Synchronisation of tongue and forepaw movements in the rat: a model of instrumental muscle synergy.

Acquisition of new skills is often implemented by the formation of coordinated movements of functionally unrelated muscles. This situation was modelled in rats trained to lick at a retractable spout which was automatically withdrawn after termination of every lick but could be returned by pressing and releasing a lever placed 4 cm below the spout. Rats (n = 12) learned to perform short presses (50 ms) synchronized with the lick cycle in a way allowing continuous drinking. When the distance of the lever from the box was increased from 5 mm to 12.5 mm or when the force required for lever operation was raised from 20 g to 90 g, the lick-press interval increased in multiples of the interlick interval. Licking ceased during the lever pressing but regular timing of subsequent licks indicated uninterrupted activity of the licking generator. It is concluded that the coordination of orofacial and forelimb muscles engaged in licking and lever pressing, respectively, is accomplished by propagation of the activity of the lick generator to cortical and subcortical motor centers.