Effects of Number of Alternatives on the Psychological Refractoriness of an Extended Movement,

The single-channel prediction that delays in RT₂ should be proportional to RT₁ was examined by varying the number of alternatives (one, two, or three) in the primary response which was an extended arm swing at maximal speed. The second response was a thumb key release. Foreperiods were 2, 3, or 4 sec., and interstimulus intervals (ISIs) ranged from .10:50 sec. in .10-sec. steps. General support for the single-channel model was found in that RT₂ paralleled changes in RT₁. However, RT₂ was greater than predicted when the second signal (S₂) occurred during the execution of the primary response. In addition, while delays in RT₂ as a function of ISI – RT₁ showed corresponding differences between choice conditions, most values were greater than predicted. The deviations from the single-channel model appeared to be explained best in terms of a decrease in readiness to respond to the second signal as ISI lengthened.