Response variability is associated with multiple projection neurons in the locust antennal lobe 348. Applying this approach for the first time to the olfactory system, I show that the information content of single projection neurons (PNs) in the locust antennal lobe Its high.
Synopsis
Towards the end, I discuss a little-explored aspect of olfaction, that of stimulus variance in space and time, and propose a speculative new hypothesis for the role of glomerular convergence of receptor neurons expressing the same receptor gene.
Olfaction as a model system for information processing and pat- tern recognition
Something similar is true for hearing, a sense used to recognize complex sequences of the single notes detected by individual hair cells in the ear. This is not meant to belittle the recognition process - we will study a great deal of the problems inherent in achieving invariance from the natural variability in the stimuli - but rather that the recognition process is not far removed from the periphery and thus can be studied without the need to treat inputs as black boxes or wait until sensory physiologists 'work their way there'.
Brief history of the beginnings of research in olfaction
Furthermore, he remarkably demonstrated the existence of spatio-temporal coding in the olfactory bulb, the clustering of these spatio-temporal responses according to the chemical similarity of the odorants, and the relative invariance of these patterns over a wide range of concentrations (Adrian, 1951). ). But this sensory system, which was one of the last to come to the attention of most of the neurophysiological community (Adrian, 1942), has also been around the longest.
Olfaction’s place in evolution
Possibly because they are part of the earliest sensory system, cells of the olfactory cortex send and receive information from more brain areas than any other sensory system (Gesteland, 1992). This underlines its importance, but also the potential for future research: many of these brain areas have not yet been physiologically characterized in terms of odor input.
Odor-mediated behavior
How fast is odor perception?
A computer controlled the delivery of the odorants from an air dilution olfactometer and measured recognition times. Laing and his colleagues (1994) showed that a time separation of 400 msec between the presentation of two odors from separate sources is sufficient to allow significant discrimination of the order in which they were presented.
Pheromones, the accessory olfactory system and chemical com- munication
The pheromone is transmitted in the large cells of the alarm system on the fish's skin. Mammalian VNO neurons use at least three different families of molecular receptors (Pantages and Dulac, 2000; Dulac and Axel, 1995; Herrada and Dulac, 1997; Matsunami and Buck, 1997; Ryba and Tirindelli, 1997), each consisting of 50 –100 genes and are unrelated to the receptor family of the main olfactory system.
The trigeminal system in the olfactory epithelium
Before concentrating on olfaction in the rest of the thesis, we stop to consider the two remaining chemical senses (§1.7 and §1.8), compare them with olfaction (§1.9), and then consider the nature of the input to the olfactory system ( §1.10). We know that about 70% of all odors are said to stimulate the trigeminal nerve, although the latter is generally several times less sensitive than olfactory receptors (Ohloff, 1994).
Taste
Why do vertebrates possess separate gustatory and olfactory systems?
Zhang and Sejnowski's theoretical results suggest that the accuracy of a 2-D code should be unaffected by the width of the tuning curves. Consistent with the large size of the family of taste receptor genes, in fish, taste appears to respond to a broad spectrum of compounds (Kotrschal, 2000).
Interactions between olfaction and taste
Descriptions of smells, on the other hand, have a much greater cultural or experiential component and are therefore impractical as descriptors outside their specific domain. Finally, odors appear to operate at low thresholds, designed for distance sensing, while tastes appear to operate at higher thresholds and mainly designed for close-distance discrimination (Kotrschal, 2000).
On the nature of the diverse chemical senses
The practicality of labeled lines when the behavioral significance of stim- uli is fixed
The synthetic nature of olfaction?
The plastic nature of olfaction
How could the system decide which two signals should be grouped under the same labeled line (e.g. which receptors should be expressed in the same neuron) if the meaning of each is subject to plasticity, and thus could ultimately be the same? or else, depending on experience. We turn our attention to the computational problem facing the generalist olfactory system.
What constitutes an odorant?
The stimulus space
In this, the olfactory system shares a generalist task with its visual and auditory counterparts—although in those systems, too, there are specialized subsystems for detecting particular behavioral stimuli, such as insect detectors in frogs (Marr, 1970). .
Smells versus images and sounds
In contrast to smell, considering a single antenna or nostril, each stimulus is given by a vector in n-dimensional space, where n is at least the number of odorant receptor types, and possibly the number of receptor neurons if differences in the activation of similar receptor neurons in different locations of the epithelium are conserved downstream of the epithelium (see Spatial codes in olfaction?, below). But is the comparison between the number of molecular photoreceptor types and the number of molecular olfactory receptors a good reflection of the size of the stimulus space for vision and smell.
Stimulus dynamics: The nature of odor plumes
Temporal resolution is significantly better in terrestrial animals: sensory neurons of the cockroach producer can reliably follow 25 ms pulses of the pure odorant 1-hexanol and 50 ms pulses of the complex. Having examined the nature of the inputs to the olfactory system, what is the function of olfaction.
Olfaction: The computational problem
The computational role of learning in olfaction
It must therefore be remembered that the function of odor identification is not to reconstruct the exact nature of the odorant, but rather to classify it as relating to the closest or most likely class of odor memories (using the term to mean learned odor templates or innate), or entirely as a new odorant. I will address this important topic in Chapter 9 and show that the width of the valley itself is plastic and subject to the influence of experience.
Convergent evolution? Insects as a model system
A brief anatomy of the olfactory system
Mitral/tufted cells also project to other parts of the cortex, including the entorhinal cortex, which sends axons to the hippocampus. Axons from the calyx project to the lobes of the mushroom body, which, like the mammalian hippocampus (Morris et al., 1982), is involved in place memory functions (Mizunami et al., 1998) and context-specific sensory filtering (Strausfeld and Hildebrand, 1999).
Getting odorants to and away from receptors
- The active and pulsed nature of olfactory sampling
- The olfactory epithelium is covered by an aqueous mucosa
- The rate of air flow through the nose influences olfactory responses dif- ferentially for odorants of different sorptions
- The bilaterality of olfactory sampling
- Flow rate through each human nostril is differentially regulated and con- tributes to differential sensitivity to different odorants
- The aqueous mucosa provides the olfactory system with invariance to volatility
- The aqueous mucosa concentrates odorants
- Odorant binding proteins (OBPs)
- Olfactory degrading enzymes (ODEs)
In the bullfrog, the rate of absorption of a specific odorant interacts with the velocity of airflow across the mucosa to produce different response amplitudes in the olfactory nerve ( Mozell et al., 1991 ). OBPs have also been shown to be expressed in the pheromonal gland of the cabbage armyworm, which has no chemosensory structure (Jacquin-Joly et al., 2001).
A broad array of generalist sensors: Olfactory receptors
Olfactory receptor (OR) genes
They then demonstrated by Northern blot analysis that members of this family are expressed only in the olfactory epithelium of the rat. The massive degeneration of the human OR repertoire may be related to our inferior sense of smell relative to other species.
Olfactory receptor neurons (ORNs)
In the honey bee, calcium imaging showed bilateral symmetry in antennal lobe activation ( Galizia et al., 1998 ). On day E21, single unit responses changed dramatically: cells became selective and responded to approximately half of the compounds in the set used (Gesteland et al., 1982).
Noise reduction, analog to digital conversion and decorrelation
The convergence to the insect antennal lobe and the vertebrate olfac- tory bulb
Glomeruli: Converting a spatial code into an identity code
The key to it lies in the projection pattern of olfactory receptor neurons to the antennal lobes and olfactory bulbs.
The convergence of like olfactory receptor neurons: Noise reduction?
Unilateral deafferentation of the left (center) or right antenna (right) shows labeling in both antennal lobes (from Vosshall et al., 2000). Why did Zhao and Reed see a depletion in mutant neurons with reduced odor-evoked activity, while Zheng et al.
The anatomy of the vertebrate olfactory bulb and insect antennal lobe
In contrast to mammals and flies, all stained locust ORNs were found to project to multiple (~2–6) glomeruli in the antennal lobe ( Hansson et al., 1996 ). The total number of glomeruli in Schistocerca gregaria was found to be more than 1000 (Hansson et al., 1996), which is significantly more than in the fly or bee brain.
Odors are represented by overlapping assemblies of PNs
Since this projection terminates in the granule cell layer, which provides inhibitory input to the outer bundle cells, this finding reveals that a set of mutually inhibitory connections connect isofunctional glomeruli in the medial and lateral maps of olfactory receptors. It has been argued that the olfactory bulb is analogous to the primary sensory cortices in the other senses ( Johnson et al., 2000 ), while the piriform cortex is analogous to the associative cortex (see below).
Oscillatory inhibition: Analog to digital conversion
Oscillations have also been found in the olfactory system of molluscs (Gelperin and Tank, 1990). When the oscillatory synchronization was disrupted by injecting a GABA antagonist, picrotoxin, into the antennal lobe, the bees' ability to discriminate between chemically similar odorants was impaired (Stopfer et al., 1997).
Temporal patterning of responses: Decorrelation
Sparsifying, digital decoding, intermodal association and learn- ing: The divergence to the insect mushroom bodies and the verte-
- The insect mushroom bodies
- The mammalian piriform cortex
Mushroom bodies are also involved in the perception of odor attractiveness, but not aversion (Want et al., 2001). This has led Johnson et al. 2000) to compare the piriform cortex with association cortices in other sensory modalities, rather than with primary cortices.
Beyond
Pyramidal cells in the piriform cortex project widely to many parts of the cortex, including areas involved in high-order functions (Johnson et al., 2000). A lack of columnar organization is also indicated by a marked disparity in the intrinsic projection patterns of adjacent injected cells (Johnson et al., 2000).
Neuromodulators and learning
Olfaction: A sense of variance
- Evaluating variance takes time…
1.19.2 …or space and convergence: Olfactory images and spatial codes in olfaction?
Architectural differences with the immune system
- Output requirements and specificity of response
- Different mechanisms to generate diversity of specificities
- Monospecificity is common to lymphocytes and olfactory receptor cells
Clearly, they can be expressed on the surface of olfactory receptor neurons; several members of the immunoglobulin superfamily are membrane associated. Could the immune system's large repertoire of specificities work in the olfactory system?
Abstract
Introduction
The olfactory epithelium is covered by an aqueous mucosa
In equilibrium, the concentration of an odorant in the mucosa is given by the fraction of its partial pressure over the vapor pressure of the pure odorant
Indeed, the odor detection thresholds for homologous series of alkanes and alcohols are to a first approximation a constant fraction of the vapor pressure of the odorant both in humans (Mullins, 1955; Cometto-Muniz and Cain, 1990; Doleman et al., 1998) and rats (Moulton and Eayrs, 1960). What then determines the fraction of an odorant's vapor pressure in the gas phase.
Transport of odorant molecules from source to nose
This relationship has also been reported for electrophysiological thresholds in the olfactory mucosa of the frog (Ottoson, 1958) and the trigeminal nerve of the rat (Silver et al., 1986). This dilution factor takes into account the inability of the vapor to reach saturation before being carried by an air stream, dilution by mixing with air while traveling from the odor source to the nose, and the inability of the odor to reach saturation in the mucous membrane if the odor is present. only temporarily present.
Invariance to volatility
Raoult’s law
Henry’s law
The concentration in the mucosa for ideal solutions
Solid-state odor sources
The pressure of a vapor in equilibrium with a solid odorant is therefore proportional to the vapor pressure in liquid form, the proportionality constantly depending on the temperature, the fusion temperature and the difference between the enthalpy of sublimation and that of evaporation. So, as in Equation 2 for liquid odorants, the vapor pressure is in the numerator of this equation, and so the odor concentration in the mucosa due to a solid source is independent of its volatility:
Discussion
This allows the mucosa to act as a gain control mechanism to maintain the concentration of different odorants in the mucosa at more similar levels than would be the case if odorant receptors detected odorant concentration in the vapor phase. Furthermore, the more the composition of the mucosa resembles that of the odor source, the more the activity coefficients for different odorants will be similar for both phases, and the less the relative concentrations in the mucosa will differ from those at the source.
Thermodynamic equlibria versus particle counters
The critical step to achieve invariance to volatility is to make the olfactory receptors not respond to the absolute concentration in the vapor, as mass spectroscopy or flame ionization detection is, but rather to the thermodynamic activity of the odorant. The former records absolute concentration in the gas phase, while the latter records activities.
Predictions and empirical support
The activities were calculated as the ratio of the partial vapor pressure at the threshold to the vapor pressure of the pure substances. Fortunately, we can use the ratio of the vapor phase thresholds to the vapor pressure of a compound to calculate the activity, since the vapor phase and solution activities are the same at equilibrium.
Introduction
Failed beginnings
We attempted to test for the presence of innate preferences for or against odors. We wondered if the large number of openings made it difficult for the locusts to identify the direction from which an odor was coming, and tried to simplify the task.
Schistocerca gregaria
One of the arms had air entering it while the other had an equal flow rate of scented air. In preliminary experiments, 6 grasshoppers were placed at the bottom of the Y-maze and each trial was scored by counting the number of grasshoppers that had to climb each branch during 5 min.
Grass Air
Conclusions
Furthermore, we show that at least one of these preferences (cherry) and one of these aversions (pentanol) are innate, as the animals were not exposed to the fragrances prior to testing.
Abstract
Introduction
Results
Odor discrimination for the named line code as a function of the size of the cell assembly used for decoding. Odor discrimination for the named line code as a function of the time scale used for decoding (T).
Discussion
The functional advantage conferred by such selectivity remains unknown and will be addressed in the first part of the next chapter. Finally, it is possible that fast time scales are useful in encoding the rapidly varying signals in natural dynamic odor plumes rather than the more uniform odor pulses used in the experiments in this thesis and in previous work from the lab.
Applications
Methods
Recent experiments have shown that synchrony plays a role in bees' discrimination of similar but not different odors (Stopfer et al., 1997). If inhibition in the accessory olfactory bulb of rats is prevented by injection of bicuculline, an olfactory memory of the present odor is formed (Brennan et al., 1990).
Theory
The main and irreconcilable difference is that #2 takes into account the continuous nature of spike trains, while #1 deforms the one-dimensional nature of spike trains by mapping time bins in Euclidean space. In contrast, in number 2, the distance between peak trains is directly related to the time difference between the corresponding peaks.
Empirical comparison
They appeared at slightly different depths on the near side of the fixation plane, as shown in the figure. The horizontal differences of the adapters were set to match the depths of adapters A and B in Figure 10 .
Who reads temporal information contained
Abstract
The behavior of a sensory system is only as rich as the set of stimuli it faces. Sensory physiologists are thus faced with the challenge of generating as rich a set of stimuli as possible in a controllable manner.
The need for a novel odor delivery system: Features
Compared to sight and hearing, where computer screens and synthesizers offer great control and flexibility, the study of olfaction has suffered from a relative lack of flexible odor delivery systems. Here, I present a computerized scent delivery system capable of delivering arbitrary real-time controlled concentrations, binary mixtures in arbitrary ratios, and the potential to deliver arbitrary discrete or continuous waveforms.
Concentration in liquid does not equate concentration in vapor
Indeed, changing the concentration in a solution can change the concentration in the vapor with a log linear relationship or a more complex one, depending on the solvent used (Brockerhoff and Grant, 1999). Changing the concentration in the solution by tenfold can change the concentration in the vapor by 1500-fold.
Short-term plasticity mandates repeatability across trials
The situation is even less desirable when the odor solution is placed on a filter paper, since the filter paper acts to some extent as a chromatographic column that separates solute from solvent and thus makes the concentration of the odor in the vapor phase more independent of the amount of added odorant, to the extreme that if the separation is complete, the concentration of the odor in the vapor phase will be the vapor pressure of the odorant, independent of the amount of added odorant1. For direct control of the concentration in the vapor phase, and especially for its quantification, gaseous dilution is preferred over liquid dilution.
The capability to deliver arbitrary concentrations
Given that one of our interests lies in understanding how the olfactory system achieves invariance to concentration, we were particularly interested in the ability to vary concentration continuously until we encountered a change in a neuron's response. However, many of the previously described odor delivery systems allow only a few discrete concentration steps.
Real-time online stimulus choice
The system described here allows dilution to practically any concentration value between the minimum and maximum allowed, its resolution limited by the computer's ability to control voltage: a 12-bit card then allows control to better than 1/1000 of the dynamic range, and a 16-bit cards give better than 1/16,000. As described below, this ability to deliver similar yet different concentrations proved critical for the discovery of abrupt transitions in neuronal responses to concentration.
Stationary vs. non-stationary flow
This happens because in the period between odor pulses the air in the flask reaches equilibrium (in the case of a pure liquid odorant it becomes saturated with odor), but because the volume of odor released during a pulse is greater than its capacity of the bottle. In the flask, the initial phase of a high odor concentration is followed by a subsequent phase of a lower concentration, the concentration of which is not determined by thermodynamics, but rather by the dynamics of a process that is out of equilibrium. The composition of the vapor of solutions of mixtures is not stable with time or concentration.
The composition of the vapor of solutions of mixtures is not stable over time or concentration
Our system solves this problem by achieving a steady flow before the start of the first pulse and by bubbling incoming air through long enough columns of liquid odorant that the air emerges saturated in odor regardless of the amount of time since the previous pulse .
Long-term stability in concentration delivered
Design
Mechanical artifact prevention
Gaseous dilution to generate arbitrary concentrations of one of several pure odors or a combinatorial diversity of binary mixtures
Of the eight valves in each manifold, one is open at any given time to determine the odor that the corresponding airflow will carry. The air stream enters the bubbler at the bottom of the liquid column of odorant and leaves it at the top, saturated in the odor.
Constant flow to eliminate non-stationarities
