Biomedical subjects

Christine Tong

Publications and source records attributed to Christine Tong.

5 recordsLinked to original sources

PubMed2006

Columnar self-assembled ureido crown ethers: an example of ion-channel organization in lipid bilayers.

The self-assembly of ureido crown-ether derivatives has been examined in homogeneous solution, in the solid state, and in planar bilayer membranes. The self-assembly is driven by head-to-tail hydrogen bonding between the urea functional groups. Dimers and higher oligomers are formed in CDCl3 solution as assessed by the change in the ureido NH chemical shift as a function of concentration. Single-crystal X-ray diffraction shows that an antiparallel association of the ureas produces columnar channels composed of face-to-face crown ethers. Powder X-ray diffraction studies also show the presence of a minor phase based upon a parallel urea association leading to an alternative columnar arrangement of the crown ethers. In bilayer membranes at low concentration of ureido crown ether added, membrane disruption is observed together with rare single-channel openings, but at higher concentration, a rich array of interconverting channel conductance states is observed. The channel results are interpreted as arising from discreet stacks of ureido crown ethers where the transport of cations would occur via the macrocycles, admixed with larger pores formed by association of the crown ether headgroups around a central large pore.

Journal Article↗

PubMed2005

Learning and recall of incremental kinematic and dynamic sensorimotor transformations.

Numerous studies have shown that when people encounter a sudden and novel sensorimotor transformation that alters perceived or actual movement, they gradually adapt and can later recall what they have learned if they encounter the transformation again. In this study, we tested whether retention and recall of learning is also observed when kinematic and dynamic transformations are introduced incrementally such that participants never experience large movement errors. Participants adapted their reaching movements to either a visuomotor rotation of hand position (kinematic transformation) or a rotary viscous force-field applied to the hand (dynamic transformation). These perturbations were introduced either incrementally or instantaneously. Thus, four groups of participants were tested with an incremental and an instantaneous group for both the kinematic and dynamic perturbations. To evaluate retention of learning, participants from all four groups were tested a day later on the same kinematic or dynamic perturbation presented instantaneously (at full strength). As expected, we found that subjects in the instantaneous group retained learning across days. We also found that, for both kinematic and dynamic perturbations, retention was equally good or better when the transformation was introduced incrementally. Because large and clearly detectable movement errors were not observed during adaptation to incremental perturbations, we conclude that such errors are not required for the learning and retention of internal models of kinematic and dynamic sensorimotor transformations.

Adolescent↗

PubMed2003

Task-specific internal models for kinematic transformations.

Numerous studies of motor learning have focused on how people adapt their reaching movements to novel dynamic and visuomotor perturbations that alter the actual or visually perceived motion of the hand. An important finding from this work is that learning of novel dynamics generalizes across different movement tasks. Thus adaptation to an unusual force field generalizes from center-out reaching movements to circular movements (Conditt et al. 1997). This suggests that subjects acquired an internal model of the dynamic environment that could be used to determine the motor commands needed for untrained movements. Using a task interference paradigm, we investigated whether transfer across tasks is also observed when learning visuomotor transformations. On day 1, all subjects adapted to a +30 degrees rotation while making center-out-and-back reaching movements. After a delay of 5 min, different groups of subjects then adapted to a -30 degrees rotation while performing either a continuous tracking task, a figure-eight drawing task, or the center-out-and-back reaching task. All subjects were then retested the next day on the +30 degrees rotation in the reaching task. As expected, subjects who experienced the opposing rotations while performing the same reaching tasks showed no retention of learning for the first rotation when tested on day 2 (Krakauer et al. 1999). In contrast, such retrograde interference was not observed in the two groups of subjects who experienced the opposing rotations while performing different tasks. In fact, their performance on day 2 was similar to that of control subjects who never experienced the opposite rotation. This lack of interference suggests that memory resources for visuomotor rotations are task specific.

Adult↗

PubMed2002

Kinematics and dynamics are not represented independently in motor working memory: evidence from an interference study.

Our capacity to learn multiple dynamic and visuomotor tasks is limited by the time between the presentations of the tasks. When subjects are required to adapt to equal and opposite position-dependent visuomotor rotations (Krakauer et al., 1999) or velocity-dependent force fields (Brashers-Krug et al., 1996) in quick succession, interference occurs that prevents the first task from being consolidated in memory. In contrast, such interference is not observed between learning a position-dependent visuomotor rotation and an acceleration-dependent force field. On the basis of this finding, it has been argued that internal models of kinematic and dynamic sensorimotor transformations are learned independently (Krakauer et al., 1999). However, these findings are also consistent with the perturbations interfering only if they depend on the same kinematic variable. We evaluated this hypothesis using kinematic and dynamic transformations matched in terms of the kinematic variable on which they depend. Subjects adapted to a position-dependent visuomotor rotation followed 5 min later by a position-dependent rotary force field either with or without visual feedback of arm position. The force field tended to rotate the hand in the direction opposite to the visuomotor rotation. To assess learning, all subjects were retested 24 hr later on the visuomotor rotation, and their performance was compared with a control group exposed only to the visuomotor rotation on both days. Adapting to the position-dependent force field, both with and without visual feedback, impaired learning of the visuomotor rotation. Thus, interference between our kinematic and dynamic transformations was observed, suggesting that the key determinant of interference is the kinematic variable on which the transformation depends.

Adolescent↗

PubMed2002

Visuomotor rotations of varying size and direction compete for a single internal model in motor working memory.

When participants adapt to equal and opposite visuomotor rotations in close temporal proximity, memory of the 1st is not consolidated. The authors investigated whether this retrograde interference depends on the use of equal and opposite rotations. On Day 1, different groups of participants adapted to a -30 degrees rotation followed 5 min later by rotations of +30 degrees, +60 degrees, or -60 degrees. On Day 2, all groups were retested on the -30 degrees rotation. Either retrograde interference (in groups who adapted to rotations of opposite sign on Day 1) or retrograde facilitation (in the remaining group) was observed. In all groups, learning of the 2nd rotation resulted in unlearning of the first, indicating that all visuomotor rotations compete for common working memory resources.

Arm↗