Timing in the Absence of Supraspinal Input: Effects of Temporally Regular Stimulation on Spinal Plasticity
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Prior work has shown that spinal neurons are capable of discriminating between temporally regular and temporally irregular stimulation. These effects have been observed using an in vivo assay of spinal plasticity based on an instrumental learning task, in which response-contingent leg shock produces an increase in flexion duration. Exposure to temporally regular stimulation (fixed spaced stimulation; FT) promotes learning, and temporally irregular stimulation produces a learning deficit. The experiments in this dissertation were designed to test other properties of fixed spaced shock that promote spinal plasticity and the structure responsible for the FT effect. Experiment 1 focused on the minimum number of stimulations necessary to re-establish the capacity to learn (a component of the “FT effect”), finding that180-360 shocks produced a learning deficit and that additional training (540-900 shocks) allowed learning. Experiment 2 found that shock number, not duration of exposure determined whether the FT effect emerged. Experiment 3 investigated if the FT effect emerges after shock was presented in two sessions separated by 24 hrs, and showed that two bouts of 360 shocks yielded the FT effect. Further, the initial bout of fixed spaced shock had a long-term benefit (Experiment 4). The results of Experiment 5 suggested that omitting shocks from a train of FT stimulation has little effect on the benefit of fixed spaced shock treatment. Experiment 6 replicated this observation, showing that randomly deleting half of the shocks (from a 720 FT shock series) had no effect on learning. Further, this schedule also induces a lasting protective effect, blocking the learning deficit produced by variable spaced shock (Experiment 7). To explore whether a central system or a peripheral filter mediates the FT effect, Experiment 8 challenged spinal neurons by phase shifting the relation between fixed spaced stimulation applied to two dermatomes. The FT effect only emerged when stimuli occurred in an alternating pattern across dermatomes, implying regularity is abstracted by a central system. Experiment 9 surgically isolated central pattern generator (L1-L2) from the portion of the spinal cord that mediates instrumental learning (L4-S2), finding that disrupting the connections between these two regions eliminated the FT effect.
Lee, Kuan Hsien (2013). Timing in the Absence of Supraspinal Input: Effects of Temporally Regular Stimulation on Spinal Plasticity. Doctoral dissertation, Texas A & M University. Available electronically from