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 table.
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
(a) 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.
(b) Compute effect size using estimated Cohen's d.