Peripheral Flicker Fusion at High Luminance: Beyond the Ferry–Porter Law by Fernandez-Alonso M, Innes W, Read JCA, FernandezAlonsoInnesRead2023.pdf (0.9 MiB) - The relationship between luminous intensity and the maximum frequency of flicker that can be detected defines the limits of the temporal-resolving ability of the human visual system, and characterizing it has important theoretical and practical applications; particularly for determining the optimal refresh rate for visual displays that would avoid the visibility of flicker and other temporal artifacts. Previous research has shown that this relationship is best described by the Ferry–Porter law, which states that critical flicker fusion (CFF) increases as a linear function of log retinal illuminance. The existing experimental data showed that this law holds for a wide range of stimuli and up to 10,000 Trolands; however, beyond this, it was not clear if the CFF continued to increase linearly or if the function saturated. Our aim was to extend the experimental data available to higher light intensities than previously reported in the literature. For this, we measured the peripheral CFF at a range of illuminances over six orders of magnitude. Our results showed that for up to 104 Trolands, the data conformed to the Ferry–Porter law with a similar slope, as previously established for this eccentricity; however, at higher intensities, the CFF function flattens and saturates at ~90 Hz for a target size of 5.7 degrees, and at ~100 Hz for a target of 10 degrees of angular size. These experimental results could prove valuable for the design of brighter visual displays and illumination sources that are temporally modulated.
Reduced surround suppression in monocular motion perception by Arranz-Paraiso S, Read JCA, Serrano-Pedraza I, ArranzParaisoReadSerranoPedraza2021.pdf (0.7 MiB) - Motion discrimination of large stimuli is impaired at high contrast and short durations. This psychophysical result has been linked with the center-surround suppression found in neurons of area MT. Recent physiology results have shown that most frontoparallel MT cells respond more strongly to binocular than to monocular stimulation. Here we measured the surround suppression strength under binocular and monocular viewing. Thirty-nine participants took part in two experiments: (a) where the nonstimulated eye viewed a blank field of the same luminance (n = 8) and (b) where it was occluded with a patch (n = 31). In both experiments, we measured duration thresholds for small (1 deg diameter) and large (7 deg) drifting gratings of 1 cpd with 85% contrast. For each subject, a Motion Suppression Index (MSI) was computed by subtracting the duration thresholds in logarithmic units of the large minus the small stimulus. Results were similar in both experiments. Combining the MSI of both experiments, we found that the strength of suppression for binocular condition (MSIbinocular = 0.249 ± 0.126 log10 (ms)) is 1.79 times higher than under monocular viewing (MSImonocular = 0.139 ± 0.137 log10 (ms)). This increase is too high to be explained by the higher perceived contrast of binocular stimuli and offers a new way of testing whether MT neurons account for surround suppression. Potentially, differences in surround suppression reported in clinical populations may reflect altered binocular processing.
Second-order cues to figure motion enable object detection during prey capture by praying mantises by Nityananda V, O’Keeffe J, Umeton D, Simmons A, Read JCA, NityanandaOKeeffeUmetonSimmonsRead2019.pdf (1.6 MiB) - Detecting motion is essential for animals to perform a wide variety of functions. In order to do so, animals could exploit motion cues, including both first-order cues—such as luminance correlation over time—and second-order cues, by correlating higher-order visual statistics. Since first-order motion cues are typically sufficient for motion detection, it is unclear why sensitivity to second-order motion has evolved in animals, including insects. Here, we investigate the role of second-order motion in prey capture by praying mantises. We show that prey detection uses second-order motion cues to detect figure motion. We further present a model of prey detection based on second-order motion sensitivity, resulting from a layer of position detectors feeding into a second layer of elementary-motion detectors. Mantis stereopsis, in contrast, does not require figure motion and is explained by a simpler model that uses only the first layer in both eyes. Second-order motion cues thus enable prey motion to be detected, even when perfectly matching the average background luminance and independent of the elementary motion of any parts of the prey. Subsequent to prey detection, processes such as stereopsis could work to determine the distance to the prey. We thus demonstrate how second-order motion mechanisms enable ecologically relevant behavior such as detecting camouflaged targets for other visual functions including stereopsis and target tracking.
Pattern and Speed Interact to Hide Moving Prey by Umeton D, Tarawneh G, Fezza E, Read JCA, Rowe C, UmetonTarawnehFezzaReadRowe2019.pdf (1.1 MiB) - Evolutionary biologists have long been fascinated by camouflage patterns that help animals reduce their chances of being detected by predators. However, patterns that hide prey when they remain stationary, such as those that match their backgrounds, are rendered ineffective once prey are moving. The question remains: can a moving animal ever be patterned in a way that helps reduce detection by predators? One long-standing idea is that high-contrast patterns with repeated elements, such as stripes, which are highly visible when prey are stationary, can actually conceal prey when they move fast enough [11, 12, 13, 14]. This is predicted by the “flicker fusion effect,” which occurs when prey move with sufficient speed that their pattern appears to blur, making them appear more featureless and become less conspicuous against the background [2, 8]. However, although this idea suggests a way to camouflage moving prey, it has not been empirically tested, and it is not clear that it would work at speeds that are biologically relevant to a predator [13]. Combining psychophysics and behavioral approaches, we show that speed and pattern interact to determine the detectability of prey to the praying mantis (Sphodromantis lineola) and, crucially, that prey with high-contrast stripes become less visible than prey with background-matching patterns when moving with sufficient speed. We show that stripes can reduce the detection of moving prey by exploiting the spatiotemporal limitations of predator perception, and that the camouflaging effect of a pattern depends upon the speed of prey movement.
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.
The optomotor response of the praying mantis is driven predominantly by the central visual field by Nityananda V, Tarawneh G, Errington S, Serrano-Pedraza I, Read JCA, NityanandaTarawnehErringtonSerranoPedrazaRead.pdf (1.9 MiB) - The optomotor response has been widely used to
investigate insect sensitivity to contrast and motion. Several
studies have revealed the sensitivity of this response
to frequency and contrast, but we know less about the
spatial integration underlying this response. Specifically,
few studies have investigated how the horizontal angular
extent of stimuli influences the optomotor response. We
presented mantises with moving gratings of varying horizontal
extents at three different contrasts in the central or
peripheral regions of their visual fields. We assessed the
relative effectivity of different regions to elicit the optomotor
response and modelled the dependency of the response
on the angular extent subtended by stimuli at these different
regions. Our results show that the optomotor response
is governed by stimuli in the central visual field and not
in the periphery. The model also shows that in the central
region, the probability of response increases linearly with
increase in horizontal extent up to a saturation point. Furthermore,
the dependency of the optomotor response on the
angular extent of the stimulus is modulated by contrast. We
discuss the implications of our results for different modes
of stimulus presentation and for models of the underlying
mechanisms of motion detection in the mantis.
Unravelling the illusion of flicker fusion by Umeton D, Read JCA, Rowe C, UmetonReadRowe2017.pdf (0.7 MiB) - For over 150 years, researchers have investigated the anti-predator function of animal patterns. However, this work has mainly focused on when prey remain still, and has only recently started to incorporate motion into the study of defensive coloration. As motion breaks camouflage, a new challenge is to understand how prey avoid predators while moving around their environment, and if a moving prey can ever be camouflaged. We propose that there is a solution to this, in that a ‘flicker fusion effect’ can change the appearance of the prey in the eyes of their predators to reduce the chances of initial detection. This effect occurs when a high contrast pattern blurs at speed, changing the appearance of the prey, which may help them better match their background. Despite being widely discussed in the literature, the flicker fusion effect is poorly described, there is no clear theoretical framework for testing how it might reduce predation, and the terminology describing it is, at best, rather confusing. Our review addresses these three key issues to enable researchers to formulate precise predictions about when the flicker fusion effect occurs, and to test how it can reduce predation.
All Pulfrich-like illusions can be explained without joint encoding of motion and disparity. by Read JCA, Cumming BG , ReadCumming05c.pdf (2.8 MiB) - The final step was to build a neuronal model, and show that it experienced the illusion. We modelled a neuronal population constructed of neurons which either encoded motion, or depth (not both), and showed that a very simple way of "reading out" this activity, so as to convert it to a perception of depth, would be subject to the Pulfrich illusion. We also examined other evidence which had been put forward in support of the joint motion/depth idea, such as the illusion of swirling motion which occurs in dynamic noise with an interocular delay. We found that this, too, could be experienced by a brain which encoded motion and depth entirely separately. So, while there certainly are primate neurons which jointly encode motion and depth (notably in MT), there is no reason to suppose that these play a privileged role in supporting the Pulfrich effect and related illusions.
This series of three papers (Read & Cumming 2005abc) has recently attracted some criticism from Ning Qian and Ralph Freeman, in a paper entitled "Pulfrich phenomena are coded effectively by a joint motion-disparity process" (J Vis, 9(5): 1-16). My take on it is that we are all basically in agreement, but the situation is obscured by the lack of a clear agreed definition of "joint" vs "separate" encoding of motion and disparity. For example, we said that to be called a motion detector, a cell not only had to be tuned to speed, it also had to respond differently to opposite directions of motion, whereas Qian & Freeman required only speed tuning. I want to clear up one other point. Qian & Freeman say that our model is "non-causal", apparently because it responds to the disparity between a stimulus in one eye and a stimulus which arrives in the other eye at a later time. At the time that stimulus 1 occurs, stimulus 2 is still in the future. However, at the time the neuron responds to the disparity between the two stimuli, both stimuli have already occurred. Thus, the model is firmly causal. Indeed, our derivation of its properties explicitly sets the temporal kernel to zero for future times.
