Humans often change their views or opinions while interacting with each other. This often leads to behavioral changes. Such changes are often reciprocal and ultimately lead to an agreement or conclusion. One way to experimentally study human reciprocity would be to offer participants to take part in collective problem solving. This study analyzed feedback-related negativity (FRN) components of visual event-related potentials (ERP) in order to examine how the brain activity changes during joint performance of a task aimed at identifying a genuine image of a famous masterpiece as opposed to its mirror reflection as a function of a number of matched answers. The results of our electroencephalographic analysis showed that both erroneous and mismatched choices evoked comparable FRN responses in the brain activity of jointly working participants, possibly reflecting individual learning process based on action-monitoring and error-detection. When the subjects were asked to judge the stimuli for the second time following the peer’s feedback, the number of matched answers significantly increased while the amplitude of prediction error signal and FRN decreased, indicating conformity changes, possibly underlying the attunement. Our results agree with previous FRN findings supporting the neurobiological model of reinforcement learning, offering a possible neural mechanism of behavioral reciprocity and social attunement.