Fig. 2. Leg coordination rules 1 and 2. (A) Leg coordination rules 1 and 2, sensu Cruse et al. (1995), act ipsilaterally in an anterior direction and contralaterally between intrasegmental leg pairs (arrows point from sender to receiver leg). Rule 1 supposedly does not act between middle and front legs (broken arrows). Legs are labelled according to the standardised clockwise turning direction. (B) Quantification of coupling strength associated with rules 1 and 2. Step cycle timing of a leg is considered a sequence of alternating, mutually exclusive states S over time t, assuming value 0 for stance and value 1 for swing. In an ipsilateral leg pair, rule 1 inhibits stance–swing transition in the anterior leg (dotted line, receiver leg) whenever the posterior leg (broken line, sender leg) is in state 1 (large grey arrows). In the same leg pair, rule 2 excites stance–swing transition in the anterior leg soon after swing–stance transition of the posterior leg. Thus, if rules 1 and 2 were effective, the receiver leg should undergo a stance–swing transition at the time of a swing–stance transition of the sender leg (trans10). Coupling strength is calculated by summing the state of the receiver leg for a given time bin (t–t_trans10) within a time window (horizontal arrows between stops) for each one of N steps belonging to the same stimulus period. Division by N gives the likelihood of the receiver leg to be in state 1, given a particular time delay relative to the swing–stance transition in the sender leg. If coupling according to rules 1 and 2 is strong, values are expected to be close to zero before t_trans10 (rule 1) and close to unity after t_trans10 (rule 2).