PHYSIOLOGY OF CHEMORECEPTION 233 



olfactory bulbs and secondary projection areas have a quite uniform and 

 conservative structure and circuitry in all vertebrate groups. Consequently, 

 it is reasonable to use comparative data when observations are lacking on 

 a particular species. This does not imply, of course, that the effective 

 chemical stimuli are the same or that the ultimate behavioral responses 

 are similar throughout the elasmobranchs and other vertebrates. 



Limitations of Techniques 



Ideally, experimentation on the chemical senses of any species would span 

 the whole range of phenomena, from primary receptor cell recordings to 

 the behavior of freely moving animals. Practically, experimenters always 

 have to settle for only part of this range. With elasmobranchs, the closest 

 it has been possible to come to recording of initial excitatory states are 

 EEG-type records from olfactory bulbs. Such records have notable limita- 

 tions: (1) they are not from the actual chemosensory cells, but from second- 

 order neurons, and (2) the EEG cannot discriminate between stimuli that 

 evoke very different types of behavioral responses. Therefore, it is abso- 

 lutely necessary to return to behavioral studies, or to carry out the electro- 

 physiological and behavioral studies in parallel, if the EEGs are to be 

 interpreted in terms of the positive or negative effects of chemical stimuli. 



A striking illustration of this need was recently reported from studies 

 on salmon (Oncorhynchus). A chemical stimulus (1-serine) washed off 

 mammalian skin has been shown to be strongly aversive to sockeye salmon 

 (Idler, Fagerlund, and May oh 1956); it elicits EEG responses as large as 

 or larger than the home stream waters, to which the salmon orient very 

 strongly (Bodznick 1975). State of sexual maturity and recent experience 

 of the fish have also been suggested as influences on EEG size and changes 

 (Oshima et al. 1973, Bodznick 1975). It has become quite clear that EEG 

 size is affected by some factors other than odor qualities of stimulus waters, 

 leading some investigators to claim that there is ". . . an inherent lack of 

 precision associated with evoked bulbar recordings" (Dizon, Horrall, and 

 Hasler 1973). Some of this variability may be accounted for by the known 

 integration of olfactory EEGs with visual, auditory, and gustatory senses 

 (Harada and Takagi 1961), which further emphasizes the need for carefully 

 controlled behavioral studies to accompany electrophysiological experi- 

 ments involving any kind of EEG recordings. 



In our own studies, detailed below, this necessity for combining electro- 

 physiological and behavioral analyses has been applied to most of the work 

 with elasmobranchs. Few chemical stimuli, among the wide variety tested, 

 are without some effects on the sharks tested, at least on initial exposure. 

 The effects may show up in EEG recordings, or they may consist of be- 

 havioral changes that indicate "awareness" of the stimulus by the shark. 

 The presence of EEG changes alone is not predictive of a shark's behavior. 

 The difficulties of a single approach can be largely overcome by combining 

 EEG studies with behavioral studies. 



