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How strong a signal is required for an observer to say that they have seen it? The answer depends not only on the strength of the signal, but also the sensitivity of the observer, (which is influenced by noise and focus of attention) and the criterion adopted by the observer, (influenced by motivation or expectation).
Noise is present both in the environment and in the sensory system of the observer. The observer reacts to the momentary total activation of the sensory system, which fluctuates from moment to moment as well as responding to environmental stimuli, which may include a signal.