220 VISION 



anatomical overlap seen in the retinal and tectal terminations in the dorsola- 

 teral optic complex of the thalamus. 



As I pointed out earlier, it is difficult to draw firm conclusions about the 

 exact nature of the behavioral deficits caused by telencephalic lesions with- 

 out further data from experiments that carefully separate the response re- 

 quirements of the postoperative test situation. It is equally premature to 

 discuss the deleterious effects that tectal ablation must have on some aspects 

 of visual functioning. Nevertheless, in that the deficits so far observed with 

 central visual lesions involve visually guided behavior, there is a temptation 

 to speculate about the functional differences between the two proposed cen- 

 tral visual subsystems. 



One hypothesis is raised by studies that show that visually elicited 

 behaviors can be experimentally distinguished from visually guided behaviors 

 in kittens (Hein and Held 1967, Hein and Diamond 1971). Eventually it may 

 be shown that the shark tectum is involved primarily in the control of 

 visually elicited behaviors, while forebrain visual mechanisms are concerned 

 with more voluntary, visually guided behaviors. Contrary to Schneider's 

 (1969) results with hamsters, the successful discrimination learning of the 

 tectally ablated subjects indicates that this structure is not seriously involved 

 in the shark's ability to voluntarily orient toward an object. Their 

 performance further demonstrates that the shark tectum is not responsible 

 for learning to identify visual stimuli. Where no direct data are available on 

 the effects of tectal lesions on visually elicited behaviors in sharks, 

 substantial evidence in the fish literature confirms that the tectum governs 

 such behavior in teleosts (e.g., Akert 1949). Hopefully, increased testing of 

 unlearned responses and the continued use of conditioning techniques will 

 help us further unravel the functional aspects of the shark's central visual 

 mechanisms. 



ACKNOWLEDGMENTS 



The behavioral findings on tectal and telencephalic visual mechanisms in the 

 nurse shark resulted from research conducted at the Lerner Marine Labora- 

 tory of the American Museum of Natural History, Bimini, Bahamas, while 

 the author was affiliated with the Departments of Psychology and Neurolo- 

 gical Surgery at the University of Virginia, Charlottesville. Special thanks are 

 extended to Dr. Dolores Schroeder, who trained some of the sharks with 

 telencephalic ablations and who supervised graphic reconstruction of the 

 lesions, to Dr. Sven Ebbesson and Dr. John Jane for their active collaboration 

 while conducting the experiments, and to Mr. Dean Jones for helping with 

 some of the training. The author is grateful for the histological assistance 

 provided by Ms. Lolyn Lopez, Ms. Charlain Greene, and Ms. Sarah Fuller, and 

 for the excellent support provided by the staff of the Lerner Marine Labora- 

 tory under the direction of Dr. Robert Mathewson. Most of all, I wish to 

 thank my wife Janet for her constant encouragement and assistance during 

 13 months of "island living." 



