Sentence:  [S]ynesthetes themselves can provide the indispensable cues to design a structured qualitative questionnaire: color <WSD>synesthesias</WSD> and their variations during sexual arousal, direct physical stimulation, and orgasm ... can help scientists differentiate the specific triggers and their characteristics. 
Meaning of the word marked with <WSD> tags:  A neurological or psychological phenomenon whereby a particular sensory stimulus triggers a second kind of sensation. (from late 19th c.)