Mindfulness meditation might improve the flexibility of mind-wandering, that is, the ability to shift attention from mind-wandering. Flexibility of mind-wandering could mediate the relationship between mindfulness and improvement in depression. However, there have been no studies of this relationship because of limitations in measurement methodology. Since the mindfulness-based intervention, which instructs participants to be aware of the occurrence of, and their own engagement in,

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... g, might bias self-reports of mind wandering, a measurement method that does not rely on participants' verbal report is needed. Therefore, we propose a novel method to evaluate the flexibility of mind-wandering, based on mind-wandering intensity estimation by machine-learning using electroencephalography. We estimated mind-wandering intensity using one-second electroencephalogram samples and a machine-learning model developed in previous research. Thus, we observed fluctuations in mind-wandering during a 14-minute meditation and defined the time required to shift attention from mind-wandering as an index of mind-wandering flexibility. We performed two experiments: The first targeted experienced meditators and the second assessed non-meditators before and after participating in a mindfulness-based intervention. These experiments revealed that flexibility of mind-wandering was correlated with the extent of meditation experience. In addition, the magnitude of the decrease in flexibility and severity of depression following the intervention were found to be correlated.