Centrifugal control of the olfactory bulb as revealed by induced DC potential changes

548

BRAIN RESEARCH

C E N T R I F U G A L C O N T R O L OF T H E O L F A C T O R Y BULB AS R E V E A L E D BY I N D U C E D DC P O T E N T I A L C H A N G E S

MIRKO CARRERAS*, DOMENICO MANCIA* AND MAURO MANCIA** Clinica delle Malattie Nervose e Mentali della Universit~ di Parma, lstituto di Fisiologia Umana dell' Un iversitti di Milano, ed lmpresa di Elettrofis iologia del C. N. R., Sezion i di Milano e Parma, Milan and Parma (Italy)

(Accepted May 23rd, 1967)

INTRODUCTION The D C recording technique has been extensively employed in the study of brain physiology. Several papers have appeared dealing with slow potential changes recorded from the cerebral cortex following sensory 6,33,~3, reticular6,3~,as,45, 53, thalamic 11,al, cortical 32 and cerebellar 2a stimulations, and during the sleep-wakefulness cycle 17,34,~2,56 and under various other experimental conditions 44. DC shifts have also been observed in the hypothalamus in response to thermal 26 and acoustic stimuli 27, as well as in relation to the feeding mechanism z, during the sleep-wakefulness cycle 35 and following midbrain reticular stimulation 34. The aim of this research has been to study with the aid of DC recording techniques, any slow potential change induced on a relatively simple and relatively wellknown structure such as the olfactory bulb 3,12,2° by electrical stimulation of the contralateral bulb, rhinencephalic areas, midline thalamus and midbrain reticular formation. Although the histological evidence for centrifugal fibers leading to the olfactory bulb from the latter central structures are rather scanty in the literature 21,46,47,49 (see also refs. 39 and 40) several lines of investigation indicate, that a functional control on the olfactory bulb, may indeed be exerted by several brain regions40,sL The results of this investigation have already been presented as preliminary notes14, l~ and to an international symposium la. METHODS The experiments were performed in enc6phale isol6 cats. The animals were cannulated under ether and the spinal cord was transected at C2; they were then curarized and artificially ventilated. The periorbital region was infiltrated with pro* Present address: Clinica delle Malattie Nervose e Mentali, Via del Quartiere 4, Parma. ** Present address: lstituto di Fisiologia Umana, Via Mangiagalli 32, Milan. Brain Research, 6 (1967) 548-560

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caine and the eye enucleated so that the medial wall of the orbital cavity was widely exposed. This was drilled and the olfactory bulb exposed without damage. Recording was carried out by means of a unipolar electrode consisting of a helix of silver chloride wire contained in a glass vessel which was in contact with the bulb surface through a bridge of cotton wicks soaked in Ringer solution. The reference electrode was buried under the skin over the temporal muscle of the same side as that on which the active lead was placed. The electrodes, essentially isopotential and free from drift, were connected to a DC preamplifier with a very small spontaneous drift which did not disturb the recording conditions. In some experiments, the same system was used to record the cortical potentials. A stimulating bipolar concentric electrode was placed visually into the contralateral olfactory bulb and stereotaxically in midbrain and thalamic regions. Bipolar silver electrodes were used to stimulate the prepiriform cortex of the same side as that on which the olfactory bulb was recorded. A similar type of electrode was used to stimulate the mucosa of the gum. The olfactory mucosa was stimulated with the aid of two polythene cannulae inserted in the nostrils. Care

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Fig. 1. Surface DC change induced in one bulb by the stimulation of the contralateral one with stimuli of 0.2 msec, 4 V, at various frequencies. Note the slowly rising negative shift for low-frequency stimulation and an abrupt change which is not sustained as high-frequency stimulation is maintained. In this and following figures, the period of stimulation is shown by the black line and negativity is indicated as an upward deflection. Brain Research, 6 (1967) 548-560

M. CARRERAS et al.

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Fig. 2. Surface negative changes induced in one bulb by stimulation of the contralateral one with stimuli of 0.2 msec, 100/sec, at various intensities. Note the negative aftereffect for high-intensity stimulation. was taken to maintain the conditions of the animal as steady as possible during the experiment. The activity of the bulb was regularly tested by recording the Adrian's induced waves I which followed the olfactory stimulation. Control experiments were performed in which the primary olfactory cortices were acutely ablated or inwhich the lateral olfactory tract had been chronically transected several weeks beforehand. At the end of each experiment the brain was removed, fixed in formalin and the tracks of the stimulating electrodes were histologically checked by Nissl's technique. RESULTS

Slow potential changes induced by stimulation oJ the contralateral bulb Low-frequency stimulation (square pulses of 0.1-0.5 msec duration, 2-10 V) applied to the bulb of one side induced a negative shift on the surface of the contralateral bulb already at the frequency of 5-10 p/sec (Fig. 1). This change slowly increased, progressively reaching maximal values of 150-200 ffV a few seconds after

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Fig. 3. Fast and slow potential changes induced in the olfactory bulb by single shocks (0.1 msec at various intensities) to ipsilateral prepiriform cortex in a cat with chronic sectioning of the LOT on the same side. Note positive-negative potentials superimposed on a sustained negative shift for highintensity stimulation. the beginning of stimulation. The baseline returned to prestimulus level in I - 2 sec after the stimulus was over. A similar shift followed stimulation at 15-30/sec, but showed a greater amplitude. High-frequency stimulation (50-300/sec) yielded an abrupt and pronounced negative change which reached its maximal values (0.2-0.5 mV) at the very beginning of stimulation (Fig. 1). This change, however, was not sustained but declined towards the prestimulus level as the stimulation was maintained (Fig. 1). These high-frequency induced shifts increased with the intensity of the stimulation (Fig. 2). When the high-intensity stimulation had ceased, a negative aftereffect could be seen (Figs. 1 and 2). The D C variations induced in one bulb by stimulation of the contralateral one disappeared following the acute sectioning of the anterior commissure.

Slow potential changes induced by stimulation oJ the ipsilateral prepiri[orm cortex and lateral olJactory tract (LOT) In cats with an acute or shock stimulation (0.1 msec, positive-negative potential on At supraliminal intensity the

chronic sectioning of the lateral olfactory tract, single 2-8 V) of the prepiriform cortex induced an evoked the surface of the ipsilateral olfactory bulb (Fig. 3). negative wave was superimposed on a slow negative

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10o,s.c Fig. 4. Continuing surface negative changes induced in one bulb by stimulation (5 V, 0.1 msec at different intensities) of the ipsilateral prepiriform cortex after the LOT was chronically cut on the same side. Note increase in amplitude of the shift as the frequency of stimulation was raised and the tendency to decline when the high-frequency stimulation was maintained.

potential change which had a time course from 2 to 4 sec (Fig. 3). Stimulation at low frequency (10-15/sec) evoked potentials at the same frequency, appearing on a negative shift which grew slowly, reaching a magnitude of 0.3-0.5 mV (Fig. 4). D C changes o f a greater amplitude occurred when the prepiriform cortex was stimulated at a higher frequency (Fig. 4). In the latter case, the shift was induced abruptly but was not sustained t h r o u g h o u t the period of stimulation (Fig. 4). The potential variations induced by stimulation of the prepiriform cortex were abolished when complete acute sectioning of the olfactory peduncle was carried out. In cats with chronic L O T sectioning, in which all possible L O T centrifugal fibers to the bulb 46 were presumably degenerated, single shocks to the severed end o f the L O T connected to the bulb, induced a fast positive-negative deflection similar to that reported by Von Baumgarten et al. s. D C changes were never seen following this negative deflection, even under low-frequency L O T stimulation (Fig. 5). A small negative shift, which did not outlast the stimulus, could be found only after activation of the L O T at higher frequencies (Fig. 5). Brain Research, 6 (1967) 548-560

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Fig. 5. Surface potentials evoked in the olfactory bulb by stimulation of the peripheral stump of the LOT which was chronically sectioned 2 weeks before. Stimulation used was 0.1 msec, 4 V at different frequencies. Note small shift only for high-frequency stimulation.

Slow potential changes induced by thalamic, midbrain and peripheral stimulations Low-frequency stimulation (8-10/sec, 0.1-1 msec, 2-8 V) of midline thalamic nuclei capable of evoking cortical recruiting potentials ~2 could also induce recruiting responses in both olfactory bulbs, similar to those reported by Arduini and Moruzzi 7. These waves had a shorter latency as compared with cortical recruitings, were negative in polarity and increased slowly to reach their full extent after 4-6 stimuli. A waxing and waning of their amplitude was observed. Fig. 6 shows in A that the bulbar recruitings were superimposed upon a sustained negative shift which increased gradually, reaching a maximal value of 0.3-0.6 mV only aftersome seconds of stimulation and then returned to the prestimulus level in 0.5 sec. In the same figure, it can be seen that high-frequency stimulation of the same thalamic loci elicited a negative change which abruptly reached the maximal value and declined slightly as the stimulus was maintained. Neither high- nor low-frequency thalamic shifts were

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Fig. 6. Recruiting and slow potential changes induced in one bulb by low- (10/sec) and high- (300/sec) frequency stimulation of midline thalamic nuclei before (A) and after (B) bilateral ablation of frontal cortices. Note in A and B the slowly rising shift for low-frequency,and the abrupt change for highfrequency stimulation. Both recruiting and slow changes disappeared after sectioning of the ipsilateral olfactory peduncle (C). influenced by acute ablation of the frontal cortex (Fig. 6B) or by sectioning of the LOT, but disappeared when the ipsilateral olfactory peduncle was completely severed (Fig. 6C). Acute bilateral removal of the primary olfactory cortex completely abolished both the recruiting and the slow potentials which followed thalamic stimulation (Fig. 8C). When acute ablation was limited to the contralateral olfactory areas, fast and slow potential changes were only reduced (Fig. 8B). High-frequency stimulation (300/sec, 0.5 msec, 2-8 V) of the midbrain reticular formation induced a sustained negative shift of 0.2-0.4 mV amplitude on both olfactory bulbs (Figs. 7A and E) simultaneously with a generalized EEG arousal and a negative slow potential change on the sensorimotor cortex, similar to that originally described by Arduini et al. 6. The bulbar potentials soon reached their maximal values as the stimulus was applied and returned to the prestimulus level within about 0.5-1 sec after the end of reticular activation (Fig. 7A). The shift was still present after acute suction of frontal cortices (Fig. 7B) and sectioning of the ipsilateral LOT, but disappeared when the olfactory peduncle was completely severed (Fig. 7C). This latter procedure did not alter the conditions of the bulb as revealed by the presence of Adrian's induced waves 1, following stimulation of the olfactory mucosa. A negative shift comparable to that observed during reticular activation was produced in the bulb by electrical stimulation (300/sec, 0.5 msec, 2-6 V) of the oral Brain Research, 6 (1967) 548-560

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Fig. 7. D C surface negative changes induced in the olfactory bulb by high-frequency stimulation (300/sec, 0.5 msec, 3 V) of midbrain reticular formation before (A) and after (B) bilateral ablation of frontal cortices. The effect disappears after acute sectioning of the ipsilateral olfactory peduncle (C). D and E, DC changes in the olfactory bulb of another cat during high-frequency stimulation of the oral cavity mucosa (D) and midbrain tegmentum (E). Note the similarity of these two changes, although the reticular one is more sustained.

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Fig. 8. Effect of contralateral (B) and bilateral (C) acute ablation of primary olfactory cortices on the recruiting and slow potential changes induced in the bulb by 10/sec stimulation of midline thalamic nuclei (A). D, E, F show the effect of contralateral (E) and bilateral (F) ablation of primary olfactory cortices on slow potential changes induced in the bulb of another cat by the high-frequency stimulation of the midbrain reticular formation (D). Note the enhancement of shift amplitude in E and an additional increase when the olfactory areas are bilaterally removed in F.

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cavity mucosa (Fig. 7D). Similar changes, though of lower amplitude, could follow natural stimuli, e.g. whistling. Acute removal of the contralateral primary olfactory cortex could have had an enhancing effect on the amplitude of the bulbar shift following reticular stimulation (Fig. 8E). This DC change was definitely increased by the additional ablation of the ipsilateral olfactory area (Fig. 8F). In the same Fig. 8F, it can be seen that in these latter conditions the spontaneous intrinsic activity of the bulb appeared somewhat reduced. DISCUSSION

The results reported here indicate that slow potential changes can be induced in the surface of the olfactory bulb by activation of various brain regions and by sensory stimulation. Although the hypothesis of an extraneuronic origin of the DC change in the brain cannot be discarded with certainty 44, several lines of evidence48,44,~4 point to a neuronic origin of the phenomenon. The negative shift we observed in the bulb is probably due to electrical events occurring within the bulb, since it was not influenced by ablation of frontal cortices but disappeared after the olfactory peduncle was severed (or the anterior commissure in the case in which the contralateral bulb was stimulated). An observation which deserves some attention is the absence, or the very low amplitude, of potential changes following antidromic stimulation of the LOT, whilst electrical activation of the prepiriform cortex and the contralateral bulb invariably induced a sustained and high-amplitude DC potential shift. Although in the latter case the possibility remained that antidromic fibers were also excited by the stimulus, the comparison between the antidromic LOT effects and those which followed activation of the contralateral bulb and the prepiriform cortex suggests that the latter may influence the bulb essentially through an orthodromic pathway. Similar conclusions can be drawn from the changes in DC potential induced in the bulb by highand low-frequency stimulation of midline thalamic structures. Such a shift seems to be comparable in shape and time course to that observed in the frontal cortices following similar midline thalamic stimulations 11,31. The bulbar changes, observed in the case in which the thalamus was stimulated, disappeared however, after the bilateral removal of the olfactory cortices, an effect which indicates a paleocortical relay for the thalamic effect. When the development of the slow potential changes induced in one bulb by stimulation of the contralateral bulb, the prepiriform cortex and the midline thalamus is examined, it seems that the smooth shift is built up in the bulb by a progressive summation of negative aftereffects through a mechanism already described for the cerebral cortexal, 44. A sustained negative change has also been reported by us following midbrain reticular stimulation. This effect suggests a centrifugal control exerted by the brain stem core on olfactory neurones in confirmation of results obtained by micro -41 and macrophysiologica157 experiments 4°. The reticular shift was unchanged or even increased after the bilateral removal of primary olfactory cortices, indicating that, whilst thalamic control over the olfactory bulb passes through Brain Research, 6 (1967) 548-560

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POTENTIALS

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a paleocortical relay, reticular control does not. However, such an effect might be mediated by a polysynaptic pathway as suggested by histological findings which do not indicate the existence of direct fibers leading from some subcortical structures to the bulb 5,47,49. The observation that the shift induced by reticular stimulation can be increased by ablation of olfactory cortices seems interesting. Whilst this effect would imply a functional relationship between the centrifugal action of the two structures on the bulb, no data permitting any conclusion on this point are at present available. The similarity between the reticular shift and that obtained by electrical or natural stimulation of sensory receptors suggests that the latter might be mediated through the brain stem reticular system. As to the origin, nature, and meaning of slow events recorded in the bulb following activation of various brain structures, our results may be discussed by comparison with the cerebral cortex where slow potentials of similar types have been most intensively studied. DC negative shifts have been related to graded depolarization of apical dendritesg,l°,16,18,19, 44 or to postsynaptic events of a mainly depolarizing nature~S,30,4z,44, 51. However, presynaptic depolarizationoffibersfrominterneurones in the cortex may well explain the DC shift recorded from the cortical surfaceZ4,4a, 44 in analogy with what has been found in the spinal cord 25. Other evidence indicates that slow surface negative potentials can be related to stop firing of spontaneously discharging neurones29,a7,as,50, 51 or to hyperpolarization of the postsynaptic membrane of underlying cells2, 48. As to the negative shift induced in one bulb by stimulation of the contralateral one, the hypothesis that at least this slow event is associated with some inhibitory event within the bulb itself is rather suggestive. An interruption of the spontaneous firing of an extremely high number of mitral and tufted neurones s as well as postsynaptic hyperpolarizations 58 have been shown to occur in the bulb following the stimulation of the contralateral one. A similar interpretation might be advanced to explain the negative shift produced by the stimulation of the prepiriform cortex, which is known to exert a suppressing action on Adrian's induced waves 4,36. Experimental data are still too scanty to allow a discussion o f the electrical events which underlie the slow potential changes induced in the bulb by stimulation of midline thalamus and midbrain structures. The findings at present available in the literature 4°,41 indicate, however, that the control exerted by mesencephalic regions on bulbar units is mainly deactivating in character. SUMMARY

In enc6phale isol6 and curarized cats surface negative DC changes were found in one bulb following stimulation of the contralateral one, the ipsilateral prepiriform cortex, midline thalamic nuclei and midbrain reticular formation. Similar changes were induced by electrical activation of oral cavity mucosa and by natural stimuli, e.g. whistling. Sectioning of the anterior commissure abolished the shift induced by stimulation of the contralateral bulb. Cutting of the ipsilateral LOT did not influence the changes induced by stimulation of the prepiriform cortex. The bilateral removal of primary Brain Research, 6 (1967) 548-560

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olfactory cortices completely abolished both recruiting and slow potential changes induced by thalamic stimulation, whilst having a n e n h a n c i n g effect on the negative shift which followed reticular activation. The findings suggest that the olfactory bulb is u n d e r a centrifugal control from several b r a i n regions a n d heterosensory structures. Whilst the thalamic influences require the integrity of the p r i m a r y olfactory cortex, the reticular control is p r e s u m a b l y exerted t h r o u g h a different pathway. ACKNOWLEDGEMENT This investigation was supported by funds from C.N.R., Italy.

REFERENCES 1 ADRIAN,E. D., Olfactory reactions in the brain of the hedgehog, J. Physiol. (Lond.), 100 (1942) 459--473. 2 ALADJALOVA,N. m., Slow Electrical Processes in the Brain. Progress in Brain Research. Vol. 7, Elsevier, Amsterdam, 1964, 243 pp. 3 ALLISON,A. C., The morphology of the olfactory system in the vertebrates, Biol. Rev., 28 (1953) 195-244.

4 ANGELERI, F., E CARRERAS, M., Problemi di fisiologia dell'olfatto. I. Studio elettrofisiologico delle vie centrifughe di origine paleocorticale, Riv. Neurobiol., 2 (1956) 255-274. 5 ANGELERI,F., CARRERAS,M., E MACCHI, G., Sulle connessioni delle aree rinencefaliche, Riv. Neurobiol., 2 (1956) 119-131. 6 ARDUIN1,A., MANCIA, M., AND MECHELSE, K., Slow potential changes elicited in the cerebral cortex by sensory and reticular stimulation, Arch. ital. Biol., 95 (1957) 127-138. 7 ARDUINI, A., AND MORUZZI, G., Sensory and thalamic synchronization in the olfactory bulb, Electroenceph. clin. Neurophysiol., 5 (1953)235-242. 8 BAUMGARTEN,R. VON, GREEN, J'. D., AND MANCIA, M., Recurrent inhibition in the olfactory bulb. II. Effects of antidromic stimulation of commissural fibers, J. Neurophysiol., 25 (1962) 489- 500. 9 BISHOP, G. H., AND CLARE, M. H., Responses of cortex to direct electrical stimuli applied at different depths, J. Neurophysiol., 16 (1953) 1-19. 10 BISHOP, G. H., AND CLARE, M. H., Facilitation and recruitment in dendrites, Electroenceph. clin. Neurophysiol., 7 (1955)486--489. 11 BROOKrtART,J. M., ARDUINI,A., MANCIA,M., AND MORUZZI,G., Thalamo-cortical relations as revealed by induced slow potential changes, J. Neurophysiol., 21 (1958) 499-525. 12 CAJAL, R. S., Histologie du Systdme Nerveux de I'Homrne et des Vert#br#s, Vol. H, Maloine, Paris, 1911, reprinted by Consejo Superior de lnvestigaciones Cientificas, Madrid, 1955. 13 CARRERAS,M., MANCIA,D., AND MANCIA, M., DC potential changes induced in the olfactory bulb by central and peripheral stimuli. In T. HAYASHI(Ed.), Olfaction and Taste, Pergamon, Oxford, 1967, pp. 181-191. 14 CARRERAS,M., MANCIA,D., MANCIA,M., E AVANZINI,G., Variazioni lente di potenziale indotte nel bulbo olfattivo da stimoli centrali e periferici, Atti Acc. Naz. Lincei, CI. Sci.fis. mat. nat., Serie VIII, 39 (I 965) 325-328. 15 CARRERAS,M., MANCIA, D., MANCIA, M., E AVANZINI,G., Oscillazioni lente di potenziale osservate nel bulbo olfattivo per effetto di stimoli centrali e periferici, Boll. Soc. ital. BioL sper., 42 (1966) 1831-1832. 16 CASPERS, H., Ober die Beziehungen zwischen Dendritenpotential und Gleichspannung an der Hirnrinde, pfliigers Arch. ges. Physiol., 269 (1959) 157-181. 17 CASPERS, H., Shifts of the cortical steady potential during various stages of sleep. In Aspects Anatomo-fonctionnels de la Physiologie des Etats de Sommeil, Lyon, 1963, C.N.R.S., Paris, 1965, pp. 213-229. Brain Research, 6 (1967) 548-560

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CLARE, M. H., AND BISHOP, G. H., Properties of dendrites: apical dendrites of the cat cortex, Electroenceph. clin. Neurophysiol., 7 0955) 85-98. 19 CLARE, M. H., AND BISHOP, G. H., Potential wave mechanism in cat cortex, Electroenceph. clin. Neurophysiol., 8 (1956) 583-602. 20 CLARK, W. E. LE GROS, Inquiries into the anatomical basis of olfactory discrimination, Proc. roy. Soc. B, 146 (1957) 299-319. 21 CRAG6, B. G., Centrifugal fibres to the retina and olfactory bulb, and composition of the supraoptic commissures in the rabbit, Exp. Neurol., 5 (1962) 406-427. 22 DEMPSEY,E. W., AND MORISON,R. S.,The production of rhythmically recurrent cortical potentials after localized thalamic stimulation, ,4met. J. Physiol., 135 (1942) 293-300. 23 DONDEY, M., AND SNIDER, R. S., Slow potential shifts following cerebellar stimulation, Electroenceph, clin. NeurophysioL, 7 (1955) 265- 272. 24 ECCLES, J. C., Discussion. In G. MoRuzzI, A. FESSARDAND H. H. JASPER (Eds.), Brain Mechanisms. Progress in Brain Research, Vol. l, Elsevier, Amsterdam, 1963, pp. 263-265. 25 ECCLES,J. C., The Physiology of Synapses, Springer, Berlin, 1964, XI, 316 pp. 26 EULER, C. VON, Slow 'temperature potentials' in the hypothalamus, J. cell. comp. Physiol., 36 (1950) 333-350. 27 EULER, C. VON, A preliminary note on slow hypothalamic 'osmo-potentials', ,4cta physiol, scand., 29 (1953) 133-136. 28 EULER, C. VON, GREEN, J. D., AND RICCI, G., The role of hippocampal dendrite in evoked responses and after discharge, ,4ctaphysiol. scand., 42 (1958) 87-111. 29 FROMM, G. H., AND BOND, H. W., The relationship between neuron activity and cortical steady potentials, Electroenceph. clin. NeurophysioL, 22 (1967) 159-166. 30 GLOOR, P., SPERTI, L., AND VERA, C., An analysis of hippocampal evoked responses and seizure discharges with extracellular microelectrode and DC-recordings. In Physiologie de l'Hippocampe, C.N.R.S., Paris, 1962, pp. 147-170. 31 GOLDRING,S., AND O'LEARY, J. L., Cortical DC changes incident to midline thalamic stimulation, Electroenceph. clin. Neurophysiol., 9 (1957) 577-584. 32 GOLDRIN~, S., O'LEARV, J. L., HOLMES,T., AND JERVA, M. J., Direct response of isolated cerebral cortex of cat, J. Neurophysiol., 24 (1961) 633-650. 33 GUNMIT, R. J., D.C. potential changes from auditory cortex of cat, J. Neurophysiol.,23 (1960) 667-675. 34 HAYWARD, J. N., FAIRCHILD, M. D., AND STtJART, D. G., Hypothalamic and cortical D.C. potential changes induced by stimulation of the midbrain reticular formation, Exp. Brain Res., 1 (1966) 205-219. 35 KAWAMtJRA,H., AND SAWYER, C. H., D.C. potential changes in rabbit brain during slow wave and paradoxical sleep, Amer. J. Physiol., 207 (1964) 1379-1386. 36 KERR, D. I. B., AND HAGBARTH,K. E., An investigation of olfactory centrifugal system, J. Neurophysiol., 18 (1955) 362-374. 37 LI, C. L., AND CHOU, S. N., Cortical intracellular synaptic potentials and direct cortical stimulation, J. cell. comp. Physiol., 60 (1962) 1-16. 38 LI, C. L., AND SALr~OIRA~HI, G. C., Cortical steady potential changes: extracellular microelectrodes investigations, Nature (Lond.), 198 (1963) 858-859. 39 LOHMAN,A. H. M., AND LAMMERS,n. J., On the structure and fibre connections of the olfactory centres in mammals. In Y. ZOTTERMAN(Ed.), Sensory Mechanisms, Progress in Brain Research, Vol. 23, Elsevier, Amsterdam, 1967, pp. 65- 82. 40 MANCIA, M., Specific and unspecific influences on the olfactory bulb, Electroenceph. clin. Neurophysiol., Suppl. 24 (1963) 1-12. 41 MANCIA, M., GREEN, J. D., AND BAUMGARTEN,R. VON, Reticular control of single neurons in the olfactory bulb, Arch. ital. Biol., 100 (1962) 463-475. 42 MARSHALL,W. H., Spreading cortical depression of Le~o, Physiol. Rev., 39 (1959) 239-279. 43 O'LEARY, .J.L., Brief survey of direct current potentials of the cortex. In G. MORUZZI, A. FESSARD AND H. H. JASPER (Eds.), Brain Mechanisms, Progress in Brain Research, Vol. 1, Elsevier, Amsterdam, 1963, pp. 259-271. 44 O'LEARY, J. L., AND GOLDRING, S., D.C. potentials of the brain, Physiol. Rev., 44 (1964)91-125. 45 O'LEARY, .l'. L., KERR, F. W. L., AND GOLDRING, S., The relation between spino-reticular and ascending cephalic systems. In H. H. JASPER,L. D. PROCTOR, R. S. KNIGHTON,W. C. NOSHAYAND R. T. COSTELLO(Eds.), Reticular Formation of the Brain, Little, Brown and Co., Boston, Mass., 1958, pp. 187-202.

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