Before revision:  When gradients are shallow or noisy, URLanisms move by alternating straight motion (runs) with random reorientations (tumbles) . Navigation is achieved by transiently reducing the probability to tumble when attractant signal increases.
After revision:  When gradients are shallow or noisy, URLanisms move by alternating straight motion (runs) with random reorientations (tumbles) , transiently suppressing tumbles whenever attractant signal increases.