TASTE STIMULATION AND PREFERENCE BEHAVIOR 



261 



prior stimulation or adaptation by other ions. Perhaps the fluid electrolyte 

 balance in the rat maintains the receptors in an inhibited state for water 

 stimulation as compared with other species. The lesser decrement to weak 

 salts is systematic with concentration so that a neutral point (no change in 



rnmmimjxm 



77/ 7177/7/ /. 7771/1 



Hi I nil II r 



mtmfTMfh^/ 



Fig. 3. Summator record of chorda tympani responses to water and weak salt 

 solutions. Water reduces the level of activity; weak saline solutions cause less 

 reduction. Stronger concentrations above 0.0018 m produce increments in 



discharge. 



resting activity) is observed at about 0.001 m NaCl. Concentrations above 

 this level give the usual increment in discharge ; below, they produce a 

 decrement but the exact point where this change occurs depends upon 

 prior stimulation and the interval between stimulations. 



The above results confirm our early observations at threshold and ex- 

 tends them to suprathreshold levels of NaCl and other salts and taste 

 stimuli. It must be remembered that these animals were surgically anes- 

 thetized and so these observations do not rule out the possibility of afferent 

 modulation by reticular influences or by other neural efTerents. Further 

 examination of this possibility is needed. 



We do have evidence of receptor modulation by another factor, namely, 

 level of salivary electrolytes. Our recording results argue against any 

 intrinsic chemical change in receptor mechanism, yet the recent studies of 

 Yensen (1959), de Wardener and Herxheimer (1957), and Henkin and 

 Solomon (1960) all report changes in threshold with changes in fluid intake 

 or salt deprivation. McBurney* and 1 (1962) have recently made threshold 

 determinations for NaCl under conditions which approximated those we 

 employed in the electrophysiological studies, that is, when the tongue was 

 well rinsed with water and free of all saliva as compared with the tongue 

 bathed with saliva between stimulations. 



McBurney devised a continuous flow system resembling that used by Hahn 

 (1934) a number of years ago but without the flow chamber. The subject 

 sat with his tongue slightly extended under a spout which delivered a steady 

 stream of distilled water. Upon a signal from the experimenter, the subject 

 moved his tongue aside from the rinsing stream so that 10 ml of taste 

 stimulus could be presented by a pipette to the anterior surface of the 

 tongue. The subject then returned his tongue to the rinsing stream until 



*On a Fellowship grant from the General Foods Corporation. 



