39 research outputs found
The tetrad effect and geochemistry of apatite from the Altay Koktokay No. 3 pegmatite, Xinjiang, China: implications for pegmatite petrogenesis
In order to better constrain the evolution and petrogenesis of pegmatite, geochemical analysis was conducted on a suite of apatite crystals from the Altay Koktokay No. 3 pegmatite, Xinjiang, China and from the granitic and amphibolitic wall rocks. Apatite samples derived from pegmatite zones show convex tetrad effects in their REE patterns, extremely negative Eu anomalies and non-chondritic Y/Ho ratios. In contrast, chondritic Y/Ho ratios and convex tetrad effects are observed in the muscovite granite suggesting that different processes caused non-chondritic Y/Ho ratios and lanthanide tetrad effects. Based on the occurrence of convex tetrad effects in the host rocks and their associated minerals, we propose that the tetrad effects are likely produced from immiscible fluoride and silicate melts. This is in contrast to previous explanations of the tetrad effect; i.e. surface weathering, fractional crystallization and/or fluid-rock interaction. Additionally, we put forward that extreme negative Eu and non-chondritic Y/Ho in apatite are likely caused by the large amount of hydrothermal fluid exsolved from the pegmatite melts. Evolution of melt composition was found to be the primary cause of inter and intra-crystal major and trace element variations in apatite. Mn entering into apatite via substitution of Ca is supported by the positive correlation between CaO and MnO. Different evolution trends in apatite composition imply different crystallization environments between wall rocks and pegmatite zones. Based on the geochemistry of apatite samples, it is likely that there is a genetic relationship between the source of muscovite granite and the source of the pegmatite
Dopaminergic effects on the implicit processing ofdistracter objects in Parkinson\u2019s disease
The aim of the present study was to assess the effects of dopaminergic medication on the
selection-for-action mechanisms in Parkinson\u2019s disease (PD). PD subjects were tested after not having
taken medication for at least 12 h ("Off" state) and then retested 1-2 h after medication ("On" state). A
three-dimensional kinematic system (ELITE, BTS, Italy) was used to record reach-to-grasp
movements to a target object placed at a reaching distance of 30 cm. The target was presented alone or
in the presence of distracter objects, which could be of either the same size (compatible distracter) or a
different size (incompatible distracter). PD subjects in the Off state were significantly more affected
by the presence of the incompatible distracter than in the On state. These results indicate that
dopaminergic medication is of benefit in reducing interference effects when distracter objects evoke
motor programs that differ from the motor program elicited by the target. Results are discussed in light
of the role played by the striatal and mesocortical dopaminergic systems for response selection in basal
ganglia disorders
The reach-to-grasp movement in Parkinson\u2019s disease:response to a simultaneous perturbation of object positionand object size
This study assessed the adaptive response of
the reach-to-grasp movement of 12 Parkinson\u2019s disease
(PD) and 12 control subjects to a simultaneous perturbation
of target object location and size. The main aim was
to test further the reported dysfunction of PD subjects in
the simultaneous activation of movement components.
Participants were required to reach 30 cm to grasp a central
illuminated cylinder of either small (0.7 cm) or large
(8 cm) diameter. For a small percentage of trials
(20/100) a visual perturbation was introduced unexpectedly
at the onset of the reaching action. This consisted of
a shift of illumination from the central cylinder to a cylinder
of differing diameter, which was positioned 20\ub0 to
the left (n=10) or to the right (n=10). The subject was required
to grasp the newly illuminated cylinder. For the
Parkinson\u2019s disease subject group, the earliest response
to this \u2018double\u2019 perturbation was in the parameter of
peak reaching acceleration, which was on average 50 ms
earlier for \u2018double\u2019 perturbed than for non-perturbed trials.
The grasp component response followed more than
500 ms after the earliest transport response. For the control
subjects initial signs of a response to the \u2018double\u2019
perturbation were seen almost simultaneously in the
transport parameter of peak arm deceleration, and in the
manipulation parameter of maximum grip aperture, but
these changes were not evident until more than 400 ms
after movement onset. These results indicate that the
basal ganglia can be identified as part of a circuit which
is involved in the integration of parallel neutral pathways,
and which exercise flexibility in the degree to
which these components are \u2018coupled\u2019 functionally. With
basal ganglia dysfunction the activation of integration
centres that at first gate the flow of information to the
parallel channels of reach and grasp seems inefficient
Unilateral pallidotomy for Parkinson's disease disrupts ocular fixation
Although some motor functions of the basal ganglia have been well studied, the oculomotor functions are not well established. We studied eye movements in patients with Parkinson's disease (PD) undergoing pallidotomy to assess the role of the globus pallidus interna (GPi) in oculomotor control. Horizontal visually guided, gap and predictive saccades as well as ocular fixation were studied in patients with advanced PD before and 1 month after unilateral pallidotonny, and in healthy controls on two occasions 1 month apart. There was no difference in saccadic latency or accuracy, the number of saccadic anticipations or the ability to generate predictive saccades between the two assessments for either patients or controls. The number and amplitude of square wave jerks during ocular fixation however increased significantly in patients after pallidotomy. The results imply altered function of frontal or prefrontal cortical regions involved in ocular fixation resulting from a disruption to inhibitory pallidal influences on thalamocortical projections. The posteroventral GPi however appears not to be involved in externally controlled or predictive saccadic function. (C) 2002 Elsevier Science Ltd. All rights reserved