The P300 event-related potentials: A one-humped dromedary's saddle on a two-humped camel

Differences between target and non-target probe processing--combined evidence from fMRI, EEG and fMRI-constrained source analysis

Previous studies reported heterogeneous findings in working memory tasks when examining differences between correct recognition (targets) and correct rejection (non-targets). In the present study, twenty human participants completed a delayed match-to-sample task in two separate functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) sessions. Targets and non-target items were presented at different within-trial positions. We used fMRI-constrained source analysis to investigate the spatio-temporal neuronal dynamics of probe processing. Probe type-related differences were modulated by position in the trial or by the ratio of target stimuli to non-target stimuli at different trial positions. fMRI-constrained source analysis revealed a temporal pattern of source activities starting in occipital and temporal brain regions, followed by a simultaneous engagement of parietal and frontal brain regions and a later activity of a source in pre-SMA (supplementary motor area). Source activities demonstrated a specific involvement of left fusiform gyrus in the non-target condition compared to the target condition that might be associated with mental imagination of the target stimulus during non-target probe processing. Source activities, furthermore, showed the anterior cingulate to be particularly involved in target processing compared to non-target processing before response execution and the pre-SMA before and during response execution. These brain areas appear to be activated in different stages of conflict managing operations due to a lower stimulus frequency of target trials compared to non-target trials at different target positions in the present design.

The waltzing oddball

We investigated whether task relevance and probability interact to influence P3 amplitude. High and low tones were presented in random order with equal probability. In the control condition (standard oddball), every high tone had to be counted. In the waltz condition, high tones had to be counted only if they were preceded by two other high tones. It was predicted that the P3s evoked by targets in the waltz condition would be larger than the P3s evoked by the same sequence of targets in the oddball condition. That is, the frequency of occurrence of the targets should have an effect on P3, in addition to effects of the frequency of stimulus occurrence and stimulus task relevance (target/nontarget). This prediction was upheld. However, the largest P3s were evoked by nontargets following two high tones in the waltz condition. These P3s had a more anterior topographic maximum than usual. We contend that these anterior P3s reflect the interruption of an ongoing task and cannot be easily fit into the framework of the two concepts of task relevance and probability.