Fig. 5. Leg coordination rule 3. (A) Leg coordination rule 3, sensu Cruse et al. (1995), acts ipsilaterally in a posterior direction and contralaterally between intrasegmental leg pairs (arrows point from sender to receiver leg). Legs are labelled according to the standardised clockwise turning direction. (B) Quantification of coupling strength associated with rule 3. Step cycle sequences of state S over time t for an ipsilateral leg pair, as in Fig. 2. Rule 3 excites stance–swing transition in the posterior leg (broken line, receiver leg) if the anterior leg (broken line, sender leg) is close to a stance–swing transition. Thus, if rule 3 is in effect, the receiver leg should undergo a stance–swing transition prior to a stance–swing transition of the sender leg (trans01). The coupling strength of this rule is calculated by summing the state of the receiver leg for a given time bin (t–t_trans01) 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 stance–swing transition in the sender leg. If coupling according to rule 3 was strong, values would be expected to be close to unity before t_trans01.