Paradigm shift in sensory system - Animals do not have sensors

When temperature (T) of skin decreases, we feel that the skin is cold. Skin T is a measurable physical and physiological parameter but distinct from skin coldness in our mind. Nevertheless, there has been a dogma ("sensor theory") that cold receptors are sensors to transform skin T into impulses, which convey information of the actual skin T to the brain in order to mediate perception of coldness. However, this explanation is false, because skin coldness is not skin T. Thus, we do not have sensors. Alternatively, we have comparators. We have clarified that receptors are comparators ("comparator theory") with no signal transduction from outer world to the brain. When skin T is lower than threshold T, a receptor acting as comparator evokes impulses as the trigger to activate target neurons. Then, we feel that the skin is cold. This leads us to conclude that information for skin coldness is pre-stored in a target neuron. Thus, the site of information for evoking sensation has shifted from outer world to the brain. "comparator theory" therefore causes a paradigm shift in sensory system. (c) 2005 Elsevier Ltd. All rights reserved.