Negative polarity in quantifiers evokes greater activation in language-related regions compared to negative polarity in adjectives.

The processing of sentences with negative quantifiers (e.g., few) is more costly than of sentences that contain their positive counterparts (e.g., many). While this polarity effect is robust and reliably replicable, its neurological bases are not well understood. In this study, we use functional magnetic resonance imaging (fMRI) paradigm for 30 participants to assess the polarity effect in sentences with polar quantifiers, and compare it with the polarity effect of polar adjectives. Both in quantifiers and in adjectives, the polarity effect manifests in the anterior insula bilaterally. The polarity effect in quantifiers, however, shows greater activation in the left hemisphere than it does for adjectives. In particular, left inferior frontal gyrus (IFG) and left superior temporal sulcus (STS) show increased activation for polarity in quantifiers than in adjectives, which is the evidence for the specific involvement of the language network in this type of polarity processing. Using the polarity effect in adjectives as a control, we provide further evidence for the linguistic complexity that negative quantifiers implicate on processing.

Evaluating arcuate fasciculus laterality measurements across dataset and tractography pipelines.

The arcuate fasciculi are white-matter pathways that connect frontal and temporal lobes in each hemisphere. The arcuate plays a key role in the language network and is believed to be left-lateralized, in line with left hemisphere dominance for language. Measuring the arcuate in vivo requires diffusion magnetic resonance imaging-based tractography, but asymmetry of the in vivo arcuate is not always reliably detected in previous studies. It is unknown how the choice of tractography algorithm, with each method's freedoms, constraints, and vulnerabilities to false-positive and -negative errors, impacts findings of arcuate asymmetry. Here, we identify the arcuate in two independent datasets using a number of tractography strategies and methodological constraints, and assess their impact on estimates of arcuate laterality. We test three tractography methods: a deterministic, a probabilistic, and a tractography-evaluation (LiFE) algorithm. We extract the arcuate from the whole-brain tractogram, and compare it to an arcuate bundle constrained even further by selecting only those streamlines that connect to anatomically relevant cortical regions. We test arcuate macrostructure laterality, and also evaluate microstructure profiles for properties such as fractional anisotropy and quantitative R1. We find that both tractography choice and implementing the cortical constraints substantially impact estimates of all indices of arcuate laterality. Together, these results emphasize the effect of the tractography pipeline on estimates of arcuate laterality in both macrostructure and microstructure.

The robust and independent nature of structural STS asymmetries.

The superior temporal sulcus (STS) is an important region for speech comprehension. The greater language network is known to exhibit asymmetries in both structure and function, and consistent with that theory are reports of STS structural asymmetry in MRI-based, morphological measures such as mean thickness and sulcal depth. However, it is not known how these individual STS structural asymmetries relate to each other, or how they interact with the broader language asymmetry that manifests in other brain regions. In this study, we assess the interrelations of STS asymmetries in the human brain in vivo, using four independent datasets to validate our findings. For morphological measurements, we identify STS laterality effects consistent between our datasets and with the literature: leftward for surface area, and rightward for sulcal depth and mean thickness. We then add two more measurements of STS asymmetry: in T1, a quantitative index of the tissue's underlying biophysical properties; and in the projections to the STS from the arcuate fasciculus, a left-lateralized white-matter bundle that connects temporal regions (including STS) with frontal regions (including Broca's area). For these two new measurements, we identify no effect for T1 and a leftward effect for arcuate projections. We then test for correlations between these STS asymmetries, and find associations mainly between measurements of the same type (e.g., two morphological measurements). Finally, we ask if STS asymmetry is preferentially related to Broca asymmetry, as these are both important language regions and connected via the arcuate fasciculus. Using a linear model with cross-validation, we find that random regions are as successful as Broca's area in predicting STS, and no indication of a hypothesized leftward asymmetry. We conclude that although these different STS asymmetries are robust across datasets, they are not trivially related to each other, suggesting different biological or imaging sources for different aspects of STS lateralities.