Progressive degradation of serial grooming chains by descending decerebration.

Rule-governed behavioral chains occur predictably within the grooming sequences of rats. Descending levels of decerebration were used to identify the minimum brain substrate needed to generate the sequential structure of a chain that connects up to 25 actions into a stereotyped grooming pattern. Full brain transections in the coronal plane isolated the decerebrate brainstem of rats at one of 3 different levels: mesencephalic (above the midbrain), metencephalic (above the hindbrain), and myelencephalic (above the medulla oblongata). Complete chain sequences were produced successfully by higher decerebrates, demonstrating that brainstem circuitry suffices for the basic generation of this sequential pattern. The pattern of sequential degradation across lower transection levels was gradual and continuous, raising the possibility that the generating circuitry for this chain may not be localized at a single level within the brainstem but rather may be distributed across the hindbrain as a degenerate or parallel network. The competence of this network appears to be reduced merely in increments by descending transections. This possibility is compared to localized generator alternatives.