Ensemble Response in Mushroom Body Output Neurons of the Honey Bee Outpaces Spatiotemporal Odor Processing Two Synapses Earlier in the Antennal Lobe

Abstract

Neural representations of odors are subject to computations that involve sequentially convergent and divergent anatomical connections across different areas of the brains in both mammals and insects. Furthermore, in both mammals and insects higher order brain areas are connected via feedback connections. In order to understand the transformations and interactions that this connectivity make possible, an ideal experiment would compare neural responses across different, sequential processing levels. Here we present results of recordings from a first order olfactory neuropile – the antennal lobe (AL) – and a higher order multimodal integration and learning center – the mushroom body (MB) – in the honey bee brain. We recorded projection neurons (PN) of the AL and extrinsic neurons (EN) of the MB, which provide the outputs from the two neuropils. Recordings at each level were made in different animals in some experiments and simultaneously in the same animal in others. We presented two odors and their mixture to compare odor response dynamics as well as classification speed and accuracy at each neural processing level. Surprisingly, the EN ensemble significantly starts separating odor stimuli rapidly and before the PN ensemble has reached significant separation. Furthermore the EN ensemble at the MB output reaches a maximum separation of odors between 84–120 ms after odor onset, which is 26 to 133 ms faster than the maximum separation at the AL output ensemble two synapses earlier in processing. It is likely that a subset of very fast PNs, which respond before the ENs, may initiate the rapid EN ensemble response. We suggest therefore that the timing of the EN ensemble activity would allow retroactive integration of its signal into the ongoing computation of the AL via centrifugal feedback.

Parallel Processing via a Dual Olfactory Pathway in the Honeybee

“…Air exhaust was provided by the building's ventilation system. We adapted electrode fabrication from Okada et al (1999Okada et al ( , 2007 and Strube-Bloss et al (2011). Each electrode shank consisted of three copper wires ( polyurethane-coated copper wire with 15 m diameter; Elektrisola) glued together with melted dental wax (diameter, Ͻ50 m; Fig.…”

“…Therefore, we took special care in this task. For spike sorting, we used a well established commercial software (Spike 2, v7.4, Cambridge Electronic Design) that has been used successfully in several other studies (Brierley et al, 2003;Hoare et al, 2008;Strube-Bloss et al, 2011Payton et al, 2012). We preprocessed each channel using the implemented "smooth" algorithm, which finite impulse response filters the input with a time constant of 80 s (comparable to a low-pass filter of 6250 Hz) and "DC-remove" with a time constant of 3.2 ms leading to offset adjustment (comparable to a high-pass filter of 312.5 Hz).…”

“…Tuning measurements were performed according to recent publications (Krofczik et al, 2008;Strube-Bloss et al, 2011;. For each single unit, we computed the lifetime sparseness (LS) as follows:…”

Simultaneous Long-term Recordings at Two Neuronal Processing Stages in Behaving Honeybees

“…Extracellular multi-unit recordings became a favorable tool to achieve high temporal resolution combined with spatial information. In our case, these are either parallel neuronal tracts 2 or two different neuropils 1 . Multiple neurons can be recorded and analyzed at the single neuron level in parallel and at high temporal resolution.…”

“…The described electrodes are suitable for recording both single unit and population activity which is especially useful for latency measurements and other temporal response properties of different neurons and different neuropils within a single specimen (for details see 1,2,25 ). Additionally we have shown how to permanently implement the micro wire electrodes to allow stable long-term recordings in behaving honeybees that last for hours up to days.…”