Effects of significance of auditory location changes on event related brain potentials and pitch discrimination performance.

We examined effects of significance of task irrelevant changes in the location of tones on the mismatch negativity (MMN) and P3a event related brain potentials. The participants were to discriminate between two frequency modulated tones differing from each other in the direction of frequency glide. Each tone was delivered through one of five loudspeakers in front of the participant. On most trials, a tone was presented from the same location as the preceding tone, but occasionally the location changed. In the Varying Location Condition, these changes, although irrelevant with regard to pitch discrimination, were still significant for performance as the following tones were presented from the new location where attention had to be therefore shifted. In the Fixed Location Condition, the location changes were less significant as the tones following a location change were presented from the original location. In both conditions, the location changes were associated with decreased hit rates and increased reaction times in the pitch discrimination task. However, the hit rate decrease was larger in the Fixed Location Condition suggesting that in this condition the location changes were just distractors. MMN and P3a responses were elicited by location changes in both conditions. In the Fixed Location Condition, a P3a was also elicited by the first tone following a location change at the original location while the MMN was not. Thus, the P3a appeared to be related to shifting of attention in space and was not tightly coupled with MMN elicitation.