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Olfactory systems must detect and discriminate odors while maintaining a degree of perceptual invariance of identity in a highly variable environment. Studies have suggested that olfactory systems of both vertebrates and invertebrates detect odors based on the interaction of specific molecular features of odorant molecules with olfactory receptor neurons. It has also been shown that changing stimulus concentration can elicit substantive changes in spatial representations of odorants. The first goal of our studies was to psychophysically characterize the ability of the moth Manduca sexta to detect and discriminate odors as a function of concentration using a Pavlovian olfactory-learning paradigm. Our results indicate that odor detection thresholds varied widely with different odors and that discrimination between odors occurred at one or more orders of magnitude above detection. Counterbalancing the reinforced odors produced asymmetric discrimination thresholds which were predictable by the degree of asymmetry in the detection thresholds. Previous studies have also shown that blockade of GABAergic transmission within the insect antennal lobe disrupts discrimination of monomolecular odors with similar molecular features. These studies used generalization protocols, which in principle could falsely imply that discrimination of molecularly different odors is unaffected. Using Manduca sexta and Pavlovianbased stimulus generalization protocols, we confirm that GABAA blockade within the antennal lobe increases generalization of a conditioned response to only similar odors as previously demonstrated. However, using differential conditioning protocols, we show that GABAA blockade disrupts discrimination of similar and dissimilar odors equally as indicated by increased discrimination thresholds. We also observed an effect on concentration-response functions, suggesting odor detection was also effected. To establish whether disruption of discrimination thresholds could be attributed to disrupted detection thresholds, we investigated the effect of GABAA blockade on detection threshold measures. Results show that detection thresholds increased, indicating that disrupted discrimination is due to impairment of the ability to detect; because physiological measures of antennal lobe output indicate that GABAA blockade increases output from the antennal lobe, we conclude that our effects are attributable to a loss of ability to extract the neural signal from background activity.

Psychophysical measures of detection & discrimination and effects of GABA blockade in the moth Manduca sexta