Figure 1.

Itch coding models. (A) Intensity theory hypothesizes individual neurons that signal either itch or pain through lower or higher intensity stimuli (represented by low and high frequency spiking), respectively. (B) Labeled line theory hypothesizes entirely separate neurons and circuitry for detecting pain versus itch. (C) Occlusion theory hypothesizes dual itch/pain-sensing neurons that signal itch when they are selectively activated. Noxious stimuli will activate both this and the pain-only population, leading to inhibition of the itch circuit.