Synesthetic Metaphors

TL;DR: Synesthetic metaphors map figurative relationships between multiple subjects using multiple human senses.

In terms of medicine, "synesthesia" is the phenomenon in which your mind links different sensory/cognitive pathways together, such as associating certain colors with letters or numbers, seeing visual hallucinations when hearing music, or tasting a color when seen.

In terms of poetics, synesthetic metaphors bridge the gap between non-similar objects using multiple human senses.

They can be simple: "That apple is a loud red."

Or complex: "The deafeningly red arterial blood spurting out of his carotids as he lay motionless enraged me. I started to hear beating war drums, each one encouraging me to seek vengeance."

The simple example uses "loud" as an adjective to describe the brightness of the apple's color, bridging together the visual and auditory senses.

The complex example also contains the simple example by describing the blood as "deafeningly red" but then bridges the adverb "deafeningly" with the sound of the beating war drums.