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.