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. Author manuscript; available in PMC: 2012 Dec 4.
Published in final edited form as: Cell Rep. 2012 Nov 15;2(5):1143–1150. doi: 10.1016/j.celrep.2012.09.035

Coding Odorant Concentration Through Activation Timing Between the Medial and Lateral Olfactory Bulb

Zhishang Zhou 1, Leonardo Belluscio 1,*
PMCID: PMC3513620  NIHMSID: NIHMS418691  PMID: 23168258

SUMMARY

In mammals, each olfactory bulb (OB) contains a pair of mirror-symmetric glomerular maps organized to reflect odorant receptor identity. The functional implication of maintaining these symmetric medial-lateral maps within each OB remains unclear. Here, using in vivo multi-electrode recordings to simultaneously detect odorant-induced activity across the entire OB, we reveal a timing difference in the odorant-evoked onset latencies between the medial and lateral halves. Interestingly, the latencies in the medial and lateral OB decreased at different rates as odorant concentration increased, causing the timing difference between them to also diminish. As a result, output neurons in the medial and lateral OB fired with greater synchrony at higher odorant concentrations. Thus, we propose that temporal differences in activity between the medial and lateral OB can dynamically code odorant concentration, which is subsequently decoded in the olfactory cortex through the integration of synchronous action potentials.

INTRODUCTION

The sense of smell is critical for the survival of most animals since it is necessary for both feeding and reproductive behaviors. Odorant identity and concentration are two fundamental aspects of olfactory information, representing the quality and quantity of an odorant signal, respectively. Animals may follow increasing odorant concentrations to locate food and mates, while avoiding predators by moving toward decreasing concentrations. Due to the importance of odorant concentration in olfactory function, its role is widely studied behaviorally, but little is known about the basic mechanism by which concentration is encoded.

At the periphery, sensory intensity is generally represented by the frequency of neural discharges (see review Gardner and Martin, 2000). However, within the central nervous system, there is extensive convergence and divergence of both excitatory and inhibitory signals, which complicates the response. In the olfactory bulb (OB), increasing odorant concentration has been shown to evoke both increases and decreases in neural firing (Meredith, 1986) as well as no changes at all (Chalansonnet and Chaput, 1998). Imaging studies that focused on the surface of the OB have shown that increasing odorant concentration leads to glomerular recruitment (Rubin and Katz, 1999; Uchida et al, 2000). However, other studies report that glomerular patterns change very little with increasing concentration (Xu et al, 2000; Wachowiak et al., 2002). Whether or not temporal information such as changes in response latency plays a more important role than neural recruitment or firing rate is also controversial (Bathellier et al., 2008; Junek et al., 2010). These discrepancies may be due, in part, to differences in animal species and recording methods but more likely due to inconsistencies in olfactory nerve activation resulting from the molecular diversity of the peripheral neurons.

In mice, there are ~1000 different odorant receptors (ORs) expressed by olfactory sensory neurons (OSNs) in the nasal epithelium, with each OSN expressing only one of the 1000 possible receptors (Buck and Axel, 1991). While the OSNs expressing a given odorant receptor are broadly distributed in the olfactory epithelium, their axons converge onto two glomeruli in the olfactory bulb (OB), usually one in the medial and the other in the lateral side of each OB. This produces a pair of symmetric medial-lateral glomerular maps (Buck, 2000). A precise link has been revealed between the medial-lateral maps, mediated by second-order neurons (Belluscio et al., 2002).

Genetically, each OSN expresses only one OR but, functionally, an OSN is broadly tuned such that individual ORs are activated by many odorants and simple odorants can activate multiple types of ORs (Malnic et al., 1999). This genetic and functional diversity in ORs may help to explain some of the variation associated with previous studies of the OB response to varying odorant concentrations. Thus, to more effectively uncover the fundamental principles for encoding concentration in the OB, we focused on a single OR by using the mouse line, UBI7. In this transgenic line, all OSNs express the same OR, the I7 receptor, in addition to their endogenous ORs (Zhao and Reed, 2001; Figure 1; Figure S1). Since the I7 odorant receptor has an identified ligand, octanal (Zhao et al., 1998), we could therefore reliably activate the same odorant response, from the same OR, in all of our experiments (Figure 1). Using UBI7 mice, we investigated the temporal and spatial coding of odorant concentration in the two halves of the OB with in vivo multi-channel recordings (Figure 2A).

Figure 1. Broad activation of OSNs and Olfactory Bulbs using UBI7 mice.

Figure 1

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(A) Targeted insertion of the mouse I7 odorant receptor and IRES sequence downstream of the OMP coding region.

(B) Olfactory bulb schematic depicting uniform expression of I7 in addition to endogenous ORs (pair color coded) in the mirror–symmetric maps.

(C) Perforated patch recordings on dendritic knobs of randomly selected OSNs show that each functionally responds to 1% octanal (top trace), a ligand for the I7 receptor, but not to 1% butanal (bottom trace). Middle trace shows onset of octanal response in top trace (indicated by white box).

(D) Intrinsic signal imaging of dorsal OB (blood vessel image, top panel) showing broad activation by 1% octanal (dark regions, middle panel) and a wild type like 1% butanal activity map (dark spots, bottom panel) demonstrating normal functional response of endogenous ORs. Scale bar, 200 μm.

Figure 2. Medial olfactory bulb was activated earlier than the lateral OB.

Figure 2

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(A) Schematic of in vivo muti-channel recording and odorant delivery. The electrode penetrates from lateral to medial across the OB. Inset shows the tilted stereotaxis platform which enables the electrode to pass perpendicular to the line of symmetry (shown by dashed lines) between the medial and lateral maps.

(B) Local field potentials in response to 0.001% octanal recorded with a 16-channel electrode spanning the medial and lateral OB (channel number listed on right) show that activation appeared earlier on the medial side than on the lateral side. Red traces are the recordings in medial and blue traces the recordings in lateral OB. Green traces are granule layer activities. Vertical broken line across recordings of all channels indicates time of the first exhale trough of the respiration cycle in which there was an odorant response on both sides.

(C) Neural spikes (sorted from the recordings in B) across all OB layers of both medial and lateral sides. Different colors show waveforms of individual neural units.

(D) Peri-stimulus time histogram (PSTH) shows the response of the first breath with response on both medial and lateral side (from C). The neurons fired earlier in medial than in lateral side. The histogram is binned with 5 ms time interval. Short red bars indicate onset of odorant response in each channel, with the onset threshold set to the first 2 consecutive bins (a 10 ms window).

(E) Cross-correlation of field potentials from B, comparing signals of similar layers between the medial and lateral OB from superficial glomerular layer (Ch 16 vs. Ch 1) to deep mitral cell layer (Ch 13 vs. Ch 4) layers, shows that medial is activated earlier than lateral in all layers (65 ms, 118 ms, 167 ms, and 71 ms, respectively).

(F) Quantification of the odorant-response onset-latencies shows that mitral cells in the medial OB were activated significantly earlier than in the lateral OB (60 ± 42 ms vs. 113 ± 37 ms, mean ± SD; n = 22 mice; **p < 0.0001).

RESULTS AND DISCUSSION

Multi-electrode Recording Reveals Inter-hemispheric Timing Difference

In UBI7 mice octanal exposure produced distinct signals that were recorded across the entire OB from the lateral to the medial side (Figure 2B). These odorant-evoked local field potentials (LFPs) were generally locked to the breath cycle of about 2 Hz. By comparing the signals between the two halves of the OB, we observed that the odorant responses arose earlier in the medial side than in the lateral side, revealing a medial to lateral timing difference (MLTD) (Figure 2B). This MLTD was first demonstrated by correlation analysis of the LFP signals between the two halves of the OB (Figure 2E). The correlation also showed obvious theta oscillations at about 2 Hz on both sides. Gamma oscillations were also induced by stimulation at high odorant concentration and alpha/beta oscillations appeared in centrifugal signals (data not shown), similar to the previous reports (Kay et al., 2009). Since LFPs are broad neural signals and OB information is transmitted to higher order neurons specifically via action potentials we next examined the spike patterns generated by odorant exposure. Again, our recordings revealed clear breath-locked neural spike discharges across the layers of the OB (Figure 2C). We focused our analysis on the signals generated during a single breath cycle since previous studies have shown that animals can identify an odorant within a single sniff (Uchida and Mainen, 2003). For consistency, we selected the first cycle that produced spikes on both medial and lateral sides (Figure 2D). Notably, subsequent breath responses often showed suppressed or persistent firing (Figure 2C, Figure S5), due to modulation by the bulbar network.

Analysis of the spike patterns (Figure 2C and Figure 2D) demonstrated that the neural ensembles on the medial side were activated earlier than their counterparts on the lateral side. Our multi-electrode recording approach enabled clear distinction of the OB layers by electrode depth (Figure S2), current source-density (CSD) analysis (Figure S3), and neural firing properties. As a result, we were able to focus our analysis on the activity of mitral cells, which are confined to a distinct layer and are the primary output neurons of the bulb. For these cells there was a significant time delay between the medial and lateral populations (60 ± 42 ms vs. 113 ± 37 ms; p < 0.0001; n = 22), with the medial bulb always activated before the lateral bulb (Figure 2F). Spike sorting and clustering analysis revealed that the response latencies varied among individual mitral cells (Figure S4) and therefore the MLTD was most apparent at the local population level (Figure 2). Response latencies have been reported to vary substantially among different odorants and among different neurons, but to remain relatively constant for a single odorant and a given neuron (Carey and Wachowiak, 2011; Junek et al., 2010; Shusterman et al., 2011). Using UBI7 mice we avoided this potential complication, thus insuring that differences in response latencies could not be explained by activation of different ORs, but rather were due entirely to the difference in their medial-lateral location. This finding further suggests a functional rationale for the presence of two mirror symmetric glomerular maps as a means to encode odorant information via MLTDs. A recent study showed that mice can discern time differences in stimulus as short as 10 ms (Smear et al., 2011). This illustrates that the olfactory system is not only capable of detecting timing differences within the range of the MLTDs but also that such signals may provide function to a specific aspect of the stumuli. Therefore, we next investigated which aspect of the odorant signal could be encoded in the MLTD.

Concentration Dependence of the Medial–Lateral Timing Difference

To determine if the MLTD was concentration-dependent, we examined if changes in odorant concentrations could alter the temporal aspects of the firing patterns between the medial and lateral OB. Increasing odorant concentration decreased the onset latencies of neural populations across all layers in both halves of the OB (Figure 3A). This finding is consistent with previous reports that increasing stimulus intensity decreases the response latency in the olfactory system (Cang and Isaacson, 2003; Kauer and Shepherd, 1977; Spors and Grinvald, 2002) and in other sensory systems (Eggermont, 1998; Oram, 2002). Interestingly, the latency changes on the medial and lateral side decreased at different rates, with the lateral side decreasing faster than the medial side (−0.038 sec/log(C) vs. – 0.020 sec/log(C), Figure 3B). This produced smaller medial-lateral timing differences at high odorant concentrations (10 ± 6 ms) than at low concentrations (59 ± 21 ms; n=22) (Figure 3D). This MLTD shift is bidirectional, dynamically responding to both increases and decreases in odorant concentrations, as observed even within a single odorant application (Figure S5). Due to this concentration-dependent shift in the MLTD, mitral cells in the two halves of the OB also fired more synchronously as odorant concentrations increased. Interestingly, synchronization has been shown to be critical for fine sensory discrimination (Stopfer et al., 1997). Since mitral cells are the primary projections neurons of the OB, modulation of their activation timing could be used to synchronize their output and thus affect integration of odorant signals within the olfactory cortex.

Figure 3. Odorant concentrations modulate onset latencies and time differences between medial and lateral OB.

Figure 3

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(A) Peri-stimulus time histograms of odorant responses show the decreasing onset latencies of both medial (pink bars) and lateral (blue bars) OB neural ensembles to increasing odorant concentrations (from left to right panels). Arrows point to onset time of activity in medial and lateral mitral cells, respectively. At high concentrations (1% octanal, right panel), medial and lateral neurons fire nearly in synchrony.

(B) Plot of onset latencies of the medial (red) and lateral (blue) mitral cells against odorant concentrations (in log unit) shows a linear decrease of the latencies to increasing odorant concentrations (n = 7 mice, indicated by different symbols). Linear fitting for individual mice (dashed lines, see Methods) shows that latencies in the lateral side (average indicated by thick blue line) decrease faster than their counterparts in the medial side (average indicated by thick red line) (slope of −0.038 ± 0.005 sec/log(C) vs. −0.020 ± 0.006 sec/log(C), mean ± SD, **p = 0.00014).

(C) Time differences between lateral and medial OB for individual animals (linear fitting indicated by dashed lines) decrease with increasing odorant concentrations (data from B). The averaged slope (indicated by thick line) is –0.018 ± 0.006 (in unit of sec/log(C).

(D) Graph comparing average medial-lateral time difference (MLTD) at low (0.0001%, 0.001% or 0.01%) and high octanal concentrations (0.01%, 0.1% or 1%) reveals a greater MLTD at low (59 ± 21 ms, mean ± SD; n = 22 mice) than at high concentrations (10 ± 6 ms), **p < 0.0001. Data from individual animals shown by open dots.

Similarly, natural odorant intensity could also rely on MLTDs. While most natural odorants are multicomponent mixtures they would still activate distinct sets of glomeruli in the medial and lateral OB with different timing and at certain threshold concentrations. The relative timing of the output between activated sets of medial and lateral glomeruli would then determine the intensity of each chemical component, which in turn would also define the overall intensity of the odorant mixture. With increasing concentration of the mixture the intensiy of each component would increase as the MLTDs between activated glomeruli decreases and more mitral cells are drawn into synchronous activation as we showed in UBI7 mice.

Firing Rates Increase Linearly With Increasing Odorant Concentration

We have shown that the onset timing of the odorant response between the medial and lateral halves of the OB has the potential to encode odorant concentration but this does not preclude the involvement of other mechanisms for encoding concentration. We next examined the relationship between firing rates and odorant concentration. We observed that the firing rate of both medial and lateral mitral cell ensembles linearly increased with increasing odorant concentration (Figure 4A and Figure S6). Surprisingly, in contrast to the differential latency-concentration relationship described above (Figure 3B), the rate increase does not show a significant difference in the slope between the medial and lateral OB (Figure 4A). These data suggest that a temporal code based upon medial-lateral response latencies utilizing both halves of the OB would provide more information about concentration than would a code based purely on firing rate (Cury and Uchida, 2010; Hopfield, 1995; Junek et al., 2010; Margrie and Schaefer, 2003; Stopfer et al., 2003). Our data further imply that, in the olfactory system, temporal coding and rate coding information are interdependent such that when the onset latencies decrease in response to increased odorant concentration the firing rate of output neurons also increases (Figure 4B). Therefore, given the underlying framework of the glomerular maps, complete coding of odorant concentration is likely determined through a combination of temporal, spatial, and rate codes (Bathellier et al., 2008; Chalansonnet and Chaput, 1988; Cury and Uchida, 2010; Hopfield, 1995; Junek et al., 2010; Margrie and Schaefer, 2003; Meredith, 1986; Stopfer et al., 2003; Rubin and Katz, 1999; Tan et al., 2010).

Figure 4. Correlation of the firing rates with temporal spatial information in medial and lateral mitral cells suggests a mechanism for coding odorant concentration.

Figure 4

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(A) Comparison of the relationship between firing rate and odorant concentration shows, that both medial and lateral mitral cell firing rates increased linearly with increasing odorant concentrations (in log) at simlar rates with an average slope of 77 ± 40 Hz/log(C) (mean ± SD) for the medial (thick red line) and 68 ± 44 Hz/log(C) for the lateral (thick blue line), p = 0.30, n = 7. Dashed lines represent linear fitting for the medial (red) or the lateral (blue) of each individual.

(B) Comparison of mitral cell firing rate and onset latency shows a linear relationship between firing rates and onset latencies of mitral cells on both medial and lateral sides but with significantly different slopes (medial red line, −4039 ± 2228 Hz/sec vs. lateral blue line, –1732 ± 975 Hz/sec, mean ± SD, *p = 0.016, n = 7), indicating odorant concentration is synergistically represented by both firing rate and temporal information of onset latencies between medial and lateral mitral cells. Individual medial (red) and individual lateral regressions (blue) are shown with dashed lines.

(C) A systematic model for translating odorant intensity signals from the nose and olfactory bulb to the cortex. Due to anatomical heterogeneity in the nasal cavity, airflow rate and concentrations are not equally distributed throughout the epithelium, introducing large temporal differences in the activation of medial and lateral OSNs at low odorant concentrations (middle left panel). At high concentrations (bottom left panel) there is a more uniform activation as OSNs on both sides reach their firing threshold more rapidly. Similarly the corresponding medial and lateral glomeruli will receive temporally distinct inputs which could be further modulated by the intrabulbar network (Grey, Tufted cells: TC) before the output is sent to the pyriform cortex via mitral cells (MT). At low odorant concentrations (middle panels) when the time difference between medial (red) and lateral (blue) MT cells is large, only those cortical pyramidal neurons (PyNs) that receive strong medial or lateral OB inputs (Grey) would fire action poteintial while those receiving combined medial-lateral MT cell inputs (shown in magenta) may fire few or no spikes. However, at high concentrations (bottom panels) both medial and lateral OSNs would rapidly reach firing threshold, resulting in smaller medial-lateral time differences and more synchronous activity between the medial and lateral OB. The coincident signals from the synchronized MT cells would produce more spikes (PyNs in grey) and recruit additional neurons (PyN in magenta) to fire by intergration of superimposed EPSPs.

Decoding OB Timing in a Integration Model

This study clearly demonstrates a dynamic timing difference between medial and lateral halves of the OB, which raises two new questions: What is the source of the MLTD and how is it decoded by downstream neurons in the olfactory cortex?

A previous study using field potential recordings showed that, in the nasal cavity, the medial olfactory epithelium was activated ~65 ms before the epithelium in the lateral turbinates (Ezeh et al., 1995), possibley due to differential odorant penetration. Mathematical models also showed that, during respiratory cycles, the medial side of the nose experiences faster airflow and higher pressure than the lateral side (Kimbell et al., 1997; Zhao et al., 2006). This could produce differential odorant concentrations in the medial and lateral epithelium causing OSNs in different regions of the nasal cavity to fire at different timing. Since OSNs in the medial and lateral epithelium generally project their axons ipsilaterally to form their respective glomeruli in the medial or lateral regions of the OB (Buck, 2000), we propose that the MLTDs in the OB are initiated in the nasal epithelium, and modulated by the local circuitry in the OB. Studies have shown that the medial and lateral glomerular maps are connected through an intrabulbar map that is medated by tufted cells, and links iso-functional pairs of glomeruli between the medial and lateral OB. (Lodovichi et al., 2003). The anatomical precision of these connections suggests that they are involved in coordinating the signals from sets of glomeruli that respond similarly in the medial and lateral OB. Interestingly, in a previous study we showed that tufted cells can modulate the firing of mitral cells based upon the relative timing of their activation response (Zhou and Belluscio, 2008). Thus, it is possible that tufted cells use this function to regulate MLTDs for either synchronizing or de-synchronizing mitral cell output between the two halves of the OB.

Although prior studies have identified altered response latencies associated with changes in odorant concentration, it is not clear how such differences are decoded in the cortex. Such a relationship between timing of activity and concentration has been reported in many vertebrate species, including mouse and rat (Bathellier et al., 2008; Cang and Isaacson, 2003; Spors and Grinvald, 2002; Wellis et al, 1989), salamander (Kauer and Shepherd, 1977) and tadpoles (Junek et al, 2010). These studies usually define latency timing with reference to an odorant delivery pulse or breath cycles. However, it is unknown how such latency timing could be decoded in the cortex if linked to an external reference. In this study we reveal a dynamic MLTD that exists between the symmetric medial-lateral maps in the OB. This relative timing provides a simple means to represent odorant concentrations with the reference time built into the signal. Since coincident timing of OB output to piriform cortex has been shown to determine the firing probability of pyramidal neurons in the cortex, variation in the MLTD could provide a high dynamic time window for integration in these neurons (Apicella et al., 2010). Thus, we propose that a coincident timing-based mechanism within the cortex, based upon MLTDs, could be used to represent odorant concentration in the olfactory cortex (Figure 4C).

At the single OR level, we revealed a concentration-modulated timing difference in activation between the medial and lateral halves of the OB using UBI7 mice. Such a mechanism could also be utilized by endogenous ORs. We tested different odorant responses in wild-type mice and indeed observed medial-lateral time differences (Figure S7). There was some variation in activation sequence which was not surprising given that in wild-type mice we could only estimate homologous medial-lateral regions rather than record the output of glomeruli associated with a specific OR as in UBI7 mice. In addtion even simple odorants can activate multiple ORs with different activation latencies which further complicates the signal. Nonetheless, the response latencies decreased in response to increasing odorant concentrations (Figure S7), consistent with the results in the UBI7 mice and with previous reports (Junek et al, 2010; Bathellier et al., 2008; Cang and Isaacson, 2003; Spors and Grinvald, 2002; Wellis et al, 1989; Kauer and Shepherd, 1977). Furthermore, the medial-lateral timing difference between the two sides also decreased as odorant concentration increased, exactly as revealed in the UBI7 mice. Therefore, the synchrony-based integration model (Figure 4C) could also be applied to the concentration coding of complex odorants that involve multiple ORs.

The 60 ms time window that we have defined between the medial and lateral OB response forms the basis by which odorant intensity may be encoded in the olfactory cortex through a temporal integration approach. This model both supports the necessity for discriminating differences in OB activation timing as short as 10 ms (Smear et al., 2011) and presents a specific function for the mirror-symmetric maps as internal references to signal on the opposite side. Since individual odorants activate glomeruli on both sides of the OB, the symmetric maps could provide an organizational framework for comparing activation patterns and timing within the OB and thus encoding odorant intensity through the degree of synchrony between the two maps as concentrations change. Such a model also implies that the olfactory cortex maintains a strong capacity for detecting spatially derived timing differences and thus would use this function to integrate the bilateral OB signals presented in this study for decoding odorant intensities. Although future studies will be required to fully undertand the many spatio-temporal codes that link OB output to olfactory perception this study presents an important part of that process with a mechanism for the coding of odorant intensity.

EXPERIMENTAL PROCEDURES

UBI7 mice

Glomerular organization and function: Targeted insertion of the UBI7 cassette (containing an IRES +I7 coding region) into the OMP locus produced broad expression of the I7 receptor in mature OSNs at a level <1% of endogenous OR expression levels (Zhao and Reed, 2001). At this reduced level, UBI7 expression does not interfere with endogenous OR expression and does not disrupt the glomerular map as indicated by M71 (Figure S1) and P2 receptor (Zhao and Reed, 2001) expression. Odorant responses were tested with perforated patch recordings on dendritic knobs of OSNs from acute epithelial slices as previously reported (Nguyen et al., 2007). To identify the maps of odorant activation in the OB, intrinsic signal imaging was performed in live anesthetized mice as described previously (Belluscio and Katz, 2001).

Animal preparation

UBI7 mice were first anesthetized by urethane solution (1.2g/kg, i.p.) and then maintained with 0.5 – 1% isofluorane in oxygen. The animal was fixed with a stereotactic frame, in which the head was held in place by a bar to each temporal side of the skull. Animals were kept warm with hand warmers (Brabber, MI). Surgery was started when the animal showed no movement to foot pinching. A craniotomy was performed in the lateral side of the skull over the olfactory bulbs. The surface of the olfactory bulb was then perfused throughout the experiment with oxygenated ACSF and maintained at 35 °C by a temperature controller (TC-344B; Warner Instruments, CT).

In vivo multi-channel recordings

A multi-channel recording system (Multi channel Systems, Germany) was used to record the activity of neural ensembles in the olfactory bulb. The 16-channel electrode was silicon-based (NeuroNexus Technologies, INC., MI) with a resistance of approximately 1 MΩ for each channel. The distance between two neighboring electrodes was 100 μm. The 16-channel electrode shaft was inserted from lateral to medial across the entire OB. Signals were sampled at 20 kHz. The animal’s respiration was recorded by measuring chest extension with a piezoelectric strap. Timing of odorant delivery and data acquisition for olfactory bulb activity and respiration were controlled by a Master-8 timer (A.M.P.I. LTD, Israel). Odorants were dissolved in mineral oil (concentration in volume/volume) and delivered by a PV-820 Pneumatic PicoPump (World Precision Instruments, FL). All recordings were performed in an environment without noise or light contamination.

Data analysis

Multi-channel and respiration signals were analyzed using Spike-2 software (Cambridge Electronic Design, UK). Field potentials were retrieved by low-pass filtering the raw data at 300 Hz. To extract multi-units, raw data were high-pass filtered at 300 Hz and the spikes were sorted in Spike-2 by setting the threshold to five times the root mean square (RMS) of the resting signal of each channel. Correlation analysis of field potentials, principle component analysis (PCA) of spikes, peri-stimulus time histogram (PSTH) analysis, and current-source density (CSD) analysis were performed using Spike-2. Pseudo-color display of CSD signals was generated using MatLab software. PSTH analysis was in 5 ms bins. In each respiration cycle, the trough was set as the 0 time point, and thus all neuronal activity response timing was relative to this time point.

To determine the onset time of the odorant response, the threshold was set to detect the first 2 spikes within a sliding 25 ms or 10 ms time window (equal to 5 or 2 bins in PSTH) depending upon individual spontaneous activity levels.

To examine the relationship between odorant concentrations and either odorant response latencies or firing rates, linear fittings were performed on data from medial and lateral OB of individual mice, respectively. An average of the individual fittings was displayed (Figure 3B, Figure 4A and 4B) as y = a*x + b, where a is an average of slopes of either medial or lateral.

All group data were represented as mean ± SD. A paired t-test was used for comparisons, with significance set at p < 0.05.

Supplementary Material

01
02

Highlights.

  • Odorants differentially activate the medial and lateral olfactory bulb.

  • A ~60 ms time-window is defined between medial and lateral mitral cell output.

  • The medial-lateral time window is dynamically modulated by odorant concentrations.

  • High odorant concentrations synchronize medial-lateral OB output.

Acknowledgments

We thank Haiqing Zhao and Randy Reed for providing the UBI7 mice; Elakkat Gireesh and Dietmar Plenz for help establishing the multielectrode recordings; and Mark Stopfer, Nick Ryba, Rory McQuiston, and Beth Belluscio for critical comments on the manuscript. This work was supported by the National Institute of Neurological Disorders and Stroke, Intramural Research Program at the National Institutes of Health.

Footnotes

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