Load affects human odometry for travelled distance but not straight-line distance.

In a simple homing task with human participants, we disassociated the outbound distance travelled from the straight-line distance between home and target. Prior to the outbound journey, which involved a detour, participants were given one of two instructions concerning the inbound journey, which did not involve a detour: to walk the distance travelled or to walk to home. The inbound journey under each intention, made with eyes closed at a self-selected pace, was the measure of the perceived distance. We conducted two experiments that differed in whether or not the detour and target were visible during the outbound journey. In both experiments, we manipulated the load carried in the outbound journey (0% or 20% body weight) and the speed (fast or slow) of the outbound journey. The outcome of both experiments was that, indifferent to speed, participants perceived the distance travelled with load to be longer than that travelled without load, but perceived home's straight-line distance from target to be the same for both load conditions. Perceptions of travel distance and straight-line distance seem to be based on different information kinds and to refer to different animal-environment relations. In identifying neural mechanisms supportive of navigation, straight-line distance versus travelled distance may prove to be a productive distinction.