Experimental Conditions (all with 8 trials per session)
Each experimental condition consisted of a forced (i.e., ‘sample’) run, followed by a free (i.e., ‘test’) run in the T-shaped maze. The correct choice arm across the block of 8 trials was pseudorandomized so that the same choice arm was not repeated more than twice consecutively. To start each trial, both T-maze arms were baited with a quarter of a multigrain hoop before the sample run, but access to one arm was blocked at its base with an aluminium barrier.
To begin the sample run, the rat was released from the start position and ran to the junction of the T-maze, where it turned into the pre-selected arm and ate the reward. The rat was then immediately picked up and the barrier at the choice point removed. Then, the animal was carried to the start position and allowed to begin the test run. After running down the stem of the maze, the rat could choose between the left and right arms. The animal received a food reward only if it alternated, i.e., selected the arm located opposite from the baited sample arm. A test run was considered correct when the animal’s back feet crossed markings at the base of each side arm. The animal was picked up and held until, the T-maze was reset, and the next trial commenced after 10-15s. Each rat completed all 8 trials prior to running the next animal.
1. Standard T-maze (all spatial cue types available) (Figure 2A) – This condition was the same as that used in pre-training. The two phases of each trial started at the same position.
2. Start T-maze (flexible learning, all cue types available) (Figure 2B) – The start position was changed after each trial between the four arms of the maze. Importantly, the start arm remained consistent for both the sample and test runs. In all other respects, training followed the ‘Standard’ procedure. Both the selection of the start-arm and the correct test-arm were pseudorandomized, so that no start-arm or test-arm was repeated more than two consecutive times.
3. Rotation T-maze (disrupted intra-maze cues ) (Figure 2C) – The maze was rotated between the sample and the test run, by either 90°or 180° degrees with every trial. The arm on the test run, the degree of rotation and the direction of the rotation were all pseudorandomized so that the same manipulation was not repeated more than two consecutive times. The location of the start position was consistent for all trials, so that extra-maze and egocentric cues remained viable, while intra-maze cues were nullified.
4. Opposite arm T-maze (disrupted egocentric cues) (Figure 2D) –The start position of the animal was rotated 180° between the sample and the test runs. Therefore, each sample run began at the same start arm and each test run began in the arm directly opposite. Critically, the correct arm on the test run remained in the opposite room location to the position of the baited arm in the sample run. No test arm was used as the correct arm on more than two consecutive trials. For all trials, the start position remained in the South (Figure 2D).
5. Dark T-maze (visual cues removed) (Figure 2E) – The standard T-maze protocol was repeated but now in the dark. The maze was baited, and barriers put in place before each trial in dim illumination provided by a 10W red light facing away from the maze. Then, the light was turned off (~0.2lx) and the rat placed in the start position. Once each trial was completed, the rat was picked up and held while the maze was reset. Only Cohort 2 received the additional infusion-free session.