Figure 1. Scoring guidelines for paw placement on the mouse ladder beam.

For each row, the pseudo-colored bar is the one being scored. . In frame one, the arrow points to the right hind paw as it approaches the green rung; several video frames later, the right hind paw has made contact (arrow) with the green rung and the toes are visible “above” (in the direction of travel) the rung. In the last frame, the left hind paw is well past the green rung while the right hind paw has almost completed contact (arrow) with the green rung. The arrow points to the left hind paw as it makes initial contact with the green rung. In the next frame, the left hind limb is halfway through a step cycle, yet no toes are visible “above” the rung. In the last frame, there are still no toes visible as the left hind paw leaves this rung. In this example, both green rungs are scored as skips because the left hind paw completes a step cycle (starting at the first arrow) and lands on the fourth rung (arrow in last frame). Skips over 3 contiguous rungs are also frequently observed. In the first frame, it appears that the red rung should be scored as a plantar step (arrow). However, in the subsequent two frames, it is clear that the right rear paw slipped off the red rung and dropped below the horizontal plane of the ladder. A slip is differentiated from a miss by paw contact with the rung being scored. Although a rare event, the left hind paw in the first frame does not touch the red rung, (arrow), but drops below the horizontal plane of the rungs between the red rung and the next rung. The initial red rung is scored as a miss, while the subsequent rung is scored independently. The dorsal surface of the right hind paw contacts the rung preceding the red rung, and in subsequent frames, it can be seen that the dorsal surface drags across the red rung.