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A critical motivational factor in maintaining health and well-being is how we view ourselves. This thesis presented a series of studies on modulating two forms of positive biases: a self-positivity bias when forecasting future adverse life events (i.e., optimism bias) and a self-positivity bias when evaluating one’s own personality traits (i.e., self-evaluative memory bias). The first study (Chapter 2) aimed at providing EEG evidence during a belief updating task to assess optimism biases. Results showed that optimistic belief-updating biases emerged immediately and became significantly larger after 24 hours, suggesting an active role of valence-dependent offline consolidation processes in the change of optimism biases. Participants also showed optimistic memory biases: they were less accurate in remembering undesirable feedback probabilities than desirable feedback probabilities, with inferior memories of undesirable feedback associated with lower belief updating in the delayed test. Examining event-related brain potentials (ERPs) revealed the desirability of feedback-biased initial encoding: desirable feedback elicited larger P300s than undesirable feedback and the amplitudes of P300 also positively predicted belief updating and memory accuracy. To uncover the causal relationship between brain activity and optimism bias, the second study (Chapter 3) used transcranial alternating current stimulation (tACS) to target the right frontal area to probe the causal relationship between frontal alpha asymmetry (FAA) and optimism bias. Although alpha tACS did not modulate FAA or optimism bias, the gamma-tACS group modulated optimism biases. The …