The number of premature responses varied as a function of ITI duration [F(2,35)=16.2, p<0.0001], increasing above the normal baseline conditions (ITI=5 sec) at values of 10 or 20 sec (upper panel). The MCD also varied as a function of ITI duration [F(2,35)=5.2, p=0.014], and was increased only when the ITI was increased to 20 sec (upper panel: *, significantly different from ITI=5, p<0.05). The number of trials completed was significantly decreased when the ITI was increased from 5 to either 10 or 20 sec [F(2,35)=130.1, p<0.0001], but the omission rate was not changed [F(2,35)=0.6, p=0.5] (lower panel: *, significantly different from ITI = 5 sec, p < 0.001. #, significantly different from ITI=10 sec, p<0.01.) N=12.