Sebastián-Gallés et al. [The influence of initial exposure on lexical representation: Comparing early and simultaneous bilinguals. Journal of Memory and Language, 52, 240–255, 2005] contrasted highly proficient early Spanish-Catalan and Catalan-Spanish bilinguals, using Catalan materials in a lexical decision task (LDT). They constructed two types of experimental pseudowords, substituting Catalan phoneme /e/ for Catalan /ɛ/, or vice versa. Catalan-dominant bilinguals showed a performance asymmetry across experimental conditions, making more mistakes for /ɛ/→/e/ changes, than for /e/→/ɛ/ ones. This was considered evidence of a developed acceptance of mispronounced Catalan /ɛ/-words, caused by exposure to a bilingual environment where mispronunciations by Spanish-dominant bilinguals using their /e/-category abound. Although this indicated modified or added lexical representations, an open issue is whether such lexical information also modifies phoneme categories. We address this using a biophysically realistic neurodynamic model, describing neural activity at the synaptic and spiking levels. We construct a network of pools of neurons, representing phonemic and lexical processing. Carefully analyzing the dependency of network dynamics on connection strengths, by first exploring parameter space under steady-state assumptions (mean-field scans), then running spiking simulations, we investigate the neural substrate role in a representative LDT. We also simulate a phoneme discrimination task to address whether lexical changes affect the phonemic level. We find that the same network configuration which displays asymmetry in the LDT shows equal performance discriminating the two modeled phonemes. Thus, we predicted that the Catalan-dominant bilinguals do not alter their phoneme categories, although showing signs of having stored a new word variation in the lexicon. To explore this prediction, a syllable discrimination task involving the /e/-/ɛ/ contrast was set up, using Catalan-dominants displaying performance asymmetry in a repetition of the original LDT. Discrimination task results support the prediction, showing that these subjects discriminate both categories equally well. We conclude that subjects often exposed to dialectal word variations can store these in their lexicons, without altering their phoneme representations.