H. ENGER ROSVOLD AND MORTIMER MISHKIN 563 



on motor activity. The frontal animal's mipairment in inhibiting condi- 

 tioned motor responses might then be considered a symptom analogous 

 to the locomotor hyperactivity 'released' by frontal lobe damage. A 

 theory somewhat similar to this was, ni fact, proposed some time ago by 

 Stanley and Jaynes (1949). 



LOSS OF INHIBITION 



Fic. 7 

 The behavioural manifestations and neural mechanisms associated with three proposed types 

 of inhibitory defects. 



Alternatively, if an inhibitory defect were assigned to the motivational 

 sphere, the neural mechanism could be a loss of suppression of hypothala- 

 mic appetitive systems. By combining the data ot Anand and Brobeck 

 (1951) and Dclagdo and Anand (1953) demonstrating hypothalamic 

 control over food intake with the results of Ward and McCulloch (1947) 

 demonstrating frontal projections to the hypothalamus, it might be 

 proposed that frontal cortex normally regulates alimentary inhibition. 

 Increased appetite, another commonly reported symptom of frontal 

 damage (Morgan and Stellar, 1950), could then provide the explanation 

 for the frontal animal's ditikulty in inhibiting a conditioned food-getting 

 response. 



Finally, if an inhibitory defect were postulated on the sensory side it 

 could be ascribed perhaps to a loss of suppression of the diffuse activating 

 systems. Magoun (195S) has reviewed the evidence indicating that the 

 brain stem may inhibit as well as facilitate sensory input and the results of 

 Adey, Segundo and Livingston (1957) suggest that frontal cortex may 

 exert an important regulating influence on brain stem mechanisms. If 

 frontal damage caused a loss of suppression of the diffuse sensory input, 

 this would account first of all for the common observation that the 

 frontal animal is hyper-reactive to, anc^ easily distracted by extraneous 

 stimuli (Kcnnard, Spencer and Fountain, 1941; Malmo, 1942). That it 

 could also account for the frontal animal's impairment on discrimination 



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