Given the current fixation position s t , the text X, and the chosen saccadic type u t+1 , the amplitude is constrained to fall within a specific interval-for instance, within the characters of the currently fixated word for refixations.
we model the distribution over the saccade amplitude given the saccadic type (Equation 4) as truncated Gamma distributions, given by 1 is the Gamma distribution with shape parameter α and scale parameter β, and Γ is the Gamma function.