For example, the P300 component is well known to appear in response to emotional stimuli
, and shows greater amplitudes in response to negative stimuli
than to positive ones (e.g., Chapman, McCrary, Chapman, & Martin, 1980; Conroy & Polich, 2007; Cuthbert, Schupp, Bradley, Birbaumer, & Lang, 2000; Delplanque, Silvert, Hot, Rigoulot, & Sequeira 2006a; Delplanque, Silvert, Hot, & Sequeira, 2006b; Ito, Larsen, Smith, & Cacioppo, 1998; Keil, Bradley, Hauk, Rockstroh, Elbert, & Lang, 2002; Naumann, Bartussek, Diedrich, & Laufer, 1992).
Therefore, it is usually inferred from the ability of animals to discriminate between the exposed stimuli
after one of them has been conditioned (i.e., Honey & Hall, 1989; Mackintosh, Kaye, & Bennett, 1991; Rodriguez & Alonso, 2011; Wills, Wells, & Mackintosh, 1994).