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Priming contributes to concentration invariance in early olfactory circuits

Object recognition requires both specificity, to ensure that stimuli with distinct behavioral relevance are distinguished, and invariance, to ensure that different instances of the same stimulus are recognized as the same under varied conditions (intensity, pitch, position, etc.). In olfaction, the question of stimulus specificity has received considerable attention1-13. Psychophysical studies also show that individual odors can be perceived as identical over significant ranges of concentrations14,15. Whether concentration invariance results, at least in part, from low-level neural phenomena rather than cognitive grouping is so far unknown. Using locusts, we found that projection neurons in the antennal lobe (the first olfactory relay) are sensitive to odor concentration differences, but often in a history-dependent manner. In ~50% of recordings, exposure to a high concentration modified subsequent responses to lower concentrations such that responses to lower concentrations became more similar to responses to the higher con-