The process of separating useful sensory information from the many thousands of stimuli present in the external environment, so that only potentially useful information is sent to the brain. Many sensory organs are adapted so that they receive stimuli only within a certain range. For example, the human eye only detects colours in the visible region of the spectrum and not in the ultraviolet or infrared regions. Another method of stimulus filtering involves sign stimuli, which are more important than other stimuli in eliciting a response and, once detected, may lead to other stimuli being ignored.