BACKGROUND: This study investigates the modulatory effect of stimulus sequence on neural responses to novel stimuli. A group of 34 healthy volunteers underwent event-related functional magnetic resonance imaging while performing a three-stimulus visual oddball task, involving randomly presented frequent stimuli and two types of infrequent stimuli - targets and distractors. NEW METHOD: We developed a modified categorization of rare stimuli that incorporated the type of preceding rare stimulus, and analyzed the event-related functional data according to this sequence categorization; specifically, we explored hemodynamic response modulation associated with increasing rare-to-rare stimulus interval. RESULTS: For two consecutive targets, a modulation of brain function was evident throughout posterior midline and lateral temporal cortex, while responses to targets preceded by distractors were modulated in a widely distributed fronto-parietal system. As for distractors that follow targets, brain function was modulated throughout a set of posterior brain structures. For two successive distractors, however, no significant modulation was observed, which is consistent with previous studies and our primary hypothesis. COMPARISON WITH EXISTING METHODS: The addition of the aforementioned technique extends the possibilities of conventional oddball task analysis, enabling researchers to explore the effects of the whole range of rare stimuli intervals. CONCLUSION: This methodology can be applied to study a wide range of associated cognitive mechanisms, such as decision making, expectancy and attention.

Behavioural and Social Neuroscience Research Group CEITEC Central European Institute of Technology Masaryk University Brno Czech Republic

Department of Neurology St Anne's University Hospital and Medical Faculty of Masaryk University Brno Czech Republic

Multimodal and Functional Neuroimaging Research Group CEITEC Central European Institute of Technology Masaryk University Brno Czech Republic

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