Responses of cockroach antennal lobe projection neurons to pulsatile olfactory stimuli

Behavioral evidence indicates that insects preferentially orient toward pulses of odorants as they occur downwind from a point source. Our recent results have shown that cockroach olfactory receptor neurons are able to reliably resolve 10-Hz pulses of the general "green' odorant 1-hexanol, but it is unknown to what extent the central olfactory pathway is able to resolve temporal aspects of a general odor stimulus. In the present study, temporal response characteristics were measured in antennal lobe projection neurons of female American cockroaches, Periplaneta americana in response to series of short odor pulses (2.5-20 Hz). Odor pulses were delivered to olfactory sensilla in a moving airstream controlled by electromagnetic valves and quantified by replacing the odorant with oil smoke and measuring the concentration of smoke passing through a light beam. The responses of projection neurons were recorded with an intracellular microelectrode placed in the projection neuron cell body. A variety of time courses of responses were recorded. Response patterns were consistent among identical stimuli within a neuron and varied among neurons. Some neurons increased spike frequency with stimulus onset while others decreased spike frequency. The latency to the change in spike frequency and the duration of the response also varied among neurons. Regardless of the temporal characteristics of the responses, nearly all projection neurons were able to resolve pulses of 1-hexanol presented at 5 Hz and some could resolve 10-Hz pulses. Thus, responses of antennal lobe projection neurons can reflect fine structures of non-uniform distributions of general odorants in a turbulent odor plume. In addition, the variety of temporal response characteristics to identical stimuli suggests that odor quality is coded by a temporal code expressed across a population of projection neurons.