Stimuli might arise from the individual’s internal or external
environment. Roy puts environmental stimuli into three classes:
focal, contextual and residual. Focal stimulus is the one which is
confronted immediately by the individual and which affects the
individual most. All the other stimuli that contribute to the effects
of focal stimuli are contextual stimuli. Residual stimuli are the ones
which are present in the environment but whose impact on
behavior has yet to be explained.