Physical Effort Pre-Crastination Determines Preference in an Isometric Task

Abstract

How the brain decides when to invest effort is a central question in neuroscience. When asked to walk a mile to a destination, would you choose a path with a hill at the beginning or the end? The traditional view of effort suggests we should be indifferent-all joules are equal so long as it does not interfere with accomplishing the goal. Yet when total joules are equal, the brain's sensitivity to the temporal profile of effort investment throughout a movement remains poorly understood. Here, we sought to parse out the interaction of time and physical effort by comparing subjective preferences in an isometric arm-pushing task that varied the duration and timing of high and low effort. Subjects were presented with a series of two-alternative forced choices, where they chose the force profile they would rather complete. Subjects preferred to pre-crastinate physical effort but were idiosyncratic about preference for task timing. A model of subjective utility that includes physical effort costs, task costs, and independent temporal sensitivity factors described subject preferences best. Interestingly, deliberation time and response vigor are best described by the same subjective utility model that won for preference, further validating this model of subjective utility. These results suggest physical effort costs are temporally sensitive, with earlier investment of effort preferred to later investment. These findings demonstrate that the representation of effort is based not only on the total energy required but its timing as well, and offer an alternative hypothesis for why animals pre-crastinate in physical tasks.