| Abstract No.: | A-D1129 |
| Country: | Canada |
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| Title: | DISSECTING THE LONG-LATENCY REFLEX REVEALS TWO COMPONENTS: ONE FIXED AND ONE FLEXIBLE |
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| Authors/Affiliations: | 1 J. Andrew Pruszynski*; 1 Isaac Kurtzer; 1 Stephen Scott;
1 Queens University, Kingston, ON, Canada
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| Content: | Objectives: We have previously demonstrated that long-latency reflexes of the human upper-limb are modulated by the metrics of a visuo-spatial task; that is, they are systematically increased or decreased in proportion to the voluntary muscular activity required to attain the spatial target. In the present experiment we were interested in determining whether the source of target-dependent activity was (1) the multiplicative scaling of a default pattern of activity or (2) the addition of an independent component on top of a default pattern. We separated these possibilities by manipulating the known scaling of reflexes with tonic muscle activity whereby the same magnitude of perturbation results in a larger default reflex pattern when the background load is increased. If the target-dependency was due to constant multiplicative scaling then we should observe a systematic increase in target-dependency with increased background load. In contrast, if the target-dependency was caused by the addition of an independent component then we should observe no change in target-dependent activity as a function of background load.
Materials and Methods: The task was performed with a robotic device that permits combined flexion and extension movements/perturbations of the shoulder and elbow in the horizontal plane. Subjects (N = 8) held their hand in a small central target while countering a background load and were instructed to respond to a step-torque perturbation by placing their hand in a visually-defined spatial target placed such that the same elbow perturbation displaced the hand either into (IN) or out of (OUT) the spatial target. Notably, the background load could excite or inhibit the subsequently stretched muscle.
Results: The two background loads resulted in different default patterns of activity. When muscles were excited by the background load and subsequently stretched by the perturbation they exhibited multi-phasic responses classically known as short- and long-latency reflexes. In contrast, when muscles were inhibited prior to the perturbation this phasic activity was essentially absent. Critically, the target-dependent activity was strikingly similar in timing and magnitude despite the differences in default activity caused by the background loads; target-dependent activity began at the onset of the long-latency reflex window (~50 ms) and progressively increased through the subsequent reflex and voluntary epochs.
Conclusion: Our present observations demonstrate that an inhibitory background load attenuates the phasic component of the long-latency reflex yet it does not substantially alter the differential response to target position. This suggests that the long-latency reflex is the sum of two components that overlap in time; (1) a phasic component which does not respond in a target-dependent manner and is sensitive to background load and (2) a gradually accumulating component which is insensitive to background load and contributes all the observed target-dependency.
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