Binocular vision is widely recognized as the most reliable source of 3D information within the peripersonal space, where grasping takes place. Since grasping is normally successful, it is often assumed that stereovision for action is accurate. This claim contradicts psychophysical studies showing that observers cannot estimate the 3D properties of an object veridically from binocular information. In two experiments, we compared a front-to-back grasp with a perceptual depth estimation task and found that in both conditions participants consistently relied on the same distorted 3D representation. The subjects experienced (a) compression of egocentric distances: objects looked closer to each other along the z-axis than they were, and (b) underconstancy of relative depth: closer objects looked deeper than farther objects. These biases, which stem from the same mechanism, varied in magnitude across observers, but they equally affected the perceptual and grasping task of each subject. In a third experiment, we found that the visuomotor system compensates for these systematic errors, which are present at planning, through online corrections allowed by visual and haptic feedback of the hand. Furthermore, we hypothesized that the two phenomena would give rise to estimates of the same depth interval that are geometrically inconsistent. Indeed, in a fourth experiment, we show that the landing positions of the grasping digits differ systematically depending on whether they result from absolute distance estimates or relative depth estimates, even when the targeted spatial locations are identical.

Keywords: tRNS1, perception2, stimulation3, noise4, psychophysics5, vision6. (Min.5-Max. 8) 26 27 tRNS improves vision-in-noise 2 Abstract (250 words) 28 29Transcranial random noise stimulation (tRNS), a relatively recent addition to the field of non-30 invasive, electrical brain stimulation, has been shown to improve perceptual and cognitive functions 31 across a wide variety of tasks. However, the underlying mechanisms of visual improvements caused 32 by tRNS remain unclear. To study this question, we employed a well-established, equivalent-noise 33 approach, which measures perceptual performance at various levels of external noise and is 34 formalized by the Perceptual Template Model (PTM). This approach has been used extensively to 35 infer the underlying mechanisms behind changes in visual processing, including those from 36 perceptual training, adaptation and attention. Here, we used tRNS during an orientation 37 discrimination task in the presence of increasing quantities of external visual white noise and fit the 38 PTM to gain insights into the effects of tRNS on visual processing. Our results show that tRNS 39 improves visual processing when stimulation is applied during task performance, but only at high 40 levels of external visual white noise-a signature of improved external noise filtering. There were no 41 significant effects of tRNS on task performance after the stimulation period. Of interest, the reported 42 effects of tRNS on visual processing mimic those previously reported for endogenous spatial 43 attention, offering a potential area of investigation for future work. 44 45 46 47 tRNS improves vision-in-noise 3

Humans have a remarkable capacity to make intuitive predictions about physical scenes. Recent studies suggest that this capacity recruits a general-purpose "physics engine" that reliably simulates how scenes will unfold. Here, we complicate this picture by introducing knots to the study of intuitive physics. Three experiments reveal that even basic judgments about knots strain human physical reasoning. Experiment 1-2 presented photographs of simple knots and asked participants to judge each knot's relative strength. Strikingly, observers reliably ranked weaker knots as strong and stronger knots as weak. Experiment 3 presented photographs of tangled strings and asked participants whether the tangle forms a knot when pulled. When shown a tangle that would not form a knot when pulled taut, subjects were at or near chance discriminating knots from non-knots. These failures challenge the domain-generality of physical reasoning mechanisms, and perhaps suggest that soft-body phenomena recruit different cognitive processes than rigid-body physics.

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