Is Effort Encoded in Motor Imagery?

Though moment-to-moment navigation through the environment is largely accomplishedunconsciously, we can also consciously imagine spaces and locomotion through them even without overt movement. We have found consistent differences between imagery for locomotion and the actual physics of locomotion. Chronometric studies provide strong support that mental imagery recruits perceptual processes (Shepard & Metzler, 1970; Shepard & Cooper, 1971). An emerging trend suggests that information on effort is intrinsic to perceptual coding (Proffitt, 2003). If imagery recruits perceptual processes, and perception is influenced by anticipated effort, then imagery should exhibit effects of anticipated effort. Two experiments examined the role of effort in mental imagery. In Experiment 1, participants imagined self-rotation through a right angle from a route perspective across two conditions of distance (Room / Field). Simulated rotation required 150 ms more in the larger setting, suggesting that the length of the resultant arc is incorporated when rotating through a given angle, even though the imagined rotation is the same in each condition. Experiment 2, a replication with an effect size of 188 ms for larger settings, added a variable for imagined load (Heavy / Light). Subjects wore a heavy backpack on a walk through a set of hallways, then did the imagery task without overt rotation. Subjects responded with 198 ms longer latency for imagined heavy loads. The results suggest that both spatial metrics and anticipated effort play a role in coding of mental imagery, but further research is required to ascertain whether the information on anticipated effort is due to sensorimotor processes or semantically based knowledge.