202 VISION 



Clark (1959) was the first to argue against this belief by demonstrating that 

 sharks can be instrumentally conditioned to respond to visual cues (i.e., 

 bump a target). Several subsequent studies (Aronson et al. 1967, Clark 1961, 

 Graeber and Ebbesson 1972a, Tester and Kato 1966) have confirmed her 

 basic finding and have extended the results to include discrimination learning. 

 However, caution should be exercised in interpreting some of these later re- 

 ports. In particular, Clark (1961) trained lemon sharks on color and shape 

 discrimination tasks but did not control adequately for brightness differences, 

 left— right target positions, or transfer effects among the five target pairs. 

 Aronson and his colleagues (1967) reported some intriguing results based on 

 operant light— dark discrimination, but they tested only one animal. Never- 

 theless, the feasibility of using instrumental visual discrimination as a tool 

 for the neurological testing of elasmobranchs has been adequately demon- 

 strated. Sharks can at least learn light— dark and simple form discriminations, 

 including upright vs inverted triangles (Graeber 1972), without much dif- 

 ficulty. Whether they can also learn to discriminate hue and more complex 

 visual patterns remains to be seen. 



MIDBRAIN VISUAL MECHANISMS 

 The "Dominant" Tectum 



The mesencephalic optic tectum traditionally has been considered the 

 predominant, if not sole, recipient of retinal afferents in both teleosts and 

 elasmobranchs. Consequently, it is often referred to as "the visual center" 

 for all fish (Aronson 1963, Healey 1957, Ten Cate 1935). 



The results of previous behavioral studies supported this concept and led 

 Aronson (1963, p. 205) to state that "Most investigators agree that removal 

 of the optic lobes causes blindness in both elasmobranchs and teleosts." 

 Upon closer examination, however, the empirical evidence for sharks is 

 extremely limited compared to that for teleosts. Steiner (1888, cited by 

 Ten Cate 1935), using general observations, was the first to report that blind- 

 ness follows removal of the optic lobes in sharks. Since then only Polimanti 

 (1913) and Rizzolo (1929) have reported similar results. 



The behavior Polimanti attributed to blindness in Scyllium may not have 

 resulted from the removal of the tectal visual center, but rather from the 

 severe oculomotor, locomotor (forced circling), and pupillary deficits, which 

 he also reported. The latter effects suggest that the surgical damage extended 

 below the tectum into the underlying tegmentum. The most complete des- 

 cription of blindness is that of Rizzolo, who stated that subjects (Galeus 

 canis) with bilateral optic lobe ablations failed to avoid the sides of the 

 aquarium. The validity of his conclusion is also questionable, however, in that 

 all of the operated sharks died within two or three days after the operation. 

 This suggests that the "blindness" was due not to the tectal lesions per se but 

 to other factors, such as possible ionic imbalance caused by incomplete pro- 

 tection of the wound. 



