1. A method of second language learning, comprising: a) providing a user model of latent traits that represent different semantic and syntactic second language learning skills including word meanings and grammatical structures, a state of each said latent trait representing the learner's mastery of that second language learning skill; b) presenting a lesson of second language learning materials including error-free text, text that includes syntax errors and text that includes semantic errors to test the one or more latent traits on a display to elicit a response from a learner, said materials presented on the display so that the learner is allowed to move his or her eyes freely in response to the displayed materials; c) measuring EEG data of the learner's brain activity from a plurality of electrodes placed on the learner's scalp; d) tracking the learner's eye movements to determine fixations on the materials; for each fixation, e) locking a window to the fixation and applying that fixation-locked window to the EEG data to generate a time segment of EEG data; f) extracting one or more features from the time segment of EEG data; g) presenting said one or more features to a classifier to detect an event-related potential (ERP) to generate a fixation-locked cue indicative of whether the learner exhibited a significant cognitive response to the displayed materials; h) identifying the text in the second language learning materials associated with the fixation; i) retrieving from the user model the one or more latent traits tested by the identified text including the one or more words at the point of fixation and the grammatical structure that contains the one or more words; j) using the fixation-locked cue to update the state of the one or more latent traits retrieved from the user model; k) monitoring the states of the latent traits to assess mastery or difficulty the learner is having with specific learning skills; and l) customizing a subsequent lesson based on the states of one or more latent traits in the user model.