Are express saccades generated under natural viewing conditions?

To assess whether express saccades are generated under everyday conditions, we collected eye movement data from Rhesus monkeys engaged in free viewing under a variety of conditions. The durations of the fixation periods that occurred between saccades were calculated. The results show that while short-duration fixations within the range of express saccades occur quite commonly, the overall distribution is unimodal. This is the case even when all the object elements in the visual scene have the same contrast. The findings suggest that while saccades that fall within the express range occur commonly under natural viewing conditions, bimodal distributions of saccadic latencies are obtainable only under laboratory conditions.