Systematic errors of planar arm movements provide evidence for space categorization effects and interaction of multiple frames of reference

Healthy humans performed arm movements in a horizontal plane, from an initi al position toward remembered targets, while the movement and the targets w ere projected on a vertical computer monitor. We analyzed the mean error of movement endpoints and we observed two distinct systematic error patterns. The first pattern resulted in the clustering of movement endpoints toward the diagonals of the four quadrants of an imaginary circular area encompass ing all target locations (oblique effect). The second pattern resulted in a tendency of movement endpoints to be closer to the body or equivalently lo wer than the actual target positions on the computer monitor Cv-effect). Bo th these patterns of systematic error increased in magnitude when a time de lay was imposed between target presentation and initiation of movement. In addition, the presence of a stable visual cue in the vicinity of some targe ts imposed a novel pattern of systematic errors, including minimal errors n ear the cue and a tendency for other movement endpoints within the cue quad rant to err away from the cue location. A pattern of systematic errors simi lar to the oblique effect has already been reported in the literature and i s attributed to the subject's conceptual categorization of space. Given the properties of the errors in the present work, we discuss the possibility t hat such conceptual effects could be reflected in a broad variety of visuom otor tasks. Our results also provide insight into the problem of reference frames used in the execution of these aiming movements. Thus. the oblique e ffect could reflect a hand-centered reference frame while the y-effect coul d reflect a body or eye-centered reference frame. The presence of the stabl e visual cue may impose an additional cue-centered (allocentric) reference frame.