Tallahassee, Florida, U.S.A.

JI. Phy8iol. (1964), 172, pp. 439448 439 With 2 textfigures Printed in Great Britain THE TEMPERATURE SENSITIVITY OF FURRED SKIN OF CATS BY D. R. KENSHALO From the Department of Psychology, Florida State University, Tallahassee, Florida, U.S.A. (Received 4 February 1964) A question is raised as to whether infrahuman mammals, e.g. rat, cat or dog, possess sensitivity to warm stimuli applied to their furry skin. This question has its origin mainly in evidence obtained from electrophysiological studies of activity in nerve fibres that respond to thermal stimuli. Other evidence which employs behavioural techniques, suggests a similar question. Hensel & Zotterman (1951) describe afferent nerve fibres of the lingual nerve of the cat which show, among other characteristics, an increase in the frequency of discharge when the tongue is cooled and a decrease in frequency when it is warmed. They refer to these as 'cool' fibres. Other fibres which supply the tongue respond in the opposite manner. These nerve fibres show an increased frequency of discharge when the tongue is warmed and a decreased frequency when it is cooled. These are labelled 'warm' fibres. Both types of temperatureresponsive fibres belong to the large myelinated fibres of the A delta group. The 'warm' fibres are slightly larger in diameter (255,u) than the 'cool' fibres (153 4). The generality of distribution of the 'warm' A fibres throughout the skin of rats, cats and dogs has been questioned by Boman (1958). Numerous A fibres which show the functional characteristics of 'cool' fibres, as described by Hensel & Zotterman (1951), were found in the infraorbital nerve of these animals when the skin of the face was cooled. However, he was unable to find fibres which showed the characteristic response of 'warm' fibres when the skin of the face was warmed. Similar findings have been reported by Witt & Hensel (1959) in their investigations of thermally responsive A fibres of the saphenous and clunium nerves of the cat Ȯther investigators have also suggested that cats and dogs do not possess warm receptors in their furry skin. Wall (1960), by recording the activity of primary central cells of the spinal cord of cats in response to temperature increases applied to the skin, concluded that this system is insensitive when compared to the sensitivity shown by fibres from the tongue as described by Hensel & Zotterman (1951). Murgatroyd, Keller & Hardy (1958) were

440 D. R. KENSHALO able to make dogs discriminate between warmth and no warmth applied to the face region in order to obtain food. The threshold, at the 75 % level of discrimination, was 10 times higher than that which had been found in humans. In order to explain the results of these studies both Wall and Murgatroyd et at. cite Hardy (1955) as stating that warm receptors may exist only in the nose, conjunctiva and sclera of the eye but not under the hairy portions of the skin. In contrast with the evidence for the lack of 'warm' fibres of the A group supplying hairy skin, Hensel, Iggo & Witt (1960), Iruchijima & Zotterman (1960) and Douglas, Ritchie & Straub (1960) have described C fibres, in the saphenous nerve of dogs and cats and the infraorbital nerve of rats, which respond specifically to warming and cooling the skin. These fibres show responses to thermal stimulation which are similar to those of the A fibres in the lingual nerve, as described by Hensel & Zotterman (1951). The evidence appears fairly definite that of the afferent C fibres supplying the hairy skin some give a positive response, e.g. an increased frequency of discharge, to warm and others to cool stimuli. There is some doubt, however, that afferent fibres of the A group, capable of specific responses to warming, exist elsewhere than in the tongue. This investigation was undertaken to provide data, using behavioural methods, concerning the cat's sensitivity to mild thermal stimuli applied to hairy skin and to determine the effects of altering the skin temperature upon the threshold response to warm and cool stimuli. METHODS The investigation consists of three experiments. The first experiment used instrumental avoidance training techniques (Hilgard & Marquis, 1940) to obtain measures of the cat's thermal thresholds after the shaved skin of the back had been adapted to temperatures ranging from 29 to 440 C. The procedure was designed to be as similar as possible to that used by Kenshalo, Nafe & Brooks (1961) for the measurement of thermal thresholds of human subjects. In the second experiment of the investigation, different cats were given training with the procedures employed by Rice & Kenshalo (1962) to determine the nociceptivethreshold on the same area of skin from which thermal threshold measurements had been obtained. In the third experiment, thermal threshold measurements were made on the backs of human subjects with the same apparatus and procedures used with the cats. Subjects. The experimental animals were four nonpedigree cats approximately 2 years old. These cats had been maintained in the cat colony at Florida State University for more than a year before the start of the observations. They received daily handling and attention from the animal caretaker and the experimenters. The human subjects were one male and one female undergraduate student. They were thoroughly experienced as subjects in other experiments involving thermal threshold measurements. Apparatus. The catrestraining apparatus has been described in detail by Rice & Kenshalo (1962). Aluminum Flexoframe rods were arranged to provide a head stock for the cats. Plaster casts were made to fit the body of each cat, and these could be anchored to the rack by adhesive tape. Leather restraining straps were fastened to the platform of the rack for all but the right rear leg, which was strapped to a movable lever. The leather strap on the lever contained two copper electrodes through which a mild electric shock was administered.

TEMPERATURE SENSITIVITY OF CAT SKIN 441 Strain gauges were attached to the lever so that its movement was recorded on a Sanborn Model no. 296 recorder. The maximum lever movement was 7 cm and was set by a limit bar attached to the conditioning frame. Provision was also made to record respiration. A pneumograph made of 25 mm surgical drain was inserted between the cast and the thorax of the cat. Pressure changes in the pneumograph could be recorded instead of the leg lever movement. Thermal stimuli were presented by a new type of stimulator which has been described in detail by Kenshalo (1963). This stimulator, the skincontact surface of which measures 24 x 30 mm, consists of junctions of n or p bismuth telluride and copper. It operates on the Peltier principle so that when direct current is passed through the junctions in one direction, one surface cools while the other surface warms. A change in the polarity of the current reverses the direction of the temperature change on each side. Through appropriate circuitry, the stimulator can be maintained at any temperature within the physiological range with an accuracy of +0.0120 C. Its temperature may be changed in either direction, at will, by a predetermined amount, from as little as 005 to as much as 300 C at rates up to 2' C/sec. The stimulator has the additional advantage that cues other than a temperature change do not occur since its operation is completely electrical and no moving parts are involed. The temperature of the surface of the stimulator which was in contact with the skin was recorded continuously on the second channel of the Sanborn recorder. The sequence and timing of the presentation of temperature changes and electric shock were controlled by three RC decade timers (Hunter & Brown, 1949) connected so that the durations of the temperature change, the interval between the onset of the temperature change and the electric shock, and the duration of the electric shock could be adjusted independently of one another. The occurrence of these events was recorded by an event marker on the Sanbom recorder. The timer which regulated the duration of temperature change also provided a timed 6 VAC source so that other stimuli, e.g. a buzzer or vibrator, might be used as conditioned stimuli (CS) in place of the temperature change. A mercury switch mounted on the leg lever was connected into the timing circuits so that a leg lift response interrupted the timning sequence when it occurred. The experimenter started each trial by pushing a button. The animalrestraining apparatus, transducers and stimulus sources were located in a small room adjacent to the room which contained the recording and control apparatus. The experimenter viewed the animal through a oneway vision window between the two rooms. All relays of the control apparatus were mounted in polystyrene foam and the entire controlrecording console was placed in an acoustic tilelined box. The experimenters could not detect any sounds associated with the control of a trial above the ambient noise level of the laboratory when they stood immediately adjacent to the controlrecording console. Procedure. The procedure employed in obtaining measurements of the thermal threshold may be divided into three parts: first, habituation to handling and to the restraint apparatus; second, conditioned avoidance training; third, measurement of thermal thresholds after the skin had been adapted to temperatures which ranged from 29 to 440 C. As noted by Rice & Kenshalo (1962), the general care and handling of cats in their home cage area is important in obtaining consistent results. All cats were handled and petted daily throughout the course of the experiment. Habituation to the restraint apparatus consisted of placing the cat in its cast and securing the cast to the conditioning frame for a period of 4560 min each day. During these periods the pneumograph was connected to the recorder and periodic samples of respiration rate and wave form were taken. When the breathing record became stable and the animal no longer struggled in the cast, usually after 1015 sessions, avoidance conditioning training was started. Conditioning training consisted of teaching the animal to make a shock avoidance response (right rear leg lift) to an auditory (buzzer) conditioned stimulus (CS). This response was then transferred to a tactile CS (vibration) and finally it was transferred to a thermal CS. In the

442 D. R. KENSHALO initial avoidance conditioning, a high frequency buzzer was connected to the 6 VAC circuit of the CS timer. The buzzer was sounded for 25 sec and a mild electric shock (UCS) of 02 sec duration overlapped the final 02 sec of the CS. The intertrial interval was varied randomly from 45 to 120 sec. Daily sessions consisted of twenty trials. The UCS intensity was adjusted throughout each session so that it would evoke a leg flexion sufficient to cause the leg lift lever to hit its limit bar. A leg flexion (CR) of 12 mm interrupted the buzzershock sequence of that trial. When the animal had achieved and maintained at least a 90 % level of CR's for five sessions, the CR was transferred to a vibratory CS. The vibrator (Bice, 1961) was secured to the shaved back, where the thermal stimulator would be applied later, by a 25 mm elastic band around the thorax. The buzzer and vibrator were used together as a CS for one session of twenty trials, after which the buzzer was discontinued. When the animal had again maintained a level of 90 % CR's for two consecutive sessions, the thermal stimulator was placed on the shaved back about 20 mm to the left of the spinal column over the scapula. It was held in place by a 25 mm elastic and adhesive strap. The skin temperature was measured just before applying the thermal stimulator. During the initial training trials, the stimulator was maintained at skin temperature between each trial. Throughout training the rate6of the temperature change of the CS was set at 20 C/sec. The intensity of the temperature change was determined by the duration set on the CS timer. Thus, a 10 sec CS duration resulted in a 200 C temperature rise. The procedure used to measure the threshold to temperature increases and decreases consisted of presenting the same intensity CS on three consecutive trials. If the animal responded to two of the three stimuli, the intensity of the CS was decreased by 0.20 C and three more trials were given. This sequence was repeated until the cat failed to respond to two of the three stimuli in a three trial block. This constituted a descending series. The threshold was taken as the intensity of CS midway between the blocks of three trials in which the animal gave a 666 % response rate and a 333 % response rate. Ascending series alternated with descending series until four measurements of the threshold were obtained. During the threshold measurement trials, the rate of temperature change was set at 2.00 C/sec. Measurements of the thresholds to temperature increases and decreases were made after the skin had been adapted to temperatures between 29 and 440 C. When the adapting temperature was other than that of the normal skin (about 350 C), the cats were adapted to the temperature for 20 min before starting the threshold measurements. The nociceptive thresholds of cats were also measured. The threshold for the noxious quality of a stimulus has been defined as the minimum intensity of a stimulus which will consistently evoke the learned response that terminates it (Rice & Kenshalo, 1962). In this series of measurements two additional cats were adapted to the casting and restraint procedures. No conditioning training was given. The thermal stimulator was placed on the cat's shaved back and the temperature controls were set to produce a large temperature change from the adapting temperature. After 150 training trials the noxious threshold was measured on ten trials at each adapting temperature. The threshold was the stimulator temperature at the moment that the cat stopped the temperature rise by lifting his right rear leg. Cats used in these measurements had never been subjected to electric shock nor was it used in these measurements. This technique for measuring nociceptive thresholds will be referred to as the escape method. The thermal thresholds of the human subjects were measured on a corresponding site of the back. The procedure was the same as that used to obtain thermal thresholds of the cats, except that instead of electric shock, the subjects were instructed to lift the leg lever when they felt a sensation of warmth or coolness. Also, because the human thresholds were small, the rate of change of the stimulus temperature was set at 0.40 C/sec to allow a longer time for a response.

TEMPERATURE SENSITIVITY OF CAT SKIN 443 RESULTS Two cats were given complete conditioning training and subsequently used in obtaining measurements of thresholds to temperature changes. Figure 1 presents the results of the conditioning training of cat 2 and is considered typical of the course of training of cat 1. When the buzzer was used as a CS, both cats showed an abrupt change from practically no CR's to almost perfect performance after sixty to eighty trials. Transfer to the vibratory stimulus on the back was relatively easy after one session of 100 C 80 C~~~~~~~~~~~ 60 O40 20 20 0 5 10 15 20 25 Sessions Fig. 1. The development of a conditioned avoidance right rear leg lift response to a buzzer, a vibrator and a temperature change of the shaved skin of a cat's back. Each session represents about twenty pairings of the CS and UCS (mild electric shock) applied to the right rear leg. Cat 2. 9, buzzer; A, buzzer and vibrator; 0, vibrator; *, temperature increase. twenty trials in which the buzzer and the vibrator were presented simultaneously as a CS. Pairing of the vibrator and the thermal CS was impossible. Thus, improvement in the performance was slower when the CS was changed from the vibrator to a temperature increase than when it had been changed from the buzzer to the vibrator. Throughout training of the conditioned response to a temperature increase, the adapting temperature was set at skin temperature, about 350 C. During the training period, the increase in the stimulus temperature required to elicit a CR seemed high, approximately 160 C. Various methods were tried in order to improve the efficiency of the conditioning and to reduce the size of the threshold. By

444 D. R. KENSHALO the time the procedure finally used to measure the thresholds was adopted, each cat had received well over 3000 trials of avoidance training. The warm and cool thresholds for human subjects, the thresholds to produce a CR for temperature increases and decreases for cats 1 and 2, the nociceptive threshold to temperature increases for cats 3 and 4, and to temperature decreases for cat 3 at adapting temperatures ranging from 29 to 440 C are shown in Fig. 2. The ordinate shows the extent oftemperature change, from the adapting temperature, required to produce a threshold response. The midpoint of each bar represents the mean and its height represents the standard error of the mean threshold. Each threshold point to temperature increases is the mean of at least four ascending and four descending series of measurements made on each subject, cat or human. Similarly, each threshold point to temperature decreases is the mean of at least four ascending and four descending series on each subject. The thresholds to temperature increases, as measured by the avoidance method, appear to be a linear function of the temperatures to which the skin was adapted. The equation for a line fitted to these points by the least squares method (Lewis, 1960) is Y = 1.20X+ 5612. The nociceptive threshold, measured by the escape method, using a rate of temperature increase of 10 C/sec, also appears to be a linear function of the adapting temperature. This line of best fit is expressed by Y = *98X + 51*3. When the rate of temperature rise was increased to 20 C/sec, somewhat larger nociceptive thresholds were obtained. The expression for this function is Y = 06X + 5874. The avoidance method gives measurements Qf the threshold to temperature increases at the various adapting temperatures which are statistically significantly smaller than those obtained by the escape method. Also, the lower rate of the temperature increase gave smaller thresholds than did the faster rate with the escape method. However, over the range of adapting temperatures employed in this study, the slopes of the lines fitted to the points are not statistically different. The warm thresholds of human subjects are not a linear function of the temperature to which the skin of the back was adapted. The curve drawn through the points was fitted by eye. It is similar in shape to that reported by Kenshalo et al. (1961) which was obtained with the forearm as the site of stimulation. However, the warm thresholds of the back are about 0.50 C larger than those obtained from the forearm in the earlier study. The human cool thresholds measured on the back are also similar to those reported by Kenshalo et al. (1961) from the forearm. Cats 1 and 2 showed similar changes in threshold as a function of the temperature to which the skin was adapted. The cats' threshold is approximately 40 C larger than that of the humans.

TEMPERATURE SENSITIVITY OF CAT SKIN 445 Several attempts were made to obtain a nociceptive threshold to temperature decreases on cats 3 and 4. Cat 3 gave mean escape thresholds to temperature decreases ranging from 180 to 300 C change from the adapting temperatures. These appeared to follow the same general function as the avoidance thresholds; however, they were highly variable. It was not 2 18 u 12 =.=..' 6 6 0. O 0 E 3 0 12 00~~~~~~~~~~~~~~~~~~~~~~ 518 I 24 30 1 29 32 35 38 41 44 Adapting temperature ( C) Fig. 2. Temperature thresholds of cats and humans. The midpoint of each bar represents the mean threshold in terms of degrees change from the adapting temperature. The height of the bar represents the standard error of each mean. * *, human, 0.40 C/sec; O El cats 1 and 2, 20 C/sec; *, noc. cats 3 and 4, 10 C/sec; OlO, noc. cat 3, 20 C/sec. possible to replicate them on cat 4. Large temperature decreases from an adapting temperature of 290 C failed to produce any sort of response, respiratory change or escape, in cat 4. DISCUSSION The results of this study indicate that cats do not respond to mild increases in skin temperature applied to the furry skin of their back. Rather they seem to sense temperature increases only when these stimuli become noxious. Several considerations support this conclusion. First, there is a close agreement between the thresholds to temperature

446 D. R. KENSHALO increases as measured by the avoidance and escape methods. Both are linear functions of the temperature to which the skin is adapted. The differences which occur between them can be accounted for, in part, by the inferiority of escape conditioning compared to avoidance conditioning as a method of measuring thresholds (Hilgard & Marquis, 1940). In addition, the animals used in the escape method received only 100 training trials whereas those used in the avoidance threshold measurements had received well over 3000 training trials. When temperature decreases were used, the avoidance and escape methods gave very different thresholds. In the one animal from which measurements were obtained, the escape threshold was about 180 C larger than the avoidance threshold measured on two other animals. The second animal used in the escape method failed to give any response at all (either respiratory change or leg lift) to temperature decreases of as much as 350 C. Larger changes were not used because of the limitations of the apparatus. Secondly, if a temperature increase provided any cue which might be used as a CS before it became noxious, the avoidance thresholds should have been much smaller than the escape thresholds since the animals used in the measurement of the avoidance threshold had received more than 3000 training trials. And thirdly, there are marked differences in the size and shape of the human and cat thresholds to temperature increases as a function of the adapting temperature. When the skin was adapted to 290 C, the difference between the human and cat thresholds was almost 180 C, while at an adapting temperature of 410 C, the difference was about 6.6 C. Also, the human threshold is a curvilinear function of the adapting temperature while that of the cats is linear. The measurements presented here of the nociceptive threshold of cats to temperature increases are in good agreement with those reported by Rice & Kenshalo (1962). They used radiant energy as a stimulus and calculated that the nociceptive threshold occurred at a skin temperature of about 52.30 C. Extrapolation of the escape threshold for rate of temperature increase of 10 C/sec to the abscissa placed the nociceptive threshold at 52.340 C. The thresholds of humans and cats to temperature decreases appear to be essentially alike. The human threshold is lower, by about 40 C, than that of the cats at an adapting temperature of 290 C. This difference increases to about 60 C at the highest adapting temperature. A large part of this difference must be attributed to the insensitivity of avoidance conditioning as a means of measuring sensory thresholds. It would not be surprising, however, to find the cat less sensitive than humans to temperature decreases even under optimum conditions of threshold measure

TEMPERATURE SENSITIVITY OF CAT SKIN 447 ment, considering that the feline exhibits only to a limited degree the type of body temperature regulation found in man (Hardy, 1955). The threshold curves to temperature decreases of humans and cats are basically alike in shape. They show a constant threshold at skin adapting temperatures from 29 to 340 C. At an adapting temperature of 340 C, the thresholds of both humans and cats start to increase, the latter more than the former. These similarities in the shapes of the human and cat threshold curves support a conclusion that cats can sense mild reductions in temperature although their sensitivity is apparently not of the same order of magnitude as that of humans. The conclusions of this investigation, that cats cannot sense mild temperature increases but do sense mild temperature decreases on the furry skin of their backs, provide a behavioural basis for the inability of Boman and of Witt & Hensel to find 'warm' sensitive A fibres in the infraorbital, saphenous and clunium nerves of cats. The furry skin of cat is apparently not supplied with 'warm' fibres which are similar to the 'warm' fibres supplying the tongue. Yet, cats seem to possess the ability to make relatively fine discriminations of radiant heat flux density as demonstrated by their selection of sites in which to bask, e.g. before a fire or in a sunny window. The results of this experiment leave the role of C fibres in thermal sensitivity as a matter for conjecture. Both Iruchijima & Zotterman and Hensel et al., present evidence that these fibres, found in nerves supplying furry skin, behave much like the thermally sensitive A fibres supplying the tongue. Some of the C fibres show changes in activity in response to skin temperature increases of as little as 0.10 C. Even if their role were related to an involuntary type of body temperature regulation mediated entirely through the visceral afferent and autonomic systems, it appears likely that voluntary responses could be conditioned to them. Russian literature contains many studies in which relatively mild stimulation of the viscera was used as a CS to produce a leg flexion. Even discriminations between mild concentrations of glucose solutions placed in the lower intestinal tract have been conditioned to elicit a leg flexion response (Razran, 1961). SUMMARY 1. Thresholds to temperature increases and decreases were measured on the shaved back of cats by the conditioned avoidance and escape methods after the skin had been adapted to temperatures from 29 to 440 C. Warm and cool thresholds at an analogous site on the back of humans were also measured after the skin had been adapted to temperatures from 29 to 410 C. 2. Both the avoidance and escape methods, with temperature increases, gave similar thresholds which changed linearly as a function ofthe adapting

448 D. R. KENSHALO temperature. The human warm threshold was a curvilinear function of the adapting temperature and was from 6 to 18.50 C less than the avoidance threshold of the cats. Because of the similarity of the avoidance and escape thresholds of the cats and the dissimilarity to the human threshold, it is concluded that cats do not sense mild temperature increases on their furred skin. 3. The cool thresholds of humans and the avoidance thresholds of cats as a function of the adapting temperature have similar shapes although the human thresholds were about 3.50 C lower than the cats at adapting temperatures up to 410 C. An escape threshold to temperature decreases was obtained on one cat which was about 1315 C larger than the avoidance threshold. It is concluded that cats are able to sense mild temperature decreases before the temperature decreases become noxious. This investigation was supported by Research Grant B02992 from The National Institute of Neurological Diseases and Blindness. Acknowledgement is made of the assistance of E. S. Gallegos, Dennis Duncan and Carolyn Weyimark in handling and maintaining the cats. REFERENCES BICE, R. C. (1961). Electromechanical transducer for vibrotactile Instrum. 32, 856857. stimulation. Rev. sci. BOMAN, K. (1958). Elektrophysiologische Untersuchungen uber die Thermoreceptoren der Gesichtshaut. Acta physiol. scand. 44, 79 Pp. (Suppl. 149, vol. 44). DOUGLAS, W. W., RICHIE, J. M. & STRAUB, R. W. (1960). The role of nonmyelinated fibres in signalling cooling of the skin. J. Physiol. 150, 266283. HARDY, J. D. (1955). Control of heat loss and heat production in physiologic temperature regulation. Harvey Lect. 49, 242269. HENSEL, H., IGGO, A. & WITT, I. (1960). A quantitative study of sensitive cutaneous thermoreceptors with C afferent fibres. J. Physiol. 153, 113126. HENSEL, H. & ZOTTERMAN, Y. (1951). The effect of menthol on thermoreceptors. Acta physiol. scand. 24, 2'734. HILGARD, E. R. & MARQUIS, D. G. (1940). Con1ditioning and Learning. New York: D. AppletonCentury Co., Inc. HUNTER, T. A. & BROWN, J. S. Psychol. 62, 570575. (1949). A decade type electronic interval timer. Amer. J. IRUCHIJIMA, J. & ZOTTERMAN, Y. (1960). The specificity of afferent cutaneous C fibres in mammals. Acta physiol. scand. 49, 267278. KENSHALO, D. R. (1963). Improved method for the psychophysical study of the temperature sense. Rev. sci. Instrum. 34, 883886. KENSHALO, D. R., NAFE, J. P. & BROOKS, BARBARA. (1961). Variations in thermal sensitivity. Scientce, 134, 104105. LEWIS, D. (1960). Quantitative Methods in Psychology. New York: McGrawHill. MURGATROYD, DOROTHY, KELLER, A. K. & HARDY, J. D. (1958). Warmth discrimination in the dog after a hypothalamic ablation. Amer. J. Physiol. 195, 276284. RAZRAN, G. (1961). The observable unconscious in current Soviet psychophysiology: Interoreceptive conditioning, semantic conditioning and the orienting reflex. Psychol. Rev. 68, 81147. RICE, C. E. & KENSHALO, D. R. (1962). Nociceptive threshold measurements in the cat. J. appl. Physiol. 17, 10091012. WALL, P. D. (1960). Cord cells responding to touch, damage, and temperature of skin. J. N.europhysiol. 23, 197210. WITr, INGRID, & HENSEL, H. (1959). Afferente Impulse aus der Extremitatenhaut der Katz bie thermischer und mechanischer Reizung. Pflug. Arch. 268, 582596.