Published reports indicate that a brain region called the nucleus accumbens (NAc) is involved in interval timing, which is the perception of time in the seconds-to-minutes range. To test this, researchers investigated whether removing the NAc interferes with rats’ ability to time the presentation of a liquid reward. Using a conditioning procedure, the researchers had rats press a lever for a reward that was delivered after 16 seconds. The time that eight rats responded the most

(peak responding) was recorded before and after a surgery to remove the NAc. The peak responding times are given in the table.

1. Test whether or not the difference in peak responding changed at a .05 level of significance

(two-tailed test). State the value of the test statistic and the decision to retain or reject the null hypothesis.

2. Compute effect size using estimated Cohen’s d.

Peak Interval Timing Before NAc Surgery After NAc Surgery 15 20 14 26 16 20 16 18 17 25 18 21 15 18 16 23