Amy R. Price

Converging Evidence for the Neuroanatomic Basis of Combinatorial Semantics in the Angular Gyrus

Human thought and language rely on the brains ability to combine conceptual information. This fundamental process supports the construction of complex concepts from basic constituents. For example, both “jacket” and “plaid” can be represented as individual concepts, but they can also be integrated to form the more complex representation “plaid jacket.” Although this process is central to the expression and comprehension of language, little is known about its neural basis. Here we present evidence for a neuroanatomic model of conceptual combination from three experiments. We predicted that the highly integrative region of heteromodal association cortex in the angular gyrus would be critical for conceptual combination, given its anatomic connectivity and its strong association with semantic memory in functional neuroimaging studies. Consistent with this hypothesis, we found that the process of combining concepts to form meaningful representations specifically modulates neural activity in the angular gyrus of healthy adults, independent of the modality of the semantic content integrated. We also found that individual differences in the structure of the angular gyrus in healthy adults are related to variability in behavioral performance on the conceptual combination task. Finally, in a group of patients with neurodegenerative disease, we found that the degree of atrophy in the angular gyrus is specifically related to impaired performance on combinatorial processing. These converging anatomic findings are consistent with a critical role for the angular gyrus in conceptual combination.

Where is the anterior temporal lobe and what does it do

The anterior temporal lobe (ATL) is thought to be critical for semantic memory–our knowledge of objects, people, words, and facts. However, there is substantial disagreement over the precise role of the ATL in semantic memory, and there is considerable variability in the anatomic findings that

A Re-evaluation of the Cognitive Effects From Single-session Transcranial Direct Current Stimulation.

Awell-designedmeta-analysis can provide valuable information for researchers, clinicians, policy-makers and the general public by summarizing years of research in a field. These analyses can be highly influential, and thus it is critical that they be performed carefully. A recent meta-analysis by Horvath, Forte, and Carter (in press) [1] set out to determine whether a single-session of transcranial direct current stimulation (tDCS) can reliably affect cognition in healthy adults. Because there has been some recent debate about the reliability of tDCS effects, a carefully executed meta-analysis addressing this issue would be useful for summarizing current knowledge and guiding future studies employing this approach. The authors perform 59 analyses on behavioral data from 53 studies and conclude that single-session tDCS has no reliable effect on cognitive performance, including language, memory, and executive function. However, we performed an in-depth review of this meta-analysis and identified substantial methodological issues, including inconsistent and inappropriate data selection [2e6], mischaracterization of examined studies [7], incorrect subject number [8], and problematic statistical analyses, which systematically produce amisleading picture of tDCS findings. In light of these issues, we encourage serious caution in interpreting the results of this study. A meta-analysis is a two-stage process. First, the effect size for each study is calculated. Second, these statistics are combined into aweighted value to produce a summary effect size that informs a broader conclusion about the body of research. We structure our discussion of this meta-analysis accordingly. First, we reviewmethodological concerns related to data selection from individual studies. Because our own research is primarily concerned with the neurobiology of language, we focus on an in-depth review of each study from the language section of the meta-analysis. The individual issues from the language section are summarized in Table 1. Since there are many other studies we did not review in this level of detail, we suggest that the authors of studies outside of the language section closely examine the data used to calculate the effect sizes reported in this meta-analysis. In the last section, we review issues related to the second stage of the meta-analysis, discussing broader methodological and statistical concerns.

Causal Evidence for a Mechanism of Semantic Integration in the Angular Gyrus as Revealed by High-Definition Transcranial Direct Current Stimulation

A defining aspect of human cognition is the ability to integrate conceptual information into complex semantic combinations. For example, we can comprehend “plaid” and “jacket” as individual concepts, but we can also effortlessly combine these concepts to form the semantic representation of “plaid jacket.” Many neuroanatomic models of semantic memory propose that heteromodal cortical hubs integrate distributed semantic features into coherent representations. However, little work has specifically examined these proposed integrative mechanisms and the causal role of these regions in semantic integration. Here, we test the hypothesis that the angular gyrus (AG) is critical for integrating semantic information by applying high-definition transcranial direct current stimulation (tDCS) to an fMRI-guided region-of-interest in the left AG. We found that anodal stimulation to the left AG modulated semantic integration but had no effect on a letter-string control task. Specifically, anodal stimulation to the left AG resulted in faster comprehension of semantically meaningful combinations like “tiny radish” relative to non-meaningful combinations, such as “fast blueberry,” when compared to the effects observed during sham stimulation and stimulation to a right-hemisphere control brain region. Moreover, the size of the effect from brain stimulation correlated with the degree of semantic coherence between the word pairs. These findings demonstrate that the left AG plays a causal role in the integration of lexical-semantic information, and that high-definition tDCS to an associative cortical hub can selectively modulate integrative processes in semantic memory. SIGNIFICANCE STATEMENT A major goal of neuroscience is to understand the neural basis of behaviors that are fundamental to human intelligence. One essential behavior is the ability to integrate conceptual knowledge from semantic memory, allowing us to construct an almost unlimited number of complex concepts from a limited set of basic constituents (e.g., “leaf” and “wet” can be combined into the more complex representation “wet leaf”). Here, we present a novel approach to studying integrative processes in semantic memory by applying focal brain stimulation to a heteromodal cortical hub implicated in semantic processing. Our findings demonstrate a causal role of the left angular gyrus in lexical-semantic integration and provide motivation for novel therapeutic applications in patients with lexical-semantic deficits.

Semantics of the visual environment encoded in parahippocampal cortex

Semantic representations capture the statistics of experience and store this information in memory. A fundamental component of this memory system is knowledge of the visual environment, including knowledge of objects and their associations. Visual semantic information underlies a range of behaviors, from perceptual categorization to cognitive processes such as language and reasoning. Here we examine the neuroanatomic system that encodes visual semantics. Across three experiments, we found converging evidence indicating that knowledge of verbally mediated visual concepts relies on information encoded in a region of the ventral-medial temporal lobe centered on parahippocampal cortex. In an fMRI study, this region was strongly engaged by the processing of concepts relying on visual knowledge but not by concepts relying on other sensory modalities. In a study of patients with the semantic variant of primary progressive aphasia (semantic dementia), atrophy that encompassed this region was associated with a specific impairment in verbally mediated visual semantic knowledge. Finally, in a structural study of healthy adults from the fMRI experiment, gray matter density in this region related to individual variability in the processing of visual concepts. The anatomic location of these findings aligns with recent work linking the ventral-medial temporal lobe with high-level visual representation, contextual associations, and reasoning through imagination. Together, this work suggests a critical role for parahippocampal cortex in linking the visual environment with knowledge systems in the human brain.

Semantic Memory: Cognitive and Neuroanatomical Perspectives

This review article summarizes our current understanding of a specific type of human memory known as semantic memory. Semantic memory is our basic world knowledge of words, objects, people, and places. The first section of this article describes semantic memory within a broader context by discussing the major division between episodic memory and semantic memory. We next review cognitive and neuroanatomical perspectives of the semantic memory system, including how sensory, motor, and heteromodal regions play a role in representing modality-specific and modality-invariant semantic knowledge of the world.