Paired pulse stimulation is used to estimate the refractory period of the directly stimulated substrate of intracranial self stimulation. When the delay between the conditioning (C) pulse and the test (T) pulse is shorter than the refractory period, the T pulse elicits very little behaviour. The T pulse is ineffective because the substrate has not had time to recover from refractoriness. Conversely, when the C-T delay is longer than the refractory period, one would expect the T pulse to be fully effective. In practice, however, T pulses often fall 20% short of full effectiveness. This anomalous shortfall has received scant attention. The present study documented this anomaly, and tested a theory about its cause. Experiment 1 tested at very long C-T delays and at closely spaced C-T delays. It was found that T pulses always became fully effective, but often only at C-T delays of 30 ms. Experiment 2 determined that the shortfall is not an undersampling or scaling artifact. T pulse effectiveness was calculated using standard 0.1 log$\sb $ unit gradations, and then again with 0.05 log$\sb $ unit gradations, the finest practical scaling. The fine scaling had no effect on the shortfall. T pulse effectiveness is usually estimated with six replications per C-T delay. This limits statistical power, and estimates are necessarily approximate. Experiment 3 used an automated testing apparatus to generate very large numbers of replications: as many as 120 per C-T delay. The resulting effectiveness curves are extremely precise. They show that effectiveness usually rises to 80% within 5 ms. Effectiveness stays near that level until 25 ms. Then it gradually rises to 100% at 35 ms. Effectiveness remains steady near 100% until at least 50 ms, the longest C-T interval that was tested. Experiment 4 tested the theory that the shortfall is caused by a relative refractory period, a subnormal period, or a supernormal period by using T pulse currents that were 40% larger than the C pulse currents. Large T pulses eliminate relative refractory, subnormal, and supernormal effects, but they did not eliminate the shortfall. It follows that the shortfall is not caused by any of these factors.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/10025 |
| Date | January 1996 |
| Creators | Fournier, Thomas A. J. |
| Contributors | Fouriezos, George, |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
| Format | 218 p. |
Page generated in 0.0024 seconds