Motor "laziness" constrains fixation selection in real-world tasks

Abstract

AO_SCPLOWBSTRACTC_SCPLOWPeople coordinate their eye, head, and body movements to gather information from a dynamic environment while maximizing reward and minimizing biomechanical and energetic costs. Such natural behavior is not possible in a laboratory setting where the head and body are usually restrained and the tasks and stimuli used often lack ecological validity. Therefore, its unclear to what extent principles of fixation selection derived from lab studies, such as inhibition-of-return (IOR), apply in a real-world setting. To address this gap, participants performed nine real-world tasks, including driving, grocery shopping, and building a lego set, while wearing a mobile eye tracker (169 recordings; 26.6 hours). Surprisingly, spatial and temporal IOR were absent in all tasks. Instead, participants most often returned to what they just viewed, and saccade latencies were shorter preceding return than forward saccades. We hypothesized that participants minimize the time their eyes spend in an eccentric position to conserve eye and head motor effort. Correspondingly, we observed center biases in the distributions of fixation location and duration, relative to the heads orientation. A model that generates scanpaths by randomly sampling these distributions reproduced the spatial and temporal return phenomena seen in the data, including distinct 3-fixation sequences for forward versus return saccades. The amount of the orbit used in each task traded off with fixation duration, as if both incur costs in the same space. Conservation of effort ("laziness") explains all these behaviors, demonstrating that motor costs shape how people extract and act on relevant visual information from the environment. Significance StatementHumans display remarkably precise yet flexible control of eye and body movements, allowing for a wide range of activities. However, most studies of gaze behavior use the same setup: a head-restrained participant views small images on a computer. Such lab studies find that people avoid looking at the same thing twice, and hesitate in cases when they do. We had people perform nine everyday activities while wearing glasses with embedded eye tracking, and surprisingly found that they did the opposite, often returning to what they just viewed and expediting these "return" eye movements over others. A tendency to keep the eyes centered in the head, which we speculate helps to conserve motor effort, explained these behaviors for all tasks.