J. OLDS AND M. E. OLDS 179 



time course, an idea possibly forced on us by the fact that the hnal condi- 

 tioned unit activity was often accompanied by some minute movement of 

 an extremity. One animal learned to move his tail to the right while 

 remaining, otherwise, absolutely still. 



In contrast to the conditioning ot cortical units which sometimes seemed 

 like conditioning the muscle output itself, the conditioning of subcortical 

 units often led us to teel that we were observing the basic mechanism of 

 instrumental conditioning itself. This was the case first in the conversion 

 of dentate responses to seizure-like tempos as in type i. But also the type 

 3 modification, observed in the hippocampus and diencephalon, appeared 

 to involve some basic mechanism of learning. 



With microclectrodes here, the stimulus had quite different effects when 

 given immediately after a unit response from those it might have against 

 a background of silence (Figs. 19, and 20 IV). When it followed the 

 unit, the stimulus would invoke either a further burst of response in the 

 same unit, or a burst of very high potential activity not easily characterized. 

 If the same stimulus was applied at some temporal distance from a single- 

 unit response, there were no similar effects. 



It may be relevant that stimulation did sometimes elicit in hippocampal 

 electrodes a high-voltage, repetitive, positive discharge of somewhat 

 longer duration than the single-unit response. 



Finally, there is of course the type 4 modification: the stimulus causes 

 an elicited effect so that reinforcement cannot be tested. It is hard to 

 estimate how frequent this occurrence is because these effects have been 

 quickly by-passed in our experiments in search of areas where elicited 

 effects are absent. It is certainly clear that these cases increase in number 

 as the microelectrode approaches the point of stimulation in the posterior 

 hypothalamus (Fig. 20 V). 



SPECULATIONS 



It is certainly tempting to speculate at this point, so long as it is under- 

 stood that no scientific factual import should be attached to these specula- 

 tions. 



Ashby (1953) has suggested that perhaps each neurone is in itself a 

 negative feedback system, at least a system that modifies its output 

 depending on the feedback it gets from previous outputs. We have perhaps 

 shown something of this kind here. 



There is of course nothing in our work to determine that the unit is the 

 conditioned entity; perhaps it is simply our method for identifying a 



N 



