Fig. 2. Test trials and expected homing directions in experiment I. The large circles with a small rectangle represent the experimental arena and peripheral nest box, respectively. The dotted arrows along the arena circle indicate that the arena has been rotated before the start of the trial. The continuous lines with an arrow within arena circles represent the outward journey to the food source. The small circles indicate that the animal walks in circles (diameter30 cm). Trials without a fix (left) take place under infrared light only, and the subjects can only home through path integration (PI) that is anchored to the current nest position (dotted arrows). In trials with a fix (right), the room lights are turned on for 10–12 s during or at the end of the outward trip, when the subject is located at P1. During a fix, the homing directions based on PI on the one hand, and on visual references on the other hand, diverge by 98°. The arrows starting at P1 or P2 represent the expected homing directions depending on PI that is reset by a visual fix (dashed arrows) or is not reset (dotted arrows). (In controlled reference trials, the two expected homing directions lead to the nest at 0°). In uncontrolled reference trials, the arena is not rotated and the animal is guided from the nest exit at 0° to any point on the arena floor, where it is offered food, and then returns to the nest. In controlled reference trials, the arena and nest box are rotated by 180° and then rotated back to the standard 0° position. From the nest exit at 0°, the hamster is led to one of five different locations (dots), where it walks along 2.5 full circles (small circle) in the dark, and then along a further circle in the opposite direction, the room lights being turned on. The animal is then offered food and returns home, in darkness. In both reference trials, homing is considered to be successful if the animal deviates by no more than 45° from the correct homing direction when reaching the arena periphery. Before the start of each experimental trial, the arena (with nest box) is rotated by 135° clockwise or counterclockwise. Only clockwise trials are presented here. In baseline trials, the hamster is guided to P1, where it picks up some food, and then to P2, where it fills its pouches completely and initiates the homing trip. Baseline-with-circling trials follow the same pattern, but the animal walks along 2.5 full circles near P1 instead of pouching food. In fix-without-translation trials, the animal is led to P1, walks along 2.5 full circles in the dark and then along a further circle in the opposite direction in the fully lit environment, fills its cheek pouches at P1 and returns towards the arena periphery. Fix-with-translation trials follow the same procedures; however, after the fix interval, the animal is led from P1 to P2, where it fills its pouches and starts the homing trip.