Appraisal of definition of baseline length for somatosensory evoked magnetic fields

•Stimulus onset should not be used for baseline segment in estimating dipoles.•Mean value of prestimulus periods could be used for analysis of evoked activities.•Mean value of whole raw data is available as baseline segment for evoked activities. The baseline (BL) segment in the prestimulus period is generally assigned as a reference of evoked activities. However, an experimenter empirically defines its length in each condition. So far, the criterion for the length of a BL segment has not been established. We evaluated the effect of the length of the BL segment by recording somatosensory evoked magnetic fields (SEFs) under fixed stimulus onset asynchrony (SOA). For the evaluation of the length of the BL segment in the prestimulus period, five proportions in relation to SOA were used as the BL segment. In addition, we adopted other two types of BL segment which were the single data point measured from the value of stimulus onset (BL0) and the mean value of the whole raw data throughout the recording (DC mean). We investigated the influence of the BL segments on SEFs by utilizing two indicators: normalized N20 m amplitudes and estimated locations of corresponding equivalent current dipoles (ECDs). Both indicators did not show any significant differences, based on the factor of BL segments, in any SOA conditions. The BL0 had by far the largest variation in the ECD locations.Therefore, utilizing stimulus onset as the BL segment should be avoided. In addition, considering that other BL segments provided comparable values by the two indicators, the DC mean can reasonably be adopted. We suggest that utilizing the DC mean could be employed as the BL segment.