Apparent Motion Perception in the Praying Mantis: Psychophysics and Modelling by Tarawneh G, Jones L, Nityananda V, Rosner R, Rind C, Read JCA, TarawnehNityanandaJonesNityanandaRosnerRindRead2018.pdf (0.9 MiB) - Apparent 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.

Contrast thresholds reveal different visual masking functions in humans and praying mantises by Tarawneh G, Nityananda V, Rosner R, Errington S, Herbert W, Arranz-Paraíso S, Busby N, Tampin J, Read JCA, Serrano-Pedraza I, TarawnehNityanandaRosnerErringtonHerbertArranzParaisoBusbyTampinSerranoSerranoPedraza2018.pdf (2.3 MiB) - Recently, we showed a novel property of the Hassenstein–Reichardt detector, namely that insect motion detection can be masked by ‘undetectable’ noise, i.e. visual noise presented at spatial frequencies at which coherently moving gratings do not elicit a response (Tarawneh et al., 2017). That study compared the responses of human and insect motion detectors using different ways of quantifying masking (contrast threshold in humans and masking tuning function in insects). In addition, some adjustments in experimental procedure, such as presenting the stimulus at a short viewing distance, were necessary to elicit a response in insects. These differences offer alternative explanations for the observed difference between human and insect responses to visual motion noise. Here, we report the results of new masking experiments in which we test whether differences in experimental paradigm and stimulus presentation between humans and insects can account for the undetectable noise effect reported earlier. We obtained contrast thresholds at two signal and two noise frequencies in both humans and praying mantises (Sphodromantis lineola), and compared contrast threshold differences when noise has the same versus different spatial frequency as the signal. Furthermore, we investigated whether differences in viewing geometry had any qualitative impact on the results. Consistent with our earlier finding, differences in contrast threshold show that visual noise masks much more effectively when presented at signal spatial frequency in humans (compared to a lower or higher spatial frequency), while in insects, noise is roughly equivalently effective when presented at either the signal spatial frequency or lower (compared to a higher spatial frequency). The characteristic difference between human and insect responses was unaffected by correcting for the stimulus distortion caused by short viewing distances in insects. These findings constitute stronger evidence that the undetectable noise effect reported earlier is a genuine difference between human and insect motion processing, and not an artefact caused by differences in experimental paradigms.

Invisible noise obscures visible signal in insect motion detection by Tarawneh G, Nityananda V, Rosner R, Errington S, Herbert W, Cumming BG, Read JCA, Serrano-Pedraza I, TarawnehNityanandaRosnerErringtonHerbertCummingReadSerranoPedraza2017.pdf (2.6 MiB) - The motion energy model is the standard account of motion detection in animals from beetles to
humans. Despite this common basis, we show here that a difference in the early stages of visual
processing between mammals and insects leads this model to make radically different behavioural
predictions. In insects, early filtering is spatially lowpass, which makes the surprising prediction that
motion detection can be impaired by “invisible” noise, i.e. noise at a spatial frequency that elicits
no response when presented on its own as a signal. We confirm this prediction using the optomotor
response of praying mantis Sphodromantis lineola. This does not occur in mammals, where spatially
bandpass early filtering means that linear systems techniques, such as deriving channel sensitivity from
masking functions, remain approximately valid. Counter-intuitive effects such as masking by invisible
noise may occur in neural circuits wherever a nonlinearity is followed by a difference operation.