Heterogeneity and Convergence of Olfactory First-Order Neurons Account for the High Speed and Sensitivity of Second-Order Neurons
[['In insects and vertebrates the first two neuronal layers of the olfactory system present the same organization where many ORNs in the first layer converge to a small number of output neurons in the second layer – PNs in insects and mitral cells in vertebrates [1] , [2] .', 'The ORNs that project onto a single glomerulus express a single type of olfactory receptors .', 'Yet , they present heterogeneous dose–response properties [3] .', 'The functional consequence of this convergence has been the subject of much interest .', 'Theory [4] predicts and experiments [5] confirm that pooling N ORN inputs should increase the firing rate of output neurons by N and improve the signal-to-noise ratio by √N .', 'Experiments in the fruit fly reveal that firing rates rise more rapidly in PNs than in ORNs and that weak odor inputs are more amplified than strong inputs [6]-[8] .', 'Such a non-linear transformation leads to an efficient use of coding capacity [6] and a maximum preservation of information on odor quality [9] .', 'Although previous studies investigated the change in firing rate when sensory information passes from first- to second-order neurons , they did not consider the latency of the response .', 'This is restrictive given that environmental conditions put strong constraints on the behavioral response time .', 'In natural odor plumes , encounters with the stimulus are brief and intermittent , with up to five contacts per 1 s and each contact lasting down to under 20 ms [10]–[12] .', 'Consequently , behavioral responses to odors are fast ( <100 ms in rodents [13]–[15] and insects [16] , [17] ) and the olfactory system , like other sensory systems [18] , [19] , may rely on response latencies for fast odor discrimination [10] , [12] , [20] .', 'In this work , we compared the transformations in latency and firing rate from first- to second-order neurons and assessed whether the cell-to-cell heterogeneity contributes to this transformation .', 'We addressed these issues in a favorable model , the male moth subsystem that processes the sex pheromone .', 'Each pheromone component activates with high specificity a single ORN type [5] whose axons project to one of a few glomeruli – the macroglomerular complex ( MGC ) .', 'The second-order neurons ( PNs and local neurons LNs ) in the largest MGC glomerulus – the cumulus – receive their sole olfactory input ( homotypic ) from the most abundant ORN type sensitive to the major component , with no lateral olfactory input from other glomeruli ( heterotypic ) ( e . g . [21] , [22] ) .', 'This is a significant advantage for experimental analysis with respect to glomeruli sensitive to general odors that receive both homotypic and heterotypic inputs because of the lack of specificity of generalist ( non-pheromonal ) ORNs [7] .', 'Apart from this difference , which allowed us to record responses with well-defined input , the isomorphic glomeruli involved in general odor processing and the cumulus share the same functional organization , so that the main properties found here for the pheromonal system should apply also to the general odorant system .', 'We found that both firing rates and latencies of ORNs and PNs are strongly dose-dependent , that PNs respond with a higher firing rate and a shorter latency than most ORNs , and that the sensitivity and speed of a given neuron are not correlated .', 'We found also that dose-response curves are variable among ORNs ( and PNs ) and that this variability of essentially biological origin arises more from the variability across neurons ( heterogeneity ) than within single neurons ( irregularity ) .', 'For firing rate , the ORN-to-PN amplification mechanism is non-linear , which augments sensitivity to weak odor signals in intensity coding and enhances distinction between different general odors in quality coding [6]; it can be explained by the convergence of many ORNs on single PNs .', 'For latencies , on the contrary , the ORN-to-PN transformation is linear , which might favor concentration-invariant coding of odor blends , and requires ORN heterogeneity .'], ['When stimulated with the components of the pheromone , the cumulus of the male moth Agrotis ipsilon was activated only by the main pheromone component , cis-7-dodecenyl acetate ( Z7-12:Ac ) ( Fig . 1A ) .', 'Conversely , the other glomeruli in the MGC were activated only by the other pheromone components ( Fig . 1B–C ) .', 'In electrophysiological recordings , the Z7-12:Ac-responsive ORNs displayed phasic-tonic responses ( Fig . 2A , C ) whereas the second-order neurons we studied shared a common multiphasic response pattern with an initial excitation followed by an inhibition ( Fig . 2B , D ) and frequently a final rebound ( Fig . 2E ) .', 'All stained multiphasic neurons were found to be PNs with dendritic trees in the cumulus and axons in the inner antenno-cerebral tract .', 'The rare stained LNs we found ( 3 among 67 stained cells ) were monophasic .', 'Although these observations do not rule out the existence of LNs with a multiphasic response pattern , they support the contention that multiphasic LNs ( if they exist ) are rare in our recording conditions , which means that most if not all recorded neurons were PNs .', 'For this reason , in the following , we used the more common term PN .', 'Even in the absence of pheromone delivery , the Z7-12:Ac-responsive ORNs and PNs spiked tonically .', 'This spontaneous activity is stationary ( Fig . 3A ) with a median firing rate lower in ORNs than in PNs ( Fig . 3A , C ) .', 'The distributions of spontaneous firing rates Fsp ( all symbols are defined in S1 Table ) are well fitted to lognormal distributions with a longer tail in PNs than in ORNs ( Fig . 3C; S2 Table ) .', 'To determine whether the PN activity is influenced by ORNs at rest , the antenna was sectioned .', 'The PN firing rate began to decrease ∼10 s after the section and reached a stable regime ( ∼70% lower , range 58-85% ) after less than 5 min ( Fig . 3B ) .', 'The present study is restricted to two aspects of the olfactory code – the initial firing rate F ( as defined in S1B , D Figure ) and response latency L ( S1A , B Figure ) and their dose-dependent transformations – without ignoring that other aspects of the responses , e . g . action potentials after stimulus offset [23] or correlated activity in different neurons [24] , may also provide useful information .', 'A feature of the studied response variables is immediately noticeable .', 'The pairs ( F , L ) of a given response recorded from ORNs and PNs are quite distinct , especially at low doses .', 'For example at dose C\u200a=\u200a−1 log ng , PNs fire with higher rates and shorter latencies than ORNs so that most pairs ( F , L ) from ORNs and PNs do not overlap ( Fig . 2F ) .', 'This may seem paradoxical because one would expect that the shortest latencies be a little longer in PNs than in ORNs on account of axonal conduction and synaptic transmission .', 'This apparent paradox is the main theme of this paper and its resolution required to analyze how the neuron responses depend ( or not ) on the dose , the ORN and PN variability and the ORN-to-PN convergence ratio .', 'In order to document this feature and to provide an overview of how the two neuron populations studied respond to a given dose of Z7-12:Ac the firing rates and latencies of all recorded neurons at each applied dose were pooled .', 'It was found in this way that the firing rate F presents four distinct properties ( Fig . 4 , top row; Fig . 5 , left column ) :', '( i ) The firing rates across neurons stimulated at the same dose follow Gaussian distributions in ORNs ( Figs . 4A , 5A ) and PNs ( Figs . 4C , 5C ) .', '( ii ) The mean of the distributions increases with the dose ( Fig . 5A , C ) .', 'At the lowest dose applied ( −1 log ng for ORNs , −3 for PNs ) the distributions are not significantly different from the control stimulations with pure air or hexane ( Fig . 4A , C ) .', 'The frequencies at the two highest doses tested ( 3 and 4 log ng for ORNs , 0 and 1 log ng for PNs ) are also not significantly different ( Fig . 4A , C ) .', 'Therefore , when measured at the population level , the dynamic ranges of ORNs extend from −1 to 3 log ng and for PNs from −3 to 0 log ng .', '( iii ) For doses C≤1 , PNs respond more strongly than ORNs ( Fig . 3E , G ) .', '( iv ) The standard deviation of the distributions increases linearly with their mean for F<∼100 AP/s , with the same slope in ORNs and PNs ( i . e . same coefficient of variation CV≈0 . 33 ) , notwithstanding the very different doses evoking the same variability in the two populations ( Fig . 5E ) .', 'Above ∼100 AP/s , before saturation of the mean ORN firing rate , the variability of ORNs and PNs becomes constant ( Fig . 5E ) .', 'Similarly , the latencies at each dose follow Gaussian distributions ( Fig . 5B , D ) with standard deviations proportional to their means ( Fig . 5F ) .', 'However , contrary to frequencies ,', '( i ) the variability decreases when the dose increases;', '( ii ) the slopes ( CVs ) for ORNs and PNs are different , indicating a higher variability in PNs ( Fig . 5F ) ;', '( iii ) no discontinuity in slope is seen at high doses; and', '( iv ) , at the same dose , the PN distribution is always shifted to the left of the ORN distribution ( Fig . 4F , H ) which shows that , at all doses , the PN latencies are shorter than the ORN latencies , thus confirming that the paradox noted above holds at all doses .', 'However , the mere statistical pooling of all available responses at a given pheromone dose , as done in the previous section , is not sufficient to describe adequately the neuron properties .', 'A more detailed view of the system requires that responses to stimulations at increasing doses be analyzed on individual cells , not only cell populations .', 'The main features of dose-response curves in single neurons are examined: first their overall “shape” in the present subsection , then their location along the dose axis and their correlations in the next subsections .', 'Dose-response plots were established for 38 ORNs and 47 PNs .', 'Sigmoid Hill functions ( see eqs . 3 and 4 in Methods ) were fitted to the dose-firing rate C-F plots ( Fig . 6 ) .', 'From the fitted parameters – maximum firing rate FM , efficient dose 50% ( ED50 ) C1/2 , and Hill coefficient n – we also derived three characteristics: the doses at threshold C0 and at saturation CS and their difference , the dynamic range ΔC ( eqs . 5–7; ) .', 'The firing rate responses to the lowest doses are not significantly different from controls ( Fig . 4A , C ) and those to the two highest doses are nearly equal ( within 15% in 87% of ORNs and 80% of PNs , showing that the observed maximum firing rates were close to the asymptotic FM ) which guarantees that the parameters were correctly estimated .', 'Latencies were analyzed the same way .', 'Decreasing linear functions with a lower bound were fitted to the dose-latency C-L plots ( Fig . 7 ) .', 'Each neuron was characterized by its maximum latency LM at threshold C0 , its minimum latency Lm and their difference ΔL = LM − Lm .', 'Complementary aspects of the distributions of these coding properties such as averages , variability and correlations were analyzed .', 'As far as the “shape” parameters are concerned , we found that the typical ORN , reconstructed from the median values of the fitted parameters ( S3 Table ) , has a high maximum firing rate FM ( 163 AP/s ) and a wide dynamic range ΔC ( 3 . 6 log units ) , whereas the typical PN , reconstructed in the same way , has a lower FM ( 62 AP/s ) and a smaller ΔC ( 2 . 5 log units ) .', 'For latencies ( S4 Table ) , the maximum LM ( 164 vs . 107 ms ) and the range ΔL ( 104 vs 64 ms ) are greater for the typical ORN than the typical PN .', 'Except for FM , the variability ( SD or interquartile range ) of all these properties is slightly higher in PNs than in ORNs ( Figs . 6E , F and 7E , F ) .', 'Although the dose-response curves of ORNs and PNs are basically similar in shape , the essential difference between them is that they are not located identically on the dose axis , which means that ORNs and PNs respond with similar firing rates and latencies but at very different doses .', 'This calls for a clear distinction of the dose-independent properties ( like FM , ΔC , LM , ΔL ) , analyzed above and the dose-dependent properties ( C0 , C1/2 , Cs ) which are examined now .', 'PNs are clearly more sensitive than ORNs .', 'For example , the recruitment of PNs starts at lower doses as half of the PNs were activated at −3 log ng and half of the ORNs only at −0 . 5 log ng ( not shown ) .', 'PNs approach saturation at doses 3 orders of magnitude lower than ORNs .', 'These changes testify that major transformations take place in the neural network of the cumulus when the sensory signal passes from ORN to PN .', 'The ORN-to-PN transformations can be represented in two complementary ways: either by pairs of dose-response curves ( Fig . 7A , B ) , or by transfer functions linking the latencies ( or frequencies ) of the ORNs and PNs at the same doses ( Fig . 8C , D ) .', 'Interestingly these curves ( and the transfer functions derived from them ) can be determined in two different ways , either directly from the pooled distributions of F and L at each dose ( Figs . 4 , 5 ) , or from the distributions of the parameters determined on single dose-response plots ( Figs . 6 , 7 ) .', 'Both methods give practically the same results as shown here by the median and extreme values ( quantiles 10% and 90% ) of the firing rates ( Fig . 8A ) and the latencies ( Fig . 8B ) .', 'So far only univariate distributions have been considered since the 11 properties that describe the C-F curves ( fitted parameters FM , C1/2 , n and derived characteristics C0 , Cs , ΔC ) and C-L curves ( parameters L0 , λ , Lm and characteristics LM , ΔL ) were analyzed separately .', 'This analysis must now be completed by examining the links between them .', 'For this purpose bivariate plots and their associated correlations were studied .', 'S5 Table assembles the coefficients of correlations and significance levels of the ( 112−11 ) /2\u200a=\u200a55 non-trivial pairs of properties in ORNs ( top ) and in PNs ( bottom ) .', 'It shows that 19 pairs ( 35% ) are significantly correlated at level 1% in ORNs and also 19 in PNs , indicating that the overall correlative structure is similar in both populations .', 'However , because characteristics are derived from parameters ( for example ΔC depends on FM and n , see eq . 7 ) , correlations between characteristics are expected to be more frequent and less informative than the correlations between parameters .', 'Indeed , of the ( 62−6 ) /2\u200a=\u200a15 pairs of parameters only 3 ( 20% ) are significantly correlated in ORNs and the same number in PNs .', 'Therefore , in the following , priority is given to the parameters over the characteristics .', 'Another distinction is between two main types of pairs – those associating properties of the same variable , firing rate or latency , that provide information on the level of redundancy of properties , and those crossing the two variables that provide information on the link between them .', 'In the first type , of the 3 pairs of F-parameters ( FM-C1/2 , C1/2-n and n-FM ) none is significantly correlated ( p>0 . 01 ) in ORNs ( S2A-C Figure ) and a single one in PNs ( C1/2-n , S3B Figure ) , indicating that the most sensitive PNs ( with small C1/2 ) tend to have a steeper slope n .', 'Similarly , of the 3 pairs of L-parameters a single one is uncorrelated in both ORNs and PNs , suggesting that L-parameters are more correlated to one another than F-parameters .', 'The uncorrelated pair ( λ-Lm , S2E , S3E Figures ) indicates that neurons with longer latencies at C\u200a=\u200a0 tend to have steeper slopes λ and longer minimum latencies Lm .', 'As for the two-variable type , among the 9 pairs between the 3 F- and 3 L- parameters , a single one is significantly correlated but it is not the same in ORNs ( C1/2-L0 , S2J Figure ) and in PNs ( C0-Lm , S3P Figure ) .', 'Although the correlation C1/2-L0 suggests that fast ORNs ( small latency L0 at C\u200a=\u200a0 ) have high affinity ( small C1/2 ) , it is not confirmed by direct comparison as threshold C0 and minimum latency Lm in each ORN are not correlated ( S2P Figure ) , showing that the ORNs with the lowest thresholds are usually not the fastest .', 'As the reverse situation holds for PNs , this lack of consistency and the low proportion of significant correlations between F- and L-parameters ( 11% ) , and still more between F- and L-properties ( 10% for ORNs , 7% for PNs ) , support the overall independence of sensitivity and speed in ORNs and PNs .', 'The firing rates and latencies are variable across ORNs ( and PNs ) stimulated at the same dose ( Figs . 4 , 5 ) .', 'As shown below variability plays an essential role in the ORN-to-PN signal transformation which calls for a proper understanding of its sources and structure .', 'First , variability arises from experimental and biological sources .', 'Experimental variability in ORNs results from uncertainties on the dose and the delivery time from the stimulating device , and from the relative geometry of the airflow and the recorded sensillum .', 'It must be reduced but can never be eliminated .', 'Biological variability is more fundamental because it is an intrinsic property of the investigated system .', 'It can be known only by subtracting the experimental variability from the overall observed variability .', 'Second , variability arises from irregularities within single units and heterogeneities across units , where units can be neurons or pheromone stimuli .', 'This distinction is important .', 'The term ‘irregularity’ is used consistently throughout the paper to indicate the variability in firing rate or latency of the same neuron , or the variability on dose or delivery time of the same stimulus , following repeated stimulations with the same cartridge .', 'By extension it indicates also the variability of spontaneous firing of the same neuron over time .', 'Similarly the term ‘heterogeneity’ indicates the variability in response of different neurons , or the variability on dose and delivery time for repeated stimulations with identically prepared cartridges .', 'Systematic measurements ( see Materials and Methods ) showed that the biological component accounts for ∼90% of total variability .', 'Biological variability results more from heterogeneity across ORNs ( ∼95% for F , ∼55% for L ) than from irregularity within ORNs .', 'These observations support a relatively simple interpretation , that the observed variability of ORN responses reflects primarily across-neuron heterogeneity .', 'When compared to ORNs , the irregularity within PNs was smaller for firing rate ( 70% ) and latency ( 80% ) , and the heterogeneity across PNs was equal for firing rate ( Fig . 5E , superimposed blue and red lines ) and larger for latency ( 140% , Fig . 5F , lines with different slopes ) .', 'Of special interest from a population coding point of view are the most active and the fastest neurons .', 'To estimate the effect of the heterogeneity of ORNs and PNs and reconstruct their full range of variability , we selected the 10% most extreme values at both ends of the distributions of the parameters describing the dose-response curves ( staircases in Figs . 6 , 7 ) .', 'Fig . 8A shows that the F/FM curves of the 10% most efficient ORNs ( derived from leftmost blue dashed curve ) and the typical PN ( derived from red solid line ) are relatively close .', 'This observation is also true for the C-L curves ( Fig . 8B ) .', 'Thus , the 10% most efficient PNs are likely triggered by a small fraction of ORNs .', 'The transfer functions provide useful complements .', 'The function of the 10% most ( respectively least ) efficient neurons ( Fig . 8C , dashed and dash-dot lines ) is less ( respectively more ) curved than the average function ( solid line ) .', 'The function of the 10% fastest neurons is a steep line that decreases to shorter latencies ( Fig . 8D ) than the median function .', 'Not all ORNs in the population contribute equally to the PN response .', 'The major contribution comes from the ORNs whose latency is shorter or equal to the PN latency , since no PN can respond faster than its presynaptic ORNs .', 'In order to determine the fraction of contributing ORNs we relied on a model based on the C-F and C-L curves and the distributions of their parameters established above ( see last section “Model of the signal delivered by the ORN population” in Materials and Methods ) .', 'The model predicts the firing rates and latencies observed in the ORN population ( Fig . 9A , B ) and allows us to simulate the spike trains fired by this population when stimulated ( Fig . 9C ) and in the absence of stimulation ( Fig . 9D ) .', 'From these simulations , we calculated at any dose C the proportion of ORNs that respond with a given latency L1 or shorter .', 'This proportion , as shown in Fig . 9E when L1 is the latency of the typical PN reconstructed from the median values of the fitted parameters , decreases with the dose and only 5±2% of the ORNs are enough to activate the typical PN .', 'The proportion is greater ( 16±7% ) for the slow PNs and smaller ( 2±0 . 3% ) for the fast ones .', 'The response kinetics were obtained from the model by simulating the total firing of the heterogeneous population of ∼7000 Z7-12:Ac ORNs knowing the statistical distributions of their experimental C-L and C-F curves .', 'At all doses , the simulated PSTH shows that the instantaneous firing rate of the ORN population increases and reaches a maximum ∼200 ms after the stimulation onset ( Fig . 9F ) .', 'The initial growth results from the gradual recruitment of new ORNs .', 'Then , the pheromone dose at threshold can be determined .', 'The ORN population spontaneously fires ∼27 000± SD √27000 AP/s ( Fig . 9D ) .', 'To be detected by PNs , the firing rate must be greater than 27000+r√27000 , where r is the signal-to-noise ratio .', 'In another moth species , Bombyx mori , Kaissling [25] showed that , in the range of pheromone loads eliciting a behavioral response in 40% to 80% of male moths ( 0 . 01 to 0 . 1 ng per cartridge in his experimental conditions ) , r varies from 3 to 31 .', 'Assuming that the same range of ratios applies to Agrotis , the ORN population must fire from 27 500 AP/s ( for r\u200a=\u200a3 ) to 32 000 AP/s ( for r\u200a=\u200a31 ) .', 'Fig . 9F shows that the lowest stimulus doses evoking 275–320 AP per 10 ms are in the range −4 . 6 to −3 . 1 log ng .', 'At this dose the typical PN fires only 0 . 3–3 . 3 AP/s .'], ['Single ORNs and PNs encode the pheromone concentration in the same way , their dose-firing rate curves being well fitted to Hill functions – a classical fit for ORNs [25] , [26] .', 'The maximum firing rate FM of ORNs is usually much larger in insects ( 100–300 AP/s; e . g . [6] , [27] with an exception <60 AP/s , [28] ) than in vertebrates ( 13–50 AP/s , [26] ) .', 'The same remark holds for PNs in insects ( 170–250 AP/s; [6] , this work ) and the analogous mitral cells in vertebrates ( ∼20 AP/s in frog; [29] ) .', 'Also , the dynamic range ΔC is much wider in pheromone-sensitive ORNs than in vertebrate generalist ORNs ( usually <2 log units , [26] ) .', 'Because , for a given Hill coefficient , ΔC depends on FM , it is tempting to speculate that the reason why the firing rate of pheromone-responsive neurons is so high , despite its large energetic cost , comes from the importance of detecting pheromones at very low concentration .', 'The high firing rate would then be the price to pay to have a low threshold and a wide dynamic range .', 'The median latencies LM at threshold in A . ipsilon in ORNs ( 164 ms ) and PNs ( 107 ms ) are consistent with those in non-pheromonal honeybee PNs ( ∼125–150 ms; [12] ) , and much shorter than those in frog ORNs ( 0 . 7–1 . 9 s; [25] ) .', 'A possible interpretation of the faster response of insect ORNs is that insect ORs were proposed to be ionotropic [30] , [31] whereas vertebrates ORs are metabotropic [32] .', 'These observations raise the question of whether latency actually contributes to encode pheromone concentration because the stimulus onset defining time zero is not known by the brain .', 'However , it has been shown that relative latencies , i . e . patterns of spikes , can be used as a replacement of stimulus onset [11] , [15] , [33] .', 'This applies when successive stimuli are well separated ( as in pheromone plumes where molecules remain grouped in clumps and filaments separated by clean air [34] ) and even when they are not , for example in an ensemble of mitral cells [20] .', 'Moreover , latency coding is consistent with temporal coding mechanisms in which the precise timing of action potentials carries information on the stimulus [35] .', 'It is now recognized that temporal coding is widely used in all sensory systems , whether olfactory [36] , tactile [37] , [38] , auditory [39] , or visual [40] .', 'The overall transformations of the olfactory code from ORNs to PNs can be determined in two different ways , either directly at the population level from their pooled responses at a given dose ( Figs . 4 , 5 ) or indirectly from responses of individual neurons across doses ( Figs . 6 , 7 ) .', 'The two approaches are consistent as the same transformations were found in both ( Fig . 8A , B ) strengthening a posteriori the more complex but more informative single-cell analyses .', 'Coding properties at the system level depend on the many-to-one ORN-to-PN convergence and individual differences between neurons in the ORN and PN populations .', 'The ORN-to-PN transformations , which entail stronger and faster responses , considerably differ quantitatively , the transformations at low doses being linear in latency and highly nonlinear in firing rate .', 'These effects may be partly interpreted within the “efficient coding hypothesis” [56] , [57] stating that the sensory neurons , including the moth pheromonal ORNs [58] , are adapted , through both evolutionary and developmental processes , to the statistics of their natural stimulus .', 'The effect of the nonlinear ORN-to-PN firing rate transfer function is to give more weight to the low doses than to the high doses .', 'The typical PN reaches saturation at a relatively low dose ( CS ≈ 0 log ng ) with respect to ORNs ( Fig . 8A ) and so cannot discriminate doses above 0 log ng .', 'Such a transformation is reasonable from an efficient coding point of view because the high pheromone concentrations found within filaments far from the source [59] are rare and not informative for localizing the source .', 'This nonlinear transformation is reminiscent of logarithmic companders for coding waveforms with a wide dynamic range , such as voice , on a finite number of levels , favoring the most frequent signals at the expense of the least frequent ones [60] .', 'The same interpretation can be applied here to the coding of pheromone concentration ( e . g . a lognormal distribution of concentrations in nature will be encoded more uniformly by PNs ) .', 'In the fly this type of transformation was interpreted as favoring the qualitative discrimination of odorants .', 'At any dose , odorants evoke a wide range of firing rates in ORNs [61] and PNs but their distribution is more uniform in PNs [6]; the hyperbolic transfer function explains this histogram equalization .', 'Thus , the nonlinear ORN-to-PN transformation can be interpreted as favoring qualitative discrimination of odorants in the generalist pathway and detection at low doses in the pheromonal pathway .', 'In contrast , the linear latency transfer function preserves the type of statistical distribution between ORN and PN latencies ( e . g . a Gaussian remains Gaussian ) and so cannot perform histogram equalization .', 'A possible advantage of this linearity is to make the coding of pheromone identity concentration-invariant [11] .', 'In A . ipsilon , although the ratio of concentrations of one of the minor components to the major component is 4 , linear latency functions with the same slope will introduce the same constant delay between the specialized ORNs responding to the components whatever the blend concentration .', 'Any change of the ratio will produce a detectable change in the delay , signaling an inadequate pheromone blend .', 'In this view , the ORN-to-PN convergence may be more important for fast detection of the signal than for precise determination of its intensity .', 'The sensitive response of the PNs to the fastest ORNs and their saturation at relatively low doses make the PN output more stable to dose variations than the ORN output and favor information on the temporal structure of the plume over its concentration fluctuations .', 'The cumulus appears to obey the same principles as the ordinary glomeruli with a notable difference .', 'Because of a higher ORN-to-PN convergence ratio , the response of PNs in the cumulus is presumably more sensitive and faster than in ordinary glomeruli .', 'Temporal discrimination , a constraint in ordinary glomeruli , becomes apparently a major issue in the MGC .'], ['Agrotis ipsilon ( Lepidoptera: Noctuidae ) were reared on an artificial diet until pupation [62] .', 'Adults were fed with a 20% sucrose solution .', 'All experiments were performed on sexually mature virgin males 5 days after emergence .', 'A constant airflow ( 24 ml/s ) was blown over the antenna through a mixing tube .', 'It resulted from mixing a constant airflow ( 17 ml/s ) with an alternating flow ( 7 ml/s ) of clean air between stimulations or odorized air during stimulations .', 'Stimuli were delivered by means of a device ( Syntech , Kirchzarten , Germany ) blowing air during 200 ms through a Pasteur pipette containing the pheromone-impregnated filter paper and inserted into the mixing tube .', 'Successive stimulations were separated by intervals of at least 60 s in ORNs and 30 s in PNs; these intervals are sufficient for complete return to spontaneous activity ( Fig . 2E ) , except with the highest doses tested ( 3 and 4 log ng for ORNs and 1 log ng for PNs ) for which longer intervals were used .', 'In electrophysiological experiments , the main pheromone component of A . ipsilon , ( Z ) -7-dodecen-1-yl acetate ( Z7-12:Ac ) was used at different loads M ( 1 pg to 10 µg ) .', 'In calcium imaging experiments also the two minor pheromone components ( Z ) -9-tetradecen-1-yl acetate ( Z9-14:Ac ) , and ( Z ) -11-hexadecen-1-yl acetate ( Z11-16:Ac ) [63] were used for stimulation at a single load ( 10 ng ) .', 'Doses denoted C were expressed as the decimal logarithm of loads in ng .', 'All 3 components of the pheromonal blend [63] were tested ( Fig . 1 ) .', 'Recordings were performed as described in [64] .', 'Briefly , Calcium Green 2-AM was bath-applied for at least 1 hour .', 'A TILL Photonics imaging system ( Martinsried , Germany ) was coupled to an epifluorescent microscope ( Olympus BX-51WI ) equipped with a 10x ( NA 0 . 3 ) water immersion objective .', 'Signals were recorded using a 640×480-pixel , 12-bit monochrome CCD camera ( TILL Imago ) .', 'Only the major component ( Z7-12:Ac ) was tested .', 'Recordings from ORNs were performed as described in [62] .', 'Briefly , the glass recording electrode was brought into contact with a cut sensillum and the reference electrode was inserted in an adjacent antennal segment .', 'Great care was taken to cut the sensilla at the same short distance from their tip .', 'Each ORN was recorded for 10 s before and 40 s after the onset of each stimulus at 6 doses from -1 to 4 log ng .', 'Only recordings with spikes clearly attributable to a single ORN were kept for further analysis .', 'PNs were recorded from the cumulus area with two different techniques .', 'Extracellular recordings ( EPN ) were performed with glass electrodes of 5 MΩ resistance as described previously [65] .', 'The electrical activity of one or several neurons was recorded .', 'Spike-sorting was performed with the R-package SpikeOMatic [66] .', 'All EPNs were stimulated twice at five doses from −3 to 1 log ng .', 'For each stimulus , 12 s of post-stimulus activity were recorded .', 'Intracellular recordings ( IPN ) were performed with a glass electrode of 150–200 MΩ resistance filled with Lucifer Yellow or neurobiotin as described in [67] .', 'IPNs were tested at four doses , −2 , −1 , 0 and 1 log ng and recorded for 3 s after
['Introduction', 'Results', 'Discussion', 'Materials and Methods']
['In the olfactory system of male moths , a specialized subset of neurons detects and processes the main component of the sex pheromone emitted by females .', 'It is composed of several thousand first-order olfactory receptor neurons ( ORNs ) , all expressing the same pheromone receptor , that contact synaptically a few tens of second-order projection neurons ( PNs ) within a single restricted brain area .', 'The functional simplicity of this system makes it a favorable model for studying the factors that contribute to its exquisite sensitivity and speed .', 'Sensory information—primarily the identity and intensity of the stimulus—is encoded as the firing rate of the action potentials , and possibly as the latency of the neuron response .', 'We found that over all their dynamic range , PNs respond with a shorter latency and a higher firing rate than most ORNs .', 'Modelling showed that the increased sensitivity of PNs can be explained by the ORN-to-PN convergent architecture alone , whereas their faster response also requires cell-to-cell heterogeneity of the ORN population .', 'So , far from being detrimental to signal detection , the ORN heterogeneity is exploited by PNs , and results in two different schemes of population coding based either on the response of a few extreme neurons ( latency ) or on the average response of many ( firing rate ) .', 'Moreover , ORN-to-PN transformations are linear for latency and nonlinear for firing rate , suggesting that latency could be involved in concentration-invariant coding of the pheromone blend and that sensitivity at low concentrations is achieved at the expense of precise encoding at high concentrations .']
['Understanding how sensory signals are optimally encoded by nervous systems is of strong interest to neuroscientists , and also to engineers as it may lead to more efficient artificial detection systems .', 'This is particularly relevant to olfaction , because the current electronic noses are far outperformed by their biological counterparts in terms of speed and sensitivity .', 'We here use the moth sex pheromone processing system as a relatively simple model to understand early olfactory coding .', 'We found that performance increases when olfactory information passes from first- to second-order neurons .', 'Second-order neurons respond on average with shorter latency and higher sensitivity than first-order neurons .', 'We show that two critical factors , convergent architecture and neuronal heterogeneity , are needed to account for increased performance .']
['computational neuroscience', 'olfactory system', 'biology and life sciences', 'sensory systems', 'computational biology', 'neuroscience', 'coding mechanisms']