1: Exp Brain Res. 2005 Jan 15; [Epub ahead of print]

Manual-aiming bias and the Muller-Lyer illusion: the roles of position and
extent information.

Predebon J.

School of Psychology, University of Sydney, 2006, Sydney, NSW, Australia,
johnp@psych.usyd.edu.au.

Several studies have shown that rapid manual aiming movements from one to the
other wing of Muller-Lyer illusion figures are biased in a manner consistent
with their influence on perceptual judgments. Two experiments examined the role
of extent and position information in Muller-Lyer figures in biasing pointing
movements. Experiment 1 compared the effect of starting the action either close
to the non-target wing or from a position well outside the conventional
wings-out (>-<) and wings-in (<->) Muller-Lyer figures. Pointing movements were
longer to the target wing of the wings-out than of the wings-in figure but only
for actions starting from a position adjacent to the non-target wing. Experiment
2 found no effect of the direction of the target wing on pointing movements
between the outer wings of the combined Muller-Lyer illusion figure (<->-<).
These findings suggest that perceived extent (of the shaft) and perceived
position (of the wings) in Muller-Lyer figures are largely independent of each
other, and are consistent with the claim that the illusion is more likely to
influence pointing actions if the action is based on extent information than on
vertex position information.

PMID: 15654590 [PubMed - as supplied by publisher]



2: Exp Brain Res. 2005 Mar;161(4):532-40. Epub 2004 Dec 7.

Decoding of path-guided apparent motion from neural ensembles in posterior
parietal cortex.

Merchant H, Battaglia-Mayer A, Georgopoulos AP.

Department of Neuroscience, University of Minnesota, Minneapolis, MN, 55455,
USA.

We compared quantitatively the psychometric capacity of human subjects to detect
path-guided apparent motion (PAM) and the accuracy of cell ensembles in area 7a
to code the same type of stimuli. Nine human subjects performed a detection task
of PAM. They were instructed to indicate with a key-press whether they perceived
a circularly moving object when five stimuli were flashed successively at the
vertices of a regular pentagon. The stimuli were presented along a low contrast
circular path with one of 33 speeds (150-600 degrees /s). The average
psychometric curve revealed that the threshold for PAM detection was 314 degrees
/s. The minimum and maximum thresholds for individual subjects were 277 degrees
and 378 degrees /s, respectively. In addition, the activity of cells in area 7a
that were modulated by the stimulus position in real or apparent motion was used
in a multivariate linear regression analysis to recover the stimulus position
over time. Real stimulus motion was decoded successfully from neural ensemble
activity at all speeds. In contrast, the decoding of PAM was poor at low
stimulus speeds but improved markedly above 300 degrees /s: in fact, this was
very close to the threshold above for human subjects to perceive continuous
stimulus motion in this condition. These results suggest that the posterior
parietal cortex is part of a high-level system that is directly involved in the
dynamic representation of complex motion.

PMID: 15586277 [PubMed - in process]



3: Exp Brain Res. 2004 Dec 7; [Epub ahead of print]

Velocity and curvature in human locomotion along complex curved paths: a
comparison with hand movements.

Hicheur H, Vieilledent S, Richardson MJ, Flash T, Berthoz A.

Laboratoire de Physiologie de la Perception et de l'Action, CNRS College de
France, 11 place Marcelin Berthelot, 75005, Paris, France.

There is extensive experimental evidence linking instantaneous velocity to
curvature in drawing and hand-writing movements. The empirical relationship
between these characteristics of motion and path is well described by a power
law in which the velocity varies in proportion to the one-third power of the
radius of curvature. It was recently shown that a similar relationship can be
observed during locomotion along curved elliptical paths raising the possibility
that these very different motor activities might, at some level, share the same
planning strategies. It has, however, been noted that the ellipse is a special
case with respect to the one-third power law and therefore these previous
results might not provide strong evidence that the one-third power law is a
general feature of locomotion around curved paths. For this reason the
experimental study of locomotion and its comparison with hand writing is
extended here to non-elliptical paths. Subjects walked along predefined curved
paths consisting of two complex shapes drawn on the ground: the cloverleaf and
the limacon. It was found that the data always supported a close relationship
between instantaneous velocity and curvature. For these more complex paths,
however, the relationship is shape-dependent-although velocity and curvature can
still be linked by a power law, the exponent depends on the geometrical form of
the path. The results demonstrate the existence of a close relationship between
instantaneous velocity and curvature in locomotion that is more general than the
one-third power law. The origins of this relationship and its possible
explanation in the mechanical balance of forces and in central planning are
discussed.

PMID: 15586276 [PubMed - as supplied by publisher]



4: Exp Brain Res. 2004 Dec 7; [Epub ahead of print]

Development of visuomotor representations for hand movement in young children.

Contreras-Vidal JL, Bo J, Boudreau JP, Clark JE.

Department of Kinesiology, University of Maryland, College Park, MD 20742, USA.

The stability and adaptability of visuomotor representations for hand movement
in young children was investigated using a visuomotor adaptation paradigm in
which the real-time visual feedback of pen movement was rotated 45 degrees
clockwise during exposure trials. Four, six, and eight-year-old children
performed line drawings to visual targets, from a common centered position
("center-out task"), in the horizontal plane under normal (pre-, and
post-exposure), and rotated (exposure) visual feedback conditions. Analysis of
pre-exposure trials indicated that older children performed faster, straighter,
smoother, and showed more patterned movements than the younger children. Initial
direction of movement, computed at 80 ms after movement onset, showed a
progressive tuning of movement direction with increasing age. On introduction of
the screen cursor rotation, all age group children showed improvement in their
planning (initial directional error) and execution (movement time, movement
length, root mean square error, and normalized jerk) error scores from early to
late-exposure trials, but the 4-year-olds were less affected than older age
children by the distortion during the early exposure period. Moreover, only the
oldest group of children showed significant after-effects during post-exposure
trials indicating that only this age group learned the internal model of the
distorted environment. The absence of after-effects for initial movement
direction observed in the two younger age groups suggest that these children
might have less developed (i.e. more broad) internal visuomotor representations
for hand movements, and that their internal representations are sharpened (i.e.
tuned) with visuomotor experience.

PMID: 15586275 [PubMed - as supplied by publisher]



5: Exp Brain Res. 2004 Dec 4; [Epub ahead of print]

Motor control goes beyond physics: differential effects of gravity and inertia
on finger forces during manipulation of hand-held objects.

Zatsiorsky VM, Gao F, Latash ML.

Biomechanics Laboratory, Department of Kinesiology, The Pennsylvania State
University, 16802, University Park, PA, USA.

According to basic physics, the local effects induced by gravity and
acceleration are identical and cannot be separated by any physical experiment.
In contrast-as this study shows-people adjust the grip forces associated with
gravitational and inertial forces differently. In the experiment, subjects
oscillated a vertically-oriented handle loaded with five different weights (from
3.8 N to 13.8 N) at three different frequencies in the vertical plane: 1 Hz, 1.5
Hz and 2.0 Hz. Three contributions to the grip force-static, dynamic, and
stato-dynamic fractions-were quantified. The static fraction reflects grip force
related to holding a load statically. The stato-dynamic fraction reflects a
steady change in the grip force when the same load is moved cyclically. The
dynamic fraction is due to acceleration-related adjustments of the grip force
during oscillation cycles. The slope of the relation between the grip force and
the load force was steeper for the static fraction than for the dynamic
fraction. The stato-dynamic fraction increased with the frequency and load. The
slope of the dynamic grip force-load force relation decreased with frequency,
and as a rule, increased with the load. Hence, when adjusting grip force to task
requirements, the central controller takes into account not only the expected
magnitude of the load force but also such factors as whether the force is
gravitational or inertial and the contributions of the object mass and
acceleration to the inertial force. As an auxiliary finding, a complex finger
coordination pattern aimed at preserving the rotational equilibrium of the
object during shaking movements was reported.

PMID: 15580485 [PubMed - as supplied by publisher]



6: Exp Brain Res. 2005 Jan;160(2):259-263. Epub 2004 Nov 25.

Components of sensorimotor adaptation in young and elderly subjects.

Bock O.

Institute for Physiology and Anatomy, German Sport University, 50927,  Koln,
Germany, bock@dshs-koeln.de.

Previous studies have found that sensorimotor adaptation to visual distortions
is degraded in seniors compared with younger subjects, whereas after-effects on
removal of the distortion are age-independent. The latter finding was
interpreted as evidence that adaptive recalibration is not affected by old age,
and that the observed degradation is therefore due to impairment of strategic
control. However, after-effects are not a reliable measure of recalibration,
because they can be artificially inflated by perseveration, a characteristic
symptom in old age. The present work therefore introduces a test of
recalibration which is insensitive to perseveration. Twelve young and twelve old
subjects executed center-out pointing movements while visual feedback about
their fingertip was either veridical (baseline), 60-deg rotated (adaptation), or
absent (after-effect).They also executed tracking movements toward an
unpredictably moving object before and after the pointing task. Seniors adapted
less than younger subjects but their after-effects were not degraded. More
importantly, transfer of adaptation from a pointing to a tracking task was not
degraded in seniors. The latter outcome documents, in a more compelling fashion
than previous work, that recalibration in the elderly is not impaired, and that
the observed deficit of adaptation is therefore most probably because of
impaired strategic control. This conclusion is supported by two additional
findings: compared with young subjects our seniors performed less well on a
cognitive screening test and acquired no explicit knowledge about the nature of
the imposed visual distortion.

PMID: 15565436 [PubMed - as supplied by publisher]