Although sometimes used synonymously, polymeter is the use of two metric frameworks (time signatures) simultaneously, while polyrhythm refers to the simultaneous use of two or more different patterns, which may be in the same time-signature.
Research into the perception of polymeter shows that listeners often either extract a composite pattern that is fitted to a metric framework, or focus on one rhythmic stream while treating others as “noise”. This is consistent with the Gestalt psychology tenet that “the figure-ground dichotomy is fundamental to all perception”.
In “Toads Of The Short Forest” (from the album Weasels Ripped My Flesh), composer Frank Zappa explains: “At this very moment on stage we have drummer A playing in 7/8, drummer B playing in 3/4, the bass playing in 3/4, the organ playing in 5/8, the tambourine playing in 3/4, and the alto sax blowing his notes” (Mothers of Invention 1970). “Touch And Go”, a hit single by The Cars, has polymetric verses, with the drums and bass playing in 5/4, while the guitar, synthesizer, and vocals are in 4/4 (the choruses are entirely in 4/4) (The Cars 1981, 15).