The majority of deaf children have hearing parents, whose primary language is a spoken language. Children born severely or profoundly deaf do not have access to a spoken language and, thus, do not acquire a language from birth. In Germany, these children usually acquire DGS from school mates after entering primary school or even later [1, 2]. Deaf children who lack input from a language model in the family usually generate a gestural communication system called ‘homesign’. Such a system, however, is not a fully realized natural language and hence does not support language development . By contrast, language development in deaf children of deaf parents whose primary language is a sign language develops similarly to hearing children of hearing parents [4–6]. From a linguistic point of view, sign languages are complete, natural, and fully realized languages with a phonology, morphology, syntax, and semantics [7, 8]. Furthermore, studies on L1 processing have shown that neural correlates of oral language processing in native speakers and sign language processing in deaf signers are largely overlapping [9–11].
In our ERP study, we investigated the effects of a delayed L1 acquisition in a violation paradigm. Semantic violations are known to be associated with a negative ERP, the so called ‘N400 effect’ that has mostly been observed over centro-parietal electrode sites, particularly for written languages . The N400 is considered to reflect lexical semantic integration processes [13, 14]. Additionally, semantic violations within the arguments of the verb sometimes elicit a positive ERP as well, which has been related to the recruitment of additional resources for processing complex stimuli [13, 15, 16]. By contrast, syntactic processing has been associated with the left anterior negativity (LAN) and the P600 (or syntactic positive shift, SPS ). The LAN occurred for example after congruency violations with a similar latency as the N400 [18–21]. The LAN seems to reflect either relatively automatic syntactic processes [22, 23], and/or working memory load due to complex processing operations . The P600 has been observed after various syntactic anomalies, such as morphosyntactic violations, and within garden-path sentences [21, 25]. It is considered to reflect the processing costs of a re-analysing process after an anomaly detection [for example ].
L2 learners who are exposed to the L2 after puberty have shown lower performance and a higher variability in L2 processing compared to native speakers of the same language. A negative correlation between the age of onset of acquisition of an L2 and the achieved grammatical competence has been observed in many studies [for an overview see for example: [27, 28]]. Importantly, lexical-semantic aspects of a language seem to be less affected by a late language acquisition than syntactical and phonological aspects .
Many studies found that the semantic effect (N400) is robust to effects of age of acquisition (AoA) [23, 30], whereby the syntactic LAN effect was affected to a larger degree. For example, Weber-Fox et al.  found that even L2 learners with an AoA of four years did not show a native-like LAN. Chen et al.  investigated Chinese L2 learners of English: Despite accurate grammatical judgements for subject-verb agreement violations, they did not show a LAN but a negativity between 500–700 ms. These ERP differences between L1 and L2 learners have been suggested to reflect a reduced automatic language processing in L2 learners. In contrast to native speakers, L2 learners seem to explicitly recapitulate the words and phrases of the L2, resulting in an additional drain on working memory load . These differences in language processing between L1 and L2 speakers have been explained by the hypothesis of a sensitive (SP) or critical period (CP) for language acquisition in the development of the nervous system, during which learning capabilities are enhanced. According to Knudsen’s definition , a CP is characterized by an abrupt loss of learning capabilities after its expiration, whereas a SP only implies a considerable decline of learning capabilities after its expiration . However, apart from AoA, differences in L2 processing are influenced by the similarity between the L1 and L2 and/or the proficiency level of the participants [23, 35–37]. Automatic parsing processes, for example, as reflected in the LAN, have been shown in highly proficient L2 learners in several ERP studies [23, 35, 38].
By comparing the L2 processing of (1) deaf people who had acquired German Sign Language (DGS) as their L1 from their deaf parents (henceforth: ESL for Early Sign Language learners), and (2) deaf people who had not been exposed to sign language during the first years of life because they had hearing parents (henceforth: LSL for Late Sign Language learners) it is possible to study the effects of a delayed L1 acquisition compared to a timely L1 acquisition in sign language users when tested in their L2 German. German processing was assessed by analysing a group of (3) hearing German native speakers (henceforth: EGL for Early German Language learners). Both groups of deaf people, ESL and LSL, started to learn German at the time of primary school enrolment.
In this regard, Mayberry and Lock  compared the English competence of deaf native speakers of a sign language and hearing native speakers of a spoken language with deaf people with a delayed L1 acquisition (sign language). All three groups were tested in their L2 English. Native speakers of a sign language and native speakers of a spoken language performed similarly on a high level. Interestingly, deaf people with a delayed exposure to sign language performed significantly worse than both groups of native speakers. Chamberlaine and Mayberry  found further evidence for the notion that strong L1 skills in a sign language can scaffold strong skills in the written representation of a spoken L2. These authors [39, 40] suggest that children need the benefits of a natural language irrespectively of its modality for any successful language acquisition. By learning an L1 from birth, basic abstract principles of form and structure are acquired that are independent of the sensory motor modality through which a language is expressed. These principles create the lifelong ability to learn a language.
The effects of a delayed L1 acquisition on the neural processing mechanisms in a written L2 in sign language users have not been investigated yet. Using ERPs, we were able to assess the effects of a delayed L1-acquisition separately on semantic and syntactic aspects of language processing. Semantic ERPs are relatively robust to effects of AoA [23, 30], in contrast to the LAN that often differentiates between native speakers and L2 learners: L2 learners who are not highly proficient usually do not show a LAN [30, 31].
In accord with the findings of Mayberry and Lock , the LSL participants were expected to reach a lower performance level than ESL in several language tests (ATBG, see materials section) and in the task of the EEG experiment.
With regards to the ERP results of the group of EGL in accordance with previous findings in native speakers [12, 21, 22, 25], we expected an N400 with a centro-parietal topography in the semantic condition. In the syntactic condition we expected a LAN at the left anterior clusters L1 and/or L2 [see also ]. The LAN was expected to be followed by a posteriorly distributed positivity (P600) in the syntactic condition.
For ESL a similar N400 effect in the semantic condition has been predicted. Previous findings have suggested that L2 learners who are neither highly proficient nor very familiar with the grammatical phenomenon do not show a LAN and/or a reduced P600 [30, 37, 42]. However, we recently reported both a LAN and a P600 for ESL similar to hearing L2 learners .
As previous studies give reason to expect differences between ESL and LSL , we predicted the absence of a LAN effect in LSL. Since syntactic and phonological aspects of language are generally more vulnerable to AoA effects than lexical-semantic aspects, we also expected an N400 effect in the group of LSL .