The detection of visual motion and its direction is a fundamental task faced by several visual systems. The motion detection system of insects has been widely studied with the majority of studies focussing on flies and bees. Here we characterize the contrast sensitivity of motion detection in the praying mantis Sphodromantis lineola, an ambush predator that stays stationary for long periods of time while preying on fast-moving prey. In this, its visual behaviour differs from previously studied insects and we might therefore expect its motion detection system to differ from theirs. To investigate the sensitivity of the mantis we analyzed its optomotor response in response to drifting gratings with different contrasts and spatio-temporal frequencies. We find that the contrast sensitivity of the mantis depends on the spatial and temporal frequencies present in the stimulus and is separably tuned to spatial and temporal frequency rather than specifically to object velocity. Our results also suggest that mantises are sensitive to a broad range of velocities, in which they differ from bees and are more similar to hoverflies. We discuss our results in relation to the contrast sensitivities of other insects and the visual ecology of the mantis.Electronic supplementary materialThe online version of this article (doi:10.1007/s00359-015-1008-5) contains supplementary material, which is available to authorized users.

Target tracking behaviour of the praying mantis Sphrodromantis1 lineola (Linnaeus) is driven by looming-type motion-detectors. 2 3 4 F.

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5Apparent motion is the perception of a motion created by rapidly presenting still frames in which objects are 6 displaced in space. Observers can reliably discriminate the direction of apparent motion when inter-frame object 7 displacement is below a certain limit, Dmax. Earlier studies of motion perception in humans found that Dmax 8 scales with spatial element size, interpreting the relationship between the two as linear, and that Dmax appears 9 to be lower-bounded at around 15 arcmin. Here, we run corresponding experiments in the praying mantis 10 Sphodromantis lineola to investigate how Dmax scales with element size. We used moving random chequerboard 11 patterns of varying element and displacement step sizes to elicit the optomotor response, a postural stabilization 12 mechanism that causes mantids to lean in the direction of large-field motion. Subsequently, we calculated Dmax 13 as the displacement step size corresponding to a 50% probability of detecting an optomotor response in the same 14 direction as the stimulus. Our main findings are that mantis Dmax appears to scale as a power-law of element 15 size and that, in contrast to humans, it does not appear to be lower-bounded. We present two models to explain (frames) in which objects are displaced in small steps, tricking us into perceiving smooth motion. This illusion 22 is referred to as "apparent motion", and for it to work effectively the magnitude of each displacement step 23 must be smaller than a certain limit, referred to as Dmax. Previous studies have investigated the relationship 24 between this limit and object size in humans and found that larger objects can be displaced in larger steps 25 without affecting motion perception. In this work, we investigated the same relationship in the praying mantis 26Sphodromantis lineola by presenting them with moving chequerboard patterns on a computer monitor. Even 27 though motion perception in humans and insects are believed to be explained equally well by the same underlying 28 model, we found that Dmax scales with object size differently in mantids. These results suggest that there may 29 be qualitative differences in how mantids perceive apparent motion compared to humans. 30

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Simple SummaryComputer monitors, smart phone screens, and other forms of digital displays present a series of still images (frames) in which objects are displaced in small steps, tricking us into perceiving smooth motion. This illusion is referred to as “apparent motion”. For motion to be perceived, the magnitude of each displacement step must be smaller than a certain limit, referred to as Dmax. Previous studies have investigated the relationship between this limit and object size in humans and found that the maximum displacement is larger for larger objects than for smaller ones. In this work, we investigated the same relationship in the praying mantis Sphodromantis lineola by presenting them with moving random chequerboard patterns on a computer monitor. Even though motion perception in humans and insects are believed to be explained equally well by the same underlying model, we found that Dmax scales differently with object size in mantids. These results suggest that there may be qualitative differences in how mantids perceive apparent motion compared to humans.AbstractApparent motion is the perception of motion created by rapidly presenting still frames in which objects are displaced in space. Observers can reliably discriminate the direction of apparent motion when inter-frame object displacement is below a certain limit, Dmax. Earlier studies of motion perception in humans found that Dmax is lower-bounded at around 15 arcmin, and thereafter scales with the size of the spatial elements in the images. Here, we run corresponding experiments in the praying mantis Sphodromantis lineola to investigate how Dmax scales with the element size. We use random moving chequerboard patterns of varying element and displacement step sizes to elicit the optomotor response, a postural stabilization mechanism that causes mantids to lean in the direction of large-field motion. Subsequently, we calculate Dmax as the displacement step size corresponding to a 50% probability of detecting an optomotor response in the same direction as the stimulus. Our main findings are that the mantis Dmax scales roughly as a square-root of element size and that, in contrast to humans, it is not lower-bounded. We present two models to explain these observations: a simple high-level model based on motion energy in the Fourier domain and a more-detailed one based on the Reichardt Detector. The models present complementary intuitive and physiologically-realistic accounts of how Dmax scales with the element size in insects. We conclude that insect motion perception is limited by only a single stage of spatial filtering, reflecting the optics of the compound eye. In contrast, human motion perception reflects a second stage of spatial filtering, at coarser scales than imposed by human optics, likely corresponding to the magnocellular pathway. After this spatial filtering, mantis and human motion perception and Dmax are qualitatively very similar.