RESPONSE BIASES - TONIC NECK RESPONSE AND AFTERCONTRACTION PHENOMENON

In the present experiments, an attempt was made to document possible r esponse biases resulting from changes in head position and to determin e if this source of bias is additive or interactive in its influence o n learned motor responses when paired with response biases arising fro m the aftercontraction phenomenon (Shea, Shebilske, Kohl, & Guadagnoli , 1991). The results of Experiments 1 and 2 indicated that the influen ce of head position on response biases was dependent on head position during acquisition and on the direction in which the head was turned w ith respect to the flexors and extensors involved in the movement; the magnitude of the effect tended to increase as the force of the contra ction increased. In Experiment 3, increases in the intensity of a prio r contraction were reflected in the magnitude of the response bias and changes in head position were reflected in corresponding changes in r esponse biases. These findings are consistent with the influence of th e aftercontraction phenomenon and demonstrations of the tonic neck res ponse. The additive effects arising from precontraction and head posit ion suggest each source of bias results in independent contributions t o the performance of learned motor responses. Indeed, head position ef fects appear to combine with, or offset influences of, prior contracti ons.