Disruption of GABAA in the insect antennal lobe generally increases odor detection and discrimination thresholds.

Studies of olfactory function show that disruption of GABA A receptors within the insect antennal lobe (AL) disrupts discrimination of closely related odors, suggesting that local processing within the AL specifically enhances fine odor discrimination. It remains unclear, however, how extensively AL function has been disrupted in these circumstances. Here we psychophysically characterize the effect of GABA A blockade in the AL of the moth Manduca sexta. We used 2 GABA A antagonists and 3 Pavlovian-based behavioral assays of olfactory function. In all cases, we used matched saline-injected controls in a blind study. Using a stimulus generalization assay, we found that GABA A disruption abolished the differential response to related odors, suggesting that local processing mediates fine odor discrimination. We then assessed the effect of GABA A antagonist on discrimination thresholds. Moths were differentially conditioned to respond to one odor (reinforced conditioned stimulus [CS+]) but not a second (unreinforced conditioning stimulus [CS-]) then tested for a significant differential conditioned response between them across a series of increasing concentrations. Here, GABA A blockade disrupted discrimination of both similar and dissimilar odor pairs as indicated by generally increased discrimination thresholds. Finally, using a detection threshold assay, we established that GABA A blockade also increases detection thresholds. Because detection is a prerequisite of discrimination, this later finding suggests that disrupted discrimination may be due to impairment of the ability to detect. We conclude that the loss of ability to detect and subsequently discriminate is attributable to a loss of ability of the AL to provide a clear neural signal from background.

Characterizing psychophysical measures of discrimination thresholds and the effects of concentration on discrimination learning in the moth Manduca sexta.

What is the spatial and temporal nature of odor representations within primary olfactory networks at the threshold of an animal's ability to discriminate? Although this question is of central importance to olfactory neuroscience, it can only be answered in model systems where neural representations can be measured and discrimination thresholds between odors can be characterized. Here, we establish these thresholds for a panel of odors using a Pavlovian paradigm in the moth Manduca sexta. Moths were differentially conditioned to respond to one odor (CS+) but not another (CS-) using undiluted odorants to minimize salience-dependent learning effects. At 24 and 48 h postconditioning, moths were tested for the presence of a conditioned response (CR) with a blank, then the CS+ and CS- (pseudorandomly) across a 5-log step series of increasing concentration. Results identified discrimination thresholds and established that differential CRs to the CS+ and CS- increased with stimulus concentration. Next, 3 separate groups of moths were differentially conditioned at either one-log step below, at, or one log step above the identified discrimination threshold. At 24 and 48 h postconditioning, moths were tested sequentially with a blank, the concentration used for conditioning, and then undiluted odor. Conditioning at one log step below the discrimination threshold established a CR, indicating both stimulus detection and learning, but was insufficient to establish evidence of discrimination. Moths conditioned at the discrimination threshold were able to discriminate but only when stimulated with undiluted odors, indicating learning, but discrimination measures were hampered. When conditioned above the discrimination threshold, moths had no difficulty in discriminating. These results establish methods for psychophysical characterization of discrimination and indicate that differential conditioning at lowered concentrations biases threshold measures.

Detection versus perception: physiological and behavioral analysis of olfactory sensitivity in the moth (Manduca sexta).

The precise relationship between olfactory transduction sensitivity and sensitivity at the level of sensory perception is poorly understood. The goal of this work was to correlate neurophysiological measures of sensory transduction and psychophysical measures of salient odor perception using the moth (Manduca sexta). Moths were conditioned to respond to a single monomolecular odor and then tested across a dilution series. Resulting concentration-response functions were dependent on the conditioning odor and its concentration but not on the moth's sex. Comparison of responsiveness to odor versus an odorless blank provided a statistical indicator of detection threshold. Separate control experiments revealed that conditioned responsiveness to odor was a function of test concentration and independent of the conditioning concentration. Next, electroantennogram (EAG) responses to these odorants and concentrations were recorded. EAG responses were highly correlated with the behavioral results but typically identified lower detection thresholds. Furthermore, significant effects of sex, Sex x Odor, and Sex x Odor x Concentration were observed only within the EAG data, suggesting a mismatch across methods. Possible reasons for and implications of this mismatch are discussed.