PHYSIOLOGY OF CHEMORECEPTION 235 



lateral side, were located closest to the point of connection between the 

 olfactory tract and the telencephalon, either on the forebrain surface or at 

 a depth equivalent to the distance to the center of the olfactory tract. The 

 most effective stimuli tested in the initial experiments were extracts of tuna 

 meat, although extracts of crabs (normal food for bonnet sharks) and 

 "amine F" (an olfactory attractant for lampreys, fishes, and sharks, since 

 identified as iso-leucine-methylester (Kleerekoper 1978)) were nearly as 

 great. The exact polarities and frequencies of EEG changes, following chemi- 

 cal stimulation, depended on the species, recording sites, electrodes, time 

 relationships to initial stimulation, and several other factors in addition to 

 the chemical stimulus. Typical results are shown in Figure 3, which illus- 

 trates both the delay in onset after stimulation (latency) and the persistence, 

 at declining levels (adaptation) of the responses after initial maxima. 



Since no previous studies have been made of EEGs in this group of 

 animals, it was necessary to characterize the relevant patterns of electrical 

 activity in the forebrain and elsewhere. Spontaneous fluctuations of poten- 

 tial in the telencephalon, with a dominant frequency of 4 to 9 Hz, were 

 commonly recorded from the brain surface; both amplitudes and frequencies 

 of these potentials increased during chemical stimulation (Figure 3B). The 

 4- to 9-Hz rhythm is similar to one recorded from the telencephalon of 

 the goldfish (Schade and Weiler 1959); however, a second, more rapid 

 rhythm reported in the goldfish forebrain was not observed in the sharks. 

 Spindling was rare, and was usually associated with the pressure of surface 

 electrodes or movement of depth electrodes, as has been noted in EEG 

 studies on codfish (Enger 1957). 



Among the significant EEG patterns found in other areas of the shark 

 brain is a medullar pattern consisting of relatively large-amplitude potentials 

 (150 to 200 mV), correlated with gill movements. These potentials appear 

 to be associated with neural triggering of respiratory reflexes. Although the 

 medullar potentials were sporadic in the restrained curarized sharks while 

 they were maintained in the laboratory with seawater perfusion through the 

 gills, these particular EEG patterns were subsequently found to be quite 

 useful in recording responses to chemicals by unrestrained swimming sharks 

 (see below). 



To summarize, the initial studies of EEGs established several characteris- 

 tics of the shark brain responses during adequate chemical stimulation of 

 the olfactory sac: (1) the typical response was an increase in amplitude and 

 frequency of whatever spontaneous potentials could be recorded at a par- 

 ticular site, (2) such responses were given to extracts of normal food mate- 

 rials, as well as to several pure chemicals (amino acids and amines) present 

 in the normal food, and (3) latency and adaptation of the responses evoked 

 by sustained administrations of stimuli at constant levels were always 

 present. From the restrained and curarized sharks, nothing could be con- 

 cluded about the behavior that might normally have been associated with 

 these EEG responses. Consequently, the experimental approach was modi- 

 fied so that EEGs could be recorded from unrestrained free-swimming 

 sharks exposed to predetermined concentrations of pure chemical stimuli. 



