Somatosensory cortical unit responses to stimulation of the vibrissae in cats

Characteristics of extra- and intracellular responses of 57 neurons in the vibrissal projection zone of the first somatosensory area of the cat cortex were investigated. The intensity of both excitatory and inhibitory unit responses was found to diminish during successive stimulation of different parts of the receptive fields in the direction from center toward periphery. Usually, when central parts of receptive fields were stimulated, inhibition in the unit responses was postexcitatory, whereas when peripheral parts were stimulated inhibition could precede excitation. The possibility of an increase in the role of interaction between excitatory and inhibitory processes arising in neurons in response to vibrissal stimulation with an increase in the distance from center to periphery of receptive fields of single cortical cells is discussed. Neurons found during one insertion of the microelectrode were seen to have common center for their receptive fields, but the diameters of the receptive fields of individual neurons could differ significantly. Moreover, during such vertical insertions responses of neurons with primary inhibition to the stimuli presented were recorded.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 12, No. 2, pp. 124–130, March–April, 1980.     

Physiology and morphology of projection neurons in the antennal lobe of the male mothManduca sexta

1. We have used intracellular recording and staining, followed by reconstruction from serial sections, to characterize the responses and structure of projection neurons (PNs) that link the antennal lobe (AL) to other regions of the brain of the male sphinx moth Manduca sexta. 2. Dendritic arborizations of the AL PNs were usually restricted either to ordinary glomeruli or to the male-specific macroglomerular complex (MGC) within the AL neuropil. Dendritic fields in the MGC appeared to belong to distinct partitions within the MGC. PNs innervating the ordinary glomeruli had arborizations in a single glomerulus (uniglomerular) or in more than one ordinary glomerulus of one AL (multiglomerular) or in one case, in single glomeruli in both ALs (bilateral-uniglomerular). One PN innervated the MGC and many or all ordinary glomeruli of the AL. 3. PNs with dendritic arborizations in the ordinary glomeruli and PNs associated with the MGC typically projected both to the calyces of the ipsilateral mushroom body and to the lateral protocerebrum, but some differences in the patterns of termination in those regions have been noted for the two classes of PNs. One PN conspicuously lacked branches in the calyces but did project to the lateral protocerebrum. The PN innervating the MGC and many ordinary glomeruli projected to the calyces of the ipsilateral mushroom body and the superior protocerebrum. 4. Crude sex-pheromone extracts excited all neurons with arborizations in the MGC, although some were inhibited by other odors. One P(MGC) was excited by crude sex-pheromone extract and by a mimic of one component of the pheromone blend but was inhibited by another component of the blend. 5. PNs with dendritic arborizations in ordinary glomeruli were excited or inhibited by certain non-pheromonal odors. Some of these PNs also responded to mechanosensory stimulation of the antennae. 6. The PN with dendritic arborizations in the MGC and many ordinary glomeruli was excited by crude sex-pheromone extracts and non-pheromonal odors and also responded to mechanosensory stimulation of the antenna.     

Receptive fields of second-order neurons in the olfactory bulb of the hamster

Electrical stimulation of nerve fibers emerging from different positions of the olfactory epithelium was used to determine the receptive fields for 52 olfactory bulb neurons in the hamster. The responses of olfactory bulb neurons were recorded extracellularly with metal-filled micropipettes. Suprathreshold stimuli (500 microA) were applied to each of eight standard epithelial positions spaced approximately 250 microns apart, and the position, or positions, which, when stimulated, produced a response in the bulb were taken as an index of the neuron's receptive field. The results indicate that most bulb neurons have very localized receptive fields limited to only one or two stimulating positions. Furthermore, there was a statistically significant correlation between the location of a neuron's receptive field in the olfactory epithelium and the recording depth of the neuron in the olfactory bulb (Spearman rank correlation coefficient, rs, 0.67, P < 0.001). These findings demonstrate that in the mammalian olfactory system there exists a topographical projection of input from localized regions in the epithelium onto the second-order neurons in the olfactory bulb.     

Pheromone-evoked potentials and oscillations in the antennal lobes of the sphinx moth Manduca sexta

Using intra- and extracellular recording methods, we studied the activity of pheromone-responsive projection neurons in the antennal lobe of the moth Manduca sexta. Intracellularly recorded responses of neurons to antennal stimulation with the pheromone blend characteristically included both inhibitory and excitatory stages of various strengths. To observe the activity of larger groups of neurons, we recorded responses extracellularly in the macroglomerular complex of the antennal lobe. The macroglomerular complex is part of a specialized olfactory subsystem and the site of first-order central processing of sex-pheromonal information. Odors such as the pheromone blend and host-plant (tobacco) volatiles gave rise to evoked potentials that were reproducible upon repeated antennal stimulation. Evoked potentials showed overriding high-frequency oscillations when the antenna was stimulated with the pheromone blend or with either one of the two key pheromone components. The frequency of the oscillations was in the range of 30–50 Hz. Amplitude and frequency of the oscillations varied during the response to pheromonal stimulation. Recording intracellular and extracellular activity simultaneously revealed phase-locking of action potentials to potential oscillations. The results suggest that the activity of neurons of the macroglomerular complex was temporally synchronized, potentially to strengthen the pheromone signal and to improve olfactory perception. Accepted: 19 December 1997     

Male-specific,sex pheromone-selective projection neurons in the antennal lobes of the mothManduca sexta

A subset of olfactory projection neurons in the brain of male Manduca sexta is described, and their role in sex pheromone information processing is examined. These neurons have extensive arborizations in the macroglomerular complex (MGC), a distinctive and sexually dimorphic area of neuropil in the antennal lobe (AL), to which the axons of two known classes of antennal pheromone receptors project. Each projection neuron sends an axon from the AL into the protocerebrum. Forty-one projection neurons were characterized according to their responses to electrical stimulation of the antennal nerve as well as olfactory stimulation of antennal receptors. All neurons exhibited strong selectivity for female sex pheromones. Other behaviorally relevant odors, such as plant volatiles, had no obvious effect on the activity of these neurons. Two broad physiological categories were found: cells that were excited by stimulation of the ipsilateral antenna with pheromones (29 out of 41), and cells that received a mixed input (inhibition and excitation) from pheromone pathways (12 out of 41). Of the cells in the first category, 13 out of 29 were equally excited in response to stimulation of the antenna with either the principal natural pheromone (bombykal) or a mimic of a second unidentified pheromone ('C-15') and were similarly excited by the natural pheromone blend. The remaining 16 out of 29 cells responded selectively, and in some cases, in a dose-dependent manner, to stimulation of the antenna with bombykal or C-15, but not both. Some of these neurons had dendritic arborizations restricted to only a portion of the MGC neuropil, whereas most had arborizations throughout the MGC. Of the cells in the second category, 9 out of 12 were excited by bombykal, inhibited by C-15, and showed a mixed response to the natural pheromone blend. For the other 3 out of 12 cells, the response polarity was reversed for the two chemically-identified odors. Two additional neurons, which were not tested with olfactory stimuli, were tonically inhibited in response to electrical stimulation of the ipsilateral antennal nerve. These observations suggest that some of the male-specific projection neurons may signal general pheromone-triggered arousal, whereas a smaller number can actively integrate inputs from the two know receptor classes (Bal- and C-15-selective) and may operate as 'mixture detectors' at this level of the olfactory subsystem that processes information about sex pheromones.     

Physiological responses and central nervous projections of antennal olfactory receptor neurons in the adult desert locust, Schistocerca gregaria (Orthoptera: Acrididae)

Olfactory receptor neurons present in two morphological sensillum types on the male Schistocerca gregaria antenna were for the first time investigated physiologically when stimulated with behaviourally relevant odours. Neurons present in trichoid/basiconic sensilla showed clear excitatory responses to compounds present in the male-produced aggregation pheromone and also to a plant produced compound. Sensilla could be categorised physiologically according to the responses of their receptor neurons to the tested stimuli. Also receptor neurons present in sensilla coeloconica responded to aggregation pheromone components, but always in an inhibitory fashion. These neurons could, however, be excited by a plant produced compound and by some acids present in the nymphal odour. The antennal lobe of the male S. gregaria was observed to contain about 1000 very small glomerular structures. Single receptor neurons were stained from the antenna to the antennal lobe using a cobalt lysine technique. These stainings revealed a multi glomerular axonal branching pattern of antennal receptor neurons.Abbreviations AN antennal nerve - AL antennal lobe - RN receptor neuron     

The macroglomerular complex of the antennal lobe in the tobacco budworm moth Heliothis virescens : specified subdivision in four compartments according to information about biologically significant compounds

The functional organisation of the male specific macroglomerular complex in Heliothis virescens has been studied by tip recordings of sensilla trichodea type 1 combined with cobalt-lysine stainings and by intracellular recordings of antennal lobe projection neurons combined with neurobiotin stainings. The antennal lobe, the macroglomerular complex and the stained axons/dendrites were reconstructed by camera-lucida. Some were further computer reconstructed in three dimensions. The results showed that: 1) The macroglomerular complex consisted of four anatomically separated compartments; 2) A large compartment (the cumulus) at the entrance of the antennal nerve received input from receptor neurons responding to the major pheromone component; 3) Another large compartment, located dorso-medially of the cumulus (the dorso-medial compartment) received input from receptor neurons tuned to the second pheromone component; 4) Two ventrally located compartments received input from two receptor neuron types, co-localized in the same sensillum. Each neuron type responded strongest to one of two interspecifically acting signals, shown to interrupt the pheromone attraction. 5) The function of the dorso-medial compartment was further verified by selective arborizations in this compartment by a projection neuron showing strong response to antennal stimulation with the second pheromone component. At low concentration, the neuron responded synergistically to stimulation with the binary pheromone mixture. Accepted: 29 July 1998     

Responses of primary somatosensory cortical neurons to paired stimulation of the infraorbital nerve and vibrissae in cats

Extra- and intracellular responses of 128 neurons to paired stimulation of the infraorbital nerve and vibrissae, recorded in the projection zone of the vibrissae in cortical area SI, were studied in adult cats immobilized with tubocurarine. Conditioning stimulation completely suppressed the ability of different neurons to respond for periods of between 10 and 120 msec. The duration of the period of total suppression of test responses was shown to depend on the location of the stimulated vibrissa in the peripheral receptive field of the neurons studied. Excitatory and inhibitory responses of maximal intensity arose in the neurons to stimulation of receptive field centers. The functional role of the decrease in intensity of excitatory responses during stimulation of vibrissae located at different distances from centers of the receptive fields of cortical neurons is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 2, pp. 117–124, April, 1981.     

Responses and properties of receptive fields of neurons in the visual projection zone of the pigeon hyperstriatum

Unit responses in the hyperstriatal region of the pigeon forebrain to the action of various visual stimuli were investigated. Particular attention was paid to the discovery of retinotopic projection in the Wulst region. It was shown that as the electrode was advanced in the caudal direction in the zone of visual projection of the hyperstriatum the receptive fields of the neurons recorded shifted in the opposite direction in the visual field. The receptive fields of neurons of the ventral and dorsal hyperstriatum lie higher in the visual field and are larger in diameter than those of neurons of the accessory hyperstriatum. Unit responses in the visual projection zone of the Wulst depend on the intensity of illumination, size, and speed and direction of movement of the test objects across the receptive field. The functional role of the retino-thalamo-telencephalic system in visual interpretation in birds is discussed and it is suggested that the Wulst region is comparable with the striatal and also with the frontal regions of the mammalian cortex.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 8, No. 3, pp. 230–236, May–June, 1976.     

Receptive fields of neurons in the lateral suprasylvian area of the cat

The structure of receptive fields of single neurons in the lateral suprasylvian area of the cat's cortex was studied. Receptive fields of neurons in this area are larger (up to 2000 deg² or more) than those of the visual projection cortex. A difference was found in the sizes of these fields of the same neuron when measured by presentation of a black object and spot of light. Experimental results showed that most neurons of the area (104 of 148) that are sensitive to visual stimulation respond clearly to flashes of a stationary spot of light. Because of this feature the structure of the receptive fields of the neurons were studied by point by point testing of their whole surface. Intensities of on- and off-components of on-off neurons were found to differ. Only 16% of receptive fields had equal numbers of discharges in on- and off-components of the on-off response. Dominance of one component was observed in 84% of on-off neurons. Receptive fields with several discharge centers are a characteristic feature of neurons in this area. A concentric organization of the receptive fields was found in 11% of neurons.L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSR, Erevan, Translated from Neirofiziologiya, Vol. 14, No. 3, pp. 278–283, May–June, 1982.     

Spatial organization of olfactory receptor neurons on the antenna of the cabbage looper moth

Electrophysiological responses to stimulation with behaviorally relevant compounds were recorded from receptor neurons within specialized classes of sensilla on the antennae of male cabbage looper moths. Receptor neurons were found that specifically respond to stimulation with Z-7,12:AC, Z-7,14:AC, Z-9,14:AC, and Z-7,12:OH. Specialized receptor neurons were not found for the three remaining pheromone blend components, 12:AC, Z-5,12:AC, or 11,12:AC. However, a new class of sensillum containing a pair of neurons insensitive to all of the cabbage looper pheromone components was encountered. In addition, spatial patterns of distribution along the flagellum for identified classes of sensilla are described. Sensilla containing Z-7,12:AC-sensitive neurons are preferentially located on the proximal half of the antennal flagellum. In addition to this distribution along the length of the antenna, a pattern across individual flagellar subsegments is described. Sensilla containing neurons sensitive to Z-9,14:AC were found exclusively on the lateral margins of individual flagellar subsegments. Accepted: 19 June 1998     

Binocular depth perception mechanisms in tongue-projecting salamanders

Tongue-projecting salamanders (Bolitoglossini) combine extreme speed and high precision in prey capture. They possess all requirements for stereoscopic depth perception: frontally oriented eyes, a substantial amount of direct ipsilateral projection in addition to the contralateral one, and binocularly driven neurons. Extracellular recordings were made from retinal afferents in the tectum as well as from the somata of tectal neurons. RF-sizes of afferents and tectal neurons were determined, and the response properties of tectal neurons were tested under monocular and binocular conditions with stimuli of different size and velocity. While RF-sizes and response properties of binocular neurons during binocular and contralateral stimulation were similar, ipsilaterally stimulated neurons exhibited much smaller RFs, lower spike rates and different size preferences.Furthermore, the contralateral retinotectal projection from one eye and the ipsilateral from the other are in register. While retinal afferents are distributed linearly over the tectal surface, most tectal neurons are activated by a retinal area corresponding to the frontal visual field; this results in a magnification of this region. The two monocular receptive fields of binocular neurons exhibit zero disparities (horopter) at distances that coincide with the maximum reach of the tongue. We hypothesize that bolitoglossine salamanders (as well as amphibians in general) make use of two kinds of disparities: (1) between the maps in the left and right tectal hemisphere, coding for the lateral eccentricity of an object, and (2) between the ipsilateral and contralateral retinotectal map, coding for the distance. The presence of substantial direct ipsilateral afferents in bolitoglossine salamanders appears to be the basis for a fast computation of object distance, which is characteristic of these animals.Abbreviations Ax/Ay coordinates of a recorded afference - Nx/Ny coordinates of a recorded neuron - RF receptive field - RFc contralateral receptive field - RFi ipsilateral receptive field - RFx/RFy coordinates of a receptive field center - RGC retinal ganglion cell     

Identification of serotonin- and vasopressin immunoreactivities in the suprachiasmatic nucleus of four mammalian species

Summary Cobalt fills from small, defined regions of the antenna in D. melanogaster show that the three types of sensilla on the third segment, the flagellum, and a fourth sensillum located in the arista, project into the glomeruli of the antennal lobe. We have identified 19 glomeruli in each lobe, according to their location, shape, and size. At least ten of these represent major projection areas of flagellar or aristal sensilla. The large majority of glomeruli is innervated from both antennae, but a small group of five receive exclusively ipsilateral input. A particular sensory fiber appears to terminate only in one specific glomerulus, either in the ipsilateral or in both lobes. Fills from flagellar regions bearing a single type of sensillum, yield a specific pattern of glomeruli containing stained terminals. Aristal projections remain strictly ipsilateral, whereas those from the other sensilla consist of an ipsilateral and a bilateral component. When filling from different points in an area bearing one type of sensillum, similar projections are produced, suggesting that projection patterns observed reflect predominantly the type of sensillum rather than its location on the flagellum. Accordingly, individual glomeruli might represent functional units, each receiving antennal input in a characteristic combination.We are indebted to Dr. H. Tobler for critical comments. R.F.S. was supported by the Swiss National Foundation (Grant No. 3.541-0.79) as well as a Travel Aid by the Swiss Academy of Sciences     

Receptive fields of auditory cortical neurons in the cat

Receptive fields of auditory cortical neurons were studied by electrical stimulation of nerve fibers in different parts of the cochlea in cats anesthetized with pentobarbital. The dimensions of the receptive fields were shown to depend on the topographic arrangement of the neuron in the auditory cortex. The more caudad the neuron on the cortical projection of the cochlea in the primary auditory cortex, the more extensive its receptive field. The receptive fields were narrowest in the basal turn of the cochlea and were symmetrical with respect to their center. It is suggested that the region of finest discrimination of acoustic stimuli in cats is located in the basal region of the cochlea, i.e., in that part of its receptor system which has the narrowest receptive field and is represented by significantly more (than the middle and apical regions of the cochlea) nerve cells in the primary auditory cortex [1].A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 5, pp. 467–473, September–October, 1981.     

Intracellular responses of antennal chordotonal sensilla of the American cockroach

The responses of mechanoreceptor neurons in the antennal chordotonal organ have been examined in cockroaches by intracellular recording methods. The chordotonal organ was mechanically stimulated by sinusoidal movement of the flagellum. Stimulus frequencies were varied between 0.5 and 150 Hz. Receptor neurons responded with spike discharges to mechanical stimulation, and were classed into two groups from plots of their average spike frequencies against stimulus frequency. Neurons in one group responded to stimulation over a wide frequency range (from 0.5 to 150 Hz), whereas those in a second group were tuned to higher frequency stimuli. The peak stimulus frequency at which receptor neurons showed maximum responses differed from cell to cell. Some had a peak response at a stimulus frequency given in the present study (from 0.5 to 150 Hz), whereas others were assumed to have peak responses beyond the highest stimulus frequency examined. The timing for the initiation of spikes or of a burst of spikes plotted against each stimulus cycle revealed that spike generation was phase-locked in most cells. Some cells showed phase-independent discharges to stimulation at lower frequency, but increasing stimulus frequencies spike initiation began to assemble at a given phase of the stimulus cycle. The response patterns observed are discussed in relation to the primary process of mechanoreception of the chordotonal organ.     

Cold-receptor cells supply both cold- and warm-responsive projection neurons in the antennal lobe of the cockroach

The temperature receptor cells on the cockroach antennae are all excited by rapid cooling. In the antennal lobe, however, cold- as well as warm-responsive neurons occur. They are excited either by rapid step-like cooling or rapid step-like warming. Responses to such temperature transients do not show, however, whether antennal lobe neurons convey information on slowly changing temperatures typical of temperature gradients used for orientation. In contrast slow temperature changes permit an analysis of the effects of both instantaneous temperature and its rate of change. We compared the effect of slow temperature oscillations on the responses of antennal cold-receptors cells and cold- and warm-responsive projection neurons. In all cases the discharge rates were modulated by the temperature oscillations. They displayed a double dependence on instantaneous temperature and its rate of change. Information about cooling and warming, first contained in the output of a single cold-receptor cell diverges to form the parallel pathways of cold- and warm-responsive projection neurons, thereby in particular improving the detection of fluctuations in temperature.     

Central gustatory projections and side-specificity of operant antennal muscle conditioning in the honeybee

Gustatory stimuli to the antennae, especially sucrose, are important for bees and are employed in learning paradigms as unconditioned stimulus. The present study identified primary antennal gustatory projections in the bee brain and determined the impact of stimulation of the antennal tip on antennal muscle activity and its plasticity. Central projections of antennal taste hairs contained axons of two morphologies projecting into the dorsal lobe, which is also the antennal motor centre. Putative mechanosensory axons arborised in a dorso-lateral area. Putative gustatory axons projected to a ventro-medial area. Bees scan gustatory and mechanical stimuli with their antennae using variable strategies but sensory input to the motor system has not been investigated in detail. Mechanical, gustatory, and electrical stimulation of the ipsilateral antennal tip were found to evoke short-latency responses in an antennal muscle, the fast flagellum flexor. Contralateral gustatory stimulation induced smaller responses with longer latency. The activity of the fast flagellum flexor was conditioned operantly by pairing high muscle activity with ipsilateral antennal sucrose stimulation. A proboscis reward was unnecessary for learning. With contralateral antennal sucrose stimulation, conditioning was unsuccessful. Thus, muscle activity induced by gustatory stimulation was important for learning success and conditioning was side-specific.     

Physiology and morphology of protocerebral olfactory neurons in the male moth Manduca sexta

1. We have used intracellular recording and staining with Lucifer Yellow, followed by reconstruction from serial sections, to characterize the responses and structure of olfactory neurons in the protocerebrum (PC) of the brain of the male sphinx moth Manduca sexta. 2. Many olfactory protocerebral neurons (PCNs) innervate a particular neuropil region lateral to the central body, the lateral accessory lobe (LAL), which appears to be important for processing olfactory information. 3. Each LAL is linked by its constituent neurons to the ipsilateral lateral PC, where projection neurons from the antennal lobe terminate, as well as to other regions of the PC. The LALs are also linked to each other by bilateral neurons with arborizations in each LAL. 4. Some PC neurons showed long-lasting excitation (LLE) that outlasted the olfactory stimuli by greater than or equal to 1 s, and as long as 30 s in some preparations. LLE was more frequently elicited by the sex-pheromone blend than by individual pheromone components. All bilateral neurons that showed LLE had arborizations in the LALs. LLE responses were also recorded in a single local neuron innervating the mushroom body. 5. In some other PC neurons, pheromonal stimuli elicited brief excitations that recovered to background firing rates less than 1 s after stimulation.     

Local interneurons and information processing in the olfactory glomeruli of the moth Manduca sexta

Intracellular recordings were made from the major neurites of local interneurons in the moth antennal lobe. Antennal nerve stimulation evoked 3 patterns of postsynaptic activity: (i) a short-latency compound excitatory postsynaptic potential that, based on electrical stimulation of the antennal nerve and stimulation of the antenna with odors, represents a monosynaptic input from olfactory afferent axons (71 out of 86 neurons), (ii) a delayed activation of firing in response to both electrical- and odor-driven input (11 neurons), and (iii) a delayed membrane hyperpolarization in response to antennal nerve input (4 neurons).Simultaneous intracellular recordings from a local interneuron with short-latency responses and a projection (output) neuron revealed unidirectional synaptic interactions between these two cell types. In 20% of the 30 pairs studied, spontaneous and current-induced spiking activity in a local interneuron correlated with hyperpolarization and suppression of firing in a projection neuron. No evidence for recurrent or feedback inhibition of projection neurons was found. Furthermore, suppression of firing in an inhibitory local interneuron led to an increase in firing in the normally quiescent projection neuron, suggesting that a disinhibitory pathway may mediate excitation in projection neurons. This is the first direct evidence of an inhibitory role for local interneurons in olfactory information processing in insects. Through different types of multisynaptic interactions with projection neurons, local interneurons help to generate and shape the output from olfactory glomeruli in the antennal lobe.Abbreviations AL antennal lobe - EPSP excitatory postsynaptic potential - GABA -aminobutyric acid - IPSP inhibitory postsynaptic potential - LN local interneuron - MGC macroglomerular complex - OB olfactory bulb - PN projection neuron - TES N-tris[hydroxymethyl]methyl-2-aminoethane-sulfonic acid     

Electrophysiological evidence for a topographical projection of the nasal mucosa onto the olfactory bulb of the frog

Three olfactory nerve branches respectively subserving either a medial, an intermediate, or a lateral region of the dorsal olfactory receptor sheet of the bullfrog Rana catesbeiana were electrically stimulated with bipolar platinum hook electrodes. Extracellular single unit responses from 93 second-order cells in different regions of the olfactory bulb were recorded with metal-filled glass micropipets. The excitatory responsiveness of each unit to the stimulation of each of the three nerve branches (response profile) was determined. Some units were sensitive to stimulation of each of the three nerve branches, thus suggesting a wide projection from the entire receptor sheet. On the other hand, other units were more selective. Of this latter group, units in the lateral bulb were excited by nerve branches subserving the more lateral regions of the receptor sheet; units in the medial bulb were excited by the nerve branches subserving the more medial regions of the receptor sheet. These data provide electrophysiological evidence for a topographical projection of the olfactory receptor sheet onto the olfactory bulb, and further suggest that the projections onto different bulbar cells vary in degree of localization.