Morris water maze. (A) Learning curves for familiarization and training phases Symbols and lines represent daily latencies (mean ± SEM) to reach the platform by C57BL/6J (n = 10) and C57BL/6N (n = 10) male mice, aged 16 to 20 weeks. **P < 0.005, ***P < 0.0005 J versus N (t-test); °°P < 0.005, °°°P < 0.0001 versus day 1 (Fisher's least squares difference). (B) Probe test. Bars represent % time spent in each quadrant on day 5 during probe test. Dotted line is set at chance level (25%). *P < 0.05, **P < 0.005, ***P < 0.0005 versus correct quadrant (t-test)_. (C) Representative tracks of two C57BL/6J and C57BL/6N mice paths during probe test. Dotted circle indicates former platform location.