172 BRAIN MECHANISMS AND LEARNING 



III. THE OPERANT CONDITIONING OF SINGLE-UNIT RESPONSES 



A most important contribution of behavioural psychology to the 

 brain and behaviour field is the conception that a response is not merely 

 something to be elicited, observed, characterized, and recorded. In 

 operant-behaviour analysis, a response is more than that: it is something 

 to be conditioned. The single-unit response is, on the face of it, precisely 

 the type of categorical and clearly defined event that should be amenable 

 to operant-conditioning techniques. We report here initial successes in 

 this endeavour. 



METHODS 



In these studies, rats were prepared first with self-stimulation electrodes 

 in mcdial-forebrain-bundle regions. Preliminary tests established that 

 very high self-stimulation rates were achieved, and no tendency to escape 

 from stimulation was present. Rats which failed to meet these requirements 

 were eliminated. Each rat was then placed in a stereotaxic instrument 

 under barbiturate anaesthesia. A hole of 3-mm. diameter was drilled in the 

 skull, usually i mm. behind, and i mm. lateral to the bregma. The dura 

 was pierced repeatedly with a sharp instrument. Then tungsten micro- 

 electrodes of i-ii diameter (Hubel, 1957) were lowered imni. into the 

 cortex. 



As the animal recovered from the barbiturate anaesthesia, still in the 

 stereotaxic instrument, he was given repeated doses of isopropyl meproba- 

 mate (Soma, Wallace Laboratories). Each dose was 80 mg./kg. The dose 

 was repeated whenever any tendency of the animal to try to escape from 

 the instrument appeared. Previous tests had shown that an almost paralys- 

 ing dose of isopropyl meprobamate (100 mg. kg.) fails to block self- 

 stimulation (Olds and Travis). From this point on, the electrode was 

 advanced downwards through the cortex, hippocampal formation, thala- 

 mus and so forth, stopping whenever a clear spontaneous response 

 appeared. Output from the microelectrode was led through a cathode 

 follower into a Grass a-c amplifier and thence into a Dumont cathode-ray 

 oscilloscope. Responses of single nerve cells appeared as 200- to 500-|jv. 

 negative spikes, lasting about i msec. They were identified mainly by 

 their duration, by being repeated responses of constant amplitude, and by 

 their disappearance upon movement of the microelectrode by about 200 

 microns. 



Unit responses were not considered satisfactory for these experiments if 

 their resting frequency was more than about 2 per second ; and they were 



