Effective Threshold Intensity Research Articles

The Effect of Pulse-Modulated Thermal Radiation on the Time-Intensity Relationship for Dermal Pain

A simple experimental model was used to examine the relationship between modulation and hazard for thermal bioelectromagnetic effects. The inner forearms of 11 human volunteers were exposed to continuous and pulse-modulated thermal radiation from an incandescent light source. The time of irradiation required to produce threshold pain sensation was measured as a function of the average power density (450–2500 m W cm−2), pulse repetition frequency (continuous, 0.4 Hz, 8.0 Hz) and duty cycle (continuous, 0.33,0.50). The resulting 32 sets of data could be described by a single power function expression which relates time and power density through a regression slope. The slope was found to depend on the modulation of the radiation, but not on the age of the subject or wet bulb globe temperature (WBGT). The minimum average power density which could elicit pain within 200 sec (an effective threshold intensity) was determined to be independent of modulation, regression slope, subject age and WBGT.

The effect of pulse-modulated thermal radiation on the time-intensity relationship for dermal pain.

A simple experimental model was used to examine the relationship between modulation and hazard for thermal bioelectromagnetic effects. The inner forearms of 11 human volunteers were exposed to continuous and pulse-modulated thermal radiation from an incandescent light source. The time of irradiation required to produce threshold pain sensation was measured as a function of the average power density (450-2500 m W cm-2), pulse repetition frequency (continuous, 0.4 Hz, 8.0 Hz) and duty cycle (continuous, 0.33, 0.50). The resulting 32 sets of data could be described by a single power function expression which relates time and power density through a regression slope. The slope was found to depend on the modulation of the radiation, but not on the age of the subject or wet bulb globe temperature (WBGT). The minimum average power density which could elicit pain within 200 sec (an effective threshold intensity) was determined to be independent of modulation, regression slope, subject age and WBGT.

The effect of 5- and 14-day intertrial intervals on the 1-Hz suppression effect

Experiments were conducted to evaluate the effect of 5-day and 14-day intertrial intervals of 1-Hz brain stimulation on kindling behavior induced by 60-Hz sine-wave stimulation. The effective threshold intensity (ETI) to elicit a kindled response with 60-Hz stimulation was determined on two separate occasions with 15 brain-stimulation trials between determinations. In Experiment 1, experimental rats were stimulated with 1-Hz sine waves before and after a 60-Hz brain-stimulation trial, with a 14-day interval between each stimulation session (Group 1- 60-1). A second group (Group X-60-X) received only the 60-Hz stimulation on the same trials as those on which Group 1-60-1 was stimulated with 60-Hz current. Experiment 2 was similar to Experiment 1, but the interval was 5 days. In previous experiments with 1, 3, or 24 h between trials, Group 1-60-1 had had a gradual increase in the intensity required to elicit a kindled response with 60-Hz current from ETI1 to ETI4 (the suppression effect). However, with a 72-h interval, a decline similar to that of Group X-60-X occurred in ETI values. The results with 14-day and 5-day intervals were similar to those with a 72-h interval. Suppression of kindled behavior on daily trials was not present in Group 1-60-1, in contrast to the results with 1, 3, and 24 h. Apparently, intervals of 3 days or greater allow much of the suppression effect to dissipate.

Lack of interhemispheric transfer of the 1-Hz suppression effect in rats

Three experiments were conducted to evaluate the effect of 1-Hz brain stimulation of the amygdala on kindling behavior induced by 60-Hz sine-wave stimulation in the contralateral amygdala. In Experiment 1, the effective threshold intensity (ETI) to elicit a kindled response with 60-Hz stimulation was determined on four separate occasions with 15 trials between determinations (three per day) for each of three groups. Experimental rats in Group 1 were stimulated with 1-Hz sine waves in one amygdala, then with 60-Hz current in the opposite amygdala, followed by 1-Hz stimulation of the first site (1-60-1, OA), with 1-h interstimulation intervals. Group 2 was treated with the 1-60-1 pattern, but with all stimulation in the same amygdala (1-60-1, SA). Group 3 received only the 60-Hz stimulation trials (X-60-X), on the second trial. Group 2 showed the typical suppression result, a gradual increase in threshold over the four ETI determinations. However, Group 1’s responses were similar to those of Group 3: ETI values decreased gradually over the determinations. Later, when Group 1 received a set of trials in which 1- and 60-Hz current stimulation was to the same site, suppression responses occurred. Two further experiments were conducted with similar results. These results suggest that the suppression effect generated by 1-Hz stimulation appears to involve a local process, an intrahemispheric effect.

An evaluation of different methods of producing the 1-Hz suppression effect

A number of exploratory experiments produced results that suggested a simplified method of producing suppression of kindled behavior by 1-Hz sine-wave stimulation. This method involved determining the effective threshold intensity to produce kindled behavior (ETI1), followed 1 h later by stimulation with 1-Hz sine waves at the ETI1 intensity for 5 min. Three hours later, a second 1-Hz trial occurred. Eighteen hours later, ETI2 was determined. Four experiments with this method indicated clearly its effectiveness. However, when contrasted with the previous method used (a 1-60-1 sequence over a number of days with an intertrial interval of 1 h) both methods were equally effective.

The effect of a 72-h intertrial interval on the 1-Hz suppression effect in rats

An experiment was conducted to evaluate the effect of a 72-h intertrial interval of 1-Hz brain stimulation on kindling behavior induced by 60-Hz sine-wave stimulation. The effective threshold intensity (ETI) to elicit a kindled response with 60-Hz stimulation was determined on four separate occasions with 15 trials between determinations. Experimental rats were stimulated with 1-Hz sine waves before and after a 60-Hz brain stimulation trial with a 72-h interval between stimulation sessions (1-60-1 group). A second group received only the 60-Hz stimulation on the same trials as those on which the 1-60-1 group was stimulated with 60-Hz current (X-60-X group). In previous experiments with 1, 3, or 24 h between trials, the 1-60-1 group had a gradual increase in the intensity required to elicit a kindled response with 60-Hz current from ETI1 to ETI4 (the suppression effect). However, the 72-h interval produced a decline in ETI values similar to that of the X-60-X group, but not as great. Suppression of kindled behavior on daily trials was not present for the 1-60-1 group, in contrast to results of previous experiments. Apparently, the 72-h interval allowed much of the suppression effect to dissipate.

The effect of ultra low-frequency brain stimulation on the kindling effect in rats

An experiment was conducted with rats to evaluate the effect of various low frequencies of brain stimulation on kindling behavior induced by 60-Hz sine-wave stimulation. The effective threshold intensity (ETI) to elicit altered behavior was determined on four separate occasions, with 5 days of daily trials between determinations. On each day, experimental rats were stimulated with current of a specific frequency on the first and third trials and with 60-Hz current on the second trial. There were four experimental groups, one each for 1-, .1-, .01-, and .001-Hz stimulation. A fifth group received no stimulation on Trials 1 and 3 and 60-Hz current on Trial 2. Suppression of altered behavior induced by the 60-Hz stimulation trial was present for all ETI determinations with the four experimental groups; the mean threshold increased on each successive determination. Suppression was greatest for the .1, .01, and .001 groups. Grossly, it appears that the interference effect gradually increases with decreasing frequency.

The effect of low frequency and direct current stimulation on the kindling phenomenon in rats.

An experiment was conducted to evaluate the effect of 1-Hz or direct current brain stimulation on kindling behavior induced by 60-Hz sine wave stimulation. The effective threshold intensity to elicit a convulsion was determined on four separate occasions with 5 days of daily trials between determinations. On each day one group of experimental rats was stimulated with 1-Hz sine wave current before and after stimulation with 60-Hz sine wave current (1-60-1 group). Another group received direct current stimulation and 60-Hz current (D-60-D group). A third group received only 60-Hz stimulation. Suppression of kindling behaviour usually induced by the 60-Hz stimulation occurred with 1-Hz stimulation; the mean threshold value increased on each successive determination. Suppression was most pronounced for the direct current group; it appeared after a single trial and persisted for 32 days after the last threshold determination. In contrast, most of the rats in the 1-60-1 group had recovered from the suppression after the 32 day period of nonstimulation. A second phase of the experiment indicated that the increase in threshold values for the D-60-D group occurred after a single DC stimulation. These results are consistent with the hypothesis generated by previous research that suppression following 1-Hz stimulation is not due to tissue damage.

The effect of several intertrial intervals on the 1 Hz interference effect.

Experiments were conducted to evaluate the effect of two intertrial intervals of 1-Hz brain stimulation on kindling behavior induced by 60Hz sine wave stimulation. In two experiments, the effective threshold intensity (ETI) to elicit a convulsion was determined on four separate occasions with 5 days of daily trials between determinations. On each day experimental rats were stimulated with 1-Hz current on the first and third trials for 120 seconds duration and with 60-Hz current for 30 seconds on the second trial (1-60-1 group). A second group was stimulated with 60-Hz-current on each trial (60-60-60 group). A third group received no stimulation on Trials 1 and 3 and 60-Hz current on Trial 2 (X-60-X group). In Experiment 1, the intertrial interval was 3 hours; a 24 hour interval was used in Experiment 2. The results were similar in both experiments. For the 1-60-1 group, there was a steady increase in the intensity required to elicit a convulsion shown with 60-Hz current from EtI1 to ETI4. However, the 24 hour interval produced a lesser effect than did 3 hour interval (or the 1 hour interval used in previous experiments). Rats in the other groups maintained relatively stable values from ETI1 to ETI1, with a slight decline occurring. Suppression of convulsive behaviour on daily trials was present with the 1-60-1 groups, and nonexistent with the other groups.

Gradient of interference by various frequencies on 60 Hz kindling behavior.

An experiment was conducted to evaluate the effect of various frequencies of brain stimulation on kindling behavior induced by 60-Hz sine wave stimulation. The effective threshold intensity (ETI) to elicit a convulsion was determined on four separate occasions with 5 days of daily trials between determinations. On each day experimental rats were stimulated with current of a specific frequency on the first and third trials for 60 seconds duration and with 60-Hz current for 30 seconds on the second trial (one hour intertrial interval). There were five experimental groups, one each for 1, 5, 10, 30, and 60-Hz stimulation. A sixth group received no stimulation on trials 1 and 3 and 60-Hz current on trial 2. Suppressor of convulsive behavior induced by the 60-Hz stimulation trial was present for all ETI determinations with 1-Hz and 5-Hz stimulation; the mean ETI increased on each successive determination. Suppression was prominent also for the 10-Hz group until the ETI4 determination. Suppression was moderate for the 30-Hz and 60-Hz groups. Overall, it appeared that the interference effect gradually increased with remoteness from the 60-Hz point.

The effect of varying durations of stimulation of the 3-Hz interference effect

Experiments were conducted to evaluate the effect of various durations of 3-Hz brain stimulation on kindling behavior induced by 60-Hz sine-wave stimulation of the amygdala. In two experiments the effective threshold intensity (ETI) to elicit a convulsion was determined on four separate occasions with 5 days of daily trials interspersed between determinations. On each day experimental rats were stimulated with 3-Hz current on the first and third trials for 5, 15, 30, 60, 120, or 300 sec duration and with 60-Hz current for 30 sec on the second trial. A steady increase in the intensity required to elicit a convulsion with 60-Hz current from ETI1 to ETI4 resulted for all rats with durations of 15 sec or greater. Rats stimulated only with 60-Hz sine waves and those in the 5-sec group maintained relatively stable values from ETI1 to ETI4, with a slight decline occurring. Suppression of the expected 60-Hz-induced convulsive behavior on daily trials was modest in the 15-sec group, pronounced with the 30-sec group, and drastic with the other groups. The 300-sec group had the greatest suppressive effect operating. The suppression effect appeared not to be due to tissues damage inasmuch as many of the experimental rats (except the 300-sec group) convulsed again at previous low ETI levels following a 16-day rest at the end of the experiment. This result suggests that the suppression effect is a relatively transient event.

Interference effect of 3 Hz brain stimulation on kindling behavior induced by 60 Hz stimulation.

Experiments were conducted to evaluate the effect of 3 Hz brain stimulation on kindling behavior induced by 60 Hz sine waves stimulation. In Experiment 1, 12 rats were subjected to 40 or 60 convulsion trials with 60 Hz stimulation and then given 36 trials of 3 Hz stimulation. When these rats were stimulated again with 60 Hz sine wave current at the same brain site, none of the rats showed a convulsion in nine test trials. The intensity of stimulation had to be increased on test trial 10 to elicit convulsions for each rat. Of 10 rats in two control groups, only 1 did not convulse during the nine test trials. In Experiment 2 the effective threshold intensity (ETI) to elicit a convulsion was determined on five separate occasions with 10 days of daily trials between determinations. On each day experimental rats were stimulated with 3 Hz current on the first and third trials and with 60 Hz current on the second trial (3-60-3 group). A steady increase in the intensity required to elicit a convulsion with 60 Hz current from ETI1 to ETI5 resulted. Rats stimulated only with 60 Hz sine waves on the second trial each day (X-60-X grou,) maintained relatively stable values from ETI1 to ETI5. In the four, 10-day blocks of trials, convulsions were suppressed in 20% to 80% of the trials over the 10 day period for the 3-60-3 group, with the greatest effect occurring after about 4 days of stimulation. This suppressive effect was prominent both with rats that were at the convulsion stage prior to the first application of 3 Hz stimulation and with rats that were at preconvulsion stages. In Experiment 3 the permanency of the suppressive effect was evaluated. Eight suppressed rats from the experimental group in Experiment 2 and 4 control rats were stimulated for 90 trials over 30 days with 60 Hz current, and ETI values were determined after each set of six trials. Four of the 8 experimental rats were convulsing at ETI1 within 20 days.