Variables influencing outflow-inflow interpretations tracking performance: predictability of target motion, transfer function, and practice.

Experimental evidence can be found in support of either outflow or inflow theories of voluntary action. Accordingly, the empirical problem becomes one of ascertaining fundamental conditions under which the dichotomy plainly manifests itself. Using a within-subjects design, six undergraduates were given pursuit tracking training. The independent variables were (a) presence or absence of regularity (predictability) in the back-and-forth motion of the target, (b) type of control stick (with attendant proprioceptive feedback) used to drive the cursor, and (c) practice. The dependent variable was the time integral of error. The following results were significant (p less than .05, two-tailed): (a) A pressure stick is better than a freely moving one when tracking a random target; this finding supports a reactive force inflow model. The converse is true for a predictable target and is therefore supportive of an outflow interpretation. (b) Regardless of kind of target motion, an unloaded stick is better than an elastically loaded one at the beginning of training (outflow support) but worse by the end of training (inflow support). It is concluded that the motor outflow versus motor inflow dispute is amenable to parametric resolution.