Strategies for obstacle avoidance during walking in the cat

Abstract

Avoiding obstacles is essential for successful navigation through complex environments. This study aimed to clarify what strategies are used by a typical quadruped, the cat, to avoid obstacles during walking. Four cats walked along a corridor 2.5 m long and 25 or 15 cm wide. Obstacles, small round objects 2.5 cm in diameter and 1 cm in height, were placed on the floor in various locations. Movements of the paw were recorded with a motion capture and analysis system (Visualeyez, PTI). During walking in the wide corridor, cats' preferred strategy for avoiding a single obstacle was circumvention, during which the stride direction changed while stride duration and swing-to-stride duration ratio were preserved. Another strategy, stepping over the obstacle, was used during walking in the narrow corridor, when lateral deviations of walking trajectory were restricted. Stepping over the obstacle involved changes in two consecutive strides. The stride preceding the obstacle was shortened, and swing-to-stride ratio was reduced. The obstacle was negotiated in the next stride of increased height and normal duration and swing-to-stride ratio. During walking on a surface with multiple obstacles, both strategies were used. To avoid contact with the obstacle, cats placed the paw away from the object at a distance roughly equal to the diameter of the paw. During obstacle avoidance cats prefer to alter muscle activities without altering the locomotor rhythm. We hypothesize that a choice of the strategy for obstacle avoidance is determined by minimizing the complexity of neuro-motor processes required to achieve the behavioral goal.NEW & NOTEWORTHY In a study of feline locomotor behavior we found that the preferred strategy to avoid a small obstacle is circumvention. During circumvention, stride direction changes but length and temporal structure are preserved. Another strategy, stepping over the obstacle, is used in narrow walkways. During overstepping, two strides adjust. A stride preceding the obstacle decreases in length and duration. The following stride negotiating the obstacle increases in height while retaining normal temporal structure and nearly normal length.

Keywords: locomotion; motor control; obstacle avoidance; spatial navigation.

Fig. 1.

Experimental design. A: photograph of a cat walking along the corridor cluttered with multiple small obstacles. B: diagram of the walking chamber and location of the motion capture system. C: reconstruction of the placement of each of the 4 paws—right forepaw (black circles), left forepaw (white circles), right hindpaw (black squares), and left hindpaw (white squares)—during unobstructed walking in the wide corridor. Locations of the symbols are averages of 6 placements.

Fig. 2.

A change in the direction of the stride to circumvent an obstacle during walking in the wide corridor (experiment 1, part 1; cat SF). A: right forelimb placements during unobstructed walking along the corridor. Each placement is depicted by a gray circle. Average length (cm, numbers above arrows) and direction (°, numbers below arrows) are indicated for each stride. B: paw placements during walking along the corridor obstructed with a single object. Each placement is depicted by a black circle. The object is depicted by a white circle. C: average paw placements during unobstructed and obstructed walking superimposed. The last stride before the obstacle and a corresponding unobstructed stride are highlighted with a box. D: magnification of the highlighted fragment in C. L_o, length of obstructed stride; L_u, length of unobstructed stride; α_o, direction of obstructed stride; α_u, direction of unobstructed stride.

Fig. 5.

Adaptations of both length and direction of strides during obstacle circumvention (experiment 1, part 2; cat SF). A: right forelimb placements during unobstructed walking. B: right forelimb placements during walking along the corridor obstructed with an object. C: average placements during unobstructed and obstructed walking superimposed. A fragment showing 2 strides preceding the obstacle and corresponding unobstructed stride is highlighted with a box. D: magnification of the highlighted fragment in C. Designations as in Fig. 4.

Fig. 3.

Changes in the direction of 2 consecutive strides to circumvent an obstacle during walking in the wide corridor (experiment 1, part 1; cat FM). A: right forelimb placements during unobstructed walking. B: right forelimb placements during walking along the corridor obstructed with an object. C: average paw placements during unobstructed and obstructed walking superimposed. The last 2 strides before the obstacle and corresponding unobstructed strides are highlighted with a box. D: magnification of the highlighted fragment in C. L_o1 and L_o2, lengths of obstructed strides; L_u1 and L_u2, lengths of unobstructed strides; α_o1 and α_o2, directions of obstructed strides; α_u1 and α_u2, directions of unobstructed strides. Designations as in Fig. 2.

Fig. 4.

Changes in the direction of the strides to circumvent an obstacle while approaching it from either side (experiment 1, part 2; cat SF). A: right forelimb placements during unobstructed walking. B: right forelimb placements during 9 rounds of walking along the corridor obstructed with an object. Black circles represent paw placements during passages on the left of the obstacle; black diamonds show those of the passages on the right of the obstacle. C: average paw placements during unobstructed and obstructed walking superimposed. A fragment showing 2 strides preceding the obstacle and corresponding unobstructed stride is highlighted with a box. D: magnification of the highlighted fragment in C. Designations as in Fig. 2.

Fig. 6.

Stepping over an obstacle during walking in the narrow corridor (experiment 2; cat TI). A: right forelimb placements during unobstructed walking. B: placements during walking along the corridor obstructed with an object positioned on a site of a typical 2nd unobstructed paw placement. C: placements during walking along the corridor obstructed with an object positioned slightly before (2.5 cm) the site of the typical 2nd unobstructed paw placement. D: placements during walking along the corridor obstructed with an object positioned at a greater distance (5 cm) from the site of the 2nd unobstructed paw placement. E: placements during walking along the corridor obstructed with an object positioned at the site of the 3rd unobstructed paw placement. F: placements during walking along the corridor obstructed with an object positioned 2.5 cm before the site of the 3rd unobstructed paw placement. G: placements during walking along the corridor obstructed with an object positioned at a greater distance (5 cm) before the site of the 3rd unobstructed paw placement. H: placements during walking along the corridor obstructed with an object positioned at the site of the 4th unobstructed paw placement. I: placements during walking along the corridor obstructed with an object positioned 2.5 cm before the site of the 4th unobstructed paw placement. J: placements during walking along the corridor obstructed with an object positioned 5 cm before the site of the 4th unobstructed paw placement. In C, F, and I, gray symbols and numbers represent paw placements during passages when the stride was lengthened during stepping over the object. Other designations as in Fig. 2.

Fig. 7.

Proportions of strides, in which the length (L, areas at top left of each panel), the direction (α, areas at top right of each panel), and the paw elevation during swing (H, areas at bottom of each panel) were altered during walking in the corridor obstructed with multiple objects. Strides with and without changes in the duration and/or the swing-to-stride ratio are combined. Obstructed and unobstructed walking conditions are indicated with indices “o” and “u,” respectively. The percentage of strides, in which neither parameter changed, is indicated at bottom left of each panel. The number of comparisons between groups of obstructed and unobstructed strides is indicated at bottom right of each panel.

Fig. 8.

Selective changes in direction and length of strides during walking in the corridor cluttered with multiple objects (experiment 3; cat SF). A: right forelimb placements during unobstructed walking. B: right forelimb placements during walking along the corridor obstructed with objects. C: average placements during both unobstructed and obstructed walking. Fragments showing strides avoiding obstacles using 2 different strategies and corresponding unobstructed strides are highlighted with 2 boxes. D: magnification of the fragment in C highlighting the change in stride direction. E: magnification of the fragment in C highlighting the change in stride length. Designations as in Fig. 2.

Fig. 9.

A gross decrease in the stride length during walking in the wide corridor cluttered with multiple objects (experiment 3; cat TI). A: right forelimb placements during unobstructed walking. B: right forelimb placements during walking along the corridor obstructed with objects. C: average placements during unobstructed and obstructed walking superimposed. A fragment illustrating changes in stride length to avoid the obstacle and corresponding unobstructed stride is highlighted with a box. D: magnification of the highlighted fragment in C. Second obstructed strides of different length are designated with solid and dashed arrows. L_o2′ and L_o2ʺ, 2nd obstructed strides of different length; α_o1′ and α_o2ʺ, directions of 2nd obstructed strides of different length. Other designations as in Figs. 2 and 3.

Fig. 10.

Strategies for avoiding small obstacles during walking. A: trajectories of 2 consecutive strides (1 and 2) during unobstructed walking. B: length and height of 2 strides during unobstructed walking. C: duration of the stance (St) and swing (Sw) phases of 2 strides during unobstructed walking. D: in an unrestricting environment, obstacle avoidance is achieved by circumvention. The preferred placement of the right paw is to the left of the obstacle at a distance of about the diameter of the paw. E: during circumvention, the length and height of both strides 1 and 2 do not change. F: duration and swing-to-stride duration ratio of strides 1 and 2 do not change. G: in a restricting environment, obstacle avoidance is achieved by stepping over the obstacle. The paw is placed before the obstacle at a distance of the diameter of the paw. The obstacle is negotiated with the next stride. H: the length of the stride preceding the obstacle (1) decreases, but its height does not change. The stride negotiating the obstacle (2) increases in height. This stride (2) is longer than the preceding stride, albeit shorter than normal. I: the duration of the stride preceding the obstacle (1) reduces because of a decrease in the duration of the swing. Temporal characteristics of the stride negotiating the obstacle (2) are similar to those of the normal stride. In A, D, and G, the paw is depicted with a black pictogram. In D and G, the obstacle is depicted with a gray circle. In E, F, H, and I, the gray circle indicates the location of the obstacle. In B, E, and H, black areas show the length and height of strides. In C, F, and I, white and gray areas depict the duration of the stance (St) and swing (Sw) phases of strides. Indices “1” and “2” indicate 2 consecutive strides.