PMCCPMCCPMCC

Search tips
Search criteria 

Advanced

 
Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Neurosci. Author manuscript; available in PMC 2010 August 24.
Published in final edited form as:
PMCID: PMC2846283
NIHMSID: NIHMS181648

Smelling Sounds: Olfactory–Auditory Sensory Convergence in the Olfactory Tubercle

Abstract

Historical and psychophysical literature has demonstrated a perceptual interplay between olfactory and auditory stimuli – the neural mechanisms of which are not understood. Here we report novel findings revealing that the early olfactory code is subjected to auditory cross-modal influences. In vivo extracellular recordings from the olfactory tubercle, a tri-laminar structure within the basal forebrain, of anesthetized mice revealed that olfactory tubercle single-units selectively respond to odors – with 65% of units showing significant odor–evoked activity. Remarkably, 19% of olfactory tubercle single-units also showed robust responses to an auditory tone. Furthermore, 29% of single-units tested displayed supra-additive or suppressive responses to the simultaneous presentation of odor and tone, suggesting cross-modal modulation. In contrast, olfactory bulb units did not show significant responses to tone presentation, nor modulation of odor-evoked activity by tone – suggesting a lack of olfactory-auditory convergence upstream from the olfactory tubercle. Thus, the tubercle presents itself as a source for direct multimodal convergence within an early stage of odor processing, and may serve as a seat for psychophysical interactions between smells and sounds.

Keywords: Olfaction, Perception, Cross-modal, Olfactory bulb, Integration, Odor

Introduction

The integration of environmental information across multiple sensory channels is critical to guiding decisions and behaviors. For instance, whereas the sound of a car horn while crossing a street may evoke arousal, the sound of the same horn matched with the sight of a rapidly approaching taxi cab may result in you quickly getting out of the street. A basic tenet of multisensory integration is the ability of one sensory modality to enhance or to suppress information from another sensory modality (Calvert et al., 2004). This is distinct from modulatory interactions such as those occurring during dishabituation (Rankin et al., 2009; Smith et al., 2009) or fear-potentiated responses (Davis, 1989; Halene et al., 2009) where an intense stimulus in one modality may enhance responsiveness to stimuli in another – due to system-wide changes in neural excitability. Rather, multisensory integration involves convergence of sensory pathways, often with single neurons displaying stimulus evoked activity to both, and/or modification of the response to one sensory input while in the context of another.

While most information on the neural basis of multisensory integration stems from studies in somatosensation, audition and vision, the olfactory system is known to interact with at least two sensory systems. For instance, the percept of `flavor' is a multi-modal construct resultant from cellular interactions between olfactory and gustatory processing centers (Verhagen and Engelen, 2006). Further, it has been shown that visual cues may facilitate odor detection (Gottfried and Dolan, 2003). Integration of olfactory information with other sensory channels could have significance for a variety of critical scenarios (e.g., food selection and threat aversion).

The present study stems from a serendipitous observation of tone-evoked single-unit responses in the olfactory tubercle. However, over 150 years ago the perfumerist G.W. Piesse suggested that olfactory perception may be intimately linked with auditory processing (Piesse, 1857). In particular, Piesse postulated a strategy to catalogue odors based upon analogous auditory pitches. A recent psychophysical test of this concept by Belkin and colleagues showed that human subjects are readily capable of consistently matching odor qualities to auditory pitch (Belkin et al., 1997). Further, while rare, olfactory-auditory synesthesia has been reported in humans (Simpson and McKellar, 1955). Thus, a perceptual relationship between olfactory and auditory perception is apparent, though no neural evidence of direct olfactory-auditory convergence has been reported to our knowledge. Such convergence of olfactory and auditory channels likely requires a site wherein projections are received directly or indirectly from both principle olfactory and auditory areas.

One potential region of interest in olfactory-auditory interactions is the understudied and enigmatic olfactory tubercle. The olfactory tubercle is a tri-laminar cortical-like structure which occupies a large portion of the basal forebrain. Previous anatomical work has shown that the tubercle receives monosynaptic olfactory input from the OB and the olfactory piriform cortex (PCX) (Haberly and Price, 1977; Schwob and Price, 1984; Johnson et al., 2000). Very recent work showed that electrical stimulation of the lateral olfactory tract (LOT) evokes neural responses in the olfactory tubercle (Carriero et al., 2009). Additionally, auditory sensory information may arrive at the olfactory tubercle via general associative networks involving the hippocampus (Deadwyler et al., 1987) or ventral pallidum (Budinger et al., 2008) – both regions being interconnected with the olfactory tubercle (Ikemoto, 2007). Alternatively, auditory information from the primary auditory cortex may converge with olfactory information in the olfactory tubercle directly (Budinger et al., 2006). While the olfactory tubercle is positioned anatomically in a manner which supports a potential role in olfactory-auditory sensory integration, to our knowledge, no studies have tested the response to both odors and auditory stimuli there. Therefore, here we tested the role of the olfactory tubercle in olfactory-auditory sensory convergence.

Materials & Methods

Experimental subjects

23 adult male BL6 mice (2–6 months of age) bred and maintained within the Nathan S. Kline Institute for Psychiatric Research animal facility were used. Food and water were available ad libitum. Mice were housed on a 12:12 (light:dark) cycle with all experiments performed during the light phase. All experiments were conducted in accordance with the guidelines of the National Institutes of Health and were approved by the Nathan S. Kline Institute's Institutional Animal Care Committee.

Electrophysiology

For in vivo electrophysiology, mice were anesthetized with urethane anesthesia (2.0mg/kg, I.P.) and supplied with atropine hydrochloride (25mg/kg, I.M.) to minimize tracheal congestion. Mice were then mounted in a stereotaxic frame outfitted with a water-filled heating pad (38°C), the skin overlying the skull administered local anesthetic (1% xylocaine, S.C.) and later removed exposing the dorsal skull. To prevent closure of the ear canal, stereotaxic ear bars were secured within the posterior aspects of the orbital sockets for all preparations. Small (~1.5mm diameter) ipsilateral holes were drilled over the olfactory tubercle, and/or the OB and LOT. A tungsten stimulating electrode was lowered into the mesial region of the OB granule cell layer to aid in tubercle localization under physiologic control by OB stimulation. Due likely to the gyrating anatomy of the tubercle, clear OB stimulation-evoked positive and negative evoked potentials that are apparent in the piriform cortex, were sometimes difficult to determine. In a subset of preparations (n=5 mice), we performed recordings from mitral/tufted cells (MTs) of the OB. For these recordings, the stimulating electrode was lowered to the LOT to aid in MT localization under physiologic control by LOT stimulation. Electrode locations were verified with post-mortem histology using cresyl violet staining of slidemounted 40μm coronal sections. As shown in Figure 1, electrode tips spanned olfactory tubercle layers I through III. Preparations in which the electrode tips were found outside of the tubercle were entirely excluded from this study. Recording electrode potentials along with stimulus presentation events were acquired using Spike2 software (Cambridge Electronic Design Ltd., Cambridge, England).

Fig. 1
Electrode placement in the tubercle

Stimulus presentation

Odors were presented to anesthetized mice using an air-dilution olfactometer (Rennaker et al., 2007) at 1L/min flow using medical grade nitrogen. Stimuli included five monomolecular odorants and mixtures of monomolecular odorants as previously described (Barnes et al., 2008). The five monomolecular stimuli included 1,7-octadiene, 4-methyl-3-penten-2-one, ethyl propionate, heptanal and isoamyl acetate (Sigma Aldrich, St. Louis, MO). All odorants were `pure' in their liquid state except for 1,7-octadiene and 4-methyl-3-penten-2-one which were diluted 1:1 in mineral oil. Odor mixtures were based off of previous work in our laboratory using 10 component mixtures with individual components removed (Barnes et al., 2008). In this work we used 3 separate odor mixtures: a 10 part mixture lacking 1 component (10-1 mix), a 10 part mixture lacking 2 components (10-2 mix) and a 10 part mixture lacking 3 components (10-3 mix). All components were diluted to 100 parts-per-million in mineral oil prior to mixing. Odors were presented 2sec each, at a minimal 30sec inter-stimulus interval (ISI) and were triggered off of the animal's respiration using a piezo foil placed under the animal's chest and a window discriminator to detect peaks of respiration (World Precision Instruments, Sarasota, FL).

Auditory (`tone') stimuli were presented using a 76dB 2.8kHz piezo speaker (model 273-059, Radio Shack, Fort Worth, TX), located approximately 30cm directly anterior to the mouse (on the exterior of a faraday cage). The speaker was powered by a 5V digital output from the CED data acquisition and control board. Tone onset was triggered off of the animal's respiratory signal in the same way as done for odor stimuli. Tones were presented 2sec each, at a minimal 30sec ISI. All stimuli were presented for at least 5 counterbalanced trials for each unit (range: 5–16 trials/unit).

Data analysis

Electrophysiological data were analyzed as previously described (Wilson, 1998; Rennaker et al., 2007). Single-unit spike sorting, cluster cutting and waveform analysis were all performed in Spike2 software (Cambridge Electronic Design, Ltd.). Figures 2A & B shows example spikes, single-unit identification, and waveforms of two single-units, isolated from an individual olfactory tubercle recording. Also, the recordings were verified as single-units by a conservative inter-spike-interval threshold. No more than 1% of spikes from a single-unit could occur with an inter-spike-interval of less than 2msec. Example wave-forms and inter-spike-interval distributions for 2 units are shown in Figures 2B & 2C. Putative units which did not pass these criteria were omitted form further analysis. Both MT and olfactory tubercle units underwent the same sorting and analysis methods.

Fig. 2
In vivo spontaneous multi-unit tubercle activity in anesthetized mice

To determine responsiveness of units to stimuli, the spike magnitude (total # of spikes) during both the 2sec pre-stimulus and the 2sec during stimulus were pooled across trials for each individual stimulus. Paired 2-tail t-tests were then performed to test for an evoked response. In this manner, a unit is said to be significantly modulated by a stimulus when, across all trials, there was a significant (p > .05) effect of stimulus presentation on the spike rate. All statistical analyses were performed in StatVIEW (SAS Institute Inc., Cary, NC) or in MATLAB (The MathWorks Inc., Natick, MA). All values are reported as mean ± standard error of the mean (SEM) unless otherwise stated.

RESULTS

Odor-evoked responses in the olfactory tubercle

To begin elucidating the potential contributions of the olfactory tubercle to the multimodal convergence of odors and sounds, we recorded from a total of 62 confirmed olfactory tubercle single-units in urethane anesthetized mice (1–3 units/mouse) in response to either odors, tones or both. The majority of olfactory tubercle single-units recorded were spontaneously active, with a 14.1 ± 15Hz (mean±std) spontaneous firing rate across all units (Fig. 3A). However, the mean was biased by several very high frequency single-units as most cells recorded from had a relatively low spontaneous firing rate (mode <5Hz, Fig. 3A). Neural activity at both the population (LFP) and single-unit level were generally phasic with respiration – with the phasic component of each response beginning momentarily after inspiration (Fig. 3B). Presentation of an odorant altered the LFP and spontaneous firing rate. In the example shown in Figure 3B, the odorant 1,7-octadiene evoked both a transformation of the LFP and two action potentials upon the first inhalation. In this particular example, subsequent inhalations of the odorant evoked LFP oscillations and unit action potentials until two inhalations after odorant offset (likely reflecting lingering of the odorant around the nostrils). The odor-evoked LFP responses were qualitatively similar to those seen elsewhere in the olfactory system (Freeman, 1978; Cenier et al., 2008; Kay et al., 2009) and consist of beta (10–35 Hz) and gamma band (40–70 Hz) high-frequency oscillations riding on the downward crest of an enhanced theta (1–10 Hz) rhythm (e.g., as in `0–100Hz' trace, Fig 3B). LFP power spectrum analyses (paired t-tests for 1Hz bins) confirmed that odorant presentation evoked significant changes in theta (t(9) = −3.45, p < .01), beta (t(24) = −8.04, p < .0001) and gamma (t(28) = −8.13, p < .0001) oscillatory activity (Fig. 3C). These results demonstrate, as predicted by both anatomical (Haberly and Price, 1978; Scott et al., 1980; Schwob and Price, 1984) and physiological studies (Murakami et al., 2005; Chiang and Strowbridge, 2007; Carriero et al., 2009), that the olfactory tubercle responds to odors at both the population and single-unit level.

Fig. 3
Odorant presentation evokes neural responses in the tubercle

Previous work has demonstrated that mitral-tufted cells in the OB (Wilson and Leon, 1988; Kay and Laurent, 1999; Cang and Isaacson, 2003; Nagayama et al., 2004; Rinberg et al., 2006; Davison and Katz, 2007) and layer II/III pyramidal cells of the PCX (Wilson, 2001; Rennaker et al., 2007; Yoshida and Mori, 2007; Poo and Isaacson, 2009) are capable of discriminating between odorants. However, whether tubercle units also discriminate odorants is unknown. Therefore, we recorded odor-evoked responses to a panel of 5 monomolecular odorants (see Methods) in 17 units (n = 11 mice). Odors were presented in a counterbalanced fashion, for a minimum of 4 trials each.

We found that olfactory tubercle single-units show robust responses to odorants. In particular, approximately 64 percent (11/17) of olfactory tubercle units showed significant responses to at least one of the 5 odorants (p < .05 for each odor/unit, ≥4 trials/odor, 2-tailed t-test). In the example shown in Figure 4A, the unit on top (unit #6) responds selectively to a single odorant (ethyl propionate). The unit beneath on the other hand (unit #2) has a broader receptive range and significantly responds to 3 of the 5 odorants (Fig. 4A). We also presented the same 17 units with overlapping, complex odorant mixtures – all sharing similar components yet with some omitted (Barnes et al., 2008). These odorant mixtures were presented in a counterbalanced manner along with the odorants used in Figure 4A. Out of 17 olfactory tubercle units, 8 (47%) showed significant responses to at least one of the 3 odorant mixtures (p < .05 for each mixture/unit, ≥4 trials/mixture). Out of the 8 units which significantly responded to a mixture, only 2 (20%) showed significant responses to all 3 mixtures (Fig. 4B). More commonly a single unit responded to just a single mixture (Fig. 4B) – even though all 3 mixtures shared overlapping components. These data suggest that olfactory tubercle single-units are capable of discretely responding to individual odorants.

Fig. 4
Odorant responsivity of tubercle units

Auditory evoked responses in the tubercle

We next examined whether the olfactory tubercle responds to auditory stimuli. Auditory-associated fiber projections into early olfactory processing areas originate within the hippocampus (Deadwyler et al., 1987), the ventral pallidum (Budinger et al., 2006) and even the primary auditory cortex itself (Budinger et al., 2008). To test the hypothesis that olfactory tubercle units are sensitive to auditory input, we presented anesthetized mice with a 2sec auditory stimulus (`tone') via a simple piezo speaker during simultaneous extracellular recordings.

Tones evoked responses at both the population (LFP) and single-unit level in the olfactory tubercle (Fig. 5). In the example shown in Figure 5A, a tone (onset timed to the respiration cycle as in Figs. 3 & 4) evoked an increase in spike rate across multiple trials. We screened a total of 26 isolated single-units for tone-evoked responses. 5 of the 26 units (>19%) showed significant responses to tone presentation (Fig. 5C; p < .05 for each unit, ≥4 tone trials/unit, 2-tailed t-test). Further, LFP power spectrum analysis revealed that whereas tone presentation did not significantly increase theta (t(9) = −1.36, p = .20) oscillations, beta (t(24) = −11.0, p < .0001) and gamma (t(28) = −7.36, p < .0001) oscillations were both enhanced with tone (Fig 5D). While we limited our analysis of auditory stimuli to a simple tone for the purposes of this study, we also informally observed that alternative auditory stimuli (i.e., manual clapping, electronic buzzer) also evoked unit-level changes in activity (data not shown). These data demonstrate the functional input of auditory information into the olfactory tubercle.

Fig. 5
Tubercle units respond to an auditory stimulus

Olfactory-Auditory modulation in the olfactory tubercle

The above data show, for the first time, that single-units in the olfactory tubercle respond to both olfactory and auditory input. Therefore, finally, to provide a test of olfactory-auditory modulation in the olfactory tubercle, we presented a subset of units (n=17) with a temporally overlapping odor and tone (odor+tone) in order to ask whether the presence of a stimulus in one modality affected the response to the other. The onset of both stimuli were timed off of the animal's respiratory signal as done previously for each stimulus (Figs. 35)(see Methods). LFP and unit traces for an odor, tone and the overlapping odor+tone are shown in Figure 6A. As reported in Figures 3 and and4,4, odor presentation evoked responses phasic with each respiration cycle. Also, similar as displayed in Figure 5, a tone elicited a brief burst of action potentials (Fig. 6A, middle). Finally, in this example simultaneous presentation of odor+tone resulted, on average, in a modest supra-additive effect (Fig. 6A (lower trace) and and6B).6B). As displayed in Figure 6B, across all 17 units, the average evoked response for the odor was greater than that of the tone ((t(352) = −2.64, p < .01), n=353 total trials). Odor+tone presentation on average elicited responses greater than that of the tone ((t(353) = −2.97, p < .005), n=355 total trials) but not of odor ((t(363) = −1.49, p > .05), n=365 total trials) alone. Similar to that of the odor presentation alone (see Fig. 3C), odor+tone elicited a significant change in beta (t(24) = −8.19, p < .0001) and gamma (t(28) = −8.10, p < .0001) oscillatory activity compared to the 2sec pre stimulus (Fig. 6C). No changes in theta oscillations were observed in response to odor+tone (t(9) = −1.98, p = .07).

Fig. 6
Olfactory-auditory interactions in olfactory tubercle single-units

As shown in Figure 7, olfactory tubercle single-units possessed diverse responses towards not only the odor or tone alone, but also to the odor+tone stimulus. It is likely that averaging across all units, as done in Figure 6B, presents a less-than-representative image of the cross-modal functions within the olfactory tubercle. Looking at the response of individual units, a subpopulation (>29%) of olfactory tubercle single-units displayed cross-modal modulation in response to odor+tone. For example, one unit showed a significant response to tone, but not odor and showed response-suppression to odor+tone (Fig. 7, `#'). In another example, a unit (Fig. 7, `*') failed to show significant responses to either tone or odor alone, but was significantly excited by odor+tone. Thus, particular sub-populations of units in the olfactory tubercle functionally display olfactory-auditory convergence and cross-modal modulation.

Fig. 7
Subpopulations of tubercle single-units show olfactory-auditory cross-modal modulation

Absence of olfactory-auditory convergence in upstream OB neurons

The data so far suggest that a population of olfactory tubercle units are capable of olfactory-auditory convergence. This effect may be due to the direct convergence of both olfactory and auditory input at the level of the tubercle. Alternatively, convergence may arrive upstream (within the OB) which is simply reflected within the olfactory tubercle. Therefore, finally, we examined whether upstream MTs within the OB show responses to a tone.

We performed extracellular MT single and/or multi-unit recordings in an additional set of experiments (n=5 mice, 20 units). Similar to that done for olfactory tubercle recordings (Figs. 6 & 7), MT unit responses were assessed in the presence of either a tone, odor and an odor+tone stimulus (n=5–15 trials/stimulus/unit; see Methods). As shown in Figures 8A and 8B, individual MTs showed responses to the odor stimulus (t(214) = −10.47, p < .0001), and to the odor+tone stimulus (t(214) = −9.89, p < .0001), yet failed to respond to the tone alone (t(214) = −1.49, p > .05). In fact, 0/20 of the MT units showed a significant response to tone (p > .05 for each unit, ≥5 tone trials/unit, 2-tailed t-test). Further, the evoked responses of odor+tone stimuli did not significantly differ from that of odor alone (Fig. 8B; (t(214) = −.02, p > .05)). This was true even at the single-unit (vs. group) level. Indeed, unlike in the tubercle, no individual MT units showed significant differences in evoked-responses between odor and odor+tone (p > .05 for each unit, ≥5 trials/unit, 2-tailed t-test; data not shown). Thus, while units within the OB and the olfactory tubercle both represent odors (Fig. 3 & 8A), olfactory tubercle units respond to both olfactory and auditory information.

Fig. 8
Olfactory-auditory convergence is not detected upstream of the olfactory tubercle

DISCUSSION

To date the majority of research on the olfactory tubercle has emphasized its apparent role in reward circuitry (for review see (Ikemoto, 2007)). In contrast, despite its vast interconnectedness (Haberly and Price, 1977; Schwob and Price, 1984; Johnson et al., 2000), surprisingly little attention has been devoted to this relatively large region with regards to its functional role in sensory processing. Here we used extracellular recordings in anesthetized mice to examine a previously unexplored role of the olfactory tubercle – cross-modal sensory convergence. We found that single-units in the olfactory tubercle not only show selective odor responses, but also demonstrate apparent convergence of olfactory and auditory inputs with single-units displaying responses to both odors and tones. Remarkably, some units displayed cross-modal modulation, with supra-additive or suppressive responses to the simultaneous presentation of odor and tone. Thus, the tubercle serves cross-modal functions, likely binding smells and sounds early in the olfactory processing stream.

Olfactory coding in the olfactory tubercle

Projections from the OB target a wide-range of secondary olfactory regions - including the olfactory tubercle. Mitral/tufted cells (MTs) of the OB fasciculate to form the lateral olfactory tract which travels along the ventral-lateral aspect of the brain. Anterior to, and around the same anatomical locations where MTs project into the anterior PCX, MTs project into olfactory tubercle layer I and synapse onto primary tubercle principle neurons. The olfactory tubercle has received little attention in attempts to understand its functional roles in olfactory coding (but see (McNamara et al., 2004; Chiang and Strowbridge, 2007; Carriero et al., 2009). Indeed, to our knowledge, only one study (Murakami et al., 2005) has recorded odor-evoked unit responses in vivo. What we do know about the olfactory-processing function of the olfactory tubercle is that olfactory sensory input from the OB can arrive in the tubercle via the lateral olfactory tract or via the PCX (Carriero et al., 2009). Voltage sensitive dye imaging from in vitro guinea pig brains showed that the olfactory tubercle is homogeneously activated by lateral olfactory tract stimulation in a biphasic manner – yet with slightly differing temporal dynamics (Carriero et al., 2009). Activity spreads throughout the tubercle from lateral to medial – likely reflecting the MT input along the lateral edge (Carriero et al., 2009). Further experiments by the same group revealed that severing the lateral olfactory tract projection into the PCX reduced the tubercle's secondary biphasic response. Indeed, the tubercle is interconnected with the PCX and tenia tecta by a large association fiber network (Luskin and Price, 1983). Thus, olfactory information can enter the tubercle by two routes: directly from the OB via MTs or by PCX association fibers.

Here, in agreement with the standing anatomical (Haberly and Price, 1977; Schwob and Price, 1984; Johnson et al., 2000) and functional evidence (Murakami et al., 2005; Chiang and Strowbridge, 2007; Carriero et al., 2009), we found that the olfactory tubercle responds to odors. On the population level, olfactory tubercle LFP's showed robust odor-evoked oscillations – reflecting global activation of the olfactory tubercle network by individual odors. Further, on a single-unit level, odor-responsive units were readily observed – with almost 70% of single-units responsive to at least one of the 5 odorants we tested. Notably, screening responses to an even larger battery of odorants (including aldehydes and ketones for example) may have resulted in a greater portion of units responsive to odorants. Additionally, while not a focus of the present study, LFP and single-unit activity displayed entrainment to the respiratory rhythm. Given the importance of respiratory behavior on shaping the neural code for odors, future studies into respiratory coupling of odor information, especially in relation with PCX and/or OB activity will provide useful information on the coherence between these primary structures in conveying the early olfactory code.

Mechanisms of olfactory-auditory cross-modal convergence

Previous studies have uncovered olfactory-visual (Gottfried and Dolan, 2003) and olfactory-gustatory cross-modal convergence and integration (for review see (Verhagen and Engelen, 2006)). In the present studies we explored the olfactory tubercle for the existence of olfactory-auditory convergence. We found that ~20% of tubercle units were significant driven by an auditory tone. In contrast, 0% of upstream neurons recorded in the OB displayed significant tone-evoked responses. In the present study a single tone and intensity was tested, thus whether and how the olfactory tubercle may code for auditory stimuli of differing intensities or frequencies will be an interesting avenue for future research. Further, and especially relevant to the present study is whether olfactory-auditory interactions in the tubercle depend upon particular auditory stimuli. For instance, it is possible that spike rate in the olfactory tubercle is proportionally related with tone intensity. Finally, whether the olfactory tubercle acts alone or facilitates olfactory-auditory cross-modal interactions with other regions remains to be explored. For example, it is unclear if other olfactory cortical areas also show cross-modal convergence like that shown here in the tubercle. Furthermore, the auditory cortex and possibly other olfactory processing areas may serve a role in olfactory-auditory cross-modal convergence (Budinger and Scheich, 2009). Indeed, auditory cortex evoked potential (Halene et al., 2009) and single-unit responses are modulated by olfactory cues in behaving rats (Otazu et al., 2009). Despite these issues, the present results clearly demonstrate that the single-units in the olfactory tubercle bind information from both olfactory and auditory sensory channels.

Previous work in the olfactory system has shown extra-modal stimuli are capable of non-specific modulating olfactory pathway activity (Gray et al., 1986; Wilson and Sullivan, 1990; Bouret and Sara, 2002). Both the unit-selectivity (only ~20% of units responded to tone) and temporal precision of auditory-evoked spikes in the tubercle argue that these responses are truly cross-modal convergence, and not the result of non-specific neuromodulatory influences. Based on previous work (Wilson and Sullivan, 1990; Bouret and Sara, 2002) such responses, if due to neuromodulatory influences, may likely be more prevalent in the tubercle and likely would last for longer durations.

Cognitive and behavioral implications for olfactory-auditory convergence

The olfactory tubercle is part of the mesocorticolimbic pathway. Previous work has suggested that increased dopaminergic activity throughout this pathway, including within the olfactory tubercle, facilitates changes in behavioral strategies (Koob et al., 1978; Oades, 1981). Therefore, it is possible that the olfactory-auditory convergence and modulation in the olfactory tubercle may play a role in rapid behavioral plasticity in arousing situations. For instance, if a mouse hears the sound of an approaching predator's paw steps, this may heighten responsiveness of olfactory tubercle units to predator odor. The location of the olfactory tubercle in the mesocorticolimbic pathway positions this newly integrated information in an ideal manner for the rapid, arousal-related behavioral responses required to flee the oncoming predator. Thus, in general the olfactory tubercle may serve as a site for not only olfactory arousal-related decision making, but also odor-orientation and localization. In support of this, recent work has demonstrated that rats increase their sniffing frequency – a behavior normally implicated in olfactory behaviors – during novel tone investigation (Harrison, 1979) and during a simple auditory discrimination task (Wesson et al., 2009).

Finally, intrinsic modulation of olfactory codes based upon auditory input may underlie clinical reports of olfactory-auditory synesthesia (Simpson and McKellar, 1955) and possibly the ability for persons to relate auditory pitch with odor perceptual quality (Belkin et al., 1997). While our results demonstrate for the first time that the olfactory tubercle is a site for olfactory and auditory convergence and that such activity is susceptible to cross-modal influences, whether and how these cross-modal influences may be altered based upon cognitive task-demands and levels of arousal is unknown.

Acknowledgments

This work was supported by grant DC003906 to D.A.W. from the National Institutes of Health.

References

  • Barnes DC, Hofacer RD, Zaman AR, Rennaker RL, Wilson DA. Olfactory perceptual stability and discrimination. Nat Neurosci. 2008;11:1378–1380. [PMC free article] [PubMed]
  • Belkin K, Martin R, Kemp SE, Gilbert AN. Auditory pitch as a perceptual analogue to odor quality. Psychological Science. 1997;8:340–342.
  • Bouret S, Sara SJ. Locus coeruleus activation modulates firing rate and temporal organization of odour-induced single-cell responses in rat piriform cortex. Eur J Neurosci. 2002;16:2371–2382. [PubMed]
  • Budinger E, Scheich H. Anatomical connections suitable for the direct processing of neuronal information of different modalities via the rodent primary auditory cortex. Hearing Research. 2009 In Press, Corrected Proof. [PubMed]
  • Budinger E, Heil P, Hess A, Scheich H. Multisensory processing via early cortical stages: Connections of the primary auditory cortical field with other sensory systems. Neuroscience. 2006;143:1065–1083. [PubMed]
  • Budinger E, Laszcz A, Lison H, Scheich H, Ohl FW. Non-sensory cortical and subcortical connections of the primary auditory cortex in Mongolian gerbils: Bottom-up and top-down processing of neuronal information via field AI. Brain Research. 20081220:2–32. [PubMed]
  • Calvert GA, Spence C, Stein, editors. The handbook of multisensory processes. 1st Edition The MIT Press; Cambridge, MA: 2004.
  • Cang J, Isaacson JS. In vivo whole-cell recording of odor-evoked synaptic transmission in the rat olfactory bulb. J Neurosci. 2003;23:4108–4116. [PubMed]
  • Carriero G, Uva L, Gnatkovsky V, de Curtis M. Distribution of the Olfactory Fiber Input Into the Olfactory Tubercle of the In Vitro Isolated Guinea Pig Brain. J Neurophysiol. 2009;101:1613–1619. [PubMed]
  • Cenier T, Amat C, Litaudon P, Garcia S, Lafaye de Micheaux P, Liquet B, Roux S, Buonviso N. Odor vapor pressure and quality modulate local field potential oscillatory patterns in the olfactory bulb of the anesthetized rat. Eur J Neurosci. 2008;27:1432–1440. [PubMed]
  • Chiang E, Strowbridge BW. Diversity of neural signals mediated by multiple, burst-firing mechanisms in rat olfactory tubercle neurons. J Neurophysiol. 2007;98:2716–2728. [PubMed]
  • Davis M. Sensitization of the acoustic startle reflex by footshock. Behav Neurosci. 1989;103:495–503. [PubMed]
  • Davison IG, Katz LC. Sparse and selective odor coding by mitral/tufted neurons in the main olfactory bulb. J Neurosci. 2007;27:2091–2101. [PubMed]
  • Deadwyler SA, Foster TC, Hampson RE. Processing of sensory information in the hippocampus. CRC Crit Rev Clin Neurobiol. 1987;2:335–355. [PubMed]
  • Freeman WJ. Spatial properties of an EEG event in the olfactory bulb and cortex. Electroencephalogr Clin Neurophysiol. 1978;44:586–605. [PubMed]
  • Gottfried JA, Dolan RJ. The nose smells what the eye sees: crossmodal visual facilitation of human olfactory perception. Neuron. 2003;39:375–386. [PubMed]
  • Gray CM, Freeman WJ, Skinner JE. Chemical dependencies of learning in the rabbit olfactory bulb: acquisition of the transient spatial pattern change depends on norepinephrine. Behav Neurosci. 1986;100:585–596. [PubMed]
  • Haberly LB, Price JL. The axonal projection patterns of the mitral and tufted cells of the olfactory bulb in the rat. Brain Res. 1977;129:152–157. [PubMed]
  • Haberly LB, Price JL. Association and commissural fiber systems of the olfactory cortex of the rat. I. systems originating in the piriform cortex and adjacent areas. J Comp Neurol. 1978;178:711–740. [PubMed]
  • Halene TB, Talmud J, Jonak GJ, Schneider F, Siegel SJ. Predator odor modulates auditory event-related potentials in mice. Neuroreport. 2009;20:1260–1264. [PubMed]
  • Harrison JM. The control of responding by sounds: unusual effect of reinforcement. J Exp Anal Behav. 1979;32:167–181. [PMC free article] [PubMed]
  • Ikemoto S. Dopamine reward circuitry: Two projection systems from the ventral midbrain to the nucleus accumbens-olfactory tubercle complex. Brain Research Reviews. 2007;56:27–78. [PMC free article] [PubMed]
  • Johnson DM, Illig KR, Behan M, Haberly LB. New features of connectivity in piriform cortex visualized by intracellular injection of pyramidal cells suggest that “primary” olfactory cortex functions like “association” cortex in other sensory systems. J Neurosci. 2000;20:6974–6982. [PubMed]
  • Kay LM, Laurent G. Odor- and context-dependent modulation of mitral cell activity in behaving rats. Nat Neurosci. 1999;2:1003–1009. [PubMed]
  • Kay LM, Beshel J, Brea J, Martin C, Rojas-Líbano D, Kopell N. Olfactory oscillations: the what, how and what for. Trends in Neurosciences. 2009;32:207–214. [PMC free article] [PubMed]
  • Koob GF, Riley SJ, Smith SC, Robbins TW. Effects of 6-hydroxydopamine lesions of the nucleus accumbens septi and olfactory tubercle on feeding, locomotor activity, and amphetamine anorexia in the rat. J Comp Physiol Psychol. 1978;92:917–927. [PubMed]
  • Luskin MB, Price JL. The topographic organization of associational fibers of the olfactory system in the rat, including centrifugal fibers to the olfactory bulb. The Journal of Comparative Neurology. 1983;216:264–291. [PubMed]
  • McNamara AM, Cleland TA, Linster C. Characterization of the synaptic properties of olfactory bulb projections. Chem Senses. 2004;29:225–233. [PubMed]
  • Murakami M, Kashiwadani H, Kirino Y, Mori K. State-dependent sensory gating in olfactory cortex. Neuron. 2005;46:285–296. [PubMed]
  • Nagayama S, Takahashi YK, Yoshihara Y, Mori K. Mitral and tufted cells differ in the decoding manner of odor maps in the rat olfactory bulb. J Neurophysiol. 2004;91:2532–2540. [PubMed]
  • Oades RD. Dopaminergic agonistic and antagonistic drugs in the ventral tegmentum of rats inhibit and facilitate changes of food-search behaviour. Neurosci Lett. 1981;27:75–80. [PubMed]
  • Otazu GH, Tai L-H, Yang Y, Zador AM. Engaging in an auditory task suppresses responses in auditory cortex. Nat Neurosci. 2009;12:646–654. [PubMed]
  • Paxinos G, Franklin K. The Mouse Brain in Stereotaxic Coordinates. 2nd Edition Academic Press; San Diego: 2000.
  • Piesse G. The Art of Perfumery, And Methods of Obtaining the Odors of Plants. Lindsay & Blakiston; Philadelphia: 1857.
  • Poo C, Isaacson JS. Odor Representations in Olfactory Cortex: Sparse Coding, Global Inhibition, and Oscillations. 2009;62:850–861. [PMC free article] [PubMed]
  • Rankin CH, Abrams T, Barry RJ, Bhatnagar S, Clayton DF, Colombo J, Coppola G, Geyer MA, Glanzman DL, Marsland S, McSweeney FK, Wilson DA, Wu C-F, Thompson RF. Habituation revisited: An updated and revised description of the behavioral characteristics of habituation. Neurobiology of Learning and Memory. 2009;92:135–138. [PMC free article] [PubMed]
  • Rennaker RL, Chen C-FF, Ruyle AM, Sloan AM, Wilson DA. Spatial and Temporal Distribution of Odorant-Evoked Activity in the Piriform Cortex. J Neurosci. 2007;27:1534–1542. [PMC free article] [PubMed]
  • Rinberg D, Koulakov A, Gelperin A. Sparse odor coding in awake behaving mice. J Neurosci. 2006;26:8857–8865. [PubMed]
  • Schwob JE, Price JL. The development of axonal connections in the central olfactory system of rats. J Comp Neurol. 1984;223:177–202. [PubMed]
  • Scott JW, McBride RL, Schneider SP. The organization of projections from the olfactory bulb to the piriform cortex and olfactory tubercle in the rat. J Comp Neurol. 1980;194:519–534. [PubMed]
  • Simpson L, McKellar P. Types of Synaesthesia. Journal of Mental Science. 1955;101:141–147. [PubMed]
  • Smith JJ, Shionoya K, Sullivan RM, Wilson DA. Auditory stimulation dishabituates olfactory responses via noradrenergic cortical modulation. Neural Plasticity. 20092009:1–6. [PMC free article] [PubMed]
  • Verhagen JV, Engelen L. The neurocognitive bases of human multimodal food perception: Sensory integration. Neuroscience & Biobehavioral Reviews. 2006;30:613–650. [PubMed]
  • Wesson DW, Verhagen JV, Wachowiak M. Why Sniff Fast? The Relationship Between Sniff Frequency, Odor Discrimination, and Receptor Neuron Activation in the Rat. J Neurophysiol. 2009;101:1089–1102. [PubMed]
  • Wilson DA. Habituation of odor responses in the rat anterior piriform cortex. J Neurophysiol. 1998;79:1425–1440. [PubMed]
  • Wilson DA. Receptive fields in the rat piriform cortex. Chem Senses. 2001;26:577–584. [PubMed]
  • Wilson DA, Leon M. Spatial patterns of olfactory bulb single-unit responses to learned olfactory cues in young rats. J Neurophysiol. 1988;59:1770–1782. [PubMed]
  • Wilson DA, Sullivan RM. Olfactory associative conditioning in infant rats with brain stimulation as reward. I. Neurobehavioral consequences. Brain Res Dev Brain Res. 1990;53:215–221. [PubMed]
  • Yoshida I, Mori K. Odorant category profile selectivity of olfactory cortex neurons. J Neurosci. 2007;27:9105–9114. [PubMed]