Suprathreshold levels

Determining the threshold value is difficult because subthreshold levels of one compound may affect the threshold levels of another. Also, the flavor quality of a compound may be different at threshold level and at suprathreshold levels. The total range of perception can be divided into units that represent the smallest additional amount that can be perceived. This amount is called just noticeable difference (JND). The whole intensity scale of odor perception covers about 25 JNDs this is similar to the number of JNDs that comprise the scale of taste intensity. Flavor thresholds for some compounds depend on the medium in which the compound is dispersed or dissolved. Patton (1964) found large differences in the threshold values of saturated fatty acids dissolved in water and in oil. 

As mentioned, at suprathreshold levels subjects differ greatly both in the kind and in the number of verbal descriptors they apply to a given odor [78,79,81]. Some differences probably stem from the way subjects use labels, but some may reflect true individual differences in perceived quahty [126]. To determine exactly how the odor spaces of two subjects might differ, some test of performance (a task based on discrimination) might serve best [69]. 

One factor limiting our ability to predict the flavor of mixtures on the basis of chemical composition is an incomplete understanding of the complex processes involved in the sensory perception of these mixtures. For example, although the relationship between concentration and perceived intensity of single compounds has been well characterized at suprathreshold levels, individual components can mask or enhance the aroma of other... 

The Lyon Clinical Olfactory Test is based on thresholds, suprathres-hold detection, and identification tasks. The LCOT includes two threshold measurements, one for R-(+)-carvone and the other for tetrahydrothio-phene (THT). The same threshold determination procedure is used for both compounds. Five concentration levels are presented in increasing order with a 4-AFC paradigm. In the suprathreshold detection task 16 odors, presented at a suprathreshold level, have to be detected by following a 4-AFC procedure. The identification task consists of 16 odors to be identified, following a 4-AFC procedure and using verbal alternatives. 

Limonin has been known to be an intensely bitter substance since it was first isolated in 1841 by Bernay (23). Beginning in 1966 the development of analytical methods for limonin allowed correlations of apparent bitterness with juice limonin content to be undertaken. The various reports of relative bitterness of suprathreshold limonin levels have been reviewed by Maier et al. 

The basis for the lack of response to 3,4-DAP by the other serotypes is not well understood. At a functional level, serotype A-intoxicated neuromuscular junctions undergo an attenuated but synchronous release of ACh following stimulation preparations intoxicated by serotypes B, D, and F produce asynchronous release where the ACh quanta are dispersed and cannot summate to produce suprathreshold EPPs (Lundh et al., 1977 Molgo et al., 1980 Thesleff, 1989). It is readily apparent that the lack of synchrony would prevent 3,4-DAP from restoring transmitter release however, the factors that lead to asynchronous release are not currently understood. 

The alcohol data indicates that the bulkiness of the substituents on the carbinol moiety does not determine the level of odor intensity. The suprathreshold data also indicates that the polar effects of the groups bonded to the carbinol moiety did not effect the level of odor intensity. 

Aldehyde and ketone suprathreshold odor intensity correlated well with log P and HB as shown in Table II. No significant relationship between steric or electronic parameters with aldehyde-ketone suprathreshold data was found with the exception of the Sterimol parameter L which was highly correlated to log P (R=0.95). Aldehyde threshold data weis found to be linearly related to log P as shown in equations 10 eind 13. The same data was poorly correlated with E andvas shown in Table II (eq. 12, 15 and 16). Note that two different aldehyde threshold data sets from two different sources produced very similar equations having slopes, intercepts, correlation coefficients and standard deviations which are not statistically different at the 95% level of confidence (eq. 10 and 13). 

The use of the QSAR technique known as the Hansch Approach in the investigation of odor intensity and odorant physico-chemical properties has indicated that hydrophobic properties of homologous series of compounds, not steric or polar properties, are highly correlated to the level of odor intensity. This was shown to be the case for literature odor threshold and suprathreshold data determined at different laboratories using various media. The poor correlation between odor intensity and the steric properties of molecules (Taft Steric Constant) which had been reported earlier by this author (11) have been further verified by the use of Charton and Verloop Sterimol steric parameters. 

On the basis of the results obtained in Study 1, our team developed the European Test of Olfaetory Capabilities [28]. This test is based on a combination of a suprathreshold deteetion task and an identification task. To construct the test, some of the transeultural odors previously selected in study 1 (vanilla, clove, apple, euealyptus, dimamon, garlic, anise, orange, lemon, and mint) were used [29]. Odor intensity was adjusted to increase the level of difficulty, especially in the detection task. Alternative odor names were also carefully selected in the identification protocol, so as to adjust the difficulty of identification [30]. The test was validated in six European... 

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