Effects of salience and reward information during saccadic decisions under risk

Previous work has demonstrated that humans select visuomotor strategies maximizing expected gain during speeded hand movements under risk; see, e.g., [Trends Cogn. Sci. 12, 291 (2008)]; [Glimcher, eds., Neuroeconomics: Decision Making and the Brain (Elsevier, 2008), p. 95]. Here we report a similar study in which we recorded saccadic eye movements in a saccadic decision task in which monetary rewards and losses were associated with the final position of the eye movement. Saccades into a color-coded target region won points; saccades into a partially overlapping or abutting penalty region could yield a loss. The points won during the experiment were converted into a small monetary bonus at the end of the experiment. We compared participants' winnings to the score of an optimal observer maximizing expected gain that was calculated based on each participant's saccadic endpoint variability, similar to a recent model of optimal movement planning under risk [J. Opt. Soc. Am. A 20, 1419 (2003)]; [Spatial Vis. 16, 255 (2003)]. We used three different experimental paradigms with different interstimulus intervals (Gap, No Gap, and Overlap) to manipulate saccadic latencies and a fourth experiment (Memory) with a prolonged 500 ms delay period. Our results show that our subjects took the reward information, as specified by the different penalties, into account when making saccades and fixated onto or very close to the target region and less into the penalty region. However, the selected strategies differed significantly from optimal strategies maximizing expected gain in conditions when the magnitude of reward or penalty was changed. Furthermore, scores were notably affected by stimulus saliency. They were higher when the target region was filled and the penalty region outlined by a thin line, as compared to conditions in which the target was indicated by a less salient stimulus. Scores were particularly poor in trials with the shortest latencies (120-140 ms) mostly obtained in the Gap paradigm. At longer latencies scores improved considerably for latencies longer than 160 ms. This was in line with an improvement in accuracy for single targets up to 160 ms. Our results indicate that processing both of reward information and of stimulus saliency affect the programming of saccades, with a dominating contribution of stimulus saliency for eye movements with faster latencies.