responders and found that the administration of methyl-phenidate, a drug that is similar to d-amphetamine, reversed stop RTs only in slow-stopping rats, whereas it worsened stop RTs in fast-stopping rats. A similar pattern of response has been observed in humans (de Wit et al., 2000), where administration of d-amphetamine did not significantly change stop RT in the overall sample of participants, but when participants were divided into slow and fast stoppers by the median split, the drug improved stop RT only in participants with an initially long stop RT. With the use of genotype locus, rather than slow or fast RT, to segregate our participants into two groups, our finding that d-amphetamine improved stop RT in the rs12364283 A/A group (with initially longer stop RT) and worsened stop RT in the rs12364283 A/G + G/G group (with initially shorter stop RT) is consistent with the two previous studies.
