difference ( Nd ) event - related potential

Frontal and temporal lobe sources for electrical activity associated with auditory controlled attention (negative difference, Nd) were sought for comparison with those reported to arise from the earlier detection of stimulus-change (mismatch negativity, MMN: Jemel et al. 2002). In two sessions a month apart (T1 and T2), 14 subjects were presented with a 3-tone oddball passively, then as a discrimination. In EEG recordings (32 sites), Nd was calculated by subtraction of the event-related potential elicited by a non-attended stimulus from that after the same frequency-deviant as target. Putative generators in the 180-228 ms latency-range were modelled with brain electrical source analysis and mapped to the modified Montreal brain-atlas. Initial T1-analyses located bilateral Nd dipoles in the superior temporal gyrus (BA22) and the dorsolateral prefrontal cortex (BA8). Re-test allowed estimates of the temporal and spatial extension of activity. Peak activity occurred 14 ms later. Step-by-step analysis showed that the best spatial fit for the inverse-solutions extended 3-6 mm from the point sources, but for temporal lobe sources this increased 15 mm caudally. The right mid-frontal source (BA10) was rostral and ventral from that in the left superior frontal gyrus (BA8). T1 and T2 dipole strengths were well correlated. Nd measures of controlled attention localised to areas associated with sustained attention, problem-solving and working-memory. Temporal lobe sources were later and more posterior and medial than for automatic change-detection. Frontal Nd sources were more dorsal on the right and more rostral on the left than MMN dipoles reported for the right inferior frontal and left anterior cingulate. The sequence of information processing is reviewed.