The Smell of Green Thunder: How Does Synesthesia Differ from Hallucination? In a condition called synesthesia, the signals between the senses are often crossed. It manifests itself in a variety of fashions and in varying intensities. Synesthetes often report hearing colors or smelling sounds, which to some is an infinite source of amusement, and for others, a source of annoyance. But are these experiences hallucinations?