Real and illusory issues in the illusion debate (Why two things are sometimes better than one): Commentary on Kopiske et al. (2016)

Coming to grips with reality: Real grasps, but not pantomimed grasps, resist a simultaneous tilt illusion

Investigations of grasping real, 3D objects subjected to illusory effects from a pictorial background often choose in-flight grasp aperture as the primary variable to test the hypothesis that the visuomotor system resists the illusion. Here we test an equally important feature of grasps that has received less attention: in-flight grasp orientation. The current study tested a variant of the simultaneous tilt illusion using a mirror-apparatus to manipulate the availability of haptic feedback. Participants performed grasps with haptic feedback (real grasps) and without it (pantomime grasps), reaching for the reflection of a real, 3D bar atop a background grating that induced a 1.1° bias in the perceived orientation of the bar in a separate sample of participants. Analysis of the hand's in-flight grasp orientation at early, late, and end stages of the reach showed that at no point were the real grasps biased by the illusion. In contrast, pantomimed grasps were affected by the illusion at the late and end stages of the reach. At each stage, the effect on the real grasps was significantly weaker than the effect of the illusion as measured by the mean point of subjective equality (PSE) in a two-alternative forced-choice task. In contrast, the effect on the pantomime grasps was statistically indistinguishable from the mean PSE at all three stages of the reach. These findings reinforce the idea that in-flight grasp orientation, like grasp aperture to pictorial illusions of target size, is refractory to pictorial backgrounds that bias perceived orientation.

Pictorial depth cues always influence reaching distance

We report five experiments to test the influence of pictorial depth on reaching. Our core method is to project a wide-field background of linear perspective and/or texture gradient onto a tabletop, and to measure the amplitude of reaches made to targets within it. In 63 healthy participants performing immediate open-loop reaches across Experiments 1-4, we observed a clear effect of pictorial depth. This effect was driven specifically by the convergence of the background pattern at the target position: for each additional degree of pictorial convergence, reaching distance increased by half a millimetre. In the individual experiments, we applied manipulations that might be expected to modify the influence of pictorial depth. We found no evidence that the effect was modified with monocular viewing, or when participants responded with the left hand, or if a memory delay was inserted before the response. Nor did participants become less susceptible to pictorial depth when visual feedback of terminal reaching errors was provided, although visual feedback during the reach did mitigate the influence of pictorial depth. Finally, the visual form agnosic patient DF showed an entirely normal effect of pictorial depth cues, which leads us to question the idea that this effect emanates from visual analyses of size and shape in the ventral stream, rather than from the dorsal stream, or from earlier stages of visual processing.

Bimanual thumb-index finger indications of noncorresponding extents

Two experiments tested a prediction derived from the recent finding that the Oppel-Kundt illusion - the overestimation of a filled extent relative to an empty one - was much attenuated when the empty part of a bipartite row of dots was vertical and the filled part horizontal, suggesting that the Horizontal-vertical illusion - the overestimation of vertical extents relative to horizontal ones - only acted on the empty part of an Oppel-Kundt figure. Observers had to bimanually indicate the sizes of the two parts of an Oppel-Kundt figure, which were arranged one above the other with one part vertical and the other part tilted -45°, 0°, or 45°. Results conformed to the prediction but response bias was greater when observers had been instructed to point to the extents' endpoints than when instructed to estimate the extents' lengths, suggesting that different concepts and motor programs had been activated.

Grasping of Real-World Objects Is Not Biased by Ensemble Perception

The visual system is known to extract summary representations of visually similar objects which bias the perception of individual objects toward the ensemble average. Although vision plays a large role in guiding action, less is known about whether ensemble representation is informative for action. Motor behavior is tuned to the veridical dimensions of objects and generally considered resistant to perceptual biases. However, when the relevant grasp dimension is not available or is unconstrained, ensemble perception may be informative to behavior by providing gist information about surrounding objects. In the present study, we examined if summary representations of a surrounding ensemble display influenced grip aperture and orientation when participants reached-to-grasp a central circular target which had an explicit size but importantly no explicit orientation that the visuomotor system could selectively attend to. Maximum grip aperture and grip orientation were not biased by ensemble statistics during grasping, although participants were able to perceive and provide manual estimations of the average size and orientation of the ensemble display. Support vector machine classification of ensemble statistics achieved above-chance classification accuracy when trained on kinematic and electromyography data of the perceptual but not grasping conditions, supporting our univariate findings. These results suggest that even along unconstrained grasping dimensions, visually-guided behaviors toward real-world objects are not biased by ensemble processing.

A double dissociation between action and perception in bimanual grasping: evidence from the Ponzo and the Wundt-Jastrow illusions

Research on visuomotor control suggests that visually guided actions toward objects rely on functionally distinct computations with respect to perception. For example, a double dissociation between grasping and between perceptual estimates was reported in previous experiments that pit real against illusory object size differences in the context of the Ponzo illusion. While most previous research on the relation between action and perception focused on one-handed grasping, everyday visuomotor interactions also entail the simultaneous use of both hands to grasp objects that are larger in size. Here, we examined whether this double dissociation extends to bimanual movement control. In Experiment 1, participants were presented with different-sized objects embedded in the Ponzo Illusion. In Experiment 2, we tested whether the dissociation between perception and action extends to a different illusion, the Wundt-Jastrow illusion, which has not been previously used in grasping experiments. In both experiments, bimanual grasping trajectories reflected the differences in physical size between the objects; At the same time, perceptual estimates reflected the differences in illusory size between the objects. These results suggest that the double dissociation between action and perception generalizes to bimanual movement control. Unlike conscious perception, bimanual grasping movements are tuned to real-world metrics, and can potentially resist irrelevant information on relative size and depth.

Why some size illusions affect grip aperture

There is extensive literature debating whether perceived size is used to guide grasping. A possible reason for not using judged size is that using judged positions might lead to more precise movements. As this argument does not hold for small objects and all studies showing an effect of the Ebbinghaus illusion on grasping used small objects, we hypothesized that size information is used for small objects but not for large ones. Using a modified diagonal illusion, we obtained an effect of about 10% on perceptual judgements, without an effect on grasping, irrespective of object size. We therefore reject our precision hypothesis. We discuss the results in the framework of grasping as moving digits to positions on an object. We conclude that the reported disagreement on the effect of illusions is because the Ebbinghaus illusion not only affects size, but—unlike most size illusions—also affects perceived positions.

A visual illusion that influences perception and action through the dorsal pathway

There are two main anatomically and physiologically defined visual pathways connecting the primary visual cortex with higher visual areas: the ventral and the dorsal pathway. The influential two-visual-systems hypothesis postulates that visual attributes are analyzed differently for different functions: in the dorsal pathway visual information is analyzed to guide actions, whereas in the ventral pathway visual information is analyzed for perceptual judgments. We here show that a person who cannot identify objects due to an extensive bilateral ventral brain lesion is able to judge the velocity at which an object moves. Moreover, both his velocity judgements and his interceptive actions are as susceptible to a motion illusion as those of people without brain lesions. These findings speak in favor of the idea that dorsal structures process information about attributes such as velocity, irrespective of whether such information is used for perceptual judgments or to guide actions.

The visual guidance of action is not insulated from cognitive interference: A multitasking study on obstacle-avoidance and bisection

The Perception-Action Model (PAM) considers the visual system to be divided into two streams defined by their specific functions, a ventral stream for vision and a dorsal stream for action. In this study we investigated two behavioral paradigms which according to PAM represent the two contrasting functions of the ventral and dorsal stream, namely bisection and obstacle-avoidance, respectively. It is an assumption of PAM that while ventral stream processing is ultimately linked with processing in other cognitive systems, dorsal stream processing is insulated from cognition. Accordingly it can be expected that a secondary task will interfere with bisection but not with obstacle-avoidance. We tested this prediction using a rapid serial visual presentation task as our secondary task (RSVP). Contrary to expectations we found significant interference for both bisection and obstacle-avoidance. Our findings suggest that dorsal-stream processing is not insulated from cognitive processes.

Vision: Non-illusory Evidence for Distinct Visual Pathways for Perception and Action

When visual information about an object's distance is obscured, but its retinal size visible, the object's physical size is ambiguous to vision; however, additional proprioceptive distance information permits physical size to be estimated when grasping the object, but perceptual size estimates remain inaccurate, adding to that evidence for distinct visual pathways for perception and action.

The Sander parallelogram illusion dissociates action and perception despite control for the litany of past confounds

The two visual systems hypothesis proposes that human vision is supported by an occipito-temporal network for the conscious visual perception of the world and a fronto-parietal network for visually-guided, object-directed actions. Two specific claims about the fronto-parietal network's role in sensorimotor control have generated much data and controversy: (1) the network relies primarily on the absolute metrics of target objects, which it rapidly transforms into effector-specific frames of reference to guide the fingers, hands, and limbs, and (2) the network is largely unaffected by scene-based information extracted by the occipito-temporal network for those same targets. These two claims lead to the counter-intuitive prediction that in-flight anticipatory configuration of the fingers during object-directed grasping will resist the influence of pictorial illusions. The research confirming this prediction has been criticized for confounding the difference between grasping and explicit estimates of object size with differences in attention, sensory feedback, obstacle avoidance, metric sensitivity, and priming. Here, we address and eliminate each of these confounds. We asked participants to reach out and pick up 3D target bars resting on a picture of the Sander Parallelogram illusion and to make explicit estimates of the length of those bars. Participants performed their grasps without visual feedback, and were permitted to grasp the targets after making their size-estimates to afford them an opportunity to reduce illusory error with haptic feedback. The results show unequivocally that the effect of the illusion is stronger on perceptual judgments than on grasping. Our findings from the normally-sighted population provide strong support for the proposal that human vision is comprised of functionally and anatomically dissociable systems.

Verbalizations Affect Visuomotor Control in Hitting Objects to Distant Targets

There is a long-standing proposal for the existence of two neuroanatomically and functionally separate visual systems; one supported by the dorsal pathway to control action and the second supported by the ventral pathway to handle explicit perceptual judgments. The dorsal pathway requires fast access to egocentric information, while the ventral pathway primarily requires allocentric information. Despite the evidence for functionally distinct systems, researchers have posited important interactions. This paper examines evidence to what degree the interaction becomes more important when target-identity, the perception of which is supported by the ventral stream, is verbalized during the execution of a target-directed far-aiming movement. In the experiment reported here participants hit balls toward distant targets while concurrently making explicit perceptual judgments of target properties. The endpoint of a shaft served as the target, with conditions including illusory arrow fins at the endpoint. Participants verbalized the location of the target by comparing it to a reference line and calling out "closer" or "further" while propelling the ball to the target. The impact velocity at ball contact was compared for hits toward three shafts of lengths, 94, 100, and 106 cm, with and without verbalizations and delays. It was observed that the meaning of the expressed words modulated movement execution when the verbalizations were consistent with the action characteristics. This effect of semantic content was evident regardless of target visibility during movement execution, demonstrating it was not restricted to movements that rely on visual memory. In addition to a direct effect of semantic content we anticipated an indirect effect of verbalization to result in action shifting toward the use of context-dependent allocentric information. This would result in an illusion bias on the impact velocity when the target is embedded in a Müller-Lyer configuration. We observed an ubiquitous effect of illusory context on movement execution, and not only when verbalizations were made. We suggest that the current experimental design with a far-aiming task where most conditions required reporting or retaining spatial characteristics of targets for action over time may have elicited a strong reliance on allocentric information to guide action.