You play a catchy, silly little tune that gets stuck in the mind of the subject unless she succeeds on a Will save.

The endlessly recycling melody makes it difficult for the subject to cast spells or perform any other action that requires mental focus.

The subject takes a -4 penalty on all Intelligence-based skill checks and Concentration checks.

Whenever the target attempts to cast, concentrate on, or direct a spell, she must succeed on a Concentration check (DC equal to insidious rhythm's save DC + spell's level) or fail at the attempt.