To behave more efficiently the human brain must anticipate future events with different probabilities and prepare appropriate responses. Previous studies demonstrated that participants react faster to more probable stimuli. It has been shown that this effect in reaction time reduction is related to the response preparation process. However it is still unknown what the minimum difference in stimulus probabilities is that is sufficient to elicit response preparation as measured with the EEG. Intra-individual reaction time variability indicates the stability of an individual’s response performance and provides useful information about cognitive functioning. It’s use has become prevalent in recent clinical studies. In order to extend understanding of cognitive and neural mechanisms of response stability we hypothesized that intra-individual reaction time variability relates to the response preparation process. Specifically, we hypothesized that response preparation to more probable stimuli would result in not only faster reaction time but also in a reduction of response variability. To verify this hypothesis, we tested 14 healthy subjects using reaction time and EEG as dependent measures. Two different stimuli with probabilities of 33.3% and 66.6% were assigned to two counterbalanced responses. The results of our study showed that stimulus probabilities of 66.6% and 33.3% were sufficient to elicit response preparation. Our data also revealed that response preparation to more probable stimuli speeds RT and reduces RT variability.
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