198 VISION 



Recent Neuroanatomical Findings 



As recently as a decade ago there was little reason to question the traditional 

 view of shark neuroanatomy based on studies of normal shark brains. How- 

 ever, the situation suddenly changed when anatomists developed ways to 

 modify modern mammalian histological techniques to study neural degenera- 

 tion in poikilotherms (Ebbesson 1970). The new findings, as described in 

 this volume, have revolutionized our understanding of the structure and 

 organization of the shark brain. As a result, it is now known that only about 

 10% of the telencephalon is devoted to olfaction (Ebbesson 1972a, Ebbesson 

 and Heimer 1970). The functions of the remaining 90% are still unclear, but 

 there is no doubt that other sensory inputs are present (Cohen, Duff, and 

 Ebbesson 1973, Veselkin and Kovacevic 1973). Already extensive connec- 

 tions have been documented between the telencephalon and the thalamus, 

 optic tectum, and brainstem nuclei (Schroeder and Ebbesson 1974). At the 

 very least, these findings suggest that the neural basis of shark behavior is 

 much more complex than previously imagined. 



Unfortunately, understanding the functional organization of the shark 

 brain requires much more than a strictly anatomical blueprint of connecting 

 areas and pathways. Aronson recognized this need in his 1963 review of the 

 older neuroanatomical literature by pointing out that the behavioral evidence 

 used to interpret morphological findings was based mostly on unreliable 

 casual observations. It should now be apparent that only by a combination 

 of electrophysiological and systematic behavioral techniques will we be able to 

 determine what information these pathways transmit and how their terminal 

 areas interact. It is tempting to speculate that had these functional techniques 

 been more enthusiastically applied to sharks in the past, as they were to 

 mammals, it might not have taken over 50 years to reveal the errors of 

 traditional neuroanatomy. Yet the fruitfulness of the functional approach 

 has been recognized only recently. 



A series of studies concerning the shark central visual system has chal- 

 lenged the classic notion of visual processing being under exclusive tectal 

 control. Although electrophysiological data helped to clarify the functional 

 anatomy of this system, the emphasis in this chapter will be on the 

 complementary approach to functional analysis: the use of selective 

 neurological damage to experimentally manipulate visually mediated be- 

 havior. 



BEHAVIORAL ASSESSMENT OF VISUAL FUNCTION 



A variety of behavioral techniques have been used successfully since Lashley's 

 time to examine postoperative visual losses in mammals. The simplest of 

 these was first applied to sharks at the turn of the century, but investigators 

 have since been reluctant to employ other, more sophisticated, assess- 

 ment techniques. Much of the delay has resulted from the common view that 

 active sharks are ill suited to controlled laboratory research conditions and 

 are unable to learn standard behavioral tasks. A general consideration of the 



