I think you're misreading the study design. 85 people viewed the same photos, some of whom had taken acetaminophen, and some of whom hadn't. The researchers the

I think you're misreading the study design. 85 people viewed the same photos, some of whom had taken acetaminophen, and some of whom hadn't. The researchers then compared the emotional intensity experienced by the two groups (and, in the specific study about which you're quoting, the participants' experience of the color blue).