spacer gif spacer gif spacer gif spacer gif spacer gif
QUICK SEARCH:   [advanced]


spacer gif
     Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents    

First published online May 18, 2006
Journal of Experimental Biology 209, 2199-2214 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02251
This Article
Right arrow Summary Freely available
Right arrow Full Text
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Ebeling, W.
Right arrow Articles by Dürr, V.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Ebeling, W.
Right arrow Articles by Dürr, V.
Social Bookmarking
Add to CiteULike   Add to Complore   Add to Connotea   Add to Del.icio.us   Add to Digg   Add to Reddit   Add to Technorati   Add to Twitter  
What's this?

Perturbation of leg protraction causes context-dependent modulation of inter-leg coordination, but not of avoidance reflexes

Wiebke Ebeling* and Volker Dürr{dagger}

Abteilung für Biologische Kybernetik und Theoretische Biologie, Fakultät für Biologie, Universität Bielefeld, Postfach 10 01 31, 33501 Bielefeld, Germany


Figure 1
View larger version (32K):

[in a new window]
  		Fig. 1. Schematic drawings of the experimental setup and behavioural contexts. (A) Right front leg of a stick insect with thorax-coxa joint (ThC), coxa-trochanter joint (CTr) and femur-tibia joint (FTi) indicated. Each joint is responsible for different motion components of the leg: ThC, protraction/retraction (pro/re); CTr, levation/depression (lev/dep); FTi, extension/flexion (ex/fl). During swing movements (curved dotted arrow), the leg moves through the air from the posterior extreme position (PEP) to the anterior extreme position (AEP). During stance movements (straight broken arrow), the leg pushes the body forward, moving from AEP to PEP in body coordinates. An obstacle (grey probe) was held into the swing trajectory to examine effects of perturbation. (B) Schematic trajectories of typical swing movements of the right front leg (arrows) and approximate areas of PEPs and AEPs. In straight walking (black trajectory), front leg movements are symmetrical and approximately parallel to the body long axis, whereas in curve walking, leg movements differ between inner (orange trajectory) and outer (purple trajectory) front legs. The coordinate system used for analyses has its origin between the front leg coxae. Curve walking was elicited by means of a large-field visual motion stimulus of a rotating stripe pattern. It was always the right front leg that was perturbed.

 

Figure 2
View larger version (21K):

[in a new window]
  		Fig. 2. Representative swing trajectories and step patterns. (A) Top-view of front leg swing trajectories of two subsequent steps: unperturbed (black crosses) and perturbed (red circles) for inner front leg in curve walking (left), straight walking (centre), and for outer leg in curve walking (right). Data points were recorded at a frame rate of 50 Hz. Obstacle contact (filled circles) could last for a variable time span (here: 60-80 ms), eliciting an avoidance reflex that resulted in a deviating swing trajectory until touch-down (AEP). (B) Step patterns of all six legs in straight and curve walking. Traces from top to bottom: lFL, left front; lML, middle; lHL, hind leg; rFL, right front; rML, middle; rHL, hind leg. Arrows indicate direction of visual pattern motion and turn direction. The data belong to the same trials shown in A. The perturbation ('p') was applied during a swing movement of the right front leg. Perturbation causes only small effects in the stepping rhythm of the six legs.

 

Figure 3
View larger version (23K):

[in a new window]
  		Fig. 3. Spatial and temporal effects of perturbation. The impact of perturbation on the perturbed leg itself was investigated over a walking sequence of three subsequent steps (-1, 0, +1), step 0 being the perturbed step (white). Bars indicate mean values of step parameters ± s.d. (Inner, N=51; Straight, N=64; Outer, N=58). Perturbation affected swing distance (A), swing duration (B), and duty cycle, i.e. the fraction of the step cycle spent in stance (C). Data of the perturbed step 0 and the subsequent unperturbed step +1 (grey) were tested with reference to step -1 (black), prior to perturbation (Wilcoxon test). Perturbation caused a reduction of swing distance and duty cycle and a prolongation of the swing duration, the only exception being swing distance of inner legs, which remained unchanged (A, left). Step parameters in unperturbed walking depend on the behavioural context and differ between straight and curve walking (Mann-Whitney U-test). Significance levels: ***P<0.001; **P<0.01; *P<0.05.

 

Figure 4
View larger version (35K):

[in a new window]
  		Fig. 4. Immediate reaction of the perturbed leg to obstacle contact. (Top) Sketch of the experimental sequence. (A-C) Top view of leg movements before and after perturbation in the three behavioural contexts: inner leg (A), straight walking (B) and outer leg (C). Vectors indicate the displacement of the femur-tibia joint (FTi; grey in i, light green in ii) and tibia-tarsus joint (TiTa; black in i, dark green in ii) within 20 ms immediately before (i) and after (ii) obstacle contact. (iii) The mean vectors. Note that vectors do not indicate the action of these joints but rather their displacement caused by the movement of the joints proximal to them. The direction of the FTi vectors reflects the contribution of the ThC and CTr joints and, therefore, two motion components only (protraction/retraction, levation/depression). The direction of the TiTa vectors additionally includes a contribution of the FTi joint (extension/flexion). Vector length indicates swing velocity. Before perturbation, leg movements exhibit prominent differences between behavioural contexts, indicating that different muscle groups were active at the moment of obstacle contact. In contrast, the patterns of movement after obstacle contact were all very similar, seemingly converging to a common position.

 

Figure 5
View larger version (33K):

[in a new window]
  		Fig. 5. Motion components before and after perturbation. Joint angle velocities and motion components during the 20 ms immediately before and after obstacle contact in the three behavioural contexts. (A) Positive thorax-coxa (ThC) joint velocity causes protraction of the leg. (B) Positive coxa-trochanter (CTr) joint velocities result in levation of the femur. (C) Positive femur-tibia (FTi) joint velocities are due to extension of the tibia. Median velocities are indicated by arrowheads. Before perturbation, the combination of motion components is context-dependent. The swing phase of straight walking is characterised by protraction, depression and extension (centre), whereas in curve walking, only one motion component predominates: FTi extension in the inner leg (left) and ThC protraction in the outer leg (right). In contrast, the avoidance reflex consists of the same motion components in all three behavioural contexts: retraction, levation and flexion (Mann-Whitney U-test of median velocities against zero). Significance levels: ***P<0.001; **P<0.01; *P<0.05.

 

Figure 6
View larger version (34K):

[in a new window]
  		Fig. 6. Impact of perturbation on extreme positions of swing movements. Top view of posterior extreme position (PEP) and anterior extreme positions (AEP) in unperturbed and perturbed straight walking of the right front leg (rFL), the contralateral left front leg (lFL), and the ipsilateral right middle leg (rML). (A) PEPs and (B) AEPs of unperturbed swing movements (black crosses) and their subsequent perturbed swing movements (red circles). Red, blue and black bars indicate mean values. Both in perturbed and unperturbed steps, PEPs dispersed more widely than AEPs. Perturbation caused a posterior-lateral shift of PEPs of the right middle leg and a posterior shift of AEPs of the right front leg. (C) PEPs and (D) AEPs of unperturbed reference (black crosses) and the subsequent unperturbed swing movements after the perturbed step (blue triangles) of the same walking sequence as in A and B. Most effects of perturbation vanished in the subsequent step (for statistics, see Fig. 7 and text).

 

Figure 7
View larger version (17K):

[in a new window]
  		Fig. 7. Shifts of extreme positions due to perturbation. Top view of shift vectors, indicating the mean shift of extreme positions of the perturbed right front leg (rFL), the contralateral left front leg (lFL), and the ipsilateral right middle leg (rML). As reference, mean unperturbed PEPs (filled circles) and AEPs (open circles) are shown for the three legs. Different walking contexts are shown: (A), inner leg; (B), straight walking; (C), outer leg. Arrow length indicates the spatial extent of the shift. Statistical significance was tested for mean direction of the shift. Significance levels: ***P<0.001; **P<0.01; *P<0.05. Arcs (solid lines, PEP; dotted lines, AEP) illustrate tangential components of the shift, i.e. the contribution of the front leg and middle leg ThC joints. The impact of perturbation was strongest when the inner front leg was perturbed (A) with significant shifts in all three legs in the perturbed (red arrows) and subsequent step (blue arrows). Shifts were less pronounced if perturbation occurred in straight walking (B) or the outer front leg (C). The mean distance between the prothoracic and mesothoracic coxae was taken from Cruse (Cruse, 1976Go).

 

Figure 8
View larger version (16K):

[in a new window]
  		Fig. 8. Temporal coordination between contralateral (A) and ipsilateral (B) legs. Time delays of lift-off of two receiver legs relative to touch-down of the perturbed sender leg. (A) Delay between AEP of the perturbed front leg and PEP of its contralateral neighbour. In unperturbed reference steps (ref., filled symbols), the delay was significantly shorter if the sender leg was an outer leg (grey, right; Mann-Whitney U-test). After perturbation (pert., open symbols), delays significantly decreased if the sender leg was in the straight walking (black, middle) or inner leg context (grey, left; Wilcoxon test). (B) Delay between AEP of the perturbed front leg and PEP of its ipsilateral middle leg. In unperturbed steps, delays significantly differed between outer and inner leg contexts. In the perturbed steps, the middle leg lifts off earlier in straight walking and in outer legs. Boxes, 25-75% of the data; notch, 95% confidence interval of the median (centre of notch); whiskers, 5% and 95% percentiles. Squares, mean. Significance levels: ***P<0.001; **P<0.01; *P<0.05.

 

Figure 9
View larger version (16K):

[in a new window]
  		Fig. 9. Context-dependent targeting of middle leg AEP. Targeting of the right middle leg towards the ipsilateral front leg is illustrated by the vicinity of the middle leg AEP and the front leg PEP in straight walking. Values are means ± s.d. (Inner, N=51; Straight, N=64; Outer, N=58). (A) Inner curve-walking leg perturbed (orange symbols). (B) Straight walking leg perturbed (black symbols). (C) Outer curve-walking leg perturbed (purple symbols). Locations of front leg extreme positions (circles) and swing directions (arrows) reflect context-dependent kinematics. Squares show extreme positions of middle legs. Filled symbols, unperturbed extreme positions; open symbols illustrate the effect of perturbation. In straight walking (B), middle leg AEP was regulated to lie close to front leg PEP. In curve walking, middle leg swing direction shifted as the animals adapted to the behavioural context of curve walking. Touch-down location remained regulated, as indicated by the small s.d. (A, inner legs; C, outer legs), although not towards the front leg PEP. Perturbation of the front leg does not affect the targeting mechanism in a context-dependent manner (see text).

 

Add to CiteULike CiteULike   Add to Complore Complore   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us   Add to Digg Digg   Add to Reddit Reddit   Add to Technorati Technorati   Add to Twitter Twitter    What's this?



© The Company of Biologists Ltd 2006