Experimental setup for the determination of the threshold of heading perception from virtual optic flow and response behavior of the harbor seal.

<p>A The computer generated optic flow field was back-projected by a projector and via a mirror system onto an underwater projection screen installed inside a projection chamber. For a high-quality underwater projection, the water surface was calmed by an acrylic frame, and the experimental area in front of the projection chamber was shaded by a black curtain. Inside this area, the animal was stationing underwater in a hoop station with the eyes at 1.5 m distance to the projection. During trials, the experimenter (not displayed) stayed outside the experimental area in order to avoid giving secondary cues. Scale 0.5 m. B In a “no go”-trial, in which the superimposed cross matched the FOE, the animal had to touch the cross with its snout in order to be rewarded. C In a “go”-trial (compare with <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103555#pone-0103555-g002" target="_blank">Figure 2</a>), in which the cross deviated by a pre-programmed angular distance from the FOE, the seal had to turn away from the projection screen and touch the hoop station with its snout.