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Airlicking: Thirsty rats prefer a warm dry airstream to a warm humid airstream
Physiology and Behavior, Voi. 12, pp. 557-561. Brain Research Publications Inc., 1974. Printed in the U.S.A.
Airlicking: Thirsty Rats Prefer a Warm Dry Airstream to a Warm Humid Airstream I WILLIAM J. FREED AND JOSEPH MENDELSON
Department o f Psychology, University o f Kansas, Lawrence, Kansas 66044
(Received 20 August 1973)
FREED, W. J. AND J. MENDELSON. Airlicking: thirsty rats prefer a warm dry airstream to a warm humid airstream. PHYSIOL. BEHAV. 12(4) 557-561, 1974. - Water-deprived rats were trained to airlick in a chamber containing two drinking tubes through which air of ambient temperature and humidity was pumped. After each rat had learned to airlick and had developed a strong position preference for one of the tubes, the airstream emerging from that tube was heated to 42°C and saturated with humidity. The other airstream was heated to 42°C but its humidity was not increased. All rats developed a strong preference for licking the dry airstream and they maintained this preference throughout many position reversals. It is concluded that evaporative cooling of the orolingual tissues makes an important contribution to the reinforcement obtained from airlicking. Airlicking
Orolingual cooling
Reward
Reinforcement
HENDRY and Rasche discovered that thirsty rats would persistently lick at a stream of air emerging from a drinking tube [ 1 ]. They suspected that the capacity of the airstream to cool the mouth was crucial for reinforcing airlicking; however, they found that warming the airstream to 100°F did not diminish its capacity to maintain airlicking. This finding has been confirmed in our laboratory [2,3]. We also found that water-deprived rats, mice, guinea pigs and hamsters would lick or orally palpate a cold, dry drinking tube [2]. This finding led to a revival of the hypothesis that the sensory basis for AL is, in fact, its cooling effect. Merely warming the airstream, as was done by Hendry and Rasche [ 1 ], still would not prevent evaporative cooling. In order to more effectively reduce the cooling effect of the airstream, we warmed it to several degrees above body temperature and saturated it with humidity. This warming and humidification of the airstream did diminish airlicking (AL) to some extent [3]. However, the results of that study were complicated by the fact that it was impossible to completely eliminate condensation of moisture from the air, and small quantities of water would occasionally trickle out of the drinking tube. Furthermore, although the relative humidity of the air was very close to 100% at the point where it emerged from the drinking tube, by the time it reached the backs of the animals' mouths (where it seems to exert its major reinforcing effects [4,5]), the humidity may have somewhat decreased. These control problems presumably account for the fact that the data obtained in that experiment were neither completely consistent nor entirely convincing.
Sensory reward
Thirst
In the present study we compared the reinforcing effects of licking a warm, dry airstream with those of licking a warm, humid airstream. As we have pointed out above, we are unable to guarantee that a humidified airstream will be at 100% relative humidity at those points where it makes contact with the animals' mouths so that it would be devoid of cooling capacity. However, the cooling hypothesis of AL would still predict that, when confronted with a choice between two airstreams differing in humidity, a rat would choose to lick the drier one. On the other hand, if the major reinforcing effects of AL are dependent upon the tactual properties of the airstream, the rats should be indifferent to the humidity of the airstream. Finally, if the thirsty rats are merely trying to ingest as much moisture as possible from the air, they should prefer to lick the humid airstream. It has already been demonstrated that rats do swallow air while they airlick [6]. This fact increases the feasibility of the latter hypothesis. METHOD
Animals The animals were two naive male (HA6, HA10) and two naive female (HA7, HA9) Long-Evans rats bred in our laboratory, two naive male Holtzman Sprague-Dawley rats and two Long-Evans rats used in a previous experiment (JW4, male; JW8, female) [5]. All rats were maintained in individual cages with continuous access to Purina Lab Chow pellets. The rats' water supply was limited to 5 ml per day until they reached 80% of their ad lib weights.
1This research was supported in part by grant MH 21955 to J. M. from the National Institute of Mental Health of the U.S. Department of Health, Education and Welfare. 557
558 A ppara tus
The test chamber consisted of a Plexiglas box with a wire mesh floor, measuring 12.6 x 20.0 x 20.6 cm high, located in a ventilated, sound-attenuated chamber. A 5.5 cm wide by 11.5 cm high section was removed from each of the smaller walls, leaving openings to two compartments which were 5.5 cm wide, 5.0 cm deep, and 12.0 cm high. A glass drinking tube extended down into the center of each compartment, 2.4 cm behind the entrance to it, and terminated 5.5 cm above the floor. The diameter of the orfice of the tubes was 2 ram, and an airstream emerged from them at a pressure of about 5,000 dynes/cm 2. Access to the drinking tubes was restricted by inserting Plexiglas plates into the openings to the compartments. These plates were inserted such that the tip of each drinking tube was 15 mm behind the side of the plate facing the inside of the test chamber. Two sets of plates were prepared, with holes of 28 and 36 mm dia. When the plates were inserted, the drinking tubes extended down 8 mm from the upper edge of the holes. A beam of light struck a photocell mounted in a side wall of each compartment, just below the tip of the drinking tube. Interruption of each light beam activated a running time meter and event marker. The rats were frequently observed in order to insure that interruptions of the photocell beams coincided with airlicking behavior. Before the air reached the box it could be passed through a 500 cc bottle containing approximately 300 cc of water. This water was heated to 4 1 . 5 - 4 2 . 0 ° C with an immersion heater controlled by a variable transformer. This served to heat the air and to humidify it to approximately 100% at that temperature. The temperature of the air was maintained by being passed through 5/16 in. glass tubing containing coiled 26 ga nichrome wire. A variable transformer was used to heat this wire so that the temperature of the air at the point at which it emerged from the drinking tube was 42 +_ 1/2 ° C. The temperature of the emerging airstreams was monitored by means of a thermistor (Fenwal UUA35J1) inserted in the tip of each drinking tube. The glass tubing slanted upwards at an angle of about 35 ° and was connected at its upper end to a short piece of Tygon tubing, which bent downwards to connect to the drinking tube. The coiled nichrome wire terminated at the end of the glass tubing, so that approximately 17 cm of tubing was left unheated. This, of course, meant that the air was heated to a temperature exceeding 42°C by the time it reached the end of the glass tubing. The expedient of ending the nichrome wire coil in this manner effectively eliminated condensation from occurring in or near the drinking tube. Procedure
After the animals had been deprived to their 80% bodyweight level, six of them were given sessions of exposure to two dry (i.e. not humidified) r o o m - t e m p e r a t u r e airstreams. Each rat was run for 30 rain per day, 5 days per week, until it reached a criterion of at least 4 consecutive days on which it accumulated at least 20 min of AL time on its r u n n i n g - t i m e meter (17 min for Rat HA12). Then for the next few sessions the animal's preferred (P) side was warmed to 42°C and humidifed to 100%, and its nonpreferred (NP) side was merely warmed to 42 ° C. Once the animal changed its preference to its previously NP side, the warm dry (WD) and warm humid (WH) sides were switched in a non-systematic or an alternating sequence. In almost all
FREED AND MENDEl S()N cases, the plates with 36 mm dia holes were used to restrici access to the airstreams. Four animals started the experiment with plates bearing 28 mm holes, but were then switched to 36 mm holes because their performance was deteriorating. The other two animals, HA6 and HA7, were first trained to airlick with only one non-humidified airstream present. Each airstream was used on about one-half of the training sessions; their order of availability was unsystematically varied. These rats were then given one session with both airstreams present and at room temperature, in order to determine their P sides. RESULTS AND DISCUSSION All the rats strongly preferred the WD airstream to the WH airstream; their mean preference was 97%. Each of the 6 animals that met the criterion of at least 20 min of AL on 4 consecutive sessions (17 min for Rat HA12), developed a very strong side preference; the mean preference for one side was at least 97% for 5 of the animals and 87% for Rat HA 12. Despite these marked side preferences, when their P sides were provided with a WH airstream, 5 of them switched to their former NP side (which had WD air) during the first session, and they continued to prefer the WD side throughout a number of position reversals (Fig. 1). The sixth animal, HA13, had such a strong side preference that its AL dropped to low levels whenever its P side had a WH airstream (Fig. 2). It licked for a mean total of 176 sec for 3 sessions with WH air on its P side, whereas when WD air was on its P side for 2 sessions interspersed among these 3, its mean total AL time was 1390 sec. Subsequently this rat was encouraged to airtick on its NP side by giving it 3 sessions with no air on its P side and dry roomtemperature air on its NP side. During the first of these sessions it showed a 94% preference for its P side, licking and searching there for 180 sec, even though nothing whatever could be obtained from the drinking tube on that side. By the third session, however, its AL time had increased to 1173 sec on its former NP side, compared to 80 sec on its former P side (which is still a lot of time, considering that no airstream was available there). This rat was then given one session with dry room-temperature air on both sides in order to determine whether its P side had now changed. It hadn't; the rat continued to display a 95% preference for its old P side. On the next session WH air was made available on its P side and WD air on its NP side. On this session the rat switched its preference in order to spend most of its time with the WD airstream. It continued to prefer this airstream for 8 consecutive side reversals (Fig. 2). The remaining two animals, HA6 and HA7, had been trained to airlick with air on one side at a time and with an access hole of 28 ram. When subsequently tested with dry room-temperature air on both sides, they licked for 20 rain or more for one session. However, the airlicking of these animals became sporadic and decreased to 2 13 min per session when the WD-plus-WH condition was introduced. After 9 sessions, the access hole size was increased from 28 mm to 36 mm and both animals began to airlick consistently for 20 rain or more per session. Fig. 1 shows the data for these animals beginning with their tenth session, which was the session on which they began to airlick for 20 min or more. They both consistently showed at least a 95% preference for the WD air for 17 ( H A 6 ) a n d 16 (HAT)side reversals. One further point of interest is the fact that each animal
AIRSTREAM MOISTURE AND AIRLICKING
N 80 ,-,-
559
IFFI"
R L R L L RR RL L R L R L L RL RL L R L R
IFFT
R L R L R L R R R Li L L R
90
70 ~I00
o ..
HAl2
8O <
70
N 60 ~
20
~;,,
IFFT
R RL R L R L R R L L R L
,oo-_t
9orJw8 RIGHT SUCCESSIVESESSIONS FIG. 1. Each graph shows for each session the percentage of the animal's airlicking which was directed at the dry airstream (filled circles). On each of the graphs (except for the upper two) the left hand portion indicates the percentage of time spent airlicking at the left or right (as indicated) akstream (rifled stars). On these sessions both airstreams were dry and at room temperature. L and R on the abscissa indicate the location of the dry airstream, left or right, respectively.
560
FREED AND MENI)EI_SON
./X/X,x,x
I00 90 ~P ° e° o•
Im,,,q
80
° 0•
_. I I,,,,,,-,,I
•
•
70
•
•e
l--
:
Z
I..I.I ,-,
oo
e
o•
•
.-
~
.." °e o
60 f
l-.-
50
.z o•
I--
Z
40
Z
I i
~" 30
i
20
I0 0
b LEFT
R R R L R R L
LEFT R L R L R L R L R
SUCCESSIVE SESSIONS FIG. 2. See the legend for Fig. 1. On the sessions whose data are illustrated with squares no airstream was available on theleft side and a dry room-temperature airstream was available on the right. The dotted line indicates the percentage of session time Spent airlicking (100% = 30 min). developed a pattern of briefly sampling each airstream before settling down to steadily lick at one. More specifically, if the first airstream it sampled was the WD one, the rat would nonetheless test the other airstream before returning to the WD airstream to lick it steadily. Sometimes a rat would test each airstream a second time, but rarely would it test them more than twice. A possible problem with the interpretation of our results may arise from the fact that all of the animals were trained on non-dehumidified room-temperature air. Therefore it could be claimed that the animals chose the WD air only because it was more similar to the air to which they had become accustomed; i.e., neither the WD air nor the training air was humidified, while the WH air was humidified. However, the relative humidity of the training air was actually intermediate between the WD and the WH air (the precise humidity of the training air varied considerably from day to day; however, it should be noted that the absolute humidity was roughly the same for the training air
and the WD air). Also, in preliminary studies it proved to be almost impossible to train animals to airlick with only the WH airstream present. Additional evidence is that it was possible to overcome the strong side preferences of the animals by introducing the WH airstream on the preferred side while maintaining the WD airstream on the non-preferred side. In the case of Rat HA13, whose side prefer•nee was particularly pronounced, AL almost completely extinguished when WH air replaced WD air on its preferred side. Since all animals eventually showed a strong preference for the warm dry airstream, it is concluded that the warm humid airstream was less reinforcing. Since the humid air would provide all of the tactual sensations induced by AL as well as a greater amount of moisture, it is concluded that the greater cooling effect of the dry air was responsible for its greater reinforcing properties. This supports the contention that cooling of the orolingual tissues is a critical reinforcer for sustaining airlicking.
AIRSTREAM MOISTURE AND AIRLICKING
561
REFERENCES 1. 2. 3.
Hendry, D. P. and R. H. Rasche. Analysis of a new nonnutritive positive reinforcer based on thirst. J. comp. physiol. Psychol. 54: 477-483, 1961. Mendelson, J. and D. Chillag. Tongue cooling: a new reward for thirsty rodents. Science 170: 1418-1419, 1970. Mendelson, J., D. Chillag and M. Paramesvaran. Effects of airstream temperature and humidity on alrlicking in the rat. Behav. Biol. 8: 357-365, 1973.
4. 5. 6.
Mendelson, J. and R. Zec. Effects of lingual denervation and desalivafion on aidicking in the rat. Physiol. Behav. 8: 711-714, 1972. Mendelson, L, S. Zielke, J. S. Wemer and L. M. Freed. Effects of airstream accessibility on airlicking in the rat. Physiol. Behav. 11: 125-130, 1973. Oatley, K. and A. Dickinson. Air drinking and the measurement of thirst. Anita. Behav. 18: 259-265, 1970.
Airlicking: Thirsty Rats Prefer a Warm Dry Airstream to a Warm Humid Airstream I WILLIAM J. FREED AND JOSEPH MENDELSON
Department o f Psychology, University o f Kansas, Lawrence, Kansas 66044
(Received 20 August 1973)
FREED, W. J. AND J. MENDELSON. Airlicking: thirsty rats prefer a warm dry airstream to a warm humid airstream. PHYSIOL. BEHAV. 12(4) 557-561, 1974. - Water-deprived rats were trained to airlick in a chamber containing two drinking tubes through which air of ambient temperature and humidity was pumped. After each rat had learned to airlick and had developed a strong position preference for one of the tubes, the airstream emerging from that tube was heated to 42°C and saturated with humidity. The other airstream was heated to 42°C but its humidity was not increased. All rats developed a strong preference for licking the dry airstream and they maintained this preference throughout many position reversals. It is concluded that evaporative cooling of the orolingual tissues makes an important contribution to the reinforcement obtained from airlicking. Airlicking
Orolingual cooling
Reward
Reinforcement
HENDRY and Rasche discovered that thirsty rats would persistently lick at a stream of air emerging from a drinking tube [ 1 ]. They suspected that the capacity of the airstream to cool the mouth was crucial for reinforcing airlicking; however, they found that warming the airstream to 100°F did not diminish its capacity to maintain airlicking. This finding has been confirmed in our laboratory [2,3]. We also found that water-deprived rats, mice, guinea pigs and hamsters would lick or orally palpate a cold, dry drinking tube [2]. This finding led to a revival of the hypothesis that the sensory basis for AL is, in fact, its cooling effect. Merely warming the airstream, as was done by Hendry and Rasche [ 1 ], still would not prevent evaporative cooling. In order to more effectively reduce the cooling effect of the airstream, we warmed it to several degrees above body temperature and saturated it with humidity. This warming and humidification of the airstream did diminish airlicking (AL) to some extent [3]. However, the results of that study were complicated by the fact that it was impossible to completely eliminate condensation of moisture from the air, and small quantities of water would occasionally trickle out of the drinking tube. Furthermore, although the relative humidity of the air was very close to 100% at the point where it emerged from the drinking tube, by the time it reached the backs of the animals' mouths (where it seems to exert its major reinforcing effects [4,5]), the humidity may have somewhat decreased. These control problems presumably account for the fact that the data obtained in that experiment were neither completely consistent nor entirely convincing.
Sensory reward
Thirst
In the present study we compared the reinforcing effects of licking a warm, dry airstream with those of licking a warm, humid airstream. As we have pointed out above, we are unable to guarantee that a humidified airstream will be at 100% relative humidity at those points where it makes contact with the animals' mouths so that it would be devoid of cooling capacity. However, the cooling hypothesis of AL would still predict that, when confronted with a choice between two airstreams differing in humidity, a rat would choose to lick the drier one. On the other hand, if the major reinforcing effects of AL are dependent upon the tactual properties of the airstream, the rats should be indifferent to the humidity of the airstream. Finally, if the thirsty rats are merely trying to ingest as much moisture as possible from the air, they should prefer to lick the humid airstream. It has already been demonstrated that rats do swallow air while they airlick [6]. This fact increases the feasibility of the latter hypothesis. METHOD
Animals The animals were two naive male (HA6, HA10) and two naive female (HA7, HA9) Long-Evans rats bred in our laboratory, two naive male Holtzman Sprague-Dawley rats and two Long-Evans rats used in a previous experiment (JW4, male; JW8, female) [5]. All rats were maintained in individual cages with continuous access to Purina Lab Chow pellets. The rats' water supply was limited to 5 ml per day until they reached 80% of their ad lib weights.
1This research was supported in part by grant MH 21955 to J. M. from the National Institute of Mental Health of the U.S. Department of Health, Education and Welfare. 557
558 A ppara tus
The test chamber consisted of a Plexiglas box with a wire mesh floor, measuring 12.6 x 20.0 x 20.6 cm high, located in a ventilated, sound-attenuated chamber. A 5.5 cm wide by 11.5 cm high section was removed from each of the smaller walls, leaving openings to two compartments which were 5.5 cm wide, 5.0 cm deep, and 12.0 cm high. A glass drinking tube extended down into the center of each compartment, 2.4 cm behind the entrance to it, and terminated 5.5 cm above the floor. The diameter of the orfice of the tubes was 2 ram, and an airstream emerged from them at a pressure of about 5,000 dynes/cm 2. Access to the drinking tubes was restricted by inserting Plexiglas plates into the openings to the compartments. These plates were inserted such that the tip of each drinking tube was 15 mm behind the side of the plate facing the inside of the test chamber. Two sets of plates were prepared, with holes of 28 and 36 mm dia. When the plates were inserted, the drinking tubes extended down 8 mm from the upper edge of the holes. A beam of light struck a photocell mounted in a side wall of each compartment, just below the tip of the drinking tube. Interruption of each light beam activated a running time meter and event marker. The rats were frequently observed in order to insure that interruptions of the photocell beams coincided with airlicking behavior. Before the air reached the box it could be passed through a 500 cc bottle containing approximately 300 cc of water. This water was heated to 4 1 . 5 - 4 2 . 0 ° C with an immersion heater controlled by a variable transformer. This served to heat the air and to humidify it to approximately 100% at that temperature. The temperature of the air was maintained by being passed through 5/16 in. glass tubing containing coiled 26 ga nichrome wire. A variable transformer was used to heat this wire so that the temperature of the air at the point at which it emerged from the drinking tube was 42 +_ 1/2 ° C. The temperature of the emerging airstreams was monitored by means of a thermistor (Fenwal UUA35J1) inserted in the tip of each drinking tube. The glass tubing slanted upwards at an angle of about 35 ° and was connected at its upper end to a short piece of Tygon tubing, which bent downwards to connect to the drinking tube. The coiled nichrome wire terminated at the end of the glass tubing, so that approximately 17 cm of tubing was left unheated. This, of course, meant that the air was heated to a temperature exceeding 42°C by the time it reached the end of the glass tubing. The expedient of ending the nichrome wire coil in this manner effectively eliminated condensation from occurring in or near the drinking tube. Procedure
After the animals had been deprived to their 80% bodyweight level, six of them were given sessions of exposure to two dry (i.e. not humidified) r o o m - t e m p e r a t u r e airstreams. Each rat was run for 30 rain per day, 5 days per week, until it reached a criterion of at least 4 consecutive days on which it accumulated at least 20 min of AL time on its r u n n i n g - t i m e meter (17 min for Rat HA12). Then for the next few sessions the animal's preferred (P) side was warmed to 42°C and humidifed to 100%, and its nonpreferred (NP) side was merely warmed to 42 ° C. Once the animal changed its preference to its previously NP side, the warm dry (WD) and warm humid (WH) sides were switched in a non-systematic or an alternating sequence. In almost all
FREED AND MENDEl S()N cases, the plates with 36 mm dia holes were used to restrici access to the airstreams. Four animals started the experiment with plates bearing 28 mm holes, but were then switched to 36 mm holes because their performance was deteriorating. The other two animals, HA6 and HA7, were first trained to airlick with only one non-humidified airstream present. Each airstream was used on about one-half of the training sessions; their order of availability was unsystematically varied. These rats were then given one session with both airstreams present and at room temperature, in order to determine their P sides. RESULTS AND DISCUSSION All the rats strongly preferred the WD airstream to the WH airstream; their mean preference was 97%. Each of the 6 animals that met the criterion of at least 20 min of AL on 4 consecutive sessions (17 min for Rat HA12), developed a very strong side preference; the mean preference for one side was at least 97% for 5 of the animals and 87% for Rat HA 12. Despite these marked side preferences, when their P sides were provided with a WH airstream, 5 of them switched to their former NP side (which had WD air) during the first session, and they continued to prefer the WD side throughout a number of position reversals (Fig. 1). The sixth animal, HA13, had such a strong side preference that its AL dropped to low levels whenever its P side had a WH airstream (Fig. 2). It licked for a mean total of 176 sec for 3 sessions with WH air on its P side, whereas when WD air was on its P side for 2 sessions interspersed among these 3, its mean total AL time was 1390 sec. Subsequently this rat was encouraged to airtick on its NP side by giving it 3 sessions with no air on its P side and dry roomtemperature air on its NP side. During the first of these sessions it showed a 94% preference for its P side, licking and searching there for 180 sec, even though nothing whatever could be obtained from the drinking tube on that side. By the third session, however, its AL time had increased to 1173 sec on its former NP side, compared to 80 sec on its former P side (which is still a lot of time, considering that no airstream was available there). This rat was then given one session with dry room-temperature air on both sides in order to determine whether its P side had now changed. It hadn't; the rat continued to display a 95% preference for its old P side. On the next session WH air was made available on its P side and WD air on its NP side. On this session the rat switched its preference in order to spend most of its time with the WD airstream. It continued to prefer this airstream for 8 consecutive side reversals (Fig. 2). The remaining two animals, HA6 and HA7, had been trained to airlick with air on one side at a time and with an access hole of 28 ram. When subsequently tested with dry room-temperature air on both sides, they licked for 20 rain or more for one session. However, the airlicking of these animals became sporadic and decreased to 2 13 min per session when the WD-plus-WH condition was introduced. After 9 sessions, the access hole size was increased from 28 mm to 36 mm and both animals began to airlick consistently for 20 rain or more per session. Fig. 1 shows the data for these animals beginning with their tenth session, which was the session on which they began to airlick for 20 min or more. They both consistently showed at least a 95% preference for the WD air for 17 ( H A 6 ) a n d 16 (HAT)side reversals. One further point of interest is the fact that each animal
AIRSTREAM MOISTURE AND AIRLICKING
N 80 ,-,-
559
IFFI"
R L R L L RR RL L R L R L L RL RL L R L R
IFFT
R L R L R L R R R Li L L R
90
70 ~I00
o ..
HAl2
8O <
70
N 60 ~
20
~;,,
IFFT
R RL R L R L R R L L R L
,oo-_t
9orJw8 RIGHT SUCCESSIVESESSIONS FIG. 1. Each graph shows for each session the percentage of the animal's airlicking which was directed at the dry airstream (filled circles). On each of the graphs (except for the upper two) the left hand portion indicates the percentage of time spent airlicking at the left or right (as indicated) akstream (rifled stars). On these sessions both airstreams were dry and at room temperature. L and R on the abscissa indicate the location of the dry airstream, left or right, respectively.
560
FREED AND MENI)EI_SON
./X/X,x,x
I00 90 ~P ° e° o•
Im,,,q
80
° 0•
_. I I,,,,,,-,,I
•
•
70
•
•e
l--
:
Z
I..I.I ,-,
oo
e
o•
•
.-
~
.." °e o
60 f
l-.-
50
.z o•
I--
Z
40
Z
I i
~" 30
i
20
I0 0
b LEFT
R R R L R R L
LEFT R L R L R L R L R
SUCCESSIVE SESSIONS FIG. 2. See the legend for Fig. 1. On the sessions whose data are illustrated with squares no airstream was available on theleft side and a dry room-temperature airstream was available on the right. The dotted line indicates the percentage of session time Spent airlicking (100% = 30 min). developed a pattern of briefly sampling each airstream before settling down to steadily lick at one. More specifically, if the first airstream it sampled was the WD one, the rat would nonetheless test the other airstream before returning to the WD airstream to lick it steadily. Sometimes a rat would test each airstream a second time, but rarely would it test them more than twice. A possible problem with the interpretation of our results may arise from the fact that all of the animals were trained on non-dehumidified room-temperature air. Therefore it could be claimed that the animals chose the WD air only because it was more similar to the air to which they had become accustomed; i.e., neither the WD air nor the training air was humidified, while the WH air was humidified. However, the relative humidity of the training air was actually intermediate between the WD and the WH air (the precise humidity of the training air varied considerably from day to day; however, it should be noted that the absolute humidity was roughly the same for the training air
and the WD air). Also, in preliminary studies it proved to be almost impossible to train animals to airlick with only the WH airstream present. Additional evidence is that it was possible to overcome the strong side preferences of the animals by introducing the WH airstream on the preferred side while maintaining the WD airstream on the non-preferred side. In the case of Rat HA13, whose side prefer•nee was particularly pronounced, AL almost completely extinguished when WH air replaced WD air on its preferred side. Since all animals eventually showed a strong preference for the warm dry airstream, it is concluded that the warm humid airstream was less reinforcing. Since the humid air would provide all of the tactual sensations induced by AL as well as a greater amount of moisture, it is concluded that the greater cooling effect of the dry air was responsible for its greater reinforcing properties. This supports the contention that cooling of the orolingual tissues is a critical reinforcer for sustaining airlicking.
AIRSTREAM MOISTURE AND AIRLICKING
561
REFERENCES 1. 2. 3.
Hendry, D. P. and R. H. Rasche. Analysis of a new nonnutritive positive reinforcer based on thirst. J. comp. physiol. Psychol. 54: 477-483, 1961. Mendelson, J. and D. Chillag. Tongue cooling: a new reward for thirsty rodents. Science 170: 1418-1419, 1970. Mendelson, J., D. Chillag and M. Paramesvaran. Effects of airstream temperature and humidity on alrlicking in the rat. Behav. Biol. 8: 357-365, 1973.
4. 5. 6.
Mendelson, J. and R. Zec. Effects of lingual denervation and desalivafion on aidicking in the rat. Physiol. Behav. 8: 711-714, 1972. Mendelson, L, S. Zielke, J. S. Wemer and L. M. Freed. Effects of airstream accessibility on airlicking in the rat. Physiol. Behav. 11: 125-130, 1973. Oatley, K. and A. Dickinson. Air drinking and the measurement of thirst. Anita. Behav. 18: 259-265, 1970.