566 BRAIN MECHANISMS AND LEARNING 



example, in a 'go-iio go' discrimination in which an anmial is rewarded 

 with food for going to one stimulus, and is also rewarded with food for 

 not going to the other stimulus both stimuli should acquire positive 

 reward value. With these conditions of testing, loss of drive inhibition 

 should not affect performance. Yet this technique of symmetrical reward 

 was the one employed in Weiskrantz and Mishkin's (1958) 'go-no go' 

 auditory problem on which frontal aninials were so severely impaired. 

 Conversely, the hypothesis of a loss of drive inhibition sliould predict 

 impairment whenever a rewarded stimulus is paired with one that is 

 consistently unrewarded. Thus, in a visual discrimination in which both 

 the positive anci negative stimuli arc simultaneously present, frontal 

 animals should have greater difficulty than normals in inhibiting responses 

 to the unrewarded cue. Yet, it was the fact that frontal animals were 

 unimpaired on precisely this type of discrimination that first gave rise to 

 the erroneous generalization that frontal damage does not affect per- 

 formance on any discrimination task. 



The final hypothesis to be considered is immune to all the objections 

 that have been raised so far since it does not yield predictions with respect 

 to different cue-response-reward combinations. Predictions that would 

 follow from a loss of stimulus inhibition relate rather to the potentially 

 disruptive effects of extraneous stimuli. Thus, discrimination impairment 

 following frontal damage would bo expected to be directly related to the 

 amount of distraction in the testing situation. Unfortunately, the literature 

 on discrimination learning in frontal animals does not provide us with a 

 clear-cut test of this prediction. Nevertheless, there have been gross 

 differences betwccii studies m terms of such general testing conditions as 

 noisy v. silent apparatus, noisy c. sound-shielded testing rooms, the 

 possibility of visual exploration v. the exclusion of vision, etc. The 

 presence of certain distracting influences has been shown to have a marked 

 ciisrupting effect on frontal animals' delayed response performance 

 (Malmo, 1942; Lawicka, 1957). However, as far as can be determined, 

 there is no such relationship between the presence of extraneous stimuli 

 and their impairment on discrimination tasks. 



We do not wish to argue that our analysis of the experimental evidence 

 conclusively rules out all possibility of explaining the frontal animals' 

 impairment on discrimination tasks as a loss of some form of inhibition. 

 Perhaps further theorizing along such lines will suggest a mechanism that 

 will overcome the difficulties that have been raised. It should be pointed 

 out, however, that these ditlfculties may reflect the inadequacy of the 

 general hypothesis of a loss of inhibition from which the various specific 



