82 DAVID G. MOULTON 



DISCUSSION 

 Peripheral Activity 



The rich trigeminal and autonomic innervation of the nasal mucosa 

 makes it probable that the activity recorded from the non-olfactory 

 regions was largely derived from such fibers. However, since sites 

 especially responsive to odorants were located, some isolation of trigeminal 

 activity may be possible. In the case of recordings from electrodes 

 directed into the cribriform plate, a different source for the potentials 

 seems hkely. In this area the primary olfactory neurones form numerous 

 bundles coursing towards the bulb and they are easily accessible to elec- 

 trodes inserted from the bulbar side. The close correlation between the 

 amyl acetate stimulus-response curves for the bulbar and peripheral sites 

 (Fig. 10) and the nature of these peripheral discharges (Figs. 3 and 4), 

 indicates that contact with such bundles was probably made. It is also 

 significant that this site yielded responses to amyl acetate in concentrations 

 at least 1 log unit below the Hmit for trigeminal sensitivity to this compound 

 which Tucker (this symposium) has determined for the anaesthetized 

 rabbit. Indeed thresholds for bulbar spike discharges and the peripheral 

 activity are virtually identical (Fig. 10). 



Bulbar Activity 



The behaviour of the spike discharges in the lightly or recently anaesthe- 

 tized rabbit has been described by Adrian (1950), and Mozell and Pfaffmann 

 (1954). It appears to differ from that found in the present study mainly in 

 the degree of stability of the background discharge. In the most extreme 

 situation, such as is seen when the animal has been shocked, or the experi- 

 menter is visible (Fig. 5), bursts of activity may alternate with periods of 

 marked deactivation, in which few individual spikes appear above a con- 

 stricted baseline. These are quite distinct from the changes occurring with 

 each inspiration. It is tempting to speculate that this may be related to 

 rapid transfers of attention from one modality to another. Even in the 

 undisturbed animal apparently " spontaneous " shifts in the intensity of 

 this activity occur which maintain the baseline, sometimes for prolonged 

 periods, at higher or lower levels. The introduction of an odorant at any 

 suprathreshold concentration may initially supress this activity — with or 

 without eliciting spikes at each inspiration — or induced spikes may appear 

 to be superimposed on the resting discharge. This contrasts with the 

 peripheral discharges of the kind shown in Fig. 4, in which the response 

 to an odorant is generally an increase in activity above the baseline, which 

 remains relatively stable. 



The effect of lesions in the anterior olfactory areas is apparently to 

 shift the behaviour of the bulbar spike discharges to a condition more 

 closely approaching that seen at the periphery. Indeed, some bulbar sites 



