The present study investigated if/how individual differences in heritage language (HL) experience modulate gender agreement processing among Spanish heritage speakers (HSs). We reanalyzed the data from Luque and colleagues (2023), which reported an aggregate biphasic N400–P600. The present analysis revealed that sensitivity to morphological markedness was positively modulated by HL proficiency and exposure/use. Higher proficiency led to increased P600 across markedness conditions—the typical signature of L1-dominant processing—while increased Spanish exposure/use resulted in increased N400 for Default Errors—a signature attested only in HSs in this domain. Formal instruction led to increased N400 but reduced P600 for Feature Clash Errors. We interpret these results to suggest that the N400 reflects a morphophonological pattern-matching strategy with some HSs relying (more) on this mechanism as Spanish exposure and use increases. Markedness also modulated the relative engagement of pattern-matching (N400) versus automatic grammatical processing (P600), depending on the transparency/saliency of morphophonological patterns.
