Learning about time within the spinal cord: evidence that spinal neurons can abstract and store an index of regularity

Front. Behav. Neurosci., 2015 · DOI: 10.3389/fnbeh.2015.00274 · Published: October 21, 2015

Simple Explanation

Prior studies have shown that intermittent noxious stimulation has divergent effects on spinal cord plasticity depending upon whether it occurs in a regular (ﬁxed time, FT) or irregular (variable time, VT) manner: In spinally transected animals, VT stimulation to the tail or hind leg impaired spinal learning whereas an extended exposure to FT stimulation had a restorative/protective effect. Using spinally transected rats, it is shown that the restorative effect of FT stimulation emerges after 540 shocks; fewer shocks generate a learning impairment. The transformative effect of FT stimulation is related to the number of shocks administered, not the duration of exposure. Administration of 360 FT shocks induces a learning deﬁcit that lasts 24 h. If a second bout of FT stimulation is given a day after the ﬁrst, it restores the capacity to learn. This savings effect implies that the initial training episode had a lasting (memory-like) effect.

Study Duration

Not specified

Participants

Male Sprague-Dawley rats (n=40 in experiment 1, n=40 in experiment 2, n=48 in experiments 3 and 5, n=32 in experiment 4)

Evidence Level

Original Research

Key Findings

1
The restorative effect of fixed time (FT) stimulation on spinal learning emerges after 540 shocks in spinally transected rats.
2
The number of shocks, rather than the duration of exposure, is the critical factor for the transformative effect of FT stimulation.
3
Spinal systems encode an index of regularity rather than the specific inter-stimulus interval (ISI).

Research Summary

This study investigates the conditions under which fixed time (FT) stimulation has a transformative effect on spinal learning in spinally transected rats, focusing on the number of shocks and the duration of exposure. The research demonstrates that a restorative effect of FT stimulation emerges after 540 shocks, and that the number of shocks, not the duration of exposure, is the critical factor. The findings suggest that spinal systems abstract and store an index of regularity, rather than encoding a specific temporal interval, and that this process involves a form of learning with a savings effect across days.

Practical Implications

Rehabilitation Strategies

Step training and FT stimulation promote adaptive plasticity. Step training could allow spinal neurons to abstract a form of regularity, related to treadmill speed and step frequency.

Neuropathic Pain Treatment

Locomotor training could attenuate the development of neuropathic pain. FT stimulation opposes the adverse effect of VT shock, and also prevents and reverses both the learning impairment and EMR induced by capsaicin treatment

Therapeutic Value of FT Stimulation

FT stimulation can have therapeutic value, adding to the growing body of research demonstrating that both cutaneous and epidural stimulation can have a neuromodulatory effect.

Study Limitations

1
The study focuses on spinally transected rats, limiting generalizability to other populations.
2
The specific frequency range tested may not be fully representative of all possible stimulation parameters.
3
Further research is needed to evaluate the stimulus parameters that promote adaptive plasticity and the neurobiological mechanisms involved.

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