The BTPI: An online battery for measuring susceptibility to visual illusions.

Visual illusions provide a powerful tool for probing the mechanisms that underlie perception. While most previous studies of visual illusions focused on average group-level performance, less attention has been devoted to individual differences in susceptibility to illusions. Unlike in other perceptual domains, in which there are established, validated tools to measure individual differences, such tools are not yet available in the domain of visual illusions. Here, we describe the development and validation of the BTPI (Ben-Gurion University Test for Perceptual Illusions), a new online battery designed to measure susceptibility to the influence of three prominent size illusions: the Ebbinghaus, the Ponzo, and the height-width illusions. The BTPI also measures perceptual resolution, reflected by the just noticeable difference (JND), to detect size differences in the context of each illusion. In Experiment 1 (N = 143), we examined performance in typical self-paced tasks, whereas in Experiment 2 (N = 69), we employed a fixed presentation duration paradigm. High test-retest reliability scores were found for all illusions, with little evidence for intercorrelations between different illusions. In addition, lower perceptual resolution (larger JND) was associated with a larger susceptibility to the illusory effect. The computerized task battery and analysis codes are freely available online.

Recognition of Masked Faces in the Era of the Pandemic: No Improvement Despite Extensive Natural Exposure.

Face masks, which became prevalent across the globe during the COVID-19 pandemic, have had a negative impact on face recognition despite the availability of critical information from uncovered face parts, especially the eyes. An outstanding question is whether face-mask effects would be attenuated following extended natural exposure. This question also pertains, more generally, to face-recognition training protocols. We used the Cambridge Face Memory Test in a cross-sectional study (N = 1,732 adults) at six different time points over a 20-month period, alongside a 12-month longitudinal study (N = 208). The results of the experiments revealed persistent deficits in recognition of masked faces and no sign of improvement across time points. Additional experiments verified that the amount of individual experience with masked faces was not correlated with the mask effect. These findings provide compelling evidence that the face-processing system does not easily adapt to visual changes in face stimuli, even following prolonged real-life exposure.

The perception of food size and food shape in anorexia nervosa.

Visual perception of food size and shape in anorexia nervosa (AN) is an understudied topic, notwithstanding its relevance in approaching food, key-element in weight restoration. In addition, it is unclear how visual perception in AN is related to the age and the duration of illness. Here, we compared patients with AN to healthy controls (HCs) on their spatial resolution, biases in perceived food size, and holistic processing of food shape. A total of 122 participants were enrolled: 48 adolescents (27 AN and 21 HCs) and 74 adults (33 AN and 41 HCs). Participants at two academic sites (Israel and Italy) completed measures of psychopathology and experiments measuring visual resolution (Just Noticeable Difference), biases in food-size perception (Points of Subjective Equality), and holistic processing of food shape (indicated by the height-width illusion). Adolescents and adults with AN differed in the duration of illness and body mass index but showed comparable eating psychopathology and body measures. Patients with AN showed preserved visual resolution but distorted perception of food size, perceiving food as bigger than non-food objects, in both age groups. Patients with AN, both adolescents and adults, also processed food stimuli in a more analytic fashion, and were immune to the height-width illusion. The preserved perception of non-food stimuli in AN coupled with biases in food-size perception and in analytic processing of food shape highlight patients' real-world difficulties in approaching food. Future treatments on AN may consider taking these differences into account.

Double dissociation between perception and action in children.

Previous research has demonstrated a functional dissociation between vision for perception and vision for action. However, the developmental trajectory of this functional dissociation is not well understood. We directly compared the sensitivity of grasping and perceptual estimations within the same experimental design to the real and illusory sizes of objects positioned in the Ponzo illusion display. Two different-sized objects were placed such that the differences between their real sizes and their perceived sizes were pitted against each other. Children aged 5-8 years and adults made perceptual size discriminations and then grasped (action) or estimated (perception) one of the objects based on its perceived size. Consistent with previous results, for the action task, grasping apertures of adults were scaled with the physical differences in the objects' sizes, even in trials where their overt perceptual decisions were deceived by the illusion. In contrast, perceptual estimations were robustly modulated by the illusion. Interestingly, children outperformed adults in their perceptual discriminations but exhibited adult-like behavior in grasping and in perceptual estimations of the objects, demonstrating a dissociation between perception and action. These results suggest that although the two visual functions are not operating at fully mature levels during childhood, some key mechanisms that support a dissociation between these functions are already in place.

Consciously monitored grasping is vulnerable to perceptual intrusions.

The idea of functional differentiation between vision-for-action and vision-for-perception has been supported by evidence from different domains. According to this account, perception is based on consciously accessible, relative representations, whereas vision-for-action is performed in an analytic, automatic manner. Support for this idea comes from studies that showed that unlike perception, grasping movements are refractory to illusions and to Weber's law. Yet, interactions between the systems may occur when an action is performed in a less automated fashion. To test this idea, we asked participants to monitor their fingers apertures in flight and to halt their movement for a short duration when they felt that their aperture reached a maximum amount. The results showed that movements in the monitored condition were biased by the Ponzo illusion and showed atypical adherence to Weber's law. These results show that action and perception are more likely to interact when movements are performed in a controlled manner.

Food deprivation disrupts normal holistic processing of domain-specific stimuli.

Food deprivation has been shown to lead to a set of biological and psychological responses, including a decrease in perceptual thresholds, and an increase in attentional allocation for domain-specific, food-related stimuli. Here, we tested whether food deprivation could lead to a qualitative change in the way food is perceived. To this purpose, we tested the effect of food deprivation on a basic feature of human perception, the holistic processing of object shape. In three experiments, we examined the effect of food deprivation on participants' susceptibility to the height-width illusion, which served as a maker for holistic processing. In all experiments, food deprivation led to an abnormal, non-holistic processing of shape, which resulted in a total reduction of the illusion for food-related, but not for control stimuli. These results show that food deprivation alters the way food is perceived, and propose that motivational factors modulate people's resistance to perceptual distortions for domain-specific stimuli.

When perception intrudes on 2D grasping: evidence from Garner interference.

When participants reach out to pick up a real 3-D object, their grip aperture reflects the size of the object well before contact is made. At the same time, the classical psychophysical laws and principles of relative size and shape that govern visual perception do not appear to intrude into the control of such movements, which are instead tuned only to the relevant dimension for grasping. In contrast, accumulating evidence suggests that grasps directed at flat 2D objects are not immune to perceptual effects. Thus, in 2D but not 3D grasping, the aperture of the fingers has been shown to be affected by relative and contextual information about the size and shape of the target object. A notable example of this dissociation comes from studies of Garner interference, which signals holistic processing of shape. Previous research has shown that 3D grasping shows no evidence for Garner interference but 2D grasping does (Freud & Ganel, 2015). In a recent study published in this journal (Löhr-Limpens et al., 2019), participants were presented with 2D objects in a Garner paradigm. The pattern of results closely replicated the previously published results with 2D grasping. Unfortunately, the authors, who appear to be unaware the potential differences between 2D and 3D grasping, used their findings to draw an overgeneralized and unwarranted conclusion about the relation between 3D grasping and perception. In this short methodological commentary, we discuss current literature on aperture shaping during 2D grasping and suggest that researchers should play close attention to the nature of the target stimuli they use before drawing conclusions about visual processing for perception and action.

Active visuomotor interactions with virtual objects on touchscreens adhere to Weber's law.

Recent findings suggest that the functional separation between vision-for-action and vision-for-perception does not generalize to situations in which two-dimensional (2D), virtual objects, are used as targets. For example, unlike grasping movements directed at real, three-dimensional (3D) objects, the trajectories of grasping movements directed at 2D objects adhere to the psychophysical principle of Weber's law, indicating relative and less efficient processing of their size. Such inefficiency could be attributed to the fact that everyday interactions with touchscreens do not usually entail grasping movements. It is possible, therefore, that more typical interactions with virtual objects, which involve active manipulation of their size or location on a touchscreen, could be performed efficiently and in an absolute manner, and would violate Weber's law. We examined this hypothesis in three experiments in which participants performed active interactions with virtual objects. In Experiment 1, participants made swiping gestures to move virtual objects across the touchscreen. In Experiment 2, participants touched the edges of virtual objects to enlarge their size. In Experiment 3, participants freely enlarged the size of virtual objects, without being required to touch their edges upon contact. In all experiments, the resolution of grip aperture decreased with the size of the target object, adhering to Weber's law. These results suggest that active interactions with 2D objects on touchscreens are not performed in a natural, absolute manner which characterize visuomotor control of real objects.

Does food deprivation affect perceived size?

Exposure to food-related stimuli could lead to the triggering of a set of biological, emotional and cognitive responses. Such responses can be pronounced following food deprivation. Indeed, previous research showed that even a moderate period of food deprivation is sufficient to increase perceptual precision to detect changes along food size and to change the processing style of food-related stimuli. It is unclear, however, whether food deprivation also leads to systematic biases along the perception of food size. Here, we used two classic psychophysical methods, the method of constant stimuli and the method of adjustment, adapted to the field of food perception, to study the effect of food deprivation on average perceived food size. In two experiments, food deprived and non-deprived participants were asked to compare a series of food and non-food visual stimuli along their size. The results were inconsistent and depended upon the method used. When found, small bias effects resulted in food stimuli perceived as bigger following food deprivation. The results show that unlike the reliable effects motivational factors have on perceptual precision and on perceptual processing style, they have an inconsistent influence on average perceived food size.

Obeying the law: speed-precision tradeoffs and the adherence to Weber's law in 2D grasping.

Visually guided actions toward two-dimensional (2D) and three-dimensional (3D) objects show different patterns of adherence to Weber's law. In 3D grasping, Just Noticeable Differences (JNDs) do not scale with object size, violating Weber's law. Conversely, JNDs in 2D grasping increase with size, showing a pattern of scaler variability between aperture and JND, as predicted by Weber's law. In the current study, we tested whether such scaler variability in 2D grasping reflects genuine adherence to Weber's law. Alternatively, it could be potentially accounted for by a speed-precision tradeoff effect due to an increase in aperture velocity with size. In two experiments, we modified the relation between aperture velocity and size in 2D grasping and tested whether movement trajectories still adhere to Weber's law. In Experiment 1, we aimed to equate aperture velocities between different-sized objects by pre-adjusting the initial finger aperture to match the target's size. In Experiment 2, we reversed the relation between size and velocity by asking participants to hold their fingers wide open prior to grasp, resulting in faster velocities for smaller rather than for larger objects. The results of the two experiments showed that although aperture velocities did not increase with size, adherence to Weber's law was still maintained. These results indicate that the adherence to Weber's law during 2D grasping cannot be accounted for by a speed-precision tradeoff effect, but rather represents genuine reliance on relative, perceptually based computations in visuomotor interactions with 2D objects.

Weber's law in 2D and 3D grasping.

Visually guided grasping movements directed to real, 3D objects are characterized by a distinguishable trajectory pattern that evades the influence of Weber's law, a basic principle of perception. Conversely, grasping trajectories directed to 2D line drawings of objects adhere to Weber's law. It can be argued, therefore, that during 2D grasping, the visuomotor system fails at operating in analytic mode and is intruded by irrelevant perceptual information. Here, we explored the visual and tactile cues that enable such analytic processing during grasping. In Experiment 1, we compared grasping directed to 3D objects with grasping directed to 2D object photos. Grasping directed to photos adhered to Weber's law, suggesting that richness in visual detail does not contribute to analytic processing. In Experiment 2, we tested whether the visual presentation of 3D objects could support analytic processing even when only partial object-specific tactile information is provided. Surprisingly, grasping could be performed in an analytic fashion, violating Weber's law. In Experiment 3, participants were denied of any haptic feedback at the end of the movement and grasping trajectories again showed adherence to Weber's law. Taken together, the findings suggest that the presentation of real objects combined with indirect haptic information at the end of the movement is sufficient to allow analytic processing during grasp.

Grasping trajectories in a virtual environment adhere to Weber's law.

Virtual-reality and telerobotic devices simulate local motor control of virtual objects within computerized environments. Here, we explored grasping kinematics within a virtual environment and tested whether, as in normal 3D grasping, trajectories in the virtual environment are performed analytically, violating Weber's law with respect to object's size. Participants were asked to grasp a series of 2D objects using a haptic system, which projected their movements to a virtual space presented on a computer screen. The apparatus also provided object-specific haptic information upon "touching" the edges of the virtual targets. The results showed that grasping movements performed within the virtual environment did not produce the typical analytical trajectory pattern obtained during 3D grasping. Unlike as in 3D grasping, grasping trajectories in the virtual environment adhered to Weber's law, which indicates relative resolution in size processing. In addition, the trajectory patterns differed from typical trajectories obtained during 3D grasping, with longer times to complete the movement, and with maximum grip apertures appearing relatively early in the movement. The results suggest that grasping movements within a virtual environment could differ from those performed in real space, and are subjected to irrelevant effects of perceptual information. Such atypical pattern of visuomotor control may be mediated by the lack of complete transparency between the interface and the virtual environment in terms of the provided visual and haptic feedback. Possible implications of the findings to movement control within robotic and virtual environments are further discussed.

Three-Dimensional Representations of Objects in Dorsal Cortex are Dissociable from Those in Ventral Cortex.

An established conceptualization of visual cortical function is one in which ventral regions mediate object perception while dorsal regions support spatial information processing and visually guided action. This division has been contested by evidence showing that dorsal regions are also engaged in the representation of object shape, even when actions are not required. The critical question is whether these dorsal, object-based representations are dissociable from ventral representations, and whether they play a functional role in object recognition. We examined the neural and behavioral profile of patients with impairments in object recognition following ventral cortex damage. In a functional magnetic resonanace imaging experiment, the blood oxygen level-dependent response in the ventral, but not dorsal, cortex of the patients evinced less sensitivity to object 3D structure compared with that of healthy controls. Consistently, in psychophysics experiments, the patients exhibited significant impairments in object perception, but still revealed residual sensitivity to object-based structural information. Together, these findings suggest that, although in the intact system there is considerable crosstalk between dorsal and ventral cortices, object representations in dorsal cortex can be computed independently from those in ventral cortex. While dorsal representations alone are unable to support normal object perception, they can, nevertheless, support a coarse description of object structural information.

Numerical magnitude affects online execution, and not planning of visuomotor control.

Recent literature has established a directional influence of irrelevant numerical magnitude on actions performed toward neutral objects. For example, fingers' aperture during grasping is larger when associated with large compared with small numerical digits. This interaction between symbolic magnitude and visuomotor control has been attributed to the planning stage of the action prior to motor execution. However, this assumption has not been directly tested. In two experiments, we tested whether the effects of numerical magnitude on grasping derive from action planning or from action execution. Participants were asked to grasp an object following a short visual (Experiment 1) or auditory (Experiment 2) presentation of small (1/2) or large (8/9) digits. Grasping was performed under either closed-loop (CL) or open-loop (OL) visuomotor control, for which online vision was prevented during action execution. Digit magnitude affected grip apertures in the CL condition, when online vision was allowed. However, magnitude had no effects on grip aperture in the OL condition. This pattern of results strongly suggests that the processing of numerical magnitude originates from interactions between numerical magnitude and real object size during online motor execution. Unlike previously assumed, the findings also suggest that the effect of magnitude on visuomotor control is not likely to be attributed to the motor planning stage prior to action initiation.

Food deprivation reduces the susceptibility to size-contrast illusions.

Hunger, caused by mild amount of food deprivation, is an everyday physiological state familiar to us all. Ongoing research has pinpointed the way hunger affects people's physiological functions as well as their attitudes and allocation of attention toward domain-specific, food-related objects. Yet, little is known about the way food deprivation affects basic perceptual abilities. Here, we utilized size-contrast visual illusions commonly associated with food to explore the way deprivation affects relative processing of food size. In two experiments (Nexp1 = 32 females, mean age: 24.31; Nexpt2 = 41 females, 40 males, mean age: 23.84), we examined the effect food deprivation has on participants' susceptibility to the Delboeuf illusion, which biases the perceived size of a neutral or of a food-related object when it is placed within the context of another object (e.g., a pizza is perceived as smaller when placed on a larger plate or tray). The results showed that food deprivation reduces the illusory bias for food-related but not for neutral stimuli. Such reduction in the illusory effect indicates reliance on analytic, rather than on relative processing style, for domain-specific stimuli when in the state of hunger.

Sensitivity to object impossibility in the human visual cortex: evidence from functional connectivity.

Processing spatial configuration is a fundamental requirement for object recognition. Using fMRI, the neural basis underlying this ability was examined while human participants viewed possible and visually similar, but spatially impossible, objects presented for either long or short exposure duration. Response profiles in object-selective cortical regions exhibited sensitivity to object possibility, but only for the long exposure duration. Contrary, functional connectivity, indexed by the pairwise correlations between activation profiles across ROIs, revealed sensitivity to possibility, evident in enhanced correlations for impossible compared with possible objects. Such sensitivity was found even following a brief exposure duration, which allowed only minimal awareness of possibility. Importantly, this sensitivity was correlated with participants' general spatial ability as assessed by an independent neuropsychological test. These results suggest that the visual system is highly susceptible to objects' 3-D structural information even with minimal perceptual awareness. Such sensitivity is captured at the level of functional connectivity between object-selective regions, rather than the absolute level of within-region activity, implicating the role of interregional synchronization in the representation of objects' 3-D structure.

Impossible expectations: fMRI adaptation in the lateral occipital complex (LOC) is modulated by the statistical regularities of 3D structural information.

fMRI adaptation (fMRIa), the attenuation of fMRI signal which follows repeated presentation of a stimulus, is a well-documented phenomenon. Yet, the underlying neural mechanisms supporting this effect are not fully understood. Recently, short-term perceptual expectations, induced by specific experimental settings, were shown to play an important modulating role in fMRIa. Here we examined the role of long-term expectations, based on 3D structural statistical regularities, in the modulation of fMRIa. To this end, human participants underwent fMRI scanning while performing a same-different task on pairs of possible (regular, expected) objects and spatially impossible (irregular, unexpected) objects. We hypothesized that given the spatial irregularity of impossible objects in relation to real-world visual experience, the visual system would always generate a prediction which is biased to the possible version of the objects. Consistently, fMRIa effects in the lateral occipital cortex (LOC) were found for possible, but not for impossible objects. Additionally, in alternating trials the order of stimulus presentation modulated LOC activity. That is, reduced activation was observed in trials in which the impossible version of the object served as the prime object (i.e. first object) and was followed by the possible version compared to the reverse order. These results were also supported by the behavioral advantage observed for trials that were primed by possible objects. Together, these findings strongly emphasize the importance of perceptual expectations in object representation and provide novel evidence for the role of real-world statistical regularities in eliciting fMRIa.

Simon in action: the effect of spatial congruency on grasping trajectories.

The Simon effect, one of the well-known stimulus-response compatibility effects, is usually explained as an expression of a conflict that occurs at the response selection stage. Here, we extended previous findings to provide evidence for post-response selection expression of the Simon effect. Following a presentation of a visual stimulus, participants grasped one of two objects that differed slightly in size. The results showed that visual stimulus congruency modulated grasping trajectories. Particularly, movements were more lateralized in congruent trials. This lateralization decreased as reaction time (RT) increased and therefore this effect could not be fully dissociated from the response selection stage. However, size sensitivity, as measured by the time taken to reach the maximum grip aperture between the fingers, was decreased for incongruent trials, unrelated to RT. This finding provides novel evidence for an independent expression of the Simon effect in post-response selection stages. Overall, our findings extend previous studies and demonstrate that the spatial conflict evoked by the Simon task encompasses several components and independently affects response selection stages as well as other components of motor execution.

Cross-modal effects of auditory magnitude on visually guided grasping.

Recent research has established the role of objects' semantic properties in the planning of motor actions with respect to these objects. It has been shown that visual numerical magnitude affects visuomotor control in a similar direction to the effect of physical size: The larger the numerical value, the larger the grip aperture even when physical size remains invariant. The relationship has been attributed to a common mechanism, in particular to a neural network within the parietal lobe, which mediates the processing of magnitude across different domains. In this study, we show that the effect of magnitude on grasping is not limited to visual numerical information and is in fact cross-modal in nature; presentations of auditory signals of different types of auditory-based magnitudes affected visually guided actions in two different experiments. In Experiment 1, symbolic representations of magnitudes (numerals) affected initial grasping trajectories. In Experiment 2, a nonsymbolic presentation of magnitude, i.e., tone duration, had similar effects on grasping trajectories. We conclude that different types of magnitude representations are processed by a common mechanism that cooperates with visuomotor control.

Variability-based Garner interference for perceptual estimations but not for grasping.

Garner's speeded classification task has been used as an effective tool to probe holistic processing of object shape. This is achieved by comparing classification performance of a given object dimension between two experimental conditions. Worse performance in a "filtering" condition in which a second, irrelevant dimension of the same object varies on a trial-to-trial basis, compared to a "baseline" condition in which the irrelevant dimension is held constant, is labeled Garner interference, and indicates that the two dimensions are processed in a holistic manner. About a decade ago, we used Garner's task to provide evidence for different frames of processing mediating action and perception. Unlike perceptual estimations, visually guided grasping showed no Garner interference when subjects were asked to reach out and grasp an object along a given dimension. In other words, slower reaction times were observed in the filtering compared to the baseline condition only for perceptual estimates but not for grasping. In two experiments, we extend these findings to kinematic measures beyond simple reaction times. The results showed that Garner interference is also expressed in the variability of the response, with more variable within-subject performance in the filtering compared to the baseline condition for perceptual estimates but not for grasping. These findings provide converging evidence for the idea that, unlike perception, which processes objects holistically, visually guided action is performed in an analytic manner.