General Conclusions

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The major aim of the present thesis was to investigate how the local circuits in the honeybee AL shape the olfactory code. To this end, I selectively optically recorded the output of the olfactory glomeruli, which represents the processed odor information. Comparison of these odor-induced output patterns with the compound signals, which mainly reflect the afferent input to the AL (i.e. RN responses), allowed me to draw conclusions about the role of the AL network involved in the odor processing. How are odors represented in the output neurons compared to the afferent input? What do the two both processing levels have common and what are the differences? The studies of the present thesis showed that, like the afferent input patterns, individual glomeruli have highly predictable output responses from individual to individual (CHAPTER I). Consequently , both the input and output neurons innervating a particular glomerulus are genetically determined. Moreover, the odor representations in the dominantly responding glomeruli are very well correlated between both processing levels: Glomeruli with a strong RN input, also revealed a strong output response. The strong RN responses become even higher within the AL, based on the high convergence of RNs onto each PN, leading to an improved sensitivity on the PN level (CHAPTER IV). Contrary to the strongly-activated glomeruli, those which get a weak or intermediate RN input revealed a decreased sensitivity , leading to reduced or even no responses in their PNs (CHAPTER I, IV). This indicates specific inhibitory interactions within the AL. Thus, the strong glomerular responses are further emphasized by the specific reduction of weaker-activated glomeruli, leading to an odor-and glomerulus-specific contrast-enhancement of the spatial odor representations. Moreover, the input signals from the RNs are transformed into temporal complex glomeru-lar output patterns (CHAPTER I). PN responses to odors consisted of both excitatory and inhibitory phases. Inhibitory responses occurred as either a calcium-concentration decrease during stimulus application or as a rebound calcium increase after stimulus offset (i.e. off-response) due to the release from inhibition. The response polarity differed for PNs depending on both the glomerulus innervated and the odor used as stimulus, and was conserved among individuals. Since PNs were spontaneously active, an inhibitory response corresponds to a reduction in their firing frequency. Thus, it is likely that these inhibitory responses are also part of the olfactory code and may represent a further coding channel, which would extend the coding capacities at the output …


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