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Individual Differences in Language Lateralization among Deaf Signers
Poster Session D, Saturday, September 13, 5:00 - 6:30 pm, Field House
Maria Zimmermann1,2, Piotr Tomaszewski2, Marina Bedny1; 1Johns Hopkins University, 2University of Warsaw
A canonical trait of language neurobiology is its left-lateralization. At the same time, laterality is modified by genetic and experiential factors, suggesting the right hemisphere is equally able to support language processing at birth (Bates et al., 2001; Newport et al., 2022). Some studies suggest sign languages are as left-lateralized as spoken languages (MacSweeney, 2002; Yang et al., 2024), while others report a more bilateral profile (Hickok et al., 2002; Newman et al., 2002), possibly due to the visuo-spatial properties of sign languages. Recent evidence suggests reading in Deaf individuals also engages the right hemisphere more (Sehyr et al., 2020), possibly driven by absence of early speech or auditory experience (Poeppel,et al., 2003). Here we asked how language modality (spoken vs. signed), age of first language access (native vs. delayed), and hearing status (deaf vs. hearing) contribute to language lateralization by working with Deaf native signers (n=20), Deaf non-native signers (n=15), and hearing native signers (CODAs; n=18). We used fMRI to measure language lateralization during Polish Sign Language (PJM) processing using two paradigms: a sentence comprehension localizer (Exp1; adapted from Fedorenko et al., 2010; Newman et al., 2015) and a naturalistic story (Exp2). (Exp1) Participants passively viewed PJM sentences and a motion-matched scrambled control, responding to occasional catch trials. (Exp2) The same participants passively watched a 20-minute naturalistic PJM narrative. While prior studies focused on group-level comparisons, we measured individual-level lateralization to test whether laterality varies more across Deaf signers than hearing speakers. All groups recruited the classic fronto-temporal language network, including bilateral middle temporal gyrus (MTG) and inferior frontal gyrus (IFG) for sentences versus control condition. We quantified hemispheric dominance using laterality indices (LIs), calculated as the proportion of activated voxels in the left versus right hemisphere, as well as separately in classic language regions. At the group level, both Deaf samples showed bilateral responses to language (mean = -0.17, CI = [-0.336, -0.005]). However, this group-level result masked substantial individual variability: Deaf participants’ LIs ranged from -1 (fully right-lateralized) to 1 (fully left-lateralized), with half showing right-lateralized activation in both native and non-native signers. In contrast, CODAs showed consistent left-lateralization, with LIs significantly greater than zero and significantly less individual variability than Deaf signers (mean = 0.13, CI = [0.032, 0.240]). To assess the reliability of hemispheric dominance across the localizer and naturalistic narrative, we defined laterality in the narrative as the normalized difference between left and right hemisphere intersubject correlation (ISC). Deaf signers showed a significant correlation between narrative-based LIs and those from the sentence localizer, indicating stable individual differences. No such correlation was observed in hearing signers, likely due to more uniform laterality patterns across individuals. We identified a fronto-temporal language network engaged in sign language (PJM) comprehension. Sign language processing was left-lateralized in hearing signers. In contrast, among Deaf signers, we observed stable individual differences in hemispheric dominance with an average bilateral profile. Our results suggest bilateral responses are not inherent to sign languages, but rather related to changes in early auditory or speech experience.
Topic Areas: Signed Language and Gesture,