Structural White Matter Correlates of the Crowding Effect: Insights From a Tractography Study of the Arcuate Fasciculus Post-Hemispherotomy

Abstract

The neuropsychological crowding effect denotes the reallocation of cognitive functions within the contralesional hemisphere following unilateral brain damage, prioritizing language at the expense of nonverbal abilities. This study investigates structural white matter correlates of crowding in the arcuate fasciculus (AF), a key language tract, using hemispherotomy as a unique setting to explore structural reorganization supporting language preservation. We explore two main hypotheses. First, the contralesional right AF undergoes white matter reorganization correlated with preserved language function at the expense of nonverbal abilities following left-hemispheric damage. Second, this reorganization varies with epilepsy etiology, influencing different stages of developmental language lateralization. This retrospective study included individuals post-hemispherotomy and healthy controls. Inclusion criteria were; (1) being a native German speaker, (2) having no MRI contraindication, (3) the ability to undergo approximately 2 h of MRI scans, and (4) the ability to participate in neuropsychological assessments over two consecutive days. Neuroimaging included T1-, T2-, and diffusion-weighted imaging, alongside postoperative neuropsychological assessments, where it was taken as evidence for crowding if verbal IQ exceeded performance IQ by at least 10 points. The AF was reconstructed using advanced tractography, and CoBundleMAP was used to compare morphologically corresponding AF subsections. Statistical significance was set at , with correction for multiple comparisons applied across contiguous tract sections using Threshold-Free Cluster Enhancement. The final cohort comprised 22 individuals post-hemispherotomy (median age: $20.4$ years, range: $12.3-43.9$; 55% female; 55% with left-sided surgeries) and 20 healthy controls (median age: $23.8$ years, range: $15.5-54.0$; 55% female). Crowding was associated with significantly higher fractional anisotropy (FA) in the AF ($p=0.015$, Cohen's $d=1.69$), but only observed in individuals with left-sided hemispherotomy, localized to a subsection between Geschwind's territory and Wernicke's area ($p_{\text{corrected}}=0.02$). This region also displayed significantly higher normalized FA in AF of individuals with congenital etiology and crowding compared to acquired etiology and no crowding ($p_{\text{corrected}}=0.0189$). This study identifies previously unreported neural correlates of crowding in right contralesional AF of individuals post-hemispherotomy and highlights specific AF subsections involved in preserving language functions at the cost of nonverbal abilities. The findings suggest a link between crowding and epilepsy etiology, particularly in the region spanning Geschwind's territory and Wernicke's area.