320 ROBERT M. BENJAMIN 



to only one substance. For example, the profile at the top represents a 

 unit that responded only to the strongest concentration of sucrose. Notice 

 the NaCl units and in the bottom two profiles some units that did not res- 

 pond to any taste solution, but did fire to a water rinse after HCl. These 

 " off" responses are indicated by open bars and the letter " F ". The 

 next column contains the profiles of eight units responsive to two sub- 

 stances, in all cases, NaCl and sucrose. Notice the on-off response of the 

 bottom unit. The plus sign signifies a unit of positive polarity, with spikes 

 of extremely short duration and extremely small amplitude. In the folklore 

 of indium microelectrode enthusiasts these characteristics suggest pickup 

 from a fiber. 



In the next column are six units activated by three substances each in- 

 cluding quinine and finally, in the last column three units driven by tem- 

 perature as well as taste stimuli. Notice that a response to quinine was 

 always associated with a response to both HCl and NaCl and never with a 

 response to sucrose. 



On Fig. 10 are sample dot patterns of units subjected to stimuli of longer 



62-375-10 



0.1 HCl .01 HCl 



62-364-7 



I— 25 msec 



MJiLOi 



.01 Sue 0.1 Sue 1.0 Sue 



30sec 



Fig. 10. Interspike interval dot patterns of the responses of two thalamic 



neurons to taste solutions flowed on to the tongue of the squirrel monkey. 



Heavy bars mark periods of stimulation. 



duration. The one at the top is an HCl " off" unit which did respond to 

 HCl after about 20 sec of continuous stimulation. The " off" burst so 

 prominent after a 3 sec stimulus is attenuated but still evident. The 



