Comprehension of verb directionality in LIS and LSF

The present work reports the results of a comprehension task on verb directionality in Italian Sign Language (LIS) and French Sign Language (LSF) considering native and non-native signers. Our goals were to study age of first language exposure effects on the comprehension of verb agreement in LIS and LSF, to verify whether a significant difference between forward and backward directionality was found, and see if our results may provide insight about the nature (gestural vs. linguistic) of verb directionality in sign languages. In both languages we found that the ability to comprehend verb agreement is affected in non-native signers. This indicates that delayed first language exposure has long lasting effects in adulthood. We argue that our results support analyses of verb agreement as a fully grammatical phenomenon.