BEHAVIOR AND CNS INTEGRATION 



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times daily, to control for cues other than stripe orientation that might be 

 associated with a particular target. To rule out position learning for both 

 tasks, the left-right location of the correct target was varied quasi-randomly 

 in a predetermined fashion. A 15.2-cm-long opaque baffle separated the two 

 targets and forced the sharks to choose one or the other before swimming 

 across a choice line drawn even with the baffle's front edge. The experimenter 

 observed the responses in a mirror suspended from the roof of the pool and 

 opened the door when the correct target was chosen. 



Contrary to the findings of previous investigations, removal of the optic 

 tectum did not prevent the sharks' learning visual discriminations. They all 

 learned to discriminate black from white, and only one failed to reach 

 criterion on the horizontal-vs-vertical stripes discrimination. This failure is 

 likely due to the lack of time to train it for more than half the number of 

 trials permitted the others. 



In comparing the performances of the operated sharks with those of the 

 unoperated controls (Table 1), we find little evidence to suggest any substan- 

 tial postoperative visual discrimination deficit. In fact, some of the tectally 

 ablated sharks actually learned to discriminate faster and with fewer errors 

 than did their unoperated counterparts. The learning curves of one of the 

 operated subjects is shown in Figure 2. The apparently lengthy training 

 required by NS-184 on the black— white task is misleading in that the shark 

 performed significantly above chance (p < 0.005) after only 107 trials and 

 41 errors (Grant 1947). It continued to perform at this level for the next 10 

 sessions, after which training had to be discontinued for 16 days for un- 

 related reasons. The lack of a surgically related performance deficit is also 

 reflected in the median daily response latencies for both groups, which 

 quickly decreased to a stable level (5-10 s) after a few days of training. 



Table 1. Number of trials and errors to criterion on two visual discrimina- 

 tion tasks for sharks with and without tectal ablations 



See text for explanation. 

 'Parentheses indicate that shark did not reach criterion. 



