Sensory perceptions, differential

Differential Sensory Perceptions of Plant Compounds by Insects... 

Differential sensory sensitivity. The insect s perception of plant odours differs essentially from their discrimination of non-volatile taste substances, as phytophagous insects may already perceive the odour at some distance from the plant. In adult phytophagous insects the antennae bear a large number of olfactory sensilla in order to detect the minute concentrations of the leaf odour components in the air downwind from a plant. The overall sensitivity of the antennal olfactory receptor system can be measured by making use of the electroantennogram technique (17). An electroantennogram (EAG) is the change in potential between the tip of an antenna and its base, in response to stimulation by an odour component. Such an EAG reflects the receptor potentials of the olfactory receptor cell population in the antenna. 

Kinesthesia allows the perception of limb movement and position together with the perception of force. This sensory perception originates primarily from muscle mechanorecep-tors. The differential threshold for force perception averages 7%-10% over a force range of 0.5-200 N [26]. Discrimination deteriorates for forces smaller than 0.5 N, with the threshold increasing to 15%-27%. Forces as small as 0.14-0.2 N can still be distinguished. 

The sensory evaluation differentiates between the stimulation threshold (a just detectable level where a perceptible but not yet definable deviation of the sample from the standard is observed) and the recognition threshold, a level where the odor is identifiable or creates odor problems (a no longer tolerable quality deterioration caused by a definite off odor and/or taste). The difference between a perceptible and identifiable level is usually only one to two steps of a geometric dilution series. Therefore, only undifferentiated odor and taste thresholds are given in Table 13-6, because of the very different sensitivities of individual testers. The perceptible (stimulation) levels of a less sensitive tester can overlap with the identifiable (recognition) level of another more sensitive tester. 

Enantiomers display a sensory and biological activity differentiation. Their sensory properties are not univocal. Differences in the character of odours of enantiomers eonfirm the supposition that the enzymatic theory perception is based on the activity of enzymes and odour substances. Systemic enzymes being a chiral catalyst of the highest efficiency act in an enantioselective way, and hence the considerable differentiation in biological effects (various odours). 

There are several definitions of threshold. The detection threshold is defined as the lowest stimulus eliciting a sensorial response, even though no identification can be reached. The recognition threshold or identification threshold is the level of stimulus at which the specific stimulus can be identified. The differential threshold or just noticeable difference, is the minimum increase in a stimulus required to detect a difference in the perception. 

As noted above, there is substantial information available describing the sensory properties of aroma compounds. The classic references in this respect were not mentioned above but are the works of FenaroU [18] and Arctander [ 19]. In addition, most suppliers of aroma chemicals provide a characterization of their product. However, those directly concerned with CTeating flavoring compositions have to develop their own characterizations based on their personal evaluation of these chemicals. The descriptions provided in the literature are based on an individual s vocabulary, experience, and perception which vary as widely from individual to individual as do the sensory threshold data. Thus, those coming into this field must keep records of their own evaluations of odor and flavor characters of all aroma chemicals. This whole process is what differentiates one flavorist s creation from another. 

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