200 VISION 



Considering the rapidly growing interest in shark ethology (e.g., Allee and 

 Dickinson 1954, Myrberg and Gruber 1974), it is conceivable that more 

 detailed information will soon become available about the natural response 

 patterns sharks exhibit in reaction to specific visual stimuli. The work of 

 Nelson and his colleagues (e.g., Johnson and Nelson 1973) demonstrates, on 

 a relatively complex level, the potential contribution of this type of behav- 

 ioral analysis to future neurological studies. Studying the gray reef shark 

 (Carcharhinus menisorrah) in its natural surroundings, they have demon- 

 strated the importance of visual cues in eliciting a well-defined stereotyped 

 preattack behavior pattern in response to a diver's presence. By systematically 

 varying the relative position of diver to shark, they have also been able to 

 control the intensity of the agonistic display. It is not unreasonable to 

 assume that appropriate neurological studies of sharks in semicaptivity could 

 now be conducted to determine what central neural mechanisms process the 

 pertinent sensory information, visual or otherwise, and trigger the behavior. 



Likewise, Myrberg and Gruber (1974) have reported the triggering of less 

 complex, modal action patterns in captive young bonnethead sharks (Sphyrna 

 tiburo). They state that visual cues again appeared important, eliciting 

 stereotyped responses to companion sharks and other environmental stimuli. 

 Other studies, using chemical stimuli, indicate that once certain neural 

 mechanisms of behavior are triggered, sharks will automatically carry out 

 the corresponding behavioral response pattern to completion, even in the 

 absence of the initial stimulus (Hodgson and Mathewson, 1971; Hodgson, 

 Mathewson, and Carsten, personal communication). Unfortunately, the 

 central neural mechanisms responsible for these induced behavior patterns 

 have not yet been identified. Demski's (1977) recent attempt to apply brain 

 stimulation to freely swimming sharks is a significant step toward providing 

 such information. It is likely that other visually controlled innate response 

 patterns await discovery and, hopefully, subsequent exploitation by neuro- 

 physiological and neurophychological analysis. 



Learned Responses 



Classical Conditioning— In the mammalian literature, classical, or 

 respondent, conditioning is often used to determine whether an animal 

 suffering a neural loss can detect various types or characteristics of visual 

 stimulation. In the typical conditioning paradigm the animal is either re- 

 strained or partially paralyzed while receiving a conditioning stimulus paired 

 with a mild shock to produce an eyeblink response or a change in heart rate. 

 The passive nature of the test situation enables experimental separation of 

 sensory losses from motor-related losses that might otherwise impair the 

 animal's ability to translate perceived visual information into appropriate 

 voluntary instrumental behavior patterns. Training can usually be accom- 

 plished relatively quickly, while permitting very precise control over stimulus 

 and response characteristics. Despite these advantages, classical conditioning 

 procedures require substantial laboratory equipment and are limited to pro- 



