Neural Mechanisms of Decision Making 245 



sociated neural activity in anatomically extensive regions of the 

 nervous system causes a functional relationship to become estab- 

 lished between these regions. Subsequent to such association, 

 stimulation of one region causes a response to occur in other regions, 

 although this response did not occur before the associated activity; 

 2) such altered response relationship cannot be interpreted as 

 merely a reflection of altered threshold, since Morrell has shown 

 that the new response is differential, and is displayed only to the 

 stimulus to which the association was established and not to closely 

 similar stimuli; and 3) discharge can occur from such a representa- 

 tional system with a temporal pattern which reflects the pattern of 

 stimulation while it was established. 



TRACER STIMULI, LABELED POTENTIALS, 

 AND INFORMATION 



The bulk of the data which I wish to present here was obtained 

 in studies of changes in the electrophysiological response to inter- 

 mittent stimuli during the establishment of conditioned responses. 

 The technique, used most profitably before us by Livanov and 

 Polyakov (14), was applied by Killam and me in our studies of 

 conditioned avoidance and approach responses in cats (8, 9). We 

 reasoned that, whatever the nature of the new responses established 

 in the brain during conditioning, such responses should appear 

 fairly reliably whenever the stimulus was presented. We selected 

 an intermittent light flash, which we called a "tracer conditioned 

 stimulus" (TCS), and searched the electrical activity of the brain 

 for the appearance of waveforms at the frequency of the TCS, which 

 were called "labeled potentials." Such a procedure greatly en- 

 hances one's ability to detect stimulus-bound signal in the midst of 

 the tremendous amount of ongoing business in the nervous system. 

 The appearance of labeled potentials in a structure during the 

 presentation of a TCS is sufficient evidence to conclude that in- 

 formation about the TCS is reaching that structure. It is clear that 

 such labeled potentials are not necessary for a structure to be in- 

 fluenced. A structure which shows no labeled potentials can be 

 receiving information about a TCS. 



