Some wonder if people with synesthesia are just being metaphorical, as many people use metaphors that cross sensory modalities. a "loud shirt" doesn't actually produce a sound, a "warm color" doesn't actually evoke a change in temperature, and "bitter cold" doesn't actually produce a taste. what does cytowic friend say that suggests this isn't the case

Synesthesia is a neurological condition that an individual experience which allows her or him to perceive stimuli using a sense that is normally not associated with the stimuli. For example, people with chromesthesia, a type of synesthesia, can hear sounds when a color comes up – for example, hearing a trumpet when they see a red apple. There are various other types of synesthesia, such as seeing alphabets and at the same time seeing colors associated to them or hearing words and feeling a skin sensation with each different words said.  

Since these experiences are unique to the individual who has the condition, Cytowic’s friend should come up with a demonstration so that Cytowic can experience the same thing he does.