claim that tasks involving ToM draw upon more general semantic retrieval processes. We also identified activation specific to ToM in the right TPJ, bilateral anterior mPFC, and right precuneus. This is consistent with the view that, nested amongst more domain-general systems, there is specialised circuitry that is tuned to social processes.

A neuroanatomical and cognitive model of impaired social behaviour in frontotemporal dementia

Impaired social cognition is a core deficit in frontotemporal dementia (FTD). It is most commonly associated with the behavioural-variant of FTD, with atrophy of the orbitofrontal and ventromedial prefrontal cortex. Social cognitive changes are also common in semantic dementia, with atrophy centred on the anterior temporal lobes. The impairment of social behaviour in FTD has typically been attributed to damage to the orbitofrontal cortex and/or temporal poles and/or the uncinate fasciculus that connects them. However, the relative contributions of each region are unresolved. In this review, we present a unified neurocognitive model of controlled social behaviour that not only explains the observed impairment of social behaviours in FTD, but also assimilates both consistent and potentially contradictory findings from other patient groups, comparative neurology and normative cognitive neuroscience. We propose that impaired social behaviour results from damage to two cognitively- and anatomically-distinct components. The first component is social-semantic knowledge, a part of the general semantic-conceptual system supported by the anterior temporal lobes bilaterally. The second component is social control, supported by the orbitofrontal cortex, medial frontal cortex and ventrolateral frontal cortex, which interacts with social-semantic knowledge to guide and shape social behaviour.

Semiautomated electric source imaging determines epileptogenicity of encephaloceles in temporal lobe epilepsy

OBJECTIVE

We aimed to assess the ability of semiautomated electric source imaging (ESI) from long-term video-electroencephalographic (EEG) monitoring (LTM) to determine the epileptogenicity of temporopolar encephaloceles (TEs) in patients with temporal lobe epilepsy.

METHODS

We conducted a retrospective study involving 32 temporal lobe epilepsy patients with TEs as potentially epileptogenic lesions in structural magnetic resonance imaging scans. Findings were validated through invasive intracerebral stereo-EEG in six of 32 patients and postsurgical outcome after tailored resection of the TE in 17 of 32 patients. LTM (mean duration = 6 days) was performed using the 10/20 system with additional T1/T2 for all patients and sphenoidal electrodes in 23 of 32 patients. Semiautomated detection and clustering of interictal epileptiform discharges (IEDs) were carried out to create IED types. ESI was performed on the averages of the two most frequent IED types per patient, utilizing individual head models, and two independent inverse methods (sLORETA [standardized low-resolution brain electromagnetic tomography], MUSIC [multiple signal classification]). ESI maxima concordance and propagation in spatial relation to TEs were quantified for sources with good signal quality (signal-to-noise ratio > 2, explained signal > 60%).

RESULTS

ESI maxima correctly colocalized with a TE in 20 of 32 patients (62.5%) either at the onset or half-rising flank of at least one IED type per patient. ESI maxima showed propagation from the temporal pole to other temporal or extratemporal regions in 14 of 32 patients (44%), confirming propagation originating in the area of the TE. The findings from both inverse methods validated each other in 14 of 20 patients (70%), and sphenoidal electrodes exhibited the highest signal amplitudes in 17 of 23 patients (74%). The concordance of ESI with the TE predicted a seizure-free postsurgical outcome (Engel I vs. >I) with a diagnostic odds ratio of 2.1.

SIGNIFICANCE

Semiautomated ESI from LTM often successfully identifies the epileptogenicity of TEs and the IED onset zone within the area of the TEs. Additionally, it shows potential predictive power for postsurgical outcomes in these patients.

Structural connectivity of cytoarchitectonically distinct human left temporal pole subregions: a diffusion MRI tractography study

The temporal pole (TP) is considered one of the major paralimbic cortical regions, and is involved in a variety of functions such as sensory perception, emotion, semantic processing, and social cognition. Based on differences in cytoarchitecture, the TP can be further subdivided into smaller regions (dorsal, ventrolateral and ventromedial), each forming key nodes of distinct functional networks. However, the brain structural connectivity profile of TP subregions is not fully clarified. Using diffusion MRI data in a set of 31 healthy subjects, we aimed to elucidate the comprehensive structural connectivity of three cytoarchitectonically distinct TP subregions. Diffusion tensor imaging (DTI) analysis suggested that major association fiber pathways such as the inferior longitudinal, middle longitudinal, arcuate, and uncinate fasciculi provide structural connectivity to the TP. Further analysis suggested partially overlapping yet still distinct structural connectivity patterns across the TP subregions. Specifically, the dorsal subregion is strongly connected with wide areas in the parietal lobe, the ventrolateral subregion with areas including constituents of the default-semantic network, and the ventromedial subregion with limbic and paralimbic areas. Our results suggest the involvement of the TP in a set of extensive but distinct networks of cortical regions, consistent with its functional roles.

Diminished baseline autonomic outflow in semantic dementia relates to left-lateralized insula atrophy

In semantic dementia (SD), asymmetric degeneration of the anterior temporal lobes is associated with loss of semantic knowledge and alterations in socioemotional behavior. There are two clinical variants of SD: semantic variant primary progressive aphasia (svPPA), which is characterized by predominant atrophy in the anterior temporal lobe and insula in the left hemisphere, and semantic behavioral variant frontotemporal dementia (sbvFTD), which is characterized by predominant atrophy in those structures in the right hemisphere. Previous studies of behavioral variant frontotemporal dementia, an associated clinical syndrome that targets the frontal lobes and anterior insula, have found impairments in baseline autonomic nervous system activity that correlate with left-lateralized frontotemporal atrophy patterns and disruptions in socioemotional functioning. Here, we evaluated whether there are similar impairments in resting autonomic nervous system activity in SD that also reflect left-lateralized atrophy and relate to diminished affiliative behavior. A total of 82 participants including 33 people with SD (20 svPPA and 13 sbvFTD) and 49 healthy older controls completed a laboratory-based assessment of respiratory sinus arrhythmia (RSA; a parasympathetic measure) and skin conductance level (SCL; a sympathetic measure) during a two-minute resting baseline period. Participants also underwent structural magnetic resonance imaging, and informants rated their current affiliative behavior on the Interpersonal Adjective Scale. Results indicated that baseline RSA and SCL were lower in SD than in healthy controls, with significant impairments present in both svPPA and sbvFTD. Voxel-based morphometry analyses revealed left-greater-than-right atrophy related to diminished parasympathetic and sympathetic outflow in SD. While left-lateralized atrophy in the mid-to-posterior insula correlated with lower RSA, left-lateralized atrophy in the ventral anterior insula correlated with lower SCL. In SD, lower baseline RSA, but not lower SCL, was associated with lower gregariousness/extraversion. Neither autonomic measure related to warmth/agreeableness, however. Through the assessment of baseline autonomic nervous system physiology, the present study contributes to expanding conceptualizations of the biological basis of socioemotional alterations in svPPA and sbvFTD.

Semantic knowledge of social interactions is mediated by the hedonic evaluation system in the brain

Attaching semantic meaning to sensory information received from both inside and outside our bodies is a fundamental function of the human brain. The theory of Controlled Semantic Cognition (CSC) proposes that the formation of semantic knowledge relies on connections between spatially distributed modality-specific spoke-nodes, and a modality-general hub in the anterior temporal lobes (ATLs). This theory can also be applied to social semantic knowledge, though certain domain-specific spoke-nodes may make a disproportionate contribution to the understanding of social concepts. The ATLs have strong connections with spoke-node structures such as the subgenual ACC (sgACC) and the orbitofrontal cortex (OFC) that play an important role in predicting the hedonic value of stimuli. We hypothesized that in addition to the ATL semantic hub, a social semantic task would also require input from hedonic evaluation structures. We used voxel based morphometry (VBM) to examine structural brain-behavior relationships in 152 patients with neurodegeneration (Alzheimer's disease [N = 12], corticobasal syndrome (N = 18], progressive supranuclear palsy [N = 13], behavioral variant frontotemporal dementia [N = 56], and primary progressive aphasia (PPA) [N = 53]) using the Social Interaction Vocabulary Task (SIVT). This task measures the ability to correctly match a social term (e.g. "gossiping") with a visual depiction of that social interaction. As predicted, VBM showed that worse SIVT scores corresponded with volume loss in bilateral ATL semantic hub regions, but also in the sgACC, OFC, caudate and putamen (pFWE <0.05). These results support the CSC model of a hub-and-spoke organization of social semantic knowledge with the ATL as a domain-general semantic hub, and ventromedial and striatal structures as domain specific spoke-nodes. Importantly, these results suggest that correct comprehension of social semantic concepts requires emotional 'tagging' of a concept by the evaluation system, and that the social deficits observed in some neurodegenerative disease syndromes may be caused by the break-down of this mechanism.

A narrative review of the anatomy and function of the white matter tracts in language production and comprehension

Much is known about the role of cortical areas in language processing. The shift towards network approaches in recent years has highlighted the importance of uncovering the role of white matter in connecting these areas. However, despite a large body of research, many of these tracts’ functions are not well-understood. We present a comprehensive review of the empirical evidence on the role of eight major tracts that are hypothesized to be involved in language processing (inferior longitudinal fasciculus, inferior fronto-occipital fasciculus, uncinate fasciculus, extreme capsule, middle longitudinal fasciculus, superior longitudinal fasciculus, arcuate fasciculus, and frontal aslant tract). For each tract, we hypothesize its role based on the function of the cortical regions it connects. We then evaluate these hypotheses with data from three sources: studies in neurotypical individuals, neuropsychological data, and intraoperative stimulation studies. Finally, we summarize the conclusions supported by the data and highlight the areas needing further investigation.

Temporopolar regions of the human brain

Following prolonged neglect during the formative decades of behavioural neurology, the temporopolar region has become a site of vibrant research on the neurobiology of cognition and conduct. This turnaround can be attributed to increasing recognition of neurodegenerative diseases that target temporopolar regions for peak destruction. The resultant syndromes include behavioural dementia, associative agnosia, semantic forms of primary progressive aphasia and semantic dementia. Clinicopathological correlations show that object naming and word comprehension are critically dependent on the language-dominant (usually left) temporopolar region, whereas behavioural control and non-verbal object recognition display a more bilateral representation with a rightward bias. Neuroanatomical experiments in macaques and neuroimaging in humans show that the temporoparietal region sits at the confluence of auditory, visual and limbic streams of processing at the downstream (deep) pole of the 'what' pathway. The functional neuroanatomy of this region revolves around three axes, an anterograde horizontal axis from unimodal to heteromodal and paralimbic cortex; a radial axis where visual (ventral), auditory (dorsal) and paralimbic (medial) territories encircle temporopolar cortex and display hemispheric asymmetry; and a vertical depth-of-processing axis for the associative elaboration of words, objects and interoceptive states. One function of this neural matrix is to support the transformation of object and word representations from unimodal percepts to multimodal concepts. The underlying process is likely to start at canonical gateways that successively lead to generic (superordinate), specific (basic) and unique levels of recognition. A first sign of left temporopolar dysfunction takes the form of taxonomic blurring where boundaries among categories are preserved but not boundaries among exemplars of a category. Semantic paraphasias and coordinate errors in word-picture verification tests are consequences of this phenomenon. Eventually, boundaries among categories are also blurred and comprehension impairments become more profound. The medial temporopolar region belongs to the amygdalocentric component of the limbic system and stands to integrate exteroceptive information with interoceptive states underlying social interactions. Review of the pertinent literature shows that word comprehension and conduct impairments caused by temporopolar strokes and temporal lobectomy are far less severe than those seen in temporopolar atrophies. One explanation for this unexpected discrepancy invokes the miswiring of residual temporopolar neurons during the many years of indolently progressive neurodegeneration. According to this hypothesis, the temporopolar regions become not only dysfunctional but also sources of aberrant outputs that interfere with the function of areas elsewhere in the language and paralimbic networks, a juxtaposition not seen in lobectomy or stroke.

Neurocognitive health of older adults experiencing homelessness in Oakland, California

Background and objectives

The homeless population in the US is aging. Cognitive impairment is prevalent in this population, yet little is known about the neurologic etiologies of such impairment. Addressing this gap in knowledge is important because homeless older adults with cognitive impairment due to neurodegenerative disease may need lifelong tailored support to obtain and maintain housing. In this study, we characterized the neurocognitive health of a sample of adults who experienced homelessness for the first time after age 50 using gold standard behavioral neurology examination practices.

Methods

We conducted a descriptive cross-sectional study of older adults who first experienced homelessness after age 50. We recruited our sample purposively from an ongoing longitudinal cohort study of adults who were aged 50 and over and homeless when they entered the cohort. For this sub study, we enrolled a convenience sample from those who reported their first episode of homelessness after age 50. We did not exclude individuals based on history of substance use. Neurologists conducted a structured neurocognitive history intake, neurological examination, neuropsychological evaluation, and functional assessment between November 2020 and February 2021. We screened all participants for neurocognitive disorders using gold standard clinical research diagnostic criteria.

Results

We evaluated 25 participants, most were men (76%) and Black (84%), with a median age of 61 years. The most common neurocognitive complaints included deficits in recent episodic memory (n = 15, 60%), executive functions (n = 13, 52%), and behavior/mood, with apathy being the most common complaint (n = 20, 80%). Neuropsychological testing revealed a high prevalence of socioemotional deficits (n = 20, 80%). Common neurological examination deficits included difficulties with coordination, such as impaired Luria task (n = 16, 64%), signs of distal peripheral neuropathy (n = 8, 32%), anosmia/hyposmia (n = 4, 21%), and signs of mild Parkinsonism (n = 5, 20%). The most common diagnoses were MCI (n = 7, 28%), bvFTD (n = 4, 16%), AD (n = 4, 16%), and DLB (n = 2, 8%).

Discussion

Our findings suggest that neurocognitive concerns and examination deficits are common among older homeless adults. Specific neurocognitive disorders may be overrepresented in this population, particularly frontotemporal disorders. Longitudinal studies involving brain biomarkers are needed to characterize the neurocognitive health of this vulnerable population more precisely.

The role of the ventrolateral anterior temporal lobes in social cognition

A key challenge for neurobiological models of social cognition is to elucidate whether brain regions are specialised for that domain. In recent years, discussion surrounding the role of anterior temporal regions epitomises such debates; some argue the anterior temporal lobe (ATL) is part of a domain‐specific network for social processing, while others claim it comprises a domain‐general hub for semantic representation. In the present study, we used ATL‐optimised fMRI to map the contribution of different ATL structures to a variety of paradigms frequently used to probe a crucial social ability, namely ‘theory of mind’ (ToM). Using multiple tasks enables a clearer attribution of activation to ToM as opposed to idiosyncratic features of stimuli. Further, we directly explored whether these same structures are also activated by a non‐social task probing semantic representations. We revealed that common to all of the tasks was activation of a key ventrolateral ATL region that is often invisible to standard fMRI. This constitutes novel evidence in support of the view that the ventrolateral ATL contributes to social cognition via a domain‐general role in semantic processing and against claims of a specialised social function.

Frontotemporal dementia: A unique window on the functional role of the temporal lobes

Frontotemporal dementia (FTD) is an umbrella term covering a plethora of progressive changes in executive functions, motor abilities, behavior, and/or language. Different clinical syndromes have been described in relation to localized atrophy, informing on the functional networks that underlie these specific cognitive, emotional, and behavioral processes. These functional declines are linked with the underlying neurodegeneration of frontal and/or temporal lobes due to diverse molecular pathologies. Initially, the accumulation of misfolded proteins targets specifically susceptible cell assemblies, leading to relatively focal neurodegeneration that later spreads throughout large-scale cortical networks. Here, we discuss the most recent clinical, neuropathological, imaging, and genetics findings in FTD-spectrum syndromes affecting the temporal lobe. We focus on the semantic variant of primary progressive aphasia and its mirror image, the right temporal variant of FTD. Incipient focal atrophy of the left anterior temporal lobe (ATL) manifests with predominant naming, word comprehension, reading, and object semantic deficits, while cases of predominantly right ATL atrophy present with impairments of socioemotional, nonverbal semantic, and person-specific knowledge. Overall, the observations in FTD allow for crucial clinical-anatomic inferences, shedding light on the role of the temporal lobes in both cognition and complex behaviors. The concerted activity of both ATLs is critical to ensure that percepts are translated into concepts, yet important hemispheric differences should be acknowledged. On one hand, the left ATL attributes meaning to linguistic, external stimuli, thus supporting goal-oriented, action-related behaviors (e.g., integrating sounds and letters into words). On the other hand, the right ATL assigns meaning to emotional, visceral stimuli, thus guiding socially relevant behaviors (e.g., integrating body sensations into feelings of familiarity).

Resting functional connectivity in the semantic appraisal network predicts accuracy of emotion identification

OBJECTIVE

Structural and task-based functional studies associate emotion reading with frontotemporal brain networks, though it remains unclear whether functional connectivity (FC) alone predicts emotion reading ability. The predominantly frontotemporal salience and semantic appraisal (SAN) networks are selectively impacted in neurodegenerative disease syndromes like behavioral-variant frontotemporal dementia (bvFTD) and semantic-variant primary progressive aphasia (svPPA). Accurate emotion identification diminishes in some of these patients, but studies investigating the source of this symptom in patients have predominantly examined structural rather than functional brain changes. Thus, we investigated the impact of altered connectivity on their emotion reading.

METHODS

One-hundred-eighty-five participants (26 bvFTD, 21 svPPA, 24 non-fluent variant PPA, 24 progressive supranuclear palsy, 49 Alzheimer's disease, 41 neurologically healthy older controls) underwent task-free fMRI, and completed the Emotion Evaluation subtest of The Awareness of Social Inference Test (TASIT-EET), watching videos and selecting labels for actors' emotions.

RESULTS

As expected, patients averaged significantly worse on emotion reading, but with wide inter-individual variability. Across all groups, lower mean FC in the SAN, but not other ICNs, predicted worse TASIT-EET performance. Node-pair analysis revealed that emotion identification was predicted by FC between 1) right anterior temporal lobe (RaTL) and right anterior orbitofrontal (OFC), 2) RaTL and right posterior OFC, and 3) left basolateral amygdala and left posterior OFC.

CONCLUSION

Emotion reading test performance predicts FC in specific SAN regions mediating socioemotional semantics, personalized evaluations, and salience-driven attention, highlighting the value of emotion testing in clinical and research settings to index neural circuit dysfunction in patients with neurodegeneration and other neurologic disorders.

Characteristics of resting-state functional connectivity in older adults after the PICMOR intervention program: a preliminary report

BACKGROUND

The present study aimed to provide a basis for future research examining the neural mechanisms that underlie the beneficial effect of an intervention program, Photo-Integrated Conversation Moderated by Robots (PICMOR), on verbal fluency in older adults as identified in our previous randomized controlled trial. In this preliminary report, we conducted an additional experiment using resting-state functional magnetic resonance imaging (rsfMRI) after the intervention period. Specifically, we investigated the resting-state functional connectivity (rsFC) characteristics of the intervention group (INT) compared to the control group (CONT).

METHODS

rsfMRI data were acquired from 31 and 30 participants in INT and CONT, respectively, after the intervention. In the analyses, two of the most important regions in verbal fluency, the left inferior and middle frontal gyri, were selected as seed regions, and the rsFCs were compared between groups. We also conducted regression analyses for rsFCs using the difference in individual phonemic verbal fluency task (PVFT) scores between the pre- and post-intervention periods (i.e., post- minus pre-intervention) as an independent variable.

RESULTS

We found higher rsFC in INT than in CONT between the left inferior frontal gyrus as a seed region and the temporal pole and middle frontal gyrus. The rsFC strength between the left inferior frontal gyrus and temporal pole positively correlated with an increased PVFT score between the pre- and post-intervention periods. In contrast, we found lower rsFC in INT than in CONT between the left middle frontal gyrus as a seed region and the posterior cingulate cortex, precuneus, and postcentral gyrus.

CONCLUSIONS

Our findings suggest that the beneficial intervention effect of PICMOR on verbal fluency is characterized by enhanced rsFC of the left inferior frontal gyrus with semantic and executive control-related regions and suppressed rsFC between the left middle frontal gyrus and posterior cortical midline structures. No definitive conclusions can be made because of a lack of rsfMRI data before the intervention. However, this pilot study provides the candidates for rsFCs, reflecting the beneficial effects of PICMOR on the brain network involved in verbal fluency.

TRIAL REGISTRATION

The trial was retrospectively registered at the UMIN Clinical Trials Registry ( UMIN000036667 ) (May 7th, 2019).

From shape to meaning: Evidence for multiple fast feedforward hierarchies of concept processing in the human brain

A number of fMRI studies have provided support for the existence of multiple concept representations in areas of the brain such as the anterior temporal lobe (ATL) and inferior parietal lobule (IPL). However, the interaction among different conceptual representations remains unclear. To better understand the dynamics of how the brain extracts meaning from sensory stimuli, we conducted a human high-density electroencephalography (EEG) study in which we first trained participants to associate pseudowords with various animal and tool concepts. After training, multivariate pattern classification of EEG signals in sensor and source space revealed the representation of both animal and tool concepts in the left ATL and tool concepts within the left IPL within 250 ​ms. Finally, we used Granger Causality analyses to show that orthography-selective sensors directly modulated activity in the parietal-tool selective cluster. Together, our results provide evidence for distinct but parallel "perceptual-to-conceptual" feedforward hierarchies in the brain.

Intrinsic connectivity of anterior temporal lobe relates to individual differences in semantic retrieval for landmarks

Contemporary neuroscientific accounts suggest that ventral anterior temporal lobe (ATL) acts as a bilateral heteromodal semantic hub, which is particularly critical for the specific-level knowledge needed to recognise unique entities, such as familiar landmarks and faces. There may also be graded functional differences between left and right ATL, relating to effects of modality (linguistic versus non-linguistic) and category (e.g., knowledge of people and places). Individual differences in intrinsic connectivity from left and right ATL might be associated with variation in semantic categorisation performance across these categories and modalities. We recorded resting-state fMRI in 74 individuals and, in a separate session, examined semantic categorisation. People with greater connectivity between left and right ATL were more efficient at categorising landmarks (e.g., Eiffel Tower), especially when these were presented visually. In addition, participants who showed stronger connectivity from right than left ATL to medial occipital cortex showed more efficient semantic categorisation of landmarks regardless of modality of presentation. These results can be interpreted in terms of graded differences in the patterns of connectivity across left and right ATL, which give rise to a bilateral yet partially segregated semantic 'hub'. More specifically, right ATL connectivity supports the efficient semantic categorisation of landmarks.

Two Neural Networks for Laughter: A Tractography Study

Laughter is a complex motor behavior occurring in both emotional and nonemotional contexts. Here, we investigated whether the different functions of laughter are mediated by distinct networks and, if this is the case, which are the white matter tracts sustaining them. We performed a multifiber tractography investigation placing seeds in regions involved in laughter production, as identified by previous intracerebral electrical stimulation studies in humans: the pregenual anterior cingulate (pACC), ventral temporal pole (TPv), frontal operculum (FO), presupplementary motor cortex, and ventral striatum/nucleus accumbens (VS/NAcc). The primary motor cortex (M1) and two subcortical territories were also studied to trace the descending projections. Results provided evidence for the existence of two relatively distinct networks. A first network, including pACC, TPv, and VS/NAcc, is interconnected through the anterior cingulate bundle, the accumbofrontal tract, and the uncinate fasciculus, reaching the brainstem throughout the mamillo-tegmental tract. This network is likely involved in the production of emotional laughter. A second network, anchored to FO and M1, projects to the brainstem motor nuclei through the internal capsule. It is most likely the neural basis of nonemotional and conversational laughter. The two networks interact throughout the pre-SMA that is connected to both pACC and FO.

Regional and hemispheric susceptibility of the temporal lobe to FTLD-TDP type C pathology

Post-mortem studies show that focal anterior temporal lobe (ATL) neurodegeneration is most often caused by frontotemporal lobar degeneration TDP-43 type C pathology. Clinically, these patients are described with different terms, such as semantic variant primary progressive aphasia (svPPA), semantic dementia (SD), or right temporal variant frontotemporal dementia (FTD) depending on whether the predominant symptoms affect language, semantic knowledge for object or people, or socio-emotional behaviors. ATL atrophy presents with various degrees of lateralization, with right-sided cases considered rarer even though estimation of their prevalence is hampered by the paucity of studies on well-characterized, pathology-proven cohorts. Moreover, it is not clear whether left and right variants show a similar distribution of atrophy within the ATL cross-sectionally and longitudinally. Here we study the largest cohort to-date of pathology-proven TDP-43-C cases diagnosed during life as svPPA, SD or right temporal variant FTD. We analyzed clinical, cognitive, and neuroimaging data from 30 cases, a subset of which was followed longitudinally. Guided by recent structural and functional parcellation studies, we constructed four bilateral ATL regions of interest (ROIs). The computation of an atrophy lateralization index allowed the comparison of atrophy patterns between the two hemispheres. This led to an automatic, imaging-based classification of the cases as left-predominant or right-predominant. We then compared the two groups in terms of regional atrophy patterns within the ATL ROIs (cross-sectionally) and atrophy progression (longitudinally). Results showed that 40% of pathology proven cases of TDP-43-C diagnosed with a temporal variant presented with right-lateralized atrophy. Moreover, the findings of our ATL ROI analysis indicated that, irrespective of atrophy lateralization, atrophy distribution within both ATLs follows a medial-to-lateral gradient. Finally, in both left and right cases, atrophy appeared to progress to the contralateral ATL, and from the anterior temporal pole to posterior temporal and orbitofrontal regions. Taken together, our findings indicate that incipient right predominant ATL atrophy is common in TDP-43-C pathology, and that distribution of damage within the ATLs appears to be the same in left- and right- sided variants. Thus, regardless of differences in clinical phenotype and atrophy lateralization, both temporal variants of FTD should be viewed as a spectrum presentation of the same disease.

The Influence of Object-Color Knowledge on Emerging Object Representations in the Brain

The ability to rapidly and accurately recognize complex objects is a crucial function of the human visual system. To recognize an object, we need to bind incoming visual features, such as color and form, together into cohesive neural representations and integrate these with our preexisting knowledge about the world. For some objects, typical color is a central feature for recognition; for example, a banana is typically yellow. Here, we applied multivariate pattern analysis on time-resolved neuroimaging (MEG) data to examine how object-color knowledge affects emerging object representations over time. Our results from 20 participants (11 female) show that the typicality of object-color combinations influences object representations, although not at the initial stages of object and color processing. We find evidence that color decoding peaks later for atypical object-color combinations compared with typical object-color combinations, illustrating the interplay between processing incoming object features and stored object knowledge. Together, these results provide new insights into the integration of incoming visual information with existing conceptual object knowledge.SIGNIFICANCE STATEMENT To recognize objects, we have to be able to bind object features, such as color and shape, into one coherent representation and compare it with stored object knowledge. The MEG data presented here provide novel insights about the integration of incoming visual information with our knowledge about the world. Using color as a model to understand the interaction between seeing and knowing, we show that there is a unique pattern of brain activity for congruently colored objects (e.g., a yellow banana) relative to incongruently colored objects (e.g., a red banana). This effect of object-color knowledge only occurs after single object features are processed, demonstrating that conceptual knowledge is accessed relatively late in the visual processing hierarchy.

Dynamic activity patterns in the anterior temporal lobe represents object semantics

The anterior temporal lobe (ATL) is considered a crucial area for the representation of transmodal concepts. Recent evidence suggests that specific regions within the ATL support the representation of individual object concepts, as shown by studies combining multivariate analysis methods and explicit measures of semantic knowledge. This research looks to further our understanding by probing conceptual representations at a spatially and temporally resolved neural scale. Representational similarity analysis was applied to human intracranial recordings from anatomically defined lateral to medial ATL sub-regions. Neural similarity patterns were tested against semantic similarity measures, where semantic similarity was defined by a hybrid corpus-based and feature-based approach. Analyses show that the perirhinal cortex, in the medial ATL, significantly related to semantic effects around 200 to 400 ms, and were greater than more lateral ATL regions. Further, semantic effects were present in low frequency (theta and alpha) oscillatory phase signals. These results provide converging support that more medial regions of the ATL support the representation of basic-level visual object concepts within the first 400 ms, and provide a bridge between prior fMRI and MEG work by offering detailed evidence for the presence of conceptual representations within the ATL.

Representional and connectivity-based accounts of the cognitive consequences of atrophy of the right and left anterior temporal lobes

According to the original "hub-and-spoke" model of conceptual representations, the neural network for semantic memory requires a single convergence zone located in the anterior temporal lobes (ATLs). However, a more recent version of this model acknowledges that a graded specialization of the left and right ATLs might emerge as a consequence of their differential connectivity with language and sensory-motor regions. A recent influential paper maintained that both the format of semantic representations (representational account) and their differential connectivity (connectivity account) could contribute to the cognitive consequences of atrophy to the left versus the right ATL atrophy. That paper, however, also raised questions as to whether the distinction between representational and connectivity accounts is a meaningful question. I argue that an important theoretical difference exists between the representational and the connectivity-based models and that investigations, based on this difference, should allow to choose between these alternative accounts.

The White Matter Module-Hub Network of Semantics Revealed by Semantic Dementia

Cognitive neuroscience exploring the architecture of semantics has shown that coherent supramodal concepts are computed in the anterior temporal lobes (ATL), but it is unknown how/where modular information implemented by posterior cortices (word/object/face forms) is conveyed to the ATL hub. We investigated the semantic module-hub network in healthy adults (n = 19) and in semantic dementia patients (n = 28) by combining semantic assessments of verbal and nonverbal stimuli and MRI-based fiber tracking using seeds in three module-related cortices implementing (i) written word forms (visual word form area), (ii) abstract lexical representations (posterior-superior temporal cortices), and (iii) face/object representations (face form area). Fiber tracking revealed three key tracts linking the ATL with the three module-related cortices. Correlation analyses between tract parameters and semantic scores indicated that the three tracts subserve semantics, transferring modular verbal or nonverbal object/face information to the left and right ATL, respectively. The module-hub tracts were functionally and microstructurally damaged in semantic dementia, whereas damage to non-module-specific ATL tracts (inferior longitudinal fasciculus, uncinate fasciculus) had more limited impact on semantic failure. These findings identify major components of the white matter module-hub network of semantics, and they corroborate/materialize claims of cognitive models positing direct links between modular and semantic representations. In combination with modular accounts of cognition, they also suggest that the currently prevailing "hub-and-spokes" model of semantics could be extended by incorporating an intermediate module level containing invariant representations, in addition to "spokes," which subserve the processing of a near-unlimited number of sensorimotor and speech-sound features.

Neural basis of basic composition: what we have learned from the red-boat studies and their extensions

Language is our mind's most powerful generative system for the expression of meaning and thought. What are the neural mechanisms of our ability to compose complex meanings from simpler representations? This question is impossible to answer unless we decompose the notion of 'meaning composition' in some theoretically guided way and then begin to assess the extent to which brain activity tracks the posited subroutines. Here, I summarize results from a body of MEG research that has begun to address this question from the ground up, first focusing on simple combinations of two words. The work sets off with a hypothesis space offered by theoretical linguistics, positing syntactic and logico-semantic composition as the main combinatory routines, but then reveals that the most consistent and prominent reflection of composition, localized in the left anterior temporal cortex at 200-250 ms, cannot be described with this toolkit. Instead, this activity tracks a much more conceptually driven process, robustly sensitive to the density of the conceptual feature space of the composing items. I will describe our functional understanding of this activity and how it may operate within a broader 'combinatory network.' This article is part of the theme issue 'Towards mechanistic models of meaning composition'.

Names and their meanings: A dual-process account of proper-name encoding and retrieval

The ability to pick out a unique entity with a proper name is an important component of human language. It has been a primary focus of research in the philosophy of language since the nineteenth century. Brain-based evidence has shed new light on this capacity, and an extensive literature indicates the involvement of distinct fronto-temporal and temporo-occipito-parietal association cortices in proper-name retrieval. However, comparatively few efforts have sought to explain how memory encoding processes lead to the later recruitment of these distinct regions at retrieval. Here, we provide a unified account of proper-name encoding and retrieval, reviewing evidence that socio-emotional and unitized encoding subserve the retrieval of proper names via anterior-temporal-prefrontal activations. Meanwhile, non-unitized item-item and item-context encoding support subsequent retrieval, largely dependent on the temporo-occipito-parietal cortex. We contend that this well-established divergence in encoding systems can explain how proper names are later retrieved from distinct neural structures. Furthermore, we explore how evidence reviewed here can inform a century-and-a-half-old debate about proper names and the meanings they pick out.

A structural connectivity convergence zone in the ventral and anterior temporal lobes: Data-driven evidence from structural imaging

The hub-and-spoke model of semantic cognition seeks to reconcile embodied views of a fully distributed semantic network with patient evidence, primarily from semantic dementia, who demonstrate modality-independent conceptual deficits associated with atrophy centred on the ventrolateral anterior temporal lobe. The proponents of this model have recently suggested that the temporal cortex is a graded representational space where concepts become less linked to a specific modality as they are processed farther away from primary and secondary sensory cortices and towards the ventral anterior temporal lobe. To explore whether there is evidence that the connectivity patterns of the temporal lobe converge in its ventral anterior end the current study uses three dimensional Laplacian eigenmapping, a technique that allows visualisation of similarity in a low dimensional space. In this space similarity is encoded in terms of distances between data points. We found that the ventral and anterior temporal lobe is in a unique position of being at the centre of mass of the data points within the connective similarity space. This can be interpreted as the area where the connectivity profiles of all other temporal cortex voxels converge. This study is the first to explicitly investigate the pattern of connectivity and thus provides the missing link in the evidence that the ventral anterior temporal lobe can be considered a multi-modal graded hub.

Verbal Semantics and the Left Dorsolateral Anterior Temporal Lobe: A Longitudinal Case of Bilateral Temporal Degeneration

BACKGROUND

Semantic variant primary progressive aphasia (svPPA), a clinical syndrome characterized by loss of semantic knowledge, is associated with neurodegeneration that starts in the anterior temporal lobe (ATL) and gradually spreads towards posterior temporal and medial frontal areas. At the earliest stages, atrophy may be predominantly lateralized to either the left or right ATL, leading to different clinical profiles with greatest impairment of word comprehension or visual/social semantics, respectively.

METHODS & PROCEDURES

We report the in-depth longitudinal investigation of cognitive and neuroanatomical features of JB, an unusual case of ATL neurodegeneration with relative sparing of left lateral ATL regions.

OUTCOMES & RESULTS

Over the course of nine years, neurodegeneration progressed to involve bilateral temporo-lateral and frontal regions, resulting in a relatively symmetric and diffuse frontotemporal atrophy pattern. In parallel, JB developed greater behavioral, cognitive, and language impairments, as well as signs of motor neuron disease at her last evaluation. Episodic memory and socio-emotional processing deficits arose, likely secondary to semantic verbal deficits, while visuospatial processing, executive function, and non-semantic language abilities remained largely unaffected throughout the course of the disease.

CONCLUSIONS

The details of this rare case of early medial more than lateral ATL degeneration are consistent with a bilateral organization of the semantic system and, crucially, with a functional dissociation between medial paralimbic and lateral neocortical temporal regions. Cases of frontotemporal dementia (FTD) such as JB, who initially do not meet current clinical criteria for svPPA and instead present with some features of behavioral variant FTD, highlight the need for specific criteria for the right temporal variant of FTD that we propose could be called semantic variant FTD.

Emotion and location cues bias conceptual retrieval in people with deficient semantic control

Visuo-spatial context and emotional valence are powerful cues to episodic retrieval, but the contribution of these inputs to semantic cognition has not been widely investigated. We examined the impact of visuo-spatial, facial emotion and prosody cues and miscues on the retrieval of dominant and subordinate meanings of ambiguous words. Cue photographs provided relevant visuo-spatial or emotional information, consistent with the interpretation of the ambiguous word being probed, while miscues were consistent with an alternative interpretation. We compared the impact of these cues in healthy controls and semantic aphasia patients with deficient control over semantic retrieval following left-hemisphere stroke. Patients showed greater deficits in retrieving the subordinate meanings of ambiguous words, and stronger effects of cueing and miscuing relative to healthy controls. These findings suggest that contextual cues that guide retrieval to the appropriate semantic information reduce the need to constrain semantic retrieval internally, while miscues that are not aligned with the task increase the need for semantic control. Moreover, both valence and visuo-spatial context can prime particular semantic interpretations, in line with theoretical frameworks that argue meaning is computed through the integration of these features. In semantic aphasia, residual comprehension relies heavily on facial expressions and visuospatial cues. This has important implications for patients, their families and clinicians when developing new or more effective modes of communication.

Altered lateralization of dorsal language tracts in 6-week-old infants at risk for autism

Altered structural connectivity has been identified as a possible biomarker of autism spectrum disorder (ASD) risk in the developing brain. Core features of ASD include impaired social communication and early language delay. Thus, examining white matter tracts associated with language may lend further insight into early signs of ASD risk and the mechanisms that underlie language impairments associated with the disorder. Evidence of altered structural connectivity has previously been detected in 6-month-old infants at high familial risk for developing ASD. However, as language processing begins in utero, differences in structural connectivity between language regions may be present in the early infant brain shortly after birth. Here we investigated key white matter pathways of the dorsal language network in 6-week-old infants at high (HR) and low (LR) risk for ASD to identify atypicalities in structural connectivity that may predict altered developmental trajectories prior to overt language delays and the onset of ASD symptomatology. Compared to HR infants, LR infants showed higher fractional anisotropy (FA) in the left superior longitudinal fasciculus (SLF); in contrast, in the right SLF, HR infants showed higher FA than LR infants. Additionally, HR infants showed more rightward lateralization of the SLF. Across both groups, measures of FA and lateralization of these pathways at 6 weeks of age were related to later language development at 18 months of age as well as ASD symptomatology at 36 months of age. These findings indicate that early differences in the structure of language pathways may provide an early predictor of future language development and ASD risk.

Concrete versus abstract forms of social concept: an fMRI comparison of knowledge about people versus social terms

The anterior temporal lobes (ATLs) play a key role in conceptual knowledge representation. The hub-and-spoke theory suggests that the contribution of the ATLs to semantic representation is (a) transmodal, i.e. integrating information from multiple sensorimotor and verbal modalities, and (b) pan-categorical, representing concepts from all categories. Another literature, however, suggests that this region's responses are modality- and category-selective; prominent examples include category selectivity for socially relevant concepts and face recognition. The predictions of each approach have never been directly compared. We used data from three studies to compare category-selective responses within the ATLs. Study 1 compared ATL responses to famous people versus another conceptual category (landmarks) from visual versus auditory inputs. Study 2 compared ATL responses to famous people from pictorial and written word inputs. Study 3 compared ATL responses to a different kind of socially relevant stimuli, namely abstract non-person-related words, in order to ascertain whether ATL subregions are engaged for social concepts more generally or only for person-related knowledge. Across all three studies a dominant bilateral ventral ATL cluster responded to all categories in all modalities. Anterior to this ‘pan-category’ transmodal region, a second cluster responded more weakly overall yet selectively for people, but did so equally for spoken names and faces (Study 1). A third region in the anterior superior temporal gyrus responded selectively to abstract socially relevant words (Study 3), but did not respond to concrete socially relevant words (i.e. written names; Study 2). These findings can be accommodated by the graded hub-and-spoke model of concept representation. On this view, the ventral ATL is the centre point of a bilateral ATL hub, which contributes to conceptual representation through transmodal distillation of information arising from multiple modality-specific association cortices. Partial specialization occurs across the graded ATL hub as a consequence of gradedly differential connectivity across the region.

This article is part of the theme issue ‘Varieties of abstract concepts: development, use and representation in the brain’.

More than words: Social cognition across variants of primary progressive aphasia

Although primary progressive aphasia (PPA) is clinically typified by linguistic impairments, emerging evidence highlights the presence of early deficits in social cognition. This review systematically describes the latter patterns, specifying their relation to the characteristic linguistic dysfunctions and atrophy patterns of non-fluent, semantic, and logopenic variants of the disease (nfvPPA, svPPA, and lvPPA, respectively), relative to closely related dementia types. Whereas the evidence on lvPPA proves scant, studies on nfvPPA and svPPA patients show consistent deficits in emotion recognition, theory of mind, and empathy. Notably, these seem to be intertwined with language impairments in nfvPPA, but they prove primary and independent of language disturbances in svPPA. Also, only the profile of svPPA resembles that of behavioral variant frontotemporal dementia, probably reflecting the overlap of fronto-temporal disruptions in both conditions. In short, the neurocognitive relationship between linguistic and socio-cognitive deficits cannot be precisely predicated for PPA as a whole; instead, specific links must be acknowledged in each variant. These emergent patterns pave the way for fruitful dimensional research in the field.

Emotions and the Right Hemisphere: Can New Data Clarify Old Models?

Models advanced to explain hemispheric asymmetries in representation of emotions will be discussed following their historical progression. First, the clinical observations that have suggested a general dominance of the right hemisphere for all kinds of emotions will be reviewed. Then the experimental investigations that have led to proposal of a different hemispheric specialization for positive versus negative emotions (valence hypothesis) or, alternatively, for approach versus avoidance tendencies (motivational hypothesis) will be surveyed. The discussion of these general models will be followed by a review of recent studies which have documented laterality effects within specific brain structures, known to play a critical role in different components of emotions, namely the amygdata in the computation of emotionally laden stimuli, the ventromedial prefrontal cortex in the integration between cognition and emotion and in the control of impulsive reactions and the anterior insula in the conscious experience of emotion. Results of these recent investigations support and provide an updated integrated version of early models assuming a general right hemisphere dominance for all kinds of emotions.

A nonverbal route to conceptual knowledge involving the right anterior temporal lobe

The semantic variant of primary progressive aphasia (PPA-S) is diagnosed based on impaired single-word comprehension, but nonverbal impairments in face and object recognition can also be present, particularly in later disease stages. PPA-S is associated with focal atrophy in the left anterior temporal lobe (ATL), often accompanied by a lesser degree of atrophy in the right ATL. According to a dual-route account, the left ATL is critical for verbal access to conceptual knowledge while nonverbal access to conceptual knowledge depends upon the integrity of right ATL. Consistent with this view, single-word comprehension deficits in PPA-S have consistently been linked to the degree of atrophy in left ATL. In the current study we examined object processing and cortical thickness in 19 patients diagnosed with PPA-S, to evaluate the hypothesis that nonverbal object impairments would instead be determined by the amount of atrophy in the right ATL. All patients demonstrated inability to access conceptual knowledge on standardized tests with word stimuli: they were unable to match spoken words with their corresponding pictures on the Peabody Picture Vocabulary Test. Only a minority of patients, however, performed abnormally on an experimental thematic verification task, which requires judgments as to whether pairs of object pictures are thematically-associated, and does not rely on auditory or visual word input. The entire PPA-S group showed cortical thinning in left ATL, but atrophy in right ATL was more prominent in the subgroup with low verification scores. Thematic verification scores were correlated with cortical thickness in the right rather than left ATL, an asymmetric mapping which persisted when controlling for the degree of atrophy in the contralateral hemisphere. These results are consistent with a dual-route account of conceptual knowledge: breakdown of the verbal left hemispheric route produces an aphasic syndrome, which is only accompanied by visual object processing impairments when the nonverbal right hemispheric route is also compromised.

Controlled semantic cognition relies upon dynamic and flexible interactions between the executive 'semantic control' and hub-and-spoke 'semantic representation' systems

Built upon a wealth of neuroimaging, neurostimulation, and neuropsychology data, a recent proposal set forth a framework termed controlled semantic cognition (CSC) to account for how the brain underpins the ability to flexibly use semantic knowledge (Lambon Ralph et al., 2017; Nature Reviews Neuroscience). In CSC, the ‘semantic control’ system, underpinned predominantly by the prefrontal cortex, dynamically monitors and modulates the ‘semantic representation’ system that consists of a ‘hub’ (anterior temporal lobe, ATL) and multiple ‘spokes’ (modality-specific areas). CSC predicts that unfamiliar and exacting semantic tasks should intensify communication between the ‘control’ and ‘representation’ systems, relative to familiar and less taxing tasks. In the present study, we used functional magnetic resonance imaging (fMRI) to test this hypothesis. Participants paired unrelated concepts by canonical colours (a less accustomed task – e.g., pairing ketchup with fire-extinguishers due to both being red) or paired well-related concepts by semantic relationship (a typical task – e.g., ketchup is related to mustard). We found the ‘control’ system was more engaged by atypical than typical pairing. While both tasks activated the ATL ‘hub’, colour pairing additionally involved occipitotemporal ‘spoke’ regions abutting areas of hue perception. Furthermore, we uncovered a gradient along the ventral temporal cortex, transitioning from the caudal ‘spoke’ zones preferring canonical colour processing to the rostral ‘hub’ zones preferring semantic relationship. Functional connectivity also differed between the tasks: Compared with semantic pairing, colour pairing relied more upon the inferior frontal gyrus, a key node of the control system, driving enhanced connectivity with occipitotemporal ‘spoke’. Together, our findings characterise the interaction within the neural architecture of semantic cognition – the control system dynamically heightens its connectivity with relevant components of the representation system, in response to different semantic contents and difficulty levels.

Structural connections support emotional connections: Uncinate Fasciculus microstructure is related to the ability to decode facial emotion expressions

The Uncinate Fasciculus (UF) is an association fibre tract connecting regions in the frontal and anterior temporal lobes. UF disruption is seen in several disorders associated with impaired social behaviour, but its functional role is unclear. Here we set out to test the hypothesis that the UF is important for facial expression processing, an ability fundamental to adaptive social behaviour. In two separate experiments in healthy adults, we used high-angular resolution diffusion-weighted imaging (HARDI) and constrained spherical deconvolution (CSD) tractography to virtually dissect the UF, plus a control tract (the corticospinal tract (CST)), and quantify, via fractional anisotropy (FA), individual differences in tract microstructure. In Experiment 1, participants completed the Reading the Mind in the Eyes Task (RMET), a well-validated assay of facial expression decoding. In Experiment 2, a different set of participants completed the RMET, plus an odd-emotion-out task of facial emotion discrimination. In both experiments, participants also completed a control odd-identity-out facial identity discrimination task. In Experiment 1, FA of the right-, but not the left-hemisphere, UF was significantly correlated with performance on the RMET task, specifically for emotional, but not neutral expressions. UF FA was not significantly correlated with facial identity discrimination performance. In Experiment 2, FA of the right-, but not left-hemisphere, UF was again significantly correlated with performance on emotional items from the RMET, together with performance on the facial emotion discrimination task. Again, no significant association was found between UF FA and facial identity discrimination performance. Our findings highlight the contribution of right-hemisphere UF microstructure to inter-individual variability in the ability to decode facial emotion expressions, and may explain why disruption of this pathway affects social behaviour.

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We studied white matter microstructure correlates of facial emotion decoding skills.

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Focused on the role of a key limbic tract, the Uncinate Fasciculus (UF).

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Right UF microstructure linked to facial expression decoding skills.

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UF microstructure not related to facial identity discrimination skills.

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Right UF has a distinct role in the processing of facial expressions of emotion.

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.

The neuro-cognitive representations of symbols: the case of concrete words

We live our lives surrounded by symbols (e.g., road signs, logos, but especially words and numbers), and throughout our life we use them to evoke, communicate and reflect upon ideas and things that are not currently present to our senses. Symbols are represented in our brains at different levels of complexity: at the first and most simple level, as physical entities, in the corresponding primary and secondary sensory cortices. The crucial property of symbols, however, is that, despite the simplicity of their surface forms, they have the power of evoking higher order multifaceted representations that are implemented in distributed neural networks spanning a large portion of the cortex. The rich internal states that reflect our knowledge of the meaning of symbols are what we call semantic representations. In this review paper, we summarize our current knowledge of both the cognitive and neural substrates of semantic representations, focusing on concrete words (i.e., nouns or verbs referring to concrete objects and actions), which, together with numbers, are the most-studied and well defined classes of symbols. Following a systematic descriptive approach, we will organize this literature review around two key questions: what is the content of semantic representations? And, how are semantic representations implemented in the brain, in terms of localization and dynamics? While highlighting the main current opposing perspectives on these topics, we propose that a fruitful way to make substantial progress in this domain would be to adopt a geometrical view of semantic representations as points in high dimensional space, and to operationally partition the space of concrete word meaning into motor-perceptual and conceptual dimensions. By giving concrete examples of the kinds of research that can be done within this perspective, we illustrate how we believe this framework will foster theoretical speculations as well as empirical research.

Mapping the Multiple Graded Contributions of the Anterior Temporal Lobe Representational Hub to Abstract and Social Concepts: Evidence from Distortion-corrected fMRI

A growing body of recent convergent evidence indicates that the anterior temporal lobe (ATL) has connectivity-derived graded differences in semantic function: the ventrolateral region appears to be the transmodal, omni-category center-point of the hub whilst secondary contributions come from the peripheries of the hub in a manner that reflects their differential connectivity to different input/output modalities. One of the key challenges for this neurocognitive theory is how different types of concept, especially those with less reliance upon external sensory experience (such as abstract and social concepts), are coded across the graded ATL hub. We were able to answer this key question by using distortion-corrected fMRI to detect functional activations across the entire ATL region and thus to map the neural basis of social and psycholinguistically-matched abstract concepts. Both types of concept engaged a core left-hemisphere semantic network, including the ventrolateral ATL, prefrontal regions and posterior MTG. Additionally, we replicated previous findings of weaker differential activation of the superior and polar ATL for the processing of social stimuli, in addition to the stronger, omni-category activation observed in the vATL. These results are compatible with the view of the ATL as a graded transmodal substrate for the representation of coherent concepts.

Reading words and other people: A comparison of exception word, familiar face and affect processing in the left and right temporal variants of primary progressive aphasia

Semantic variant primary progressive aphasia (svPPA) typically presents with left-hemisphere predominant rostral temporal lobe (rTL) atrophy and the most significant complaints within the language domain. Less frequently, patients present with right-hemisphere predominant temporal atrophy coupled with marked impairments in processing of famous faces and emotions. Few studies have objectively compared these patient groups in both domains and therefore it is unclear to what extent the syndromes overlap. Clinically diagnosed svPPA patients were characterized as left- (n = 21) or right-predominant (n = 12) using imaging and compared along with 14 healthy controls. Regarding language, our primary focus was upon two hallmark features of svPPA; confrontation naming and surface dyslexia. Both groups exhibited naming deficits and surface dyslexia although the impairments were more severe in the left-predominant group. Familiarity judgments on famous faces and affect processing were more profoundly impaired in the right-predominant group. Our findings suggest that the two syndromes overlap significantly but that early cases at the tail ends of the continuum constitute a challenge for current clinical criteria. Correlational neuroimaging analyses implicated a mid portion of the left lateral temporal lobe in exception word reading impairments in line with proposals that this region is an interface between phonology and semantic knowledge.