Viewer-centered and body-centered frames of reference in direct visuomotor transformations

Abstract

It has been hypothesized that the end-point position of reaching may be specified in an egocentric frame of reference. In most previous studies, however, reaching was toward a memorized target, rather than an actual target. Thus, the role played by sensorimotor transformation could not be disassociated from the role played by storage in short-term memory. In the present study the direct process of sensorimotor transformation was investigated in reaching toward continuously visible targets that need not be stored in memory. A virtual reality system was used to present visual targets in different three-dimensional (3D) locations in two different tasks, one with visual feedback of the hand and arm position (Seen Hand) and the other without such feedback (Unseen Hand). In the Seen Hand task, the axes of maximum variability and of maximum contraction converge toward the mid-point between the eyes. In the Unseen Hand task only the maximum contraction correlates with the sight-line and the axes of maximum variability are not viewer-centered but rotate anti-clockwise around the body and the effector arm during the move from the right to the left workspace. The bulk of findings from these and previous experiments support the hypothesis of a two-stage process, with a gradual transformation from viewer-centered to body-centered and arm-centered coordinates. Retinal, extra-retinal and arm-related signals appear to be progressively combined in superior and inferior parietal areas, giving rise to egocentric representations of the end-point position of reaching.

Immediate spatial distortions of pointing movements induced by visual landmarks

“…In the no-landmark condition, the axis of maximal variability tended to align with the movement direction (Figure 3), as has been observed in previous studies (Carrozzo et al, 2002). With the introduction of landmarks, the variable error near the landmarks was reduced considerably, and its structure changed.…”

“…The axis of maximal variability was directed toward the landmarks, whereas in the no-landmark condition of Experiment 1, these were aligned toward the person. This reflects the two reference frames required by the different tasks (Carrozzo et al, 2002;Rossetti, 1998). The pattern of variable error when the landmarks did not shift appears to be a mixture of these two patterns, indicating that both egocentric and allocentric information was used.…”

“…Similarly, Carrozzo, Stratta, McIntyre, and Lacquaniti (2002) showed that if all possible target points were arranged on a line, the confidence ellipses of the movement endpoints tended to be aligned with this constructed allocentric reference frame, even though the line was never explicitly shown to the participants. Again, this influence of an allocentricreference was much more pronouncedafter a memory interval of 5 sec than after 0.5 sec.…”

Contribution of execution noise to arm movement variability in three-dimensional space

“…However, since constant errors tend to be idiosyncratic (Berkinblit et al 1995;Darling and Miller 1993;Foley and Held 1972;Soechting and Flanders 1989), we did not explore their nature in detail here. Instead, analysis focused on the variable errors, which provide more direct information about planning-and execution-related noise (McIntyre et al 1998;Carrozzo et al 1999;van Beers et al 2004). Variable errors associated with a given axis and T2 position ( t ) were calculated as follows:…”

“…For instance, patterns of variability following movements made from a starting position near the body to targets further away in depth have often been found to be significantly elongated along the depth axis (Carrozzo et al 1999;McIntyre et al 1997McIntyre et al , 1998van Beers et al 2004). These results could be interpreted as resulting from noise in execution (van Beers et al 2004), noise in visual estimation of the target and/or hand (van Beers et al 1998;Viguier et al 2001), or both processes.…”