THE INTRINSIC SYSTEMS OF THE FOREBRAIN 1 327 



group had undergone bilateral cortical resections in 

 the posterior intrinsic cortex, and those in the other 

 operated group bilateral cortical resections in the 

 frontal intrinsic cortex some 2f^ years prior to the 

 onset of the experiment (fig. 3); those in the control 

 group are unoperated. In the testing situation these 

 animals are confronted initially with two junk objects 

 placed over two holes (on a board containing 1 2 holes 

 in all) with a peanut under one of the objects. An 

 opaque screen is lowered between the monkey and the 

 objects as soon as the monkey has displaced one of the 

 objects from its hole (a trial). When the screen is 

 lowered, separating the monkey from the r2-hole 

 board, the oijjects are moved (according to a random 

 number table) to two different holes on the board. 

 The screen is then raised and the animal is again con- 

 fronted with the problem. The peanut remains under 

 the same object until the animal finds the peanut five 

 consecutive times (criterion). After a monkey reaches 

 criterion performance, the peanut is shifted to the 

 second object and testing continues (discrimination 

 reversal). After an animal again reaches criterion per- 

 formance a third object is added (fig. 4). Each of the 

 three objects in turn becomes the positive cue; testing 

 then proceeds as before — the screen separates the 

 animal from the 12-hole board, the objects are placed 

 randomly over three out of the 12 holes (with a 

 peanut concealed under one of the objects), the screen 

 is raised, the animal is allowed to pick an object (one 

 response per trial), the screen is lowered and the 

 objects are moved to different holes The testing con- 

 tinues in this fashion until the animal reaches criterion 

 performance with each of the objects positi\e in turn. 

 Then a fourth object is added and the entire pro- 

 cedure repeated. As the animal progresses, the number 

 of objects is increased serially through a total of 1 2 

 (fig. 5). The testing procedure is the same for all 

 animals throughout the experiment; however, the 

 order of the introduction of objects is i:)alanced — the 

 order being the same for only one monkey in each 

 group. 



Analysis of the problem posed by this experiment 

 indicates that solution is facilitated when a monkey 

 attains two strategies: a) during search — moving, on 

 successive trials, each of the objects until the peanut is 

 foimd; /;) after search — selecting on successive trials 

 *he object under which the peanut had been found 

 on the preceding trial. During a portion of the ex- 

 periment, searching is restricted in animals with 

 posterior intrinsic sector ablations, and selection of the 

 object under which the peanut had been found on the 

 previous trial is impaired by frontal intrinsic sector 



%' 



24 



28 



32 j^'. 



IT- 37 



"5^ 80 ' 



72 



76 





LF-5 



vu,. 3. Representative reconstructions and cross sections 

 through the cortex and thalamus showing extent of the lesions 

 in the posterior (upper figure) and frontal {lower figure) intrin- 

 sic systems. Cortical lesion and resulting thalamic degenera- 

 tion sliown in black. 



ablations. The effects of the posterior intrinsic sector 

 lesion will be dealt with first. 



Figure 6 graphs the averages of the total number 

 of repetitive errors made by each of the groups in 

 each situation. Comparison of figure 6 with figure 7, 

 representing the repetitive errors made by each group 

 in each situation during search, illustrates that the 

 deficit of the frontally operated group is not associated 

 with search (a result that is di.scusscd below); how- 

 ever, the peak and general shape of the error curves 



