Thursday, April 16, 2015

Mapping the language system: Part 1

My colleagues and I have a pair of papers coming out in Nature Communications and Neuropsychologia that I'm particularly excited about. The data came from Myrna Schwartz's long-running anatomical case series project in which behavioral and structural neuroimaging data were collected from a large sample of individuals with aphasia following left hemisphere stroke. We pulled together data from 17 measures of language-related performance for 99 participants, each of those participants was also able to provide high-quality structural neuroimaging data to localize their stroke lesion. The behavioral measures ranged from phonological processing (phoneme discrimination, production of phonological errors during picture naming, etc.) to verbal and nonverbal semantic processing (synonym judgments, Camel and Cactus Test, production of semantic errors during picture naming, etc.). I have a lot to say about our project, so there will be a few posts about it. This first post will focus on the behavioral data.

We used factor analysis to reduce the 17 measures to 4 underlying functional systems (also called principal components, or latent variables, or factors), which captured 76% of the variance in the original data:
  1. Semantic Recognition: difficulty recognizing the meaning or relationship of concepts, such as synonym judgments, semantic category discrimination, Camel and Cactus Test, and Peabody Picture Vocabulary Test.
  2. Speech Recognition: difficulty with fine-grained speech perception, such as phoneme discrimination and rhyme judgment.
  3. Speech Production: difficulty planning and executing speech actions, such as word and nonword repetition and the tendency to make phonological errors during picture naming (e.g., giraffe --> “girappe”).
  4. Semantic Errors: making semantic errors during picture naming (e.g., giraffe --> “zebra”), regardless of performance on other tasks that involved processing meaning.

These four factors may seem very intuitive and perhaps inevitable, but there are several alternative outcomes that would have been equally intuitive, so I want to highlight two ways in which these results might be surprising. First, the behavioral tests included measures of verbal short-term memory (immediate serial recall, semantic and phonological span tasks, nonword repetition), so we could have observed a STM factor. Instead, semantic STM was part of the semantic recognition factor and ISR and phonological STM was part of the speech recognition and production factors. This is not to say that STM is not important, but the domain-specific contribution (i.e., phonological or semantic processing) seems to be more important than a domain-general STM contribution. 

Second, we might have found dissociations between performance on verbal (words) and nonverbal (pictures) semantic tasks. Consider the three semantic measures mentioned above: synonym judgments, Camel and Cactus Test, and making semantic errors during picture naming. A core semantic deficit (such as semantic dementia) should affect performance on all three, which would produce a single semantic factor. A verbal semantic deficit should primarily affect just synonym judgments (and other semantic tasks that are entirely in the verbal domain) and possibly production of semantic errors (since it includes a verbal component). Instead, we found very high correlations among tasks involving extracting semantic information from either words or pictures (or both, such as word-to-picture matching), and performance on these tasks were not very correlated with production of semantic errors. We take this to mean that there is an important distinction between the functional systems involved in extracting semantic information (Semantic Recognition) and using that information to drive (verbal) behavior.

The next post (Part 2) will focus on the lesion-symptom mapping results, which identify the left hemisphere regions critical for each of the four functional systems identified in the factor analysis.

[EDIT: Added links for Neuropsychologia article.]

ResearchBlogging.orgMirman, D., Chen, Q., Zhang, Y., Wang, Z., Faseyitan, O.K., Coslett, H.B., & Schwartz, M.F. (2015). Neural Organization of Spoken Language Revealed by Lesion-Symptom Mapping. Nature Communications, 6 (6762), 1-9. DOI: 10.1038/ncomms7762.
Mirman, D., Zhang, Y., Wang, Z., Coslett, H.B., & Schwartz, M.F. (in press). The ins and outs of meaning: Behavioral and neuroanatomical dissociation of semantically-driven word retrieval and multimodal semantic recognition in aphasiaNeuropsychologia. DOI: 10.1016/j.neuropsychologia.2015.02.014

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