Sensory systems are gateways through which we receive information about the world around us. We may think that our senses indiscriminately respond to whatever happens in the environment, but sensory systems are very selective. Sensory organs themselves (and also the brain circuits that process sensory information) have an amazing ability to pick out the types of stimuli that are most important in a particular situation. Sensory neural circuits can even change the way that they process sensory cues in different behavioral states by responding to chemical signals that are released in the brain. The neuromodulator serotonin is one of these neurochemical signals.