On the natural statistics of insect locomotion: indications for two distinct mechanisms of step generation
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1
Bielefeld University, Department of Biological Cybernetics, Germany
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2
Bielefeld University, Cognitive Interaction Technology - Center of Excellence (CITEC), Germany
Assuming that central pattern generators (CPGs) and sensory mediated reflexes have different effects on step parameters, analysis of the natural statistics of stepping can help understanding their relative contribution to natural locomotion. Insect gaits may be regular on horizontal surfaces, but gaits become more variable in response to disturbances or to changes in context (curve-walking or gap-crossing stick insects: Dürr, 2005; Bläsing and Cruse, 2004; climbing cockroaches: Watson et al., 2002). Here, we search for features in step parameter distributions that are invariant across species and leg types, but differentially affected by disturbances. For this, we analyzed unrestrained walking and climbing of three stick insects species (Carausius morosus, Aretaon asperrimus and Medauroidea extradentata) on staircases of different height, using a Vicon motion capture system. Up to 20 reflective markers per animal were tracked for reconstruction of whole-body kinematics, including the three thorax segments, the head, the six legs and the two antennae. So far, our SQL-based Natural Movement Database stores about three hours of unrestrained movement sequences of up to 42 degrees of freedom.
Analysis of several thousand steps suggests that stick insects take two classes of steps during walking and climbing. Irrespective of the species or leg type, step length distributions were always bimodal, showing two distinct peaks which are well-described by a Gamma and a Logistic distribution. According to the local minimum between the two peaks, we divided the steps into two distinct classes of steps: long and short steps. The peaks remained at the same position irrespective of leg type and height of the stairs, indicating that the classification into two distinct classes of steps is context-independent. With increasing height of the staircase, the proportion of short steps increased and the distribution of long steps got wider. Several parameters indicated that the two classes of steps have different functional properties. Short steps occurred after short stance periods and were common at the edges of the staircases, whereas long steps were equally distributed across the whole walkway. Lift-off positions of short steps are more anterior than those of long steps and their swing movements were more variable, indicating that short steps may have corrective rather than propulsive function. Step direction of short steps depended on their lift-off position, revealing that they are directed movements. We conclude that long and short steps of stick insects are distinct classes of steps. Their statistical and functional differences suggest different underlying control mechanisms. Only the more regularly occurring long steps are likely to be driven by central rhythm generators.
Supported by EU-project EMICAB (FP7-ICT-270182)
References:
Bläsing B. and Cruse H. (2004). Stick insect locomotion in a complex environment: climbing over large gaps. J. Exp. Biol. 207: 1273-1286.
Dürr V. (2005). Context-dependent changes in strength and efficacy of leg coordination mechanisms. J. Exp. Biol. 208: 2253-2267.
Watson J. T., Ritzmann R. E., Zill S. N. and Pollack A. J. (2002). Control of obstacle climbing in the cockroach, Blaberus discoidalis. I. Kinematics. J Comp Physiol A 188: 39–53
Keywords:
database,
Gait,
kinematics,
Locomotion,
motion capture,
rhythm generators,
stepping pattern,
stick insect
Conference:
Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012.
Presentation Type:
Poster (but consider for participant symposium and student poster award)
Topic:
Motor Systems
Citation:
Theunissen
LM,
Bekemeier
HH and
Dürr
V
(2012). On the natural statistics of insect locomotion: indications for two distinct mechanisms of step generation.
Conference Abstract:
Tenth International Congress of Neuroethology.
doi: 10.3389/conf.fnbeh.2012.27.00150
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Received:
27 Apr 2012;
Published Online:
07 Jul 2012.
*
Correspondence:
Mr. Leslie M Theunissen, Bielefeld University, Department of Biological Cybernetics, Bielefeld, 33615, Germany, leslie.theunissen@uni-ulm.de