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32 34 36 38 40 42 

 TRAINING SESSION 



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Figure 2 Daily performance, after surgery, of tectal-lesioned shark NS-143 on the tasks 

 of discriminating black and white and horizontal and vertical stripes. 



After training, the operated sharks were sacrificed by intracardiac perfu- 

 sion with 10% formalin. After removal, the brains were frozen and sectioned 

 transversely every 50 jura. Every twelfth section was subsequently stained for 

 cell bodies with cresyl-violet (Ebbesson 1970). The extent of ablations in 

 one subject is shown in Figure 3. In the other sharks the optic tectum was 

 also totally removed except for about 5% of the lateral portion of subject 

 187 's right tectum. The lesions completely abolished both the upper and 

 lower tectal layers, which respectively receive the retinal and telencephalic 

 input. There was little, if any, damage to adjacent regions of the brain, in- 

 cluding the underlying tegmentum, except for caudal portions of the 

 pretectal area in NS-184. 



Despite the similarities in discrimination task performance between the 

 operated and unoperated sharks, one might suspect that removing such a 

 substantial amount of central visual neuropil must have disrupted visual 



