A. Bäcker*, K. MacLeod, M. Wehr and G. Laurent. Division of Biology, 139-74, California Institute of Technology, Pasadena, CA 91125, USA.

Odors appear to be represented in the antennal lobe of the locust by odor-specific, but overlapping, evolving assemblies of synchronously firing projection neurons (PNs). The synchronization across neurons can be eliminated by injection of picrotoxin (PCT) in the antennal lobe. This manipulation has been found to impair fine olfactory discrimination in bees (Stopfer et al., 1997: Nature 390:70-74). We ask here whether such an impairment may result from the loss of stimulus information in PNs, or whether the stimulus information remains after PCT injection but is unable to be read by downstream neurons. To address this question, we reconstructed the odor stimulus for each trial from the odor-evoked spike trains, using a spike-time distance method (Victor & Purpura, 1997: Network: Comput. Neural Syst. 8:127-164). Interestingly, reconstructions provide 95-98% correct identification for discrimination among 3 odors even from a single cell. In addition, for more than half of PNs, the reconstruction appears to be optimal for an integration window approximately equal to one oscillation cycle. Such accurate stimulus identification is equally possible from PN responses after PCT injection (87-93% correct). Beta-lobe cells are a third-order neuropil downstream of PNs. Their responses also carry significant information (75% correct for 5 odors) about odor identity, but this information can be greatly reduced by PCT injection (down to 27% correct). These results suggest that synchronization across PNs is critical for the correct reading of PNs' neural code by downstream neurons. Supported by the Sloan Foundation and NSF.

Keywords: OSCILLATIONS - CODING - OLFACTION - STIMULUS RECONSTRUCTION