Modulating the difficulty of a visual oddball-like task and P3m amplitude.

It is often necessary to modulate the difficulty of an experimental task without changing physical stimulus characteristics that are known to modulate event-related potentials. Here, we developed a new, oddball-like visual discrimination task with varying levels of difficulty despite using almost identical visual stimuli. Gabor patches of one orientation served as frequent standard stimuli with 75% probability. Gabor patches with a slightly different orientation served as infrequent target stimuli (25% probability). Analyzing the behavioral outcomes revealed a successful modulation of task difficulty, i.e. the hard condition revealed decreased d' values and longer reaction times for standard stimuli. In addition, we recorded MEG and computed event-related fields in response to the stimuli. In line with our expectation, the amplitude of the P3m was reduced in the hard condition. We localized the sources of the P3m with a focus on those that are modulated by changes in task difficulty. The sources of P3m modulation by difficulty were found primarily in the centro-parietal regions of both hemispheres. Additionally, we found significant differences in source activity between the easy and hard conditions in parts of the pre and post-central gyrus and inferior parietal lobe. Our findings are in line with previous research suggesting that the brain areas responsible for the conventional P3m generators also contribute to a modulation by task difficulty.