BEHAVIOR AND CNS INTEGRATION 



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typically remained on the bottom of the pool unless prodded, to which it 

 responded by suddenly swimming away erratically, often colliding with the 

 walls and other sharks. After two more sessions, this behavior became less 

 pronounced, but the shark still refused to approach the stimulus targets 

 directly and hesitated while swimming back to the start pen after each trial. 

 It usually chose a side as soon as it exited the start pen and then swam up 

 the alley while maintaining contact with one wall. The choice of sides ap- 

 peared to vary randomly and thus did not resemble the position habits seen 

 in other sharks. The animal never performed above chance on BW, even 

 though it continued to eat and swim well. 



Its subsequent performance on a nighttime light-dark discrimination task, 

 shown in the lower panel of Figure 9, suggests that the shark could detect 

 the presence of light; responding often exceeded chance but never dropped 

 below it. However, the shark never reached criterion and was finally 

 sacrificed after 48 sessions. Post-mortem examination revealed a large lesion 

 that removed about 95% of the central telencephalic areas that receive 

 thalamic fibers (Figure 10). 



NS-530 



LOA 



DOTr 



cm 



Figure 10 Transverse reconstruction of lesion for subject NS-530. See Figure 8 for ab- 

 breviations. 



Figure 11 presents behavioral results for three more sharks with lesions in- 

 volving between 10% and 50% of the central telencephalic nuclei and inflict- 

 ing no damage to their connecting pathways. While only NS-501 exhibited 

 any significant postoperative retention of either task, these subjects were less 

 severely affected than NS-530 but still had to be retrained. There was some 

 saving on BW but none, or very little, on HV. Three other nurse sharks, in 

 which 10% or less of the central nuclei was damaged, showed almost perfect 

 retention of the preoperatively acquired discrimination habits (Figure 12). 



