Normative and mechanistic model of an adaptive circuit for efficient encoding and feature extraction.

One major question in neuroscience is how to relate connectomes to neural activity, circuit function, and learning. We offer an answer in the peripheral olfactory circuit of the Drosophila larva, composed of olfactory receptor neurons (ORNs) connected through feedback loops with interconnected inhibitory local neurons (LNs). We combine structural and activity data and, using a holistic normative framework based on similarity-matching, we formulate biologically plausible mechanistic models of the circuit. In particular, we consider a linear circuit model, for which we derive an exact theoretical solution, and a nonnegative circuit model, which we examine through simulations. The latter largely predicts the ORN [Formula: see text] LN synaptic weights found in the connectome and demonstrates that they reflect correlations in ORN activity patterns. Furthermore, this model accounts for the relationship between ORN [Formula: see text] LN and LN-LN synaptic counts and the emergence of different LN types. Functionally, we propose that LNs encode soft cluster memberships of ORN activity, and partially whiten and normalize the stimulus representations in ORNs through inhibitory feedback. Such a synaptic organization could, in principle, autonomously arise through Hebbian plasticity and would allow the circuit to adapt to different environments in an unsupervised manner. We thus uncover a general and potent circuit motif that can learn and extract significant input features and render stimulus representations more efficient. Finally, our study provides a unified framework for relating structure, activity, function, and learning in neural circuits and supports the conjecture that similarity-matching shapes the transformation of neural representations.

Gross morphology, histology, and ultrastructure of the olfactory rosette of a critically endangered indicator species, the Delta Smelt, Hypomesus transpacificus.

The Delta Smelt (Hypomesus transpacificus) is a small, semi-anadromous fish native to the San Francisco Bay-Delta Estuary and has been declared as critically endangered. Their olfactory biology, in particular, is poorly understood and a basic description of their sensory anatomy is needed to advance our understanding of the sensory ecology of species to inform conservation efforts to manage and protect them. We provide a description of the gross morphology, histological, immunohistochemical, and ultrastructural features of the olfactory rosette in this fish and discuss some of the functional implications in relation to olfactory ability. We show that Delta Smelt have a multilamellar olfactory rosette with allometric growth. Calretinin immunohistochemistry revealed a diffuse distribution of olfactory receptor neurons within the epithelium. Ciliated, microvillous and crypt neurons were clearly identified using morphological and immunohistochemical features. The olfactory neurons were supported by robust ciliated and secretory sustentacular cells. Although the sense of smell has been overlooked in Delta Smelt, we conclude that the olfactory epithelium has many characteristics of macrosmatic fish. With this study, we provide a foundation for future research into the sensory ecology of this imperiled fish.

Initial Characterization of a Subpopulation of Inherent Oscillatory Mammalian Olfactory Receptor Neurons.

Published evidence suggests that inherent rhythmically active or "bursting" primary olfactory receptor neurons (bORNs) in crustaceans have the previously undescribed functional property of encoding olfactory information by having their rhythmicity entrained by the odor stimulus. In order to determine whether such bORN-based encoding is a fundamental feature of olfaction that extends beyond crustaceans, we patch-clamped bORN-like ORNs in mice, characterized their dynamic properties, and show they align with the dynamic properties of lobster bORNs. We then characterized bORN-like activity by imaging the olfactory epithelium of OMP-GCaMP6f mice. Next, we showed rhythmic activity is not dependent upon the endogenous OR by patching ORNs in OR/GFP mice. Lastly, we showed the properties of bORN-like ORNs characterized in mice generalize to rats. Our findings suggest encoding odor time should be viewed as a fundamental feature of olfaction with the potential to be used to navigate odor plumes in animals as diverse as crustaceans and mammals.

Neuronal Response Latencies Encode First Odor Identity Information across Subjects.

Odorants are coded in the primary olfactory processing centers by spatially and temporally distributed patterns of glomerular activity. Whereas the spatial distribution of odorant-induced responses is known to be conserved across individuals, the universality of its temporal structure is still debated. Via fast two-photon calcium imaging, we analyzed the early phase of neuronal responses in the form of the activity onset latencies in the antennal lobe projection neurons of honeybee foragers. We show that each odorant evokes a stimulus-specific response latency pattern across the glomerular coding space. Moreover, we investigate these early response features for the first time across animals, revealing that the order of glomerular firing onsets is conserved across individuals and allows them to reliably predict odorant identity, but not concentration. These results suggest that the neuronal response latencies provide the first available code for fast odor identification.SIGNIFICANCE STATEMENT Here, we studied early temporal coding in the primary olfactory processing centers of the honeybee brain by fast imaging of glomerular responses to different odorants across glomeruli and across individuals. Regarding the elusive role of rapid response dynamics in olfactory coding, we were able to clarify the following aspects: (1) the rank of glomerular activation is conserved across individuals, (2) its stimulus prediction accuracy is equal to that of the response amplitude code, and (3) it contains complementary information. Our findings suggest a substantial role of response latencies in odor identification, anticipating the static response amplitude code.

Protective effects of melatonin and selenium against apoptosis of olfactory sensory neurons: A rat model study.

BACKGROUND: Selenium plays a role in the prevention of oxidative damage and has been linked to regulatory functions in cell growth, apoptosis, cell survival, and cytotoxicity. Melatonin has an antioxidant effect, which protects against a number of free radical species. Given its antioxidant properties, melatonin has been widely known to inhibit neuronal apoptosis. We examined the cytoprotective effects of melatonin and selenium in rat olfactory sensory neurons after rhinosinusitis by immunohistochemical evaluation of olfactory bulb mucosa. METHODS: Rhinosinusitis was induced bilaterally in 24 animals. Twenty-four rats were randomly divided into three equal groups. The melatonin group was treated with intraperitoneal (i.p.) melatonin and ampicillin-sulbactam, the selenium group was treated with i.p. selenium and ampicillin-sulbactam, the antibiotic group was treated with i.p. ampicillin-sulbactam; all three groups were treated for 10 days. After a period of 10 days of treatment, the animals were killed for immunohistochemical analyses. All olfactory bulb mucosae were removed immediately. RESULTS: No histochemical differences were found in the three groups. Terminal deoxynucleotidyl transferase 2'-deoxyuridine 5'-triphosphate nick end labeling-positive cells were detected in each group. In the antibiotic group, the appearance of apoptotic cells was higher, whereas the number of apoptotic cells significantly decreased in the melatonin group. When compared with the selenium group, fewer terminal deoxynucleotidyl transferase 2'-deoxyuridine 5'-triphosphate nick end labeling-positive cells were observed in the melatonin group, which was not significant. In the antibiotic group, the cytoplasmic active caspase-3 and Bax immunostaining in the olfactory epithelium and glandular cells of stroma were higher when compared with the immunostaining in melatonin and selenium groups. Active caspase-3 and Bax immunostaining in the subepithelial stroma was dramatically reduced in the melatonin group. In contrast, the staining intensity and the number of Bcl-2 immunopositive cells were significantly increased in the melatonin group. In the selenium group, Bax and active caspase-3 were moderately immunopositive in the epithelium and subepithelial stroma. However, Bcl-2 immunostaining was more pronounced in the olfactory epithelium and some stromal cells. CONCLUSION: Our results indicated the possibility that the supplementation of melatonin and selenium, two antioxidant agents for the treatments in the rhinosinusitis rat model, might be reduced or prevent anosmia.

Effect of 830-nm laser phototherapy on olfactory neuronal ensheathing cells grown in vitro on novel bioscaffolds.

BACKGROUND: The purpose of this study was to analyze olfactory ensheathing cell (OEC) proliferation and growth on Biosilicate and collagen bioscaffolds, and to determine whether the application of laser phototherapy would result in increased OEC proliferation on the scaffolds. The use of bioscaffolds is considered a promising strategy in a number of clinical applications where tissue healing is suboptimal. As in vitro OEC growth is a slow process, laser phototherapy could be useful to stimulate proliferation on bioscaffolds. METHODS: OEC cells were seeded on the Biosilicate and collagen scaffolds. Seeded scaffolds were irradiated with a single exposure of 830-nm laser. Nonirradiated seeded scaffolds acted as negative controls. Cell proliferation was assessed 7 days after irradiation. RESULTS: OECs were successfully grown on discs composed of a glass-ceramic and collagen composite. Laser irradiation produced a 32.7% decrease and a 13.2% increase in OEC proliferation on glass-ceramic discs and on collagen scaffolds, respectively, compared with controls. Laser phototherapy resulted in a reduction in cell growth on the Biosilicate scaffolds and an increase in cell proliferation on collagen scaffolds. CONCLUSIONS: These results were probably due to the nature of the materials. Future research combining laser phototherapy and glass-ceramic scaffolds should take into account possible interactions of the laser with matrix compounds.

EAG responses increase of Spodoptera littoralis antennae after a single pheromone pulse.

Increased behavioral sensitivity to the pheromone after brief exposure of the whole insect to the sex pheromone has been documented in antennal lobe neurons of Spodoptera littoralis. We investigated whether a brief stimulus of the major component of the pheromone on naïve antenna separated from the head increased the electroantennographic responses after successive stimulations at different times. The response increase was clear 30 min after the first stimulation, and this effect lasted at least 60 min, the average life time of the antenna. Our results suggest that the olfactory receptor neurons, and not only the neurons in the antennal lobe, may be involved in the increased antennal response after a single pheromone pulse.

Innate responses to putative ancestral hosts: is the attraction of Western flower thrips to pine pollen a result of relict olfactory receptors?

Pollinophagy is widely documented in the order Thysanoptera, with representative individuals from six of the nine divergent families known to feed on pollen. Various pollens of the genus Pinus increase the development time, fecundity, longevity, and settling preference of Western Flower Thrips (WFT), Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae). Certain species of flower thrips discriminate among pollen types, but no studies have elucidated the olfactory cues that play a role in their pollen preferences. In this study, the volatile organic compounds emitted by pollens of the genus Pinus were elucidated. Various chemicals from pollen headspace elicited electrophysiological responses from WFT antennae. The compound (S)-(-)-verbenone, identified in pollen headspace, attracted WFT in a 4-arm olfactometer. This compound has potential for use in integrated pest management programs against the pest. We present the hypothesis that this polyphagous insect may have retained ancestral 'relict' olfactory receptors through the course of evolution, to explain this attraction to pine pollen. This attraction has allowed the insect to find and exploit an unusual nutrient source that significantly increases its fitness. The study demonstrates how fossil record analysis and subsequent evolutionary knowledge can aid in explaining possibilities as to why some insects sense and respond to chemicals that would otherwise seem peculiar to their ecology, allowing insight into the evolutionary forces that may shape insect olfactory systems over time.

Intermittency coding in the primary olfactory system: a neural substrate for olfactory scene analysis.

The spatial and temporal characteristics of the visual and acoustic sensory input are indispensable attributes for animals to perform scene analysis. In contrast, research in olfaction has focused almost exclusively on how the nervous system analyzes the quality and quantity of the sensory signal and largely ignored the spatiotemporal dimension especially in longer time scales. Yet, detailed analyses of the turbulent, intermittent structure of water- and air-borne odor plumes strongly suggest that spatio-temporal information in longer time scales can provide major cues for olfactory scene analysis for animals. We show that a bursting subset of primary olfactory receptor neurons (bORNs) in lobster has the unexpected capacity to encode the temporal properties of intermittent odor signals. Each bORN is tuned to a specific range of stimulus intervals, and collectively bORNs can instantaneously encode a wide spectrum of intermittencies. Our theory argues for the existence of a novel peripheral mechanism for encoding the temporal pattern of odor that potentially serves as a neural substrate for olfactory scene analysis.

Ionotropic glutamate receptors IR64a and IR8a form a functional odorant receptor complex in vivo in Drosophila.

Drosophila olfactory sensory neurons express either odorant receptors or ionotropic glutamate receptors (IRs). The sensory neurons that express IR64a, a member of the IR family, send axonal projections to either the DC4 or DP1m glomeruli in the antennal lobe. DC4 neurons respond specifically to acids/protons, whereas DP1m neurons respond to a broad spectrum of odorants. The molecular composition of IR64a-containing receptor complexes in either DC4 or DP1m neurons is not known, however. Here, we immunoprecipitated the IR64a protein from lysates of fly antennal tissue and identified IR8a as a receptor subunit physically associated with IR64a by mass spectrometry. IR8a mutants and flies in which IR8a was knocked down by RNAi in IR64a+ neurons exhibited defects in acid-evoked physiological and behavioral responses. Furthermore, we found that the loss of IR8a caused a significant reduction in IR64a protein levels. When expressed in Xenopus oocytes, IR64a and IR8a formed a functional ion channel that allowed ligand-evoked cation currents. These findings provide direct evidence that IR8a is a subunit that forms a functional olfactory receptor with IR64a in vivo to mediate odor detection.

Adaptation as a mechanism for gain control in cockroach ON and OFF olfactory receptor neurons.

In many sensory systems adaptation acts as a gain control mechanism that optimizes sensory performance by trading increased sensitivity to low stimulus intensity for decreased sensitivity to high stimulus intensity. Adaptation of insect antennal olfactory receptor neurons (ORNs) has been studied for strong odour concentrations, either pulsed or constant. Here, we report that during slowly oscillating changes in the concentration of the odour of lemon oil, the ON and OFF ORNs on the antenna of the cockroach Periplaneta americana adapt to the actual odour concentration and the rate at which concentration changes. When odour concentration oscillates rapidly with brief periods, adaptation improves gain for instantaneous odour concentration and reduces gain for the rate of concentration change. Conversely, when odour concentration oscillates slowly with long periods, adaptation increases gain for the rate of change at the expense of instantaneous concentration. Without this gain control the ON and OFF ORNs would, at brief oscillation periods, soon reach their saturation level and become insensitive to further concentration increments and decrements. At long oscillation periods, on the other hand, the cue would simply be that the discharge begins to change. Because of the high gain for the rate of change, the cockroach will receive creeping changes in odour concentration, even if they persist in one direction. Gain control permits a high degree of precision at small rates when it counts most, without sacrificing the range of detection and without extending the measuring scale.

¿Cómo se conecta la olfacción? Mecanismos celulares y moleculares que dirigen el desarrollo de las conexiones sinápticas desde la nariz a la corteza (I) / How is the sense of smell connected? Cellular and molecular mechanisms guiding the development

Resumen. Dentro del conjunto del sistema nervioso central, el sistema olfativo resulta fascinante por sus particularidades fisiológicas durante el desarrollo, siendo uno de los modelos más estudiados para entender los mecanismos relacionados con la guía y el crecimiento axonal hacia sus objetivos apropiados. Se conoce una constelación de mecanismos, unos mediados por contacto (lamininas, moléculas de adhesión celular, efrinas, etc.) y otros secretables (semaforinas, slits, factores de crecimiento, etc.), por desempeñar diversas funciones en el establecimiento de las interacciones sinápticas entre el epitelio olfativo, el bulbo olfativo y la corteza olfativa. También se han propuesto al respecto otros mecanismos específicos de este sistema, incluida la increíble familia de cerca de 1.000 receptores olfativos distintos. En los últimos años, diferentes revisiones se han concentrado en los elementos parciales de este sistema, sobre todo en los mecanismos implicados en la formación del nervio olfativo, echándose en falta una revisión detallada de aquellos relacionados con el desarrollo de las conexiones entre las distintas estructuras olfativas (epitelio, bulbo y corteza). En esta primera parte de la revisión, abordamos este tema desde el siguiente enfoque: los diversos mecanismos celulares y moleculares que dirigen la formación del nervio olfativo y el tracto olfativo lateral (AU)

Summary. The physiological particularities that occur during the development of the olfactory system make it one of the most fascinating parts of the central nervous system and one of models that has been most widely studied in order to understand the mechanisms related with axonal growth and guidance towards the right targets. A variety of mechanisms are known, some mediated by contact (laminins, cell adhesion molecules, ephrins, etc.) and others that are secreted (semaphorins, slits, growth factors, etc.), to play diverse roles in establishing the synaptic interactions among the olfactory epithelium, the olfactory bulb and the olfactory cortex. In relation to this, other specific mechanisms for this system have also been proposed, including the incredible family of close to 1000 different olfactory receptors. In recent years, different reviews have focused on the partial elements of this system, especially on the mechanisms involved in the formation of the olfactory nerve. However, no detailed review of those related with the development of the connections between the different olfactory structures (epithelium, bulb and cortex) has been put forward to date. In this first part of the review, we address this topic from the following perspective: the different cellular and molecular mechanisms that guide the formation of the olfactory nerve and the lateral olfactory tract (AU)

Role of the olfactory receptor neurons in the direct transport of inhaled uranium to the rat brain.

Uranium presents numerous industrial and military uses and one of the most important risks of contamination is dust inhalation. In contrast to the other modes of contamination, the inhaled uranium has been proposed to enter the brain not only by the common route of all modes of exposure, the blood pathway, but also by a specific inhalation exposure route, the olfactory pathway. To test whether the inhaled uranium enter the brain directly from the nasal cavity, male Sprague-Dawley rats were exposed to both inhaled and intraperitoneally injected uranium using the (236)U and (233)U, respectively, as tracers. The results showed a specific frontal brain accumulation of the inhaled uranium which is not observed with the injected uranium. Furthermore, the inhaled uranium is higher than the injected uranium in the olfactory bulbs (OB) and tubercles, in the frontal cortex and in the hypothalamus. In contrast, the other cerebral areas (cortex, hippocampus, cerebellum and brain residue) did not show any preferential accumulation of inhaled or injected uranium. These results mean that inhaled uranium enters the brain via a direct transfer from the nasal turbinates to the OB in addition to the systemic pathway. The uranium transfer from the nasal turbinates to the OB is lower in animals showing a reduced level of olfactory receptor neurons (ORN) induced by an olfactory epithelium lesion prior to the uranium inhalation exposure. These results give prominence to a role of the ORN in the direct transfer of the uranium from the nasal cavity to the brain.

Leukemia inhibitory factor promotes olfactory sensory neuronal survival via phosphoinositide 3-kinase pathway activation and Bcl-2.

Leukemia inhibitory factor (LIF), a neuropoietic cytokine, has been implicated in the control of neuronal development. We previously reported that LIF plays a critical role in regulating the terminal differentiation of olfactory sensory neurons (OSNs). Here, we demonstrate that LIF plays a complementary role in supporting the survival of immature OSNs. Mature OSNs express LIF, which may be elaborated in a paracrine manner to influence adjacent neurons. LIF null mice display more apoptotic immature neurons than do their wild-type littermates. LIF treatment of dissociated OSNs in vitro significantly reduces the apoptosis of immature OSNs. Double immunocytochemical analysis indicates that the survival of immature OSNs is dependent on the presence of LIF. LIF activates the phosphoinositide 3-kinase (PI3K) pathways and induces the expression of the antiapoptotic molecule Bcl-2 in OSNs, whereas inhibition of the PI3K pathway blocks LIF-dependent OSN survival and Bcl-2 induction. Thus, LIF plays a central role in maintaining the size and integrity of the population of immature neurons within the olfactory epithelium; this population is critical to the rapid recovery of olfactory function after injury. LIF may play a similar role elsewhere in the CNS and thus be important for manipulation of stem cell populations for therapeutic interventions.

High-potency olfactory receptor agonists discovered by virtual high-throughput screening: molecular probes for receptor structure and olfactory function.

The detection of diverse chemical structures by the vertebrate olfactory system is accomplished by the recognition of odorous ligands by their cognate receptors. In the present study, we used computational screening to discover novel high-affinity agonists of an olfactory G protein-coupled receptor that recognizes amino acid ligands. Functional testing of the top candidates validated several agonists with potencies higher than any of the receptor's known natural ligands. Computational modeling revealed molecular interactions involved in ligand binding and further highlighted interactions that have been conserved in evolutionarily divergent amino acid receptors. Significantly, the top compounds display robust activities as odorants in vivo and include a natural product that may be used to signal the presence of bacteria in the environment. Our virtual screening approach should be applicable to the identification of new bioactive molecules for probing the structure of chemosensory receptors and the function of chemosensory systems in vivo.

Chemical communication by behaviour-guiding olfactory signals.

Chemical Ecology is a new interdisciplinary research area with close collaborations between chemists and biologists of different descriptions. It has developed during the last 40 years because of an interest in the structure, function and evolution of chemical signalling among organisms and also because of the hope to be able to use the ubiquitous phenomenon to control organisms, like pest insects. This feature article highlights the growth of the discipline and the progress made, through examples from the author's own work on chemical communication in insects and flowering plants. The research deals with olfactory signals, i.e. volatile chemical compounds perceived by the sense of smell. Analytical techniques and methods are an important part of the work.

Functional characteristics of a tiny but specialized olfactory system: olfactory receptor neurons of carrot psyllids (Homoptera: Triozidae).

With only approximately 50 olfactory receptor neurons (ORNs), the carrot psyllid Trioza apicalis (Homoptera: Psylloidea) may have the smallest olfactory system described in adult Neopteran insects. Using single sensillum recordings (SSR) and gas chromatograph-linked SSR, we characterized 4 olfactory sensilla forming a distinct morphological type, which together house approximately 25% of all ORNs. We recorded responses to extracts and single constituents from Daucus carota ssp. sativus, from the conifers Picea abies, Pinus sylvestris, and Juniperus communis, as well as from male and female T. apicalis. Receptor neurons were highly selective; only 9 compounds in total elicited repeatable responses, and each neuron responded to at most 3 individual compounds. Chemical profiles of carrot and conifers showed significant overlap, with 4 out of 9 electrophysiologically active compounds occurring in more than one type of extract, but a carrot-specific compound elicited the most repeated responses. We identified 4 tentative neuron classes and found a rather high degree of neuronal redundancy, with 1 neuron class present in 3 and another present in all 4 of the sensilla, respectively.

Time and space are complementary encoding dimensions in the moth antennal lobe.

The contribution of time to the encoding of information by the nervous system is still controversial. The olfactory system is one of the standard preparations where this issue is empirically investigated. For instance, output neurons of the antennal lobe or the olfactory bulb display odor stimulus induced temporal modulations of their firing rate at a scale of hundreds of milliseconds. The role of these temporal patterns in the encoding of odor stimuli, however, is not yet known. Here, we use optical imaging of the projection neurons of the moth antennal lobe to address this question. First, we present a biophysically derived model that provides an accurate description of the calcium response of projection neurons. On the basis of this model, we subsequently show that the calcium response of the projection neurons displays a stimulus specific temporal structure. Finally, we demonstrate that an encoding scheme that includes this temporal information boosts classification performance by 60% as compared to a purely spatial encoding. Although the putative role of combinatorial spatio-temporal encoding strategies has been the subject of debate, our results for the first time establish quantitatively that such an encoding strategy is used by the insect brain.

Block by amiloride derivatives of odor-evoked discharge in lobster olfactory receptor neurons through action on a presumptive TRP channel.

Amiloride and its derivatives inhibit a number of sensory transduction processes, including some types of chemosensory transduction. Here, we report that pyrazine derivatives of amiloride reversibly inhibit odorant-evoked activity in lobster olfactory receptor neurons. The potency sequence is as follows-(IC50, mM): 5-(N,N-hexamethylene)amiloride (0.015) approximately 5-(N-methyl-N-isobutyl)amiloride (0.02) approximately 5-(N-ethyl-N-isopropyl)amiloride (0.03) > 5-(N,N-dimethyl)amiloride (0.48); 3',4'-dichlorobenzamil (0.4), phenamil (0.5), and amiloride itself (2) are ineffective. The same derivatives with the similar potency sequence also block a presumptive transient receptor potential (TRP) channel that is the likely downstream target of phosphoinositide signaling in these cells. Our results suggest that pyrazine derivatives of amiloride are useful probes to study more detailed mechanisms of chemosensory transduction in this system and possibly in other chemosensory systems in which TRP channels are the known or suspected downstream effector.

Olfactory receptors on the cockroach antenna signal odour ON and odour OFF by excitation.

A morphologically identifiable type of olfactory sensillum on the antenna of the American cockroach contains a pair of ON and OFF cells that responds oppositely to changes in the concentration of fruit odours. The odour of lemon oil was used to study the accuracy with which these cells can discriminate between rapid step-like, ramp-like and oscillating changes in odour concentration. The discharge rates of both cells are not only affected by the actual concentration at particular instants in time (instantaneous concentration) but also by the rate at which concentration changes. The impulse frequency of the fruit odour ON cell is high when odour concentration is high, but higher still when odour concentration is also rising. Conversely, the impulse frequency of the fruit odour OFF cell is high when odour concentration is low and higher still when odour concentration is also falling. Thus, the effect of odour concentration on the responses of both cells is reinforced by the rate of change. Sensitivity to the rate of concentration change becomes greater when the rate is low. Because of the high sensitivity to low rates of change, these cells are optimized to detect fluctuations in fruit odour concentration. Whereas the ON cell signals the arrival and presence of fruit odour, the OFF cell detects its termination and absence. These cells provide excitatory responses for both increase and decrease in fruit odour concentration and may therefore reinforce contrast information.