Recent studies using the mismatch negativity (MMN) component, an index of automatic detection of sound change, suggested that the conjunctions of auditory features are preattentively represented in the brain. These studies, however, used only sequentially presented sounds and thus are not fully comparable with related studies on vision. Hence, the present study employed simultaneous, spatially distributed sounds and tested whether the auditory features of concurrent sounds can be correctly conjoined without focal attention to the sounds.
Twelve young adults without formal musical education participated in the study. Stimulus sequences consisted of repetitive two types of sounds differing from each other in timbre and pitch. These two types of sounds were presented simultaneously through separate loudspeakers. Subjects were engaged in a visual 1-back or 3-back working-memory task and ignored the sounds. Occasional reversals of pitch- timbre conjunction elicited MMNs of a very similar amplitude and latency irrespective of the task load, thus indicating that auditory features are preattentively conjoined even when two sound objects are simultaneously presented.