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VISION 



We have been investigating the limits of brightness discrimination in the 

 lemon shark. Again using operant techniques, we trained sharks with food 

 reinforcement to swim to the brighter of two doors illuminated by an array 

 of optical fibers connected to a light source (Figure 21). Under this test situ- 

 ation sharks successfully discriminated as long as the brightness of one door 

 exceeded that of the other by 0.3 log units. Using the identical stimuli, 

 human subjects under aerial viewing were slightly better than the sharks (by 

 0.1 log units) at choosing the brighter door. 



To our knowledge the only psychophysical studies in which visual thresh- 

 olds were measured and resulting visual parameters generated are those of 



Figure 21 Underwater photograph of Negaprion breuirostris at the choice point of a 

 single-choice maze. The shark chooses the brighter stimulus obtained by differentially 

 illuminating 3136 optical fibers whose ends are embedded in one of two guillotine doors. 

 The shark receives food reinforcement behind the correct door. (From Gruber, unpub- 

 lished experiments.) 



