The problem of serial order in behavior: Lashley's legacy.

In a prescient paper Karl Lashley (1951) rejected reflex chaining accounts of the sequencing of behavior and argued instead for a more cognitive account in which behavioral sequences are typically controlled with central plans. An important feature of such plans, according to Lashley, is that they are hierarchical. Lashley offered several sources of evidence for the hierarchical organization for behavioral plans, and others afterward provided more evidence for this hypothesis. We briefly review that evidence here and then shift from a focus on the structure of plans (Lashley's point of concentration) to the processes by which plans are formed in real time. Two principles emerge from the studies we review. One is that plans are not formed from scratch for each successive movement sequence but instead are formed by making whatever changes are needed to distinguish the movement sequence to be performed next from the movement sequence that has just been performed. This plan-modification view is supported by two phenomena discovered in our laboratory: the parameter remapping effect, and the handpath priming effect. The other principle we review is that even single movements appear to be controlled with hierarchically organized plans. At the top level are the starting and goal postures. At the lower level are the intermediate states comprising the transition from the starting posture to the goal posture. The latter principle is supported by another phenomenon discovered in our lab, the end-state comfort effect, and by a computational model of motor planning which accounts for a large number of motor phenomena. Interestingly, the computational model hearkens back to a classical method of generating cartoon animations that relies on the production of keyframes first and the production of interframes (intermediate frames) second.