I'm not sure about the methodology here. Strings (and other small objects) are generally treated as prey items by cats, so playing with strings is inheritely rewarding. Also, many cats are picky about their treats, it generally takes several tastes for my cat to decide she likes a treat. For many cats playing with a string would be more rewarding then eating a treat, unlike dogs. This is why people who train their cats for feline agility generally use lures rather then food rewards. Dogs tend to be more food-driven.