et al., 2013). The amplitude of the P3 in the present study was also larger in the low-difficulty condition compared to the high-difficulty condition, perhaps due to reward expectancy being higher in the low-difficulty trials (Goldstein et al., 2006; Gruber and Otten, 2010; Wu and Zhou, 2009). Also, similar results have been found in other discrimination tasks, showing a diminished visual or auditory evoked P3 amplitude in difficult discrimination trials (Hoffman et al., 1985; Palmer et al., 1994; Polich, 1987; Senkowski and Herrmann, 2002). This has been related to decreased decision certainty (i.e. ‘equivocation’), since confidence in the decision made is reduced when discriminations are more difficult (Palmer et al., 1994; Ruchkin & Sutton, 1978). Moreover, both the reward and difficulty main effect might be partly explained by the relation of the target P3 to response execution (Doucet and Stelmack, 1999), with larger P3 amplitudes for faster responses. Hughes et al. (2012) also showed that the target-locked P3 amplitude was larger for easy compared to difficult detected target pictures in a rapid serial visual presentation task and results suggested that the P3 amplitude on single trials reflected the confidence in detecting a target. Hence, the P3 modulation probably reflects a
