Neural Mechanisms of Decision Making 265 



are the observed frequency-specific potentials a reflection of the 

 coding" and processing of information causafly related to the 

 behavioral performance, or do they merely reflect generalized 

 processes of local excitation and inhibition that are not specifically 

 informational and bear only a relationship of concomitance to 

 the behavioral performance? 



In the initial studies which we undertook to resolve these ques- 

 tions (10), an attempt was made to evaluate directly the functional 

 significance of labeled potentials observed in various brain struc- 

 tures in cats fully trained to perform diff'erential avoidance re- 

 sponses to two flicker conditioned stimuli of different frequencies. 

 We studied the behavioral effects of direct electrical stimulation 

 of the brain at frequencies concordant or discordant with the 

 frequency of the peripheral conditioned stimuli presented simul- 

 taneously. After pilot studies showed that low frequency electrical 

 stimulation was not effective, a modulation technique was devised. 

 A standard "carrier" waveform, consisting of a 100 cycle per 

 second biphasic square wave with a 2 millisecond pulse duration, 

 was modulated at the frequency of the peripheral TCS. This pro- 

 duced trains of bursts of 100 cycle per second square waves, with 

 the burst frequency identical with the flicker frequencies to which 

 the animals were conditioned. Trains at different frequencies could 

 be manipulated to achieve equal duration of constituent bursts 

 or to equate total electrical energy by selection of appropriate 

 burst durations. 



Most structures were explored both unilaterally and bilaterally. 

 For each structure, we determined the current level at which 

 central stimulation at both the reinforced (S ) and the non- 

 reinforced (S^) frequency blocked performance to concurrent 

 photic stimulation at the S frequency. This current level was 

 defined as the occlusion threshold, or cut-off. The current intensity 

 at which conditioned response perfoimance returned to concurrent 

 photic and central stimulation at one central frequency but not 

 the other was defined as the differential threshold. If a differential 

 threshold was observed, a series of trials was carried out to de- 

 termine the reliability of such an effect. Throughout such stimula- 

 tion sessions, central stimuli were presented in counterbalanced 

 frequency sequence, and each sequence was bracketed by trials 



