| 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,…). Psychophysical studies show that an odor can be perceived as identical over significant ranges of concentrations. Whether concentration invariance results, at least in part, from low-level neural phenomena rather than cognitive grouping is so far unknown.; I explore, firstly, the contribution of projection neurons (PNs) in the antennal lobe of the locust, the analog of the vertebrate olfactory bulb, to the recognition of odor identity across concentrations; and secondly, what role spike timing, neuronal identity, and synchronization among neuronal assemblies play in the encoding and decoding of odor information by downstream neurons.; I show the following: The locust can recognize odors, and shows innate olfactory preferences.; PNs solve the task of encoding both odorant concentration and odorant identity, independently of the concentration, in three ways. First, by multiplexing information in different response dimensions using a code that involves neuronal identity, spike timing and synchronization across a neuronal assembly. Second, via a novel phenomenon of experience-dependent plasticity that contributes to PNs' invariance to concentration and sensitizes PNs after exposure to an odor at high concentration, contrary to the adaptation exhibited by receptors. Third, a phenomenon of gain control, whereby excitatory and inhibitory responses balance out massive changes in receptor activity as a function of odorant concentration, maintains the output of PNs within a small dynamic range.; Response patterns sometimes exhibit stable representations over large composition ranges and then abrupt transitions as a function of concentration and mixture composition, suggesting the difference between “same” and “different” odors may be delineated by sharp boundaries in odor space.; The physical chemistry of odorant reception confers the olfactory system invariance to odorant volatility, a physical property that has hitherto been believed to play a fundamental role in an odorant's effectiveness.; Finally, although synchronization among PN assemblies does not augment stimulus information in PN temporal responses, it is necessary for the read-out of odor information by downstream neurons. |
