Four-CPG model with bilateral flexor-extensor/extensor-flexor coupling. Four CPGs were interconnected to have ipsilateral flexor-extensor/extensor-flexor, contralateral flexor-flexor/extensor-extensor, and bilateral flexor-extensor/extensor-flexor coupling. The helical symbol represents a muscle spindle that signifies sensory feedback. Representative time series output bursts for the four-CPG models indicate that the bursting patterns of contralateral CPGs (upper left and lower right, upper right and lower left) were in-phase while bilateral CPGs (upper left and upper right, lower left and lower right) were out-of-phase. Grid intersections indicate parameter combinations tested. Intersections without a symbol indicate that the output behaviour did not reach steady state or did not have alternating flexor and extensor bursts. Sensory feedback was inhibitory, hs_lo = 1.