Event-related potentials (ERPs) associated with omitted somatosensory stimuli

Late positive ERP components elicited by omitted somatosensory stimuli in passive paradigm (passive MSPP) and in active time-estimation paradigm (active MSPP) were studied in healthy subjects with single-trial analysis and conventional averages. The active MSPP was also compared with P3 component elicited by somatosensory target stimuli (target P3). Single-trial analysis showed clearly discernible passive MSPP in response to omitted stimuli of the median nerve, whereas passive MSPP was hardly detected in response to omitted stimuli of the index finger. The subjects might pay passive attention to absence of somatosensory stimuli together with absence of the thumb muscle twitches. Single-trial analysis revealed a significantly greater latency fluctuation in passive MSPP than in active MSPP. The active MSPP showed a greater latency fluctuation than the target P3. The active MSPP showed a significantly longer latency and a smaller amplitude than the target P3 in conventional averages. Topography of active MSPP showed a symmetric central-parietal maximal distribution similar to that of target P3.