Spatial odor discrimination in the hawkmoth, Manduca sexta (L.).

Flying insects track turbulent odor plumes to find mates, food and egg-laying sites. To maintain contact with the plume, insects are thought to adapt their flight control according to the distribution of odor in the plume using the timing of odor onsets and intervals between odor encounters. Although timing cues are important, few studies have addressed whether insects are capable of deriving spatial information about odor distribution from bilateral comparisons between their antennae in flight. The proboscis extension reflex (PER) associative learning protocol, originally developed to study odor learning in honeybees, was used as a tool to ask if hawkmoths, Manduca sexta, can discriminate between odor stimuli arriving on either antenna. We show moths discriminated the odor arrival side with an accuracy of >70%. Information about spatial distribution of odor stimuli may be available to moths searching for odor sources, opening the possibility that they use both spatial and temporal odor information.This article has an associated First Person interview with the first author of the paper.

Parameters of motion vision in low light in the hawkmoth Manduca sexta.

The hawkmoth Manduca sexta is nocturnally active, beginning its flight activity at sunset, and executing rapid controlled maneuvers to search for food and mates in dim light conditions. The visual system of this moth has been shown to trade off spatial and temporal resolution for increased sensitivity in these conditions. The study presented here uses tethered flying moths to characterize the flight performance envelope of the wide-field-motion-triggered steering response of M. sexta in low light conditions by measuring attempted turning in response to wide-field visual motion. Moths were challenged with a horizontally oscillating sinusoidal grating at a range of luminance, from daylight to starlight conditions. The impact of luminance on response to a range of temporal frequencies and spatial wavelengths was assessed across a range of pattern contrasts. The optomotor response decreased as a function of decreasing luminance, and the lower limit of the moth's contrast sensitivity was found to be between 1 and 5%. The preferred spatial frequency for M. sexta increased from 0.06 to 0.3 cycles deg-1 as the luminance decreased, but the preferred temporal frequency remained stable at 4.5 Hz across all conditions. The relationship between the optomotor response time to the temporal frequency of the pattern movement did not vary significantly with luminance levels. Taken together, these results suggest that the behavioral response to wide-field visual input in M. sexta is adapted to operate during crepuscular to nocturnal luminance levels, and the decreasing light levels experienced during that period changes visual acuity and does not affect their response time significantly.

Optomotor steering and flight control requires a specific sub-section of the compound eye in the hawkmoth, Manduca sexta.

While tracking odor plumes, male hawkmoths use optic flow cues to stabilize their flight movements with respect to their environment. We studied the responses of freely flying moths tracking odor plumes in a laboratory wind tunnel and tethered moths in an optomotor flight simulator to determine the locations on the compound eye on which critical optic flow cues are detected. In these behavioral experiments, we occluded specific regions of the compound eye and systematically examined the moths' behavior for specific deficits in optic flow processing. Freely flying moths with the dorsal half of the compound eye painted were unable to maintain stable flight and track the wind-borne odor plume. However, the plume tracking performance of moths with the ventral half of their compound eyes painted was the same as unpainted controls. In a matched set of experiments, we presented tethered moths with moving vertically oriented sinusoidal gratings and found that individuals with their eyes unpainted, ventrally painted and medially painted all responded by attempting optomotor-driven turns in the same proportion. In contrast, individuals with their compound eyes dorsally painted, laterally painted and completely painted showed no optomotor turning response. We decreased the contrast of the visual stimulus and found that this relationship was consistent down to a contrast level of 2.5%. We conclude that visual input from the dorso-lateral region of the moth's visual world is critical for successful maintenance of flight stability and that this species' visual environment must meet or exceed a contrast ratio of 2.5% to support visual flight control.

Laterality and symmetry in rat olfactory behavior and in physiology of olfactory input.

Many species use bilateral sampling for odor-guided navigation. Bilateral localization strategies typically involve balanced and lateralized sensory input and early neuronal processing. For example, if gradient direction is estimated by differential sampling, then any asymmetry could bias the perceived direction. Subsequent neuronal processing can compensate for this asymmetry but requires the presence of mechanisms to track changes in asymmetry. A high degree of laterality is also important for differential sampling because spillover of signals will dilute the perceived odor gradient. In apparent contradiction to this model, both symmetry and laterality of nasal air flow have been reported to be incomplete in rats. Here, we measured symmetry and laterality in early olfactory processing in the rat. We first established behavioral readouts of precisely controlled bilateral odorant stimuli. We found that rats could rapidly and accurately report the direction of a wide range of odor gradients, presented in random sequence. We then showed that nasal air flow was symmetric over an entire day in awake rats. Furthermore, odor sampling from the two nostrils in the behavioral task was highly lateralized. This lateralization extended to the receptor epithelium responses as measured by electro-olfactograms. We finally observed strong lateralization of intrinsic signal responses from the glomerular layer of the olfactory bulb. We confirmed that a differential comparison of glomerular responses was sufficient to localize odorants. Together, these results suggest that the rat olfactory system is symmetric, with highly lateralized odor flow and neuronal responses. In combination, these attributes support odor localization by differential comparison.