Learning and memory abilities of Ctsk-/- mice compared to WT controls analyzed by a novel object recognition test and by an elevated multiple choice maze.(A) Experimental setting of the novel object recognition test. Individual mice were placed in the lower left square, facing the corner. After two habituation sessions, the cube in square 6 was replaced with a shell (objects not drawn to scale). (B) Both WT (n = 13) and Ctsk-/- mice (n = 10) showed, after habituation, a marked decrease in the time spent in any object's proximity (left). During the test session, the WT mice spent significantly more time exploring the novel object than they spent with any of the familiar objects (right, open bars). Ctsk-/- mice (grey bars) did not show any preference for the new object over the familiar ones.(C) Experimental setting of the elevated multiple choice maze. Animals were placed in the starting segment of the maze, and had to make three correct decisions in order to reach the exit point, where they could descend via a wire net into a cage containing a house. The cage was placed on the floor and was not readily visible from the maze. Testing was conducted daily, over an interval of 12 days. An entry into a maze segment that was outside the correct route (0-1-4-7-E-F in this order) was counted as a reference memory error, while backtracking into any already visited segment within a given session was counted as a working memory error. (D) The learning curves for the two groups were compared on each day of the experiment. Ctsk-/- mice made significantly more working memory errors already from day 2 of testing, in contrast to WT controls (B, Ctsk-/- n = 10; WT n = 13) which showed a normal learning curve. (E) Similarly, Ctsk-/- mice made significantly more reference memory errors starting from day 5 of testing (C, Ctsk-/- n = 10; WT n = 13). Levels of significance are denoted as * for p < 0.05; ** for p < 0.01.