In vowel discrimination, commonly found discrimination patterns are directional asymmetries where discrimination is faster if differing vowels are presented in a certain sequence compared to the reversed sequence. Different models of speech sound processing try to account for these asymmetries based on either phonetic or phonological properties. In this study, we tested and compared two of those often-discussed models, namely the Featurally Underspecified Lexicon model and the Natural Referent Vowel framework. While most studies presented isolated vowels, we investigated a large (...) stimulus set of German vowels in a more naturalistic setting within minimal pairs. We conducted an mismatch negativity study in a passive and a reaction time study in an active oddball paradigm. In both data sets, we found directional asymmetries that can be explained by either phonological or phonetic theories. While behaviorally, the vowel discrimination was based on phonological properties, both tested models failed to explain the found neural patterns comprehensively. Therefore, we additionally examined the influence of a variety of articulatory, acoustical, and lexical factors but also the influence of factors beyond the well-known in depth via multiple regression analyses. The analyses revealed that the perceptual factor of perceived loudness has a greater impact than considered in the literature and should be taken stronger into consideration when analyzing preattentive natural vowel processing. (shrink)