Abstract: The main objective of the research reported here is to investigate the preattentive processing of changing from level tone to contour tone and vice versa. The long to short duration change with level to falling/rising and falling/rising to level tone changes perception elicited MMN between 196-220 msec with reference to the standard-stimulus ERPs. The long to short duration and level to falling/rising tone changes elicited a strong MMN bilaterally for both native and nonnative speakers of Thai, unlike short to long duration and falling/rising to level tone changes. Source localization analyses performed using LORETA-Key demonstrates that sources were obtained in the Middle Temporal Gyrus (MTG) of the left hemisphere and in the Superior Temporal Gyrus (STG) of the right hemisphere for both groups. The present study demonstrated that the grand enterprise of mapping language onto the human brain can be vitally enhanced by MMN studies.