Sentence:  Since electrical stimulation of the posterior hypothalamus produces the effects of wakefulness while stimulation of the anterior hypothalamus induces sleep , it may be said that the reactivity of the whole organism is altered by a <WSD>change</WSD> in the autonomic reactivity of the hypothalamus . 
Meaning of the word marked with <WSD> tags:  an event that occurs when something passes from one state or phase to another