Figure 4.

Lack of state-dependent effects of ketamine (6.25 or 12.5 mg/kg) on encoding and retention. A, Schematic illustrates presample dosing of saline or ketamine, followed by a pretest dosing of saline or ketamine. Each rat (n = 24) received each of seven treatment combinations [Sal–Sal, Ket(6.25)–Sal, Ket(12.5)–Sal, Sal–Ket(6.25), Sal–Ket(12.5), Ket(6.25)–Ket(6.25), and Ket(12.5)–Ket(12.5)] over the course of 7 weeks of testing (1 treatment per week). B, Effects of ketamine on encoding and retrieval, as well as the greater impairment when both the sample and test trials were run under the influence of ketamine. Gray band illustrates the range of mean errors per day for all 1 h delay trials the day after ketamine or saline treatments. *p < 0.05 compared with Sal–Sal treatment mean (Dunnett's t tests); **p < 0.05 compared with indicated ketamine dose (paired t test). C, Effects of collapsed dose treatments on first error by type. Note that ketamine administered before the test session or before both the sample and test session altered the distribution of first errors by reducing the number of errors to the adjacent arms and increasing the number of first errors to both the previous goal position and other arms. The distribution of previous, adjacent, and other errors during both pretest and presample/pretest ketamine trials was significantly different from all other distributions (*p < 0.001, χ² test). This distribution of errors is similar to that observed in Figure 3C (pretest ketamine doses) and further suggests that, under the influence of ketamine, rats are accessing the previously formed representation of the goal and not the newly formed representation. S₁, S₂, Start arms; G, goal arm.