208 VISION 



bumping into surrounding walls and objects when attempting to swim. After 

 a few days it ceased colliding, but, in spite of its great readiness to eat, re- 

 mained extremely reluctant to move forward in any direction, even for food. 

 Upon beginning pretraining 27 days after surgery, the shark swam sluggishly, 

 required frequent coaxing and prodding, and often hesitated before entering 

 the goal area beyond the target doors. After a day or two these behaviors dis- 

 appeared; the shark began to swim readily and successfully completed pre- 

 training within a total of 9 days. 



The behavior of NS-143 may indicate that successful discriminative 

 learning represents a recovery of visual function mediated by some other 

 area of the central visual system. However, general observation of the other 

 tectal subjects revealed no evidence of initial postoperative blindness. In 

 fact, one began successful pretraining only 9 days after surgery. The question 

 of recovery of function will not be answered until the completion of experi- 

 ments examining the effects of tectal lesions on the retention of preopera- 

 tive^ learned visual discriminations. Yet, the following evidence for telen- 

 cephalic involvement in shark vision offers a likely candidate for the recovery 

 mechanism, if one is necessary. 



The present results allow us at least to conclude that the optic tectum is 

 not necessary for visually guided behavior in the nurse shark. The similarity 

 of the retinal projections in this species to those in the two other species so 

 far examined, Negaprion brevirostris and Galeocerdo cuvieri, further increases 

 the possibility that the tectum is not vital to the performance of such be- 

 havior in most sharks. Speculation on the true role of the midbrain in shark 

 vision would be better left until after a review of forebrain involvement. 



FOREBRAIN VISUAL MECHANISMS 

 Do They Exist? 



Neuroanatomical Considerations— The extensive studies of early 

 comparative neuroanatomists (Ariens Kappers 1906, B'ackstrom 1924, 

 Herrick 1922, Houser, 1901, Johnston 1911) strongly supported the com- 

 mon belief (Aronson 1963, Nieuwenhuys 1967) that the elasmobranch 

 telencephalon serves only as an olfactory-gustatory coordination center. 

 These observations were based on the examination of "normal" material 

 from nonlesioned animals. Because the shark brain is characterized by many 

 thin and poorly myelinated axons connecting diffuse cell groups, it is very 

 difficult to distinguish its structural organization even under today's light 

 microscopes. 



In the 1950s Nauta developed the selective silver impregnation technique 

 to trace degenerating neural pathways (Nauta 1957, Nauta and Gygax 1954). 

 This technological breakthrough, coupled with Fink and Heimer's (1967) 

 new method for staining degenerating axon terminals, led the way to a host 

 of new discoveries about the structure of the mammalian brain. Similarly, 

 Ebbesson's (1970) subsequent modifications of the Nauta-Gygax and Fink- 

 Heimer histological techniques for use in fish provided the first opportunity 



