Reaction times associated with repetitive fixed interval stimuli in a given stimulus-response situation usually decrease over a short time period to some minimal value and then remain stable. explain

This is because the test subject will after a period of time will learn the pattern of the clocks or stimulus and thus allowing the reaction time to be faster or lower. However this is limited to how fast the events at synapse can occur, called synaptic delay which is about 0-10 ms.