Different scholars use different definitions of "boredom", which complicates research. Boredom has been defined by Cynthia D. Fisher in terms of its main central psychological processes: "an unpleasant, transient affective state in which the individual feels a pervasive lack of interest and difficulty concentrating on the current activity." Mark Leary et al. describe boredom as "an affective experience associated with cognitive attentional processes." In positive psychology, boredom is described as a response to a moderate challenge for which the subject has more than enough skill.