It has been shown recently that a temporal window of approximately 3 s has a modulatory effect on mismatch negativity (MMN). This special temporal window has been interpreted as representing the \"subjective present,\" and reflecting a temporal segmentation in behavioral and cognitive functions. A more detailed look into the temporal structure of the MMN appeared to be reasonable as group data might shadow the underlying mechanisms because of too-high response variance. In this study, we tested one subject on 11 successive days at the same circadian phase using a passive auditory oddball paradigm with interstimulus intervals (ISIs) ranging from 1 s to 6 s. We observed a U-shape function of MMN showing the largest amplitudes to the oddball stimuli with an ISI of 2 s and 3 s being flanked by smaller response amplitudes for shorter and longer ISIs. This result pattern can be explained with an oscillatory neural mechanism underlying the temporal modulation of MMN. Besides confirming and substantiating temporal segmentation in sensory processing, the present study also demonstrates that a single case study can be a useful and complementary tool in cognitive research.