Successful biasing of attention reveals covert information extraction contributes to saccade timing variability within and across individuals.

The decision of where to saccade during search depends on information acquired via covert processes, which takes time. Previously, we found that longer first saccade latency led to fewer eye movements per trial, possibly due to participants using more perceptual input to guide saccades. In this experiment, the probability of stimulus difficulty was varied in an attempt to increase covert processing before saccade initiation to benefit search. Task difficulty was manipulated by jittering the elements in a contour target (high, medium, and no-jitter). This varied across three blocked conditions: a neutral condition containing equal amounts of each jitter, an easy-bias condition containing medium and no-jitter, and a hard-bias condition containing medium and high jitter. Jitter bias was hypothesized to influence attentional strategies with the predominance of no-jitter trials in the easy-bias condition encouraging greater covert processing of the same medium-jitter targets. Consistent with this, the results showed longer first saccade latencies in the easy-bias condition, and these were associated with fewer total trial fixations as in previous work. Moreover, latencies declined significantly with increasing target eccentricity and this slope was steeper in the easy-bias condition, suggesting a change in covert peripheral processing. Lastly, individual differences were analyzed using random effects correlations. Positive correlations were found between bias conditions for both saccade latencies and fixation counts, indicating participants had general search strategies and outcomes. Also, mean first saccade latencies negatively correlated with eccentricity slope in the neutral and easy-bias conditions, meaning the drop in saccade latency with target eccentricity was steeper for participants who tended to wait longer. Impetuous participants showed little systematic variation in terms of target eccentricity. In conclusion, these results indicate that saccadic behavior is influenced by stable but malleable attentional strategies that determine the degree to which extrafoveal information is used to guide saccadic eye movements.