BEHAVIOR AND CNS INTEGRATION 203 



In addition to the problem of interpreting previous behavioral experi- 

 ments there have been enough substantial changes in our understanding of 

 shark neuroanatomy to warrant reexamining the behavioral effects of tectal 

 lesions. Because of evidence from selective silver impregnation studies, we 

 now know that the shark central visual system extends far beyond the optic 

 tectum. In the three species so far examined (Ginglymostoma cirratum, 

 Galeocerdo cuvieri, and Negaprion breuirostris), retinal fibers project to the 

 dorsolateral thalamus, pretectal area, and hypothalamus in addition to the 

 traditionally recognized tectum (Ebbesson and Ramsey 1968, Graeber and 

 Ebbesson 19726). The extent of these diencephalic visual projections suggests 

 that the optic tectum may not exert exclusive control over the visuomotor 

 behavior of elasmobranchs. Other areas of the brain may be equally impor- 

 tant in processing certain types of visual information. 



Tectal Ablation and Visual Discrimination 



As in past research, our work has focused on the importance of the optic 

 tectum in guiding behavior. In view of the success of Clark and her col- 

 leagues, we chose a relatively simple instrumental visual discrimination task 

 for the animals to learn. Although Clark had not attempted to train isolated 

 individual sharks on this type of task, it appeared ideal because it involves an 

 active organism but still provides relatively rigorous control over experi- 

 mental conditions. Also, in contrast to operant procedures, the fixed-trial 

 instrumental method allows response latency to be measured, thus providing 

 an index of possible motivational deficits resulting from nonspecific surgical 

 trauma. The general method and results of this study are described below, 

 but the interested reader may wish to consult the two previously published 

 reports for further details (Graeber and Ebbesson 1972a, Graeber, Ebbesson, 

 and Jane 1973). 



Juvenile nurse sharks, 2 to 4 ft long, were chosen as subjects because of 

 the species' previous use in both anatomical and conditioning studies, well- 

 documented hardiness in captivity, and ready availability. Prior to receiving 

 any training, they underwent bilateral aspiration of the optic tectum while 

 anesthetized in a 0.01% bath of tricaine methane-sulfonate and seawater. 

 Care was taken to limit the ventral extent of the lesions to that region of the 

 tectum known to receive retinal input. The brain case hole was then closed 

 with dental cement, and the skin incision sutured with silk or stainless steel 

 wire. 



After surgery the sharks were moved into one of two maintenance pens 

 (see Figure 1) where they remained until they began to eat well (9 to 27 

 days). As each one recovered, it was deprived of food for 48 h before being 

 adapted to the apparatus and taught to swim in a clockwise direction down 

 the test alley, through the goal area, and back up a corridor to the starting 

 pen. Other sharks remained unoperated and served as controls. 



Actual discrimination training began when they were required to choose 

 the correct target door in a modified-Y maze. Each shark was given six 

 consecutive trials daily, about 45 s apart, and was rewarded each time with a 



