The time period for an organism to respond to a stimulus is known as a reaction time.
A single pulse will elicit a certain response time depending on its type or nature. Some stimuli elicit faster responses especially if it involves a reflex.
However, multiple stimuli can increase the reaction time of the subject through several biological mechanism. Each consecutive pulse will increase the amount of neurotransmitter that is released and thus the reaction times will shorten. There is however a limit in the biological system.
When a certain state is reached, higher stimulus frequencies will not elicit shorter response time. If the stimuli are applied over extended periods of time, the reaction times may in fact increase as the biological system will reach a state of fatigue and no longer be able to produce the necessary amounts of neurotransmitters to elicit a fast enough reaction. Thus, the neurotransmitters may be temporarily depleted or their production time increased.