It works for counter irritants, too. The measure of sensory irritation is objective. For centuries, neurologists have observed a bipolar effect following insult to nerves. The skin served by these nerves becomes both insensitive to small stimuli and hyper-reactive to large stimuli.
Most people understand insensitivity.
By hyper-reactive, the neurologists mean that a stimulus, which is normally not painful, becomes painful. This century, skin researchers observed that skin irritation of any etiology insults the skin nerves and causes this bipolar effect. Because skin irritation insults the skin nerves, irritated skin shows both an increase in the first threshold to sensation and a decrease in the least force necessary to produce pain. As the skin is progressively irritated, the touch threshold rises and the pain threshold falls. The full model employs both thresholds, and has wide application.