Dan R. Kenshalo

Warm and cool thresholds as a function of rate of stimulus temperature change

The variables which are said to affect human temperature sensitivity are the skin temperature, the area of thermal stimulation and the rate at which the temperature of the skin is changed. Of these, the last has been only partially described. Measurements of the warm and cool thresholds were made on two males, experienced in making threshold judgments, when the rate of stimulus temperature change was varied between 0.01–0.3° C/sec. Both the warm and cool thresholds remained constant at rates of 0.1° C/sec and above but increased rapidly when slower rates were used. The effect of slow rates of stimulus temperature change was greater on the warm threshold than upon the cool threshold. It is concluded that the rate of temperature change does not represent information about the rate constants of stimulation of warm and cool receptors but that its effect is by way of the rate at which thermal adaptation occurs

The temperature sensitivity of the Type I slowly adapting mechanoreceptors in cats and monkeys

1. The sensitivity of Type I slowly adapting mechanoreceptors in cat and monkey skin to temperature and changes in temperature was investigated.

Variations in Thermal Sensitivity

In the past, confusion has resulted from the use of relative expressions indicative of the direction of temperature change, rather than positive identification of warm and cool sensations. Data are presented which show that at more extreme skin temperatures sudden changes in temperature may result in two readily discernible sensations. The occurrence of each sensation depends upon the magnitude of the change.

Multiple temperature-sensitive spots innervated by single nerve fibers.

Electrophysiological recordings were made from single nerve fibers which were specifically responsive to temperature changes of the skin of monkeys. Previous reports indicated that the receptive area on the skin of such preparations was a single small spot less than 1 millimeter in diameter. However, we found that the activity in a single thermally sensitive fiber increased when any one of eight individual spots on the skin was cooled. In other preparations two to six spots, each less than 1 millimeter in diameter, appeared to be innervated by a single fiber. The neural activity resulting from the cooling of one or several of these spots summed, and we suggest that this summation may be the neural analog of areal summation of thermal stimuli reported in psychophysical measurements.

Responses recorded from human scalp evoked by cutaneous thermal stimulation

Abstract When the glabrous palmar skin of the human hand was rapidly cooled, a change in the recorded electrical potential was localized at the contralateral scalp site that approximated the primary projection of the hand on the postcentral gyrus. Typical features of the evoked potential were an initial positive peak of large amplitude (25–36 μV), long peak latency (325 msec), long duration (200–325 msec) and it was graded to the intensity of the cooling. Little, if any, potential change was simultaneously recorded from the corresponding ipsilateral scalp site. While the evoked response is specific to skin cooling neither its receptor origin nor its relationship to thermal sensation has been established. We were not able to record an evoked potential of any form to rapid warming of the hand.

Bilateral spatial summation of cooling of symmetrical sites

The possibility of spatial summation of conducted −‡Ts (temperature decrements from the adapted skin temperature) was tested when the −‡Ts were presented simultaneously and individually at symmetrical sites on either side of the body. The use of a signal detection rating method yielded data that indicate that spatial summation is complete (area and intensity trade reciprocally) for −‡s near threshold and at clearly suprathreshold intensities of stimulation is invariant of the adapted skin temperature (AT), and the intensity of stimulation within the range of stimuli used (from as little as −0.05‡C to as much as −1.25‡C). In view of the characteristics ascribed to the thermosensory apparatus in humans, it is postulated that spatial summation occurs in the central nervous system.

The recovery of sensory function following skin flaps in humans.

Two cross-sectional studies were made of the recovery of tactile and pain sensitivity in subjects having skin flaps in the region of the chest and neck as a result of tumor excision. In experiment 1, stimuli ranging from 2.46 to 17.10 gm of force were delivered by von Frey hairs to the flaps and comparable normal sltes in 35 subjects at times ranging from 1 month to 10 years after surgery. No subjects perceived stimuli of less than 11.80 gm, thermal, or moving touch applied to flaps, whereas 21 percent perceived 11.80 gm or greater force (judged as painful applied to normal skin). The results of experiment 2 showed that these findings were not due to visual information available to subjects. Possible explanations for the fact that these results are radically different from those reported in the literature are discussed.

The Influence of Rate of Skin Indentation on Threshold and Suprathreshold Tactile Sensations

The effects of skin indentation depth and rate on threshold and suprathreshold tactile sensations were investigated. Indentation rates between 0.3 and 10 mm/sec had little effect on the absolute tactile thresholds measured in terms of indentation depth. Slower rates resulted in increased absolute thresholds. Estimates of the growth in intensity of tactile sensations were made as functions of indentation depth and rate. The fastest rate used (10 mm/sec), for a given depth of indentation, produced the most intense sensation; the slowest (0.1 mm/sec), the least intense sensation. The tactile sensation magnitude estimates, with rate as the parameter, could be described by power functions. At the slowest indentation rate the exponent of the function was 1.36. At faster indentation rates (0.4, 1.0, and 10 mm/sec), two functions of markedly different slopes were required to describe the estimates. The exponents of the power functions were between 0.38 and 0.49 for indentation depths up to about 0.9 mm, and between 1.07 and 1.43 for deeper indentation depths.

The afferent fiber population mediating the thermal evoked response to skin cooling in man

Abstract A population of cold-responding fibers with response properties similar to those innervating primate skin were determined to be mediating the thermal evoked response to skin cooling in man. Cerebral evoked responses to a 10°C cooling pulse were recorded from human scalp at a 29°C adapting temperature where primate cold-responding fibers possess a considerable dynamic sensitivity. No time-locked cortical responses were reliably recorded from the 40°C adapting temperature to the same cooling pulse. At 40°C, thermally sensitive mechanoreceptors mediated by Aβ and large Aδ fibers possess their maximal sensitivity. In primate skin, specifically sensitive cold receptors are innervated by the small Aδ fiber group. The results of this study, then, suggest the existence of a small myelinated fiber group in human radial nerve, implying that the difficulty in recording from these fibers directly using microneurographic techniques in humans involves technical-morphological considerations. The results have implications for the utility of the cerebral evoked response technique as a clinical diagnostic tool, and the organization and representation of cortical temperature information in humans.

Rhesus monkey (Macaca mulatta) cool sensitivity measured by a signal detection method

Two rhesus monkeys were trained to detect cool stimuli (decrements in skin temperature from the adapted temperature) presented to the palm of the left hand after the skin had been preadapted to a 33°C temperature. The procedure was analogous to the signal detection “yes-no” method used with human observers. Receiver operating characteristic curves were generated by varying the a priori probability of stimulus oecurrence. The proportion of hits and of false alarms increased with increases in the a priori probability of stimulation. The points of isosensitivity of both subjects for cool intensities of 0.3°C, but not at greater intensities, yielded straight lines with slopes of approximately one when plotted on normal-normal coordinates. The values of the d’s detectability index decreased from 3.52 to 0.75 for one subject and from 2.81 1to 1.28 for the other as the stimulus intensity was decreased from 0.8° to 0.15° cooling from the adapting temperature. Cool thresholds for each subject of 0.19° and 0.12°C were computed from classical psychometric functions when threshold was defined as 50% hits at a 50% probability of stimulus occurrence. These thresholds are comparable to published reports of the cool threshold for the rhesus.