238 CHEMICAL SENSES 



Motion picture films were made of the behavior of the experimental 

 animals, simultaneously with the EEG recordings before, during, and after 

 chemical stimulation. This made it possible to categorize behavioral re- 

 sponses as positive (orienting upstream, toward source of the stimulant) 

 or negative (orienting downstream, away from source of the stimulant), 

 and to separate and compare the evoked EEG potentials from behavioral 

 effects. It is useful to examine one example from the experimental records 

 in further detail, to appreciate how these EEG and behavioral responses 

 were coordinated. 



A typical EEG example is shown in Figure 5, a record made during stimu- 

 lation of a nurse shark with betaine. The upper trace indicates when the 

 stimulus could be first detected in the center of the observation tank. The 

 middle trace is from the shark's medulla, showing the neural activity asso- 

 ciated with respiratory movements of the gills. The lower trace records 

 potentials between the No. 1 and No. 2 surface recording positions on the 

 telencephalon. The horizontal time marker represents 1 s, the vertical cali- 

 bration indicates 50 uV. 



During the approximately 11 s of this sample record, changes in the 

 patterns of potentials from both forebrain and medulla start almost syn- 

 chronously, before the lead edge of the bolus of stimulus even reaches the 

 center of the observation tank. There is an increase in amplitude and fre- 

 quency of cycles in the forebrain, and an extra gill beat triggered from the 

 medulla. Motion picture records show that immediately after the extra 

 gill beat, the gills are closed for a few seconds; it is during that period, when 

 the coverings of the gills are smoothly pressed against the side of the body, 

 that the shark begins to swim forward, toward the source of the chemical 

 stimulation. 



The combined EEG and photographic records give a clearly integrated 

 picture of the beginnings of a positive reaction to a chemical stimulus. Very 

 quickly after the afferent olfactory signal arrives in the forebrain, extra 

 oxygen is taken in as a result of modified respiratory rhythms from the 

 medulla. Gills are closed for an interval, and during that period of maximum 

 body streamlining the swimming reflexes are activated, propelling the shark 

 forward toward the source of stimulation. Within a few seconds, the normal 

 cycle of gill beats resumes, and the shark continues swimming upstream. 



In some cases, recordings from the surface of the olfactory bulb could 



STIMULUS .001 BETAINE 



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Figure 5 EEG record of reaction of free-swimming nurse shark to stimulation with 

 betaine. See explanation in text. 



