216 



VISION 



learning could have resulted from any of a number of types of visual dys- 

 function. The most obvious would be a purely sensory loss causing an 

 inability to distinguish between the targets. The best way to verify this 

 would be to examine the effects of such lesions on classically conditioned 

 discriminations, thus experimentally removing any deficits due to abnormal 

 orientation or voluntary motor mechanisms. 



Because such a study was not possible at the time, a second approach was 

 chosen, aimed at determining whether the previously observed losses were 

 caused by the sharks' inability to learn to use visual cues to guide their swim- 

 ming movements (Graeber et al., in preparation). The extent of post- 

 operative training was minimized by training the subjects to criterion on the 

 visual discrimination tasks before surgery. It is possible that central 

 telencephalic lesions affect only visual learning capacity; if so, the sharks 

 would retain the learned habits after surgery. The alternative outcome is that 

 they would have to totally relearn the discriminations, this would indicate 

 that such neural damage interferes with the ability to discriminate (i.e., a 

 sensory loss) or to use visual cues to guide behavior (i.e., a visuomotor loss). 



In analyzing the results the sharks were grouped according to the extent 

 of damage to the central telencephalic nuclei. The performance of the sub- 

 ject with the most extensive damage is shown graphically in Figure 9. Before 

 undergoing surgery, NS-530 learned both tasks rapidly and retained them al- 

 most perfectly after 28 days of rest. It began eating well two days after 

 surgery and then resumed training. 



During the first three sessions the shark was very difficult to train, startled 

 easily, and refused to swim through the gates. When not being trained it 



20 30 40 50 



TRAINING SESSIONS 



Figure 9 Performance of NS-530 on visual discrimina- 

 tion tasks before and after surgery . The 28-day retention 

 period is indicated by the area between the pair of 

 vertical dashed lines, and the point at which surgery 

 was performed is indicated by the dashed line marked 

 OP. The lower graph shows performance on light-dark 

 (LD) task after postoperative BW training. 



