Even in orthodox Chrysippan epistemology, the Stoics believed that impulse can precede assent. Their doctrines on the propatheiai form a theory of temptation, in which impressions exert a force upon us to assent, just as the Academic critics of the Stoics argued. Close readings of De Fato 40-43 and Stobaeus do not actually bear out the consensus understanding of modern critics like Inwood that impulse is identical with, and can only occur with assent. Stevens collects more evidence and sets out the argument with greater clarity in his published version "Preliminary Impulse in Stoic Psychology", Ancient Philosophy 20.1 (2000) 139-168.
Stevens, John A., "Impulse and Animal Action in Stoic Psychology" (1996). The Society for Ancient Greek Philosophy Newsletter. 204.