Gal Namdar

Grasping numbers: evidence for automatic influence of numerical magnitude on grip aperture

Previous research has shown that the fingers’ aperture during grasp is affected by the numerical values of numbers embedded in the grasped objects: Numerically larger digits lead to larger grip apertures than do numerically smaller digits during the initial stages of the grasp. The relationship between numerical magnitude and visuomotor control has been taken to support the idea of a common underlying neural system mediating the processing of magnitude and the computation of object size for motor control. The purpose of the present study was to test whether the effect of magnitude on motor preparation is automatic. During grasping, we asked participants to attend to the colors of the digit while ignoring numerical magnitude. The results showed that numerical magnitude affected grip aperture during the initial stages of the grasp, even when magnitude information was irrelevant to the task at hand. These findings suggest that magnitude affects grasping preparation in an automatic fashion.

Bimanual grasping does not adhere to Weber’s law

According to Weber’s law, a fundamental principle of perception, visual resolution decreases in a linear fashion with an increase in object size. Previous studies have shown, however, that unlike for perception, grasping does not adhere to Weber’s law. Yet, this research was limited by the fact that perception and grasping were examined for a restricted range of stimulus sizes bounded by the maximum fingers span. The purpose of the current study was to test the generality of the dissociation between perception and action across a different type of visuomotor task, that of bimanual grasping. Bimanual grasping also allows to effectively measure visual resolution during perception and action across a wide range of stimulus sizes compared to unimanual grasps. Participants grasped or estimated the sizes of large objects using both their hands. The results showed that bimanual grasps violated Weber’s law throughout the entire movement trajectory. In contrast, Just Noticeable Differences (JNDs) for perceptual estimations of the objects increased linearly with size, in agreement with Weber’s law. The findings suggest that visuomotor control, across different types of actions and for a large range of size, is based on absolute rather than on relative representation of object size.

Dissociable effects of stimulus range on perception and action

We have recently reported the discovery that the ability to detect a minimum increment to a stimulus depends on the spread of the other stimuli for which this just noticeable difference (JND) is being measured (Namdar, Ganel, & Algom, 2016). In particular, the JND around a standard stimulus was larger when the other standards tested within the same experimental session spread a larger range. In this study we show that this range of standards effect (RSE) is limited to perceptual estimations and does not extend to action. The JND remained invariant when the participants grasped the objects rather than perceptually estimated their size. This difference supports the hypothesis that visual perception, on the one hand, and visually controlled action, on the other hand, are governed by separate rules and mediated by different mechanisms.

The extreme relativity of perception: A new contextual effect modulates human resolving power.

The authors report the discovery of a new effect of context that modulates human resolving power with respect to an individual stimulus. They show that the size of the difference threshold or the just noticeable difference around a standard stimulus depends on the range of the other standards tested simultaneously for resolution within the same experimental session. The larger this range, the poorer the resolving power for a given standard. The authors term this effect the range of standards effect (RSE). They establish this result both in the visual domain for the perception of linear extent, and in the somatosensory domain for the perception of weight. They discuss the contingent nature of stimulus resolution in perception and psychophysics and contrast it with the immunity to contextual influences of visually guided action.

The Size Congruity Effect Vanishes in Grasping: Implications for the Processing of Numerical Information

Judgments of the physical size in which a numeral is presented are often affected by the task-irrelevant attribute of its numerical magnitude, the Size Congruity Effect (SCE). The SCE is typically interpreted as a marker of the automatic activation of numerical magnitude. However, a growing literature shows that the SCE is not robust, a possible indication that numerical information is not always activated in an automatic fashion. In the present study, we tested the SCE via grasping by way of resolving the automaticity debate. We found results that challenge the robustness of the SCE and, consequently, the validity of the automaticity assumption. The SCE was absent when participants grasped the physically larger object of a pair of 3D wooden numerals. An SCE was still recorded when the participants perceptually indicated the general location of the larger object, but not when they grasped that object. These results highlight the importance of the sensory domain when considering the generality of a perceptual effect.

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.

A New Context Effect of Human Resolving Power Distinguishes between Perception and Action.

We report the discovery of a new effect of context that modulates the human resolving power for an individual stimulus. In particular, we show that the size of the difference threshold or the just noticeable difference around a standard stimulus depends on the range of the other standards tested simultaneously for resolution within the same experimental session. The larger this range, the poorer the resolving power for the given standard. We establish this result for the perception of linear extent, using different psychophysical methods and stimuli. We then proceed to show that this contextual influence is limited to perception and does not affect visually guided action. Notably, the difference threshold remained invariant when the participants were grasping rather than perceiving the same objects. Beyond adding a new member to the class of context effects in psychophysics, the findings lend further support for the idea that computations mediating the visual perception of object size are different from those mediating visually controlled actions toward the same objects. Meeting abstract presented at VSS 2015.