Before revision:  One drawback of this strategy is that occasional runs in the wrong direction reduce progress up the gradient. Here we discovered a positive feedback regime inherent in this strategy that strongly mitigates this problem. In an attractant field, motion up the gradient reduces tumble probability, which further boosts drift up the gradient.
After revision:  This induces a functional coupling between movement and sensation, since tumbling probability is controlled by the internal state of URLanism which, in turn, depends on previous signal levels. Although a negative feedback tends to maintain this internal state close to adapted levels, positive feedback can arise when motion up the gradient reduces tumble probability, which further boosts drift up the gradient.