In a wide variety of languages, it has been reported that vowel F0 at the onset tends to be lower after voiced than after voiceless consonants (e.g., Hombert et al., 1979). This phenomenon has been called onset voicing effect (OVE) by Kirby & Ladd (2015). Learning second language (L2) sounds is influenced by the first language (L1) sound system. This current study seeks to examine how English language learners who have a smaller L1 vowel inventory perceive English sounds. To reveal L2 sound perception, we measured the mouse trajectories related to the hand movements performed by Javanese and Sundanese language learners as they selected their responses. This paper investigates the presence of the OVE in Dutch initial labiodental fricatives. Previous studies have shown that Dutch initial fricatives are currently devoicing (e.g., Kissine et al., 2003; Pinget, 2015). We thus examine whether the OVE is maintained in the case of devoicing. The maintenance of the OVE in the case of devoicing would provide evidence for incipient tonogenesis, whereas the disappearance of the OVE would indicate that initial labiodental fricatives develop as a full merger. This study aims to map the patterns of real-time processing through compatible hand movements, to reveal any uncertainties participants have when making selections. The participants completed a categorization task, classifying stimuli as long or short vowels. Results show clear regional differences in the OVE that corresponds to the regional differences in fricative devoicing. The devoicing process seems thus to trigger a reduction of the OVE, but not (yet) its full disappearance. We argue that these results are in line with the full merger hypothesis.