Mismatch negativity to stimulus omission?

The present experiment explored the effects of stimulus omission on mismatch negativity elicitation under varying conditions of attention. Fifty subjects were divided into five groups based on attentional focus during stimulus presentation. The three experimental groups (all of whom received 10% omitted stimuli) included a Reading (ignore all stimuli) Group, a Counting (attend to omitted stimulus) Group, and a Passive Attend (attend to stimuli alone) Group. The two Control Groups (all of whom received 100% standard stimuli) comprised a Reading (ignore all stimuli) Group and a Passive Attend (attend to all stimuli alone) Group. Stimuli were presented to the subjects in a single channel binaural auditory oddball paradigm. Subjects responses to the rare tones and also to the tone prior to the rare (pre) and following the rare (post) were monitored using EEG electrodes at three vertex sites (Fz, Cz, and Pz). Results indicated, (in contrast to orienting responses to omission), that there was little evidence of any mismatch negativity to omitted stimuli in any of the attentional conditions employed within the experiment. The results did not provide any support for Näätänen's (1990) suggestion that MMN may be an integral component of, or at least associated with, the OR since both appear to be preattentive neuronal mismatch mechanisms. The results did however suggest that refractory effects occurred in Post responses to the standard stimulus immediately following an omission. In addition, there was some evidence of dishabituation in the Post responses of the Reading (Ignore) Group. Only the Counting Group exhibited a significant P300 in response to the omitted stimuli predominantly at the Cz and Pz sites. The relationship between attention and mismatch negativity and its unique response to omission are considered in light of the current findings.