A Viewer-centered frame of reference for pointing to
memorized targets in three-dimensional space
J. McIntyre, F. Stratta, and F. Lacquaniti
J. McIntyre, Istituto Scientifico S. Lucia, via Ardeatina 306, 00179 Roma,
Italy, Phone: (+39 6) 51 50 14 75 or 51 50 14 74, Fax: (+39 6) 51 50 14 77,
Email: jam@ccr.jussieu.fr
APStracts 4:0079N, 1997.
ABSTRACT
Pointing to a remembered visual target involves the transformation of
binocular visual information into an appropriate motor output. Errors
generated during pointing tasks may indicate the reference frames used by the
CNS for the transformation and storage of the target position. Previous
studies have proposed eye-, shoulder- or hand-centered reference frames for
various pointing tasks, depending on visual conditions.
We asked subjects to perform pointing movements to remembered 3D targets after
a fixed memory delay. Pointing movements were executed under dim lighting
conditions, allowing vision of the finger tip against a uniform, black
background. Subjects performed repeated movements to targets distributed
uniformly within a small (25 mm radius) workspace volume. In separate blocks
of trials, subjects pointed to different workspace regions that varied in
terms of distance and direction from the head and shoulder. Additional blocks
were performed that differed in terms of starting position, effector hand,
head rotation and memory delay duration. Final pointing positions were
quantified in terms of the constant and variable errors in three dimensions.
The orientation of these errors were examined as a function of workspace
location in order to identify the underlying reference frames.
Subjects produced anisotropic patterns of variable error, with greater
variability for end-point distances from the body. The major axes of the
variable error tolerance ellipsoids pointed toward the eyes of the subject,
independent of workspace region, effector hand (left or right), initial hand
position and head rotations. Constant errors were less consistent across
subjects, but also tended to point toward the head and body. Both overshoots
and undershoots of the target position were observed.
Increasing the duration of the memory delay period increased the size but did
not alter the orientation of the variable error ellipsoids. Variability of the
end point positions increased equally in all three Cartesian directions as the
memory delay increased from 0.5 to 8.0 seconds.
The anisotropy of variable errors indicates a viewer-centered reference frame
for pointing to remembered visual targets with vision of the finger. The
anisotropy of pointing variability stems from variability in egocentric
binocular cues, as opposed to reliance on allocentric visual references or to
specific approximations in the sensorimotor transformation. Nevertheless,
observed increases in variability with longer memory delays indicate that the
short-term storage of the target position does not simply mirror the retinal
and ocular sensory signals of the visually acquired target location. Thus,
spatial memory is carried out in an internal representation that is viewer-
centered, but that may be isotropic with respect to Cartesian space.
Received 1996 December 17; accepted in final form 1997 May 14.
APS Manuscript Number J986-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 11 June 1997