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

Use of remote data collection methodology to test for an illusory effect on visually guided cursor movements

Investigating the influence of perception on the control of visually guided action typically involves controlled experimentation within the laboratory setting. When appropriate, however, behavioral research of this nature may benefit from the use of methods that allow for remote data collection outside of the lab. This study tested the feasibility of using remote data collection methods to explore the influence of perceived target size on visually guided cursor movements using the Ebbinghaus illusion. Participants completed the experiment remotely, using the trackpad of their personal laptop computers. The task required participants to click on a single circular target presented at either the left or right side of their screen as quickly and accurately as possible (Experiment 1), or to emphasize speed (Experiment 2) or accuracy (Experiment 3). On each trial the target was either surrounded by small or large context circles, or no context circles. Participants’ judgments of the targets’ perceived size were influenced by the illusion, however, the illusion failed to produce differences in click-point accuracy or movement time. Interestingly, the illusion appeared to affect participants’ movement of the cursor toward the target; more directional changes were made when clicking the Perceived Large version of the illusion compared to the Perceived Small version. These results suggest the planning of the cursor movement may have been influenced by the illusion, while later stages of the movement were not, and cursor movements directed toward targets perceived as smaller required less correction compared to targets perceived as larger.

Graspability Modulates the Stronger Neural Signature of Motor Preparation for Real Objects vs. Pictures

The cognitive and neural bases of visual perception are typically studied using pictures rather than real-world stimuli. Unlike pictures, real objects are actionable solids that can be manipulated with the hands. Recent evidence from human brain imaging suggests that neural responses to real objects differ from responses to pictures; however, little is known about the neural mechanisms that drive these differences. Here, we tested whether brain responses to real objects versus pictures are differentially modulated by the "in-the-moment" graspability of the stimulus. In human dorsal cortex, electroencephalographic responses show a "real object advantage" in the strength and duration of mu (μ) and low beta (β) rhythm desynchronization-well-known neural signatures of visuomotor action planning. We compared desynchronization for real tools versus closely matched pictures of the same objects, when the stimuli were positioned unoccluded versus behind a large transparent barrier that prevented immediate access to the stimuli. We found that, without the barrier in place, real objects elicited stronger μ and β desynchronization compared to pictures, both during stimulus presentation and after stimulus offset, replicating previous findings. Critically, however, with the barrier in place, this real object advantage was attenuated during the period of stimulus presentation, whereas the amplification in later periods remained. These results suggest that the "real object advantage" is driven initially by immediate actionability, whereas later differences perhaps reflect other, more inherent properties of real objects. The findings showcase how the use of richer multidimensional stimuli can provide a more complete and ecologically valid understanding of object vision.

Manipulation of physical 3-D and virtual 2-D stimuli: comparing digit placement and fixation position

The visuomotor processes involved in grasping a 2-D target are known to be fundamentally different than those involved in grasping a 3-D object, and this has led to concerns regarding the generalizability of 2-D grasping research. This study directly compared participants' fixation positions and digit placement during interaction with either physical square objects or 2-D virtual versions of these objects. Participants were instructed to either simply grasp the stimulus or grasp and slide it to another location. Participants' digit placement and fixation positions did not significantly differ as a function of stimulus type when grasping in the center of the display. However, gaze and grasp positions shifted toward the near side of non-central virtual stimuli, while consistently remaining close to the horizontal midline of the physical stimulus. Participants placed their digits at less stable locations when grasping the virtual stimulus in comparison to the physical stimulus on the right side of the display, but this difference disappeared when grasping in the center and on the left. Similar outward shifts in digit placement and lowered fixations were observed when sliding both stimulus types, suggesting participants incorporated similar adjustments in grasp selection in anticipation of manipulation in both Physical and Virtual stimulus conditions. These results suggest that while fixation position and grasp point selection differed between stimulus type as a function of stimulus position, certain eye-hand coordinated behaviours were maintained when grasping both physical and virtual stimuli.

Grasping Weber's Law in a Virtual Environment: The Effect of Haptic Feedback

Recent findings suggest that the functional separation between vision-for-action and vision-for-perception does not generalize to situations in which virtual objects are used as targets. For instance, unlike actions toward real objects that violate Weber's law, a basic law of visual perception, actions toward virtual objects presented on flat-screens, or in remote virtual environments, obey to Weber's law. These results suggest that actions in virtual environments are performed in an inefficient manner and are subjected to perceptual effects. It is unclear, however, whether this inefficiency reflects extensive variation in the way in which visual information is processed in virtual environments or more local aspects related to the settings of the virtual environment. In the current study, we focused on grasping performance in a state-of-the-art virtual reality system that provides an accurate representation of the 3D space. Within this environment, we tested the effect of haptic feedback on grasping trajectories. Participants were asked to perform bimanual grasping movements toward the edges of virtual targets. In the haptic feedback condition, physical stimuli of matching dimensions were embedded in the virtual environment. Haptic feedback was not provided in the no-feedback condition. The results showed that grasping trajectories in the feedback, but not in the no-feedback condition, could be performed more efficiently, and evade the influence of Weber's law. These findings are discussed in relevance to previous literature on 2D and 3D grasping.

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.