string: An account of the present data in terms of sensitivity to reinforcement amount is consistent with previous data showing that methylphenidate can increase (Heyman, 1992) and that opiates can decrease (Egli, Schaal, Thompson, & Cleary, 1992; Lancaster & Dallery, 1999) sensitivity to reinforcement rate. Although this interpretation is possible, its application to the effects of methylphenidate in the present study is weakened by data reported by Pitts and Febbo (2004). They found that, although effects on estimates of sensitivity to reinforcement amount were somewhat inconsistent across pigeons, methamphetamine tended to decrease, rather than increase, this measure.