Stan Van Pelt | PubMed Health

PubMed2006

Gaze-centered updating of remembered visual space during active whole-body translations.

Various cortical and sub-cortical brain structures update the gaze-centered coordinates of remembered stimuli to maintain an accurate representation of visual space across eyes rotations and to produce suitable motor plans. A major challenge for the computations by these structures is updating across eye translations. When the eyes translate, objects in front of and behind the eyes' fixation point shift in opposite directions on the retina due to motion parallax. It is not known if the brain uses gaze coordinates to compute parallax in the translational updating of remembered space or if it uses gaze-independent coordinates to maintain spatial constancy across translational motion. We tested this by having subjects view targets, flashed in darkness in front of or behind fixation, then translate their body sideways, and subsequently reach to the memorized target. Reach responses showed parallax-sensitive updating errors: errors increased with depth from fixation and reversed in lateral direction for targets presented at opposite depths from fixation. In a series of control experiments, we ruled out possible biasing factors such as the presence of a fixation light during the translation, the eyes accompanying the hand to the target, and the presence of visual feedback about hand position. Quantitative geometrical analysis confirmed that updating errors were better described by using gaze-centered than gaze-independent coordinates. We conclude that spatial updating for translational motion operates in gaze-centered coordinates. Neural network simulations are presented suggesting that the brain relies on ego-velocity signals and stereoscopic depth and direction information in spatial updating during self-motion.

Adult↗

PubMed2006

Behavioral reference frames for planning human reaching movements.

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 = 10) was tested without visual information about hand position during movement planning (unseen-hand condition); another group (n = 8) was tested with hand and target position simultaneously visible before movement onset (seen-hand 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 hand-position signals can account for these results.

Adult↗