ELECTRICAL ACTIVITY IN THE OLFACTORY SYSTEM 77 



also varied, and when an odorant in medium or relatively high concentra- 

 tions evoked little activity during its presentation, a strong discharge 

 frequently followed several seconds later. At lower concentrations, 

 however, responses more often occurred within the duration of the stimulus. 

 In spite of these discrepancies, qualitative differences in the pattern of 

 summated response evoked by different odorants are apparent, and there 

 are marked variations in the discharges recorded simultaneously from 

 different points in the bulb (Fig. 7). 



Several attempts to stabilize the response were made. Of these, pre- 

 liminary results showed that pairing unavoidable shock with an odorant 

 to which the animal was habituated, resulted in an enhancement of wave 

 activity within the first 23 trials, but for a fixed stimulus, the relation of 

 the summated spike response to baseline remained inconsistent. A further 

 approach concerns the efferent fiber system. 



Spike Activity in the Bulb Following Unilateral Frontal Transection of 

 Anterior Olfactory Areas 



There is now extensive evidence that efferent fiber systems are at least 

 potentially capable of mediating central control of activity in the bulb; 

 and, indeed, it is difficult to see how the massive projection recently 

 described by Cragg can have no important function in olfaction. In 

 addition to fibers originating in rhinencephaUc or higher centers and 

 terminating in some cases as far down as the perimeter of the glomeruli, 

 there are also fine fibers interconnecting the bulbs. Although these may 

 be too few to have much functional significance, some interaction of 

 bulbar activity, possibly mediated in part by other pathways, may normally 

 occur. Thus Kerr has shown that strong olfactory stimulation of one 

 bulb can suppress afferent induced waves in the opposite bulb (Cajal, 

 1911; Adey, 1953; Cragg, 1962; Kerr and Hagbarth, 1955; Walsh, 

 1959 ; Kerr, 1960 ; Hernandez-Peon et al, 1960 ; Yamamoto and Iwama, 

 1961 ; von Baumgarten et aL 1962). However, the extra-bulbar efferent 

 pathways, particularly the autonomic supply to the nasal mucosa (Tucker 

 and Beidler, 1956), are capable of mediating powerful influences on 

 olfactory sensitivity which may account for some findings in this area. 



On the assumption that much of the variability of the bulb responses 

 in the unanaesthetized rabbit was related to the activity of bulbar efferent 

 fibers, the effect of unilaterally transecting anterior olfactory areas (includ- 

 ing the anterior commissure and prepyriform cortex) 5-9 mm posterior to 

 the caudal end of the olfactory bulb was investigated. The intention was 

 to sever all efferent fibers to one bulb and the interbulbar fibers to the other. 

 Cragg (1962) could find httle evidence of retrograde degeneration in the 

 bulb following lesions in this area. 



As Fig. 8 suggests, the effects of such a lesion were striking. On the side 



