Behavioral Reference Frames for Planning Human Reaching Movements

Abstract

. At some stage in the process of a sensorimotor transformation for a reaching movement, information about the current position of the hand and information about the location of the target must be encoded in the same frame of reference to compute the hand-to-target difference vector. Two main hypotheses have been proposed regarding this reference frame: an eye-centered and a body-centered frame. Here we evaluated these hypotheses using the pointing errors that subjects made when planning and executing arm movements to memorized targets starting from various initial hand positions while keeping gaze fixed in various directions. One group of subjects (n ϭ 10) was tested without visual information about hand position during movement planning (unseenhand condition); another group (n ϭ 8) was tested with hand and target position simultaneously visible before movement onset (seenhand condition). We found that both initial hand position and gaze fixation direction had a significant effect on the magnitude and direction of the pointing error. Errors were significantly smaller in the seen-hand condition. For both conditions, though, a reference frame analysis showed that the errors arose at an eye-or hand-centered stage or both, but not at a body-centered stage. As a common reference frame is required to specify a movement vector, these results suggest that an eye-centered mechanism is involved in integrating target and hand position in programming reaching movements. We discuss how simple gain elements modulating the eye-centered target and handposition signals can account for these results.

Cue Reliability and a Landmark Stability Heuristic Determine Relative Weighting Between Egocentric and Allocentric Visual Information in Memory-Guided Reach

“…When humans point or reach toward objects that are not aligned with gaze, the hand tends to overshoot relative to gaze (Bock 1986). This is thought to arise from some unknown error in the visuomotor transformation (Beurze et al 2006;Henriques et al 1998). McGuire and Sabes (2009) modeled this by incorporating a misestimate of gaze direction relative to the desired reach direction, as well as several other features.…”

Gaze-Centered Updating of Remembered Visual Space During Active Whole-Body Translations

“…This way we tried to minimize the error attributable to an erroneous estimate of fingertip position during pointing (Beurze et al 2006). We also allowed subjects to look where they were reaching to eliminate contributions of errors occurring otherwise, i.e., when gaze would be off the reach location ; see control experiment 2 in the following text).…”

Functional Rather than Effector-Specific Organization of Human Posterior Parietal Cortex

“…In contrast, the reference frame of a visual target is distinct from the motor reference frame of a limb (Graziano et al, 1997;Ferraina et al, 2009). Therefore, specifying a goal-directed limb movement requires additional transformations to compute the spatial relationship between target and effector position, which depends on the current body posture (Buneo et al, 2002;Beurze et al, 2006). Although effector-specific organization of PPC has been questioned on the basis of overlapping activity of eye-and hand-related activity (Hagler et al, 2007;Levy et al, 2007), experiments involving only two effectors do not allow deciding between effector specificity versus other, functionally based explanations as the reason of existing regional activity biases in PPC.…”