Neural Mechanisms of Decision Making 273 



would restore performance. Instead, we measured the occlusion 

 threshold for concurrent four per second flicker and four per second 

 visual cortex stimulation and observed it to be around 3.0 milli- 

 amperes. An experiment was then devised to explore whether the 

 disruptive eff'ect of cortical stimulation varied as a function of the 

 time that such stimulation occurred with respect to the instant 

 when the flash of light was presented. 



Specifically, we investigated the consequences of using our 

 stimulus generators (Tektronix) to delay direct cortical stimuli so 

 as to cause them to coincide either with the early or late phase of 

 the cortical response to the peripheral conditioned flash. Stimuli 

 were arranged in a counterbalanced sequence, thus: 



4 per second flicker alone 



4 per second flicker + 4 per second visual cortex stimuli (early) 



4 per second flicker + 4 per second visual cortex stimuli (late) 



4 per second flicker alone 



4 per second flicker + 4 per second visual cortex stimuli (late) 



4 per second flicker + 4 per second visual cortex stimuli (early) 



4 per second flicker alone 



All cortical stimuli were at 2.8 milliamperes. Carrier frequency 

 was 100 cycles per second with two millisecond pulse width. Bursts 

 consisted of five biphasic pulses (25 millisecond burst width). 

 "Early" stimuli were so phased as to reach visual cortex 15 milli- 

 seconds after each flash of the four per second flicker. "Late" 

 stimuli were timed to reach the cortex either 80 milliseconds or 

 110 milliseconds after each flash of the four per second flicker. 

 Shock to the feet was delivered if the avoidance response was 

 not elicited within fifteen seconds when the four per second flicker 

 was presented alone. All trials involving central stimulation were 

 under extinction conditions, i.e., no shock was delivered. Central 

 sequences were scored only if bracked by correct performance 

 of the conditioned response in less than fifteen seconds to four per 

 second flicker alone, without punishment. 



Table III summarizes the results of these experiments. As can be 

 seen, central stimuli arriving 'iate" were very much more dis- 

 ruptive than identical perturbations arriving early. This suggests 



