Many sign languages, including American Sign Language (ASL) and Nicaraguan Sign Language (NSL), employ spatial devices that involve making rotations on a horizontal plane or taking the perspective of another person. There is mounting evidence that acquiring a sign language leads to enhanced spatial cognitive skills such as mental rotation (Emmorey 1993), and that the earlier in life one acquires sign language, the better the mental rotation skills (Martin 2009, 2012). Our recent work shows that deaf children who learned sign language before age six are more accurate than deaf children who learned sign language after age six, and than hearing non-­‐signers. This suggests that there is a window for optimal development early in life, after which the cognitive advantages of learning a sign language are attenuated. Two questions remain. First, are cognitive effects equivalent for all children who began learning language before age six, or are the advantages greater with even earlier learning? Second, what specific aspects of sign language knowledge contribute to those enhancements? To address these questions, deaf children in the US and Nicaragua completed a series of mental rotation and spatial language tasks. Mental Rotation Tasks: Mental rotation was compared in three groups: children first exposed to ASL by approximately age 2 (mean 1;6, n=18, mean test age 9;2), children first exposed to NSL by approximately age 4 (mean 4;3, n=11, mean age 11;10), and children first exposed to NSL by approximately age 8 (mean 8;6, n=11, mean age 12;2). Participants were presented with an exemplar object (either a doll with one arm raised, or an asymmetrical block array) and were asked to choose which of two rotated response options was an exact match to the exemplar. Trials included two object types (doll vs. block), on two planes (horizontal vs. vertical), at rotations of 0, 90, 120, and 180 degrees (see Fig. 1 for example stimuli). Mental rotation accuracy was greatest in the two early learning groups, and significantly different from late learners’ (p<.05) (see Fig. 2). This suggests that this window may close gradually up until age 6, and more abruptly afterwards. Spatial Language Tasks: All children completed two language tasks. In a production task, children described a block and a doll figure presented at different degrees of rotation. In a comprehension task, children viewed a video of a fluent ASL or NSL signer describing each stimulus, and chose its match from a set of four options. We coded the strategy type that children used to interpret and produce spatial information (e.g. whether children interpret the signs by mentally rotating, or use alternative strategies such as directly mapping the sign to the response item). The cognitive findings underscore the importance of getting an early start on language acquisition, and also suggest that for some children there is time to catch up. By characterizing the language abilities of those late learners who do catch up, we can determine which aspects of spatial language use contribute to the enhancement of mental rotation in all signers. References Emmorey, K., Kosslyn, S., Bellugi, U. (1993). Visual imagery and visual-­‐spatial language: Enhanced imagery abilities in deaf and hearing ASL signers. Cognition, 46, 139-­‐181. Martin, A. (2009). Does age of acquisition affect the relation between American Sign Language and Mental Rotation? Unpublished doctoral dissertation. University of Minnesota. Martin, A., Senghas, A., Pyers, A. (2012). Effects of delayed first language acquisition: Evidence from Nicaraguan Sign Language. Paper presented at Boston University Conference on Language Development 37. Figure 1: Sample Mental Rotation Stimuli Figure 2: Accuracy in Mental Rotation, by Age of Exposure Group