The effect of response complexity on simple reaction time occurs even with a highly predictable imperative stimulus.

It is well known that increasing the complexity of the required response results in a corresponding increase in simple reaction time (RT). This "response complexity effect" has typically been attributed to increased time required to prepare some aspect of the response; however, most studies examining the response complexity effect have used an unpredictable foreperiod, which does not allow for optimal preparation to occur. Thus, it is conceivable that response complexity effects are influenced by an inability to predict the occurrence of the go-signal. In order to examine this possibility, participants (N = 36) were randomly assigned to one of four groups that differed in predictability of the go signal: 1) 2500-3500 ms random foreperiod; 2) 3000 ms constant foreperiod; 3) 1000 ms constant foreperiod; 4) 3000 ms constant foreperiod with a 1000 ms countdown timer. Participants performed one of three different key-press responses in a simple RT paradigm: 1) single key-press; 2) three key-presses with an equal/isochronous time interval between presses; 3) three key-presses with an unequal/non-isochronous time interval between presses. Results confirmed that while the countdown timer group had an overall reduced RT, response complexity effects were present and of similar magnitude for all groups in all testing blocks. This confirms that predictability of the go signal does not affect the response complexity effect.