Violations of Weber’s law tell us more about methodological challenges in sensorimotor research than about the neural correlates of visual behaviour

Abstract

The violation of Weber's law in grasping has been presented as evidence for the claim that grasping is guided by visual information which is distinct from the information used in perceptual tasks. Previously, we contested this claim and argued that biomechanical constraints of the hand might explain why Weber's law cannot be reliably uncovered in grasping movements. In a recent article Manzone and colleagues (2017) show that pantomime grasping follows Weber's law even with objects whose width is close to the hand's biomechanical limit. In this commentary we explain why the biomechanical account does not necessarily predict the violation of Weber's law in a pantomime grasping task and why it seems problematic to use adherence or violation of Weber's law as a criterion to assign tasks to different anatomical pathways.

A review of grasping as the movements of digits in space

“…the visuomotor-channel framework, the absence of Weber's law requires an explanation. These have been provided in terms of biomechanical constraints Schenk et al 2017;Utz et al 2015) and in terms of the two-visual-systems hypothesis Ganel et al 2008;Heath et al 2017;Manzone et al 2017). Within the digit-in-space framework, no additional explanation is required.…”

Grasping Discriminates between Object Sizes Less Not More Accurately than the Perceptual System

“…The value of the grip aperture in any given trial is likely to be the result of many factors: the estimated size of the object, the current noise in the motor system, and the intended approach angle to name just a few possible factors that can influence the maximum grip aperture. As we have argued elsewhere [23,24,32], it is in our view a mistake to treat the MGA as a perfect read-out of the motor system’s estimate of the target object’s size. Other constraints and sources of noise will contaminate this measure.…”

“…Finally, Singhal et al [20] showed that a more general cognitive phenomenon, namely the finding that the concurrent execution of two tasks creates performance costs for at least one of the two tasks, reliably occurs in tasks that can be assigned to the perceptual system but is much less prominent in tasks assigned to the visuomotor system. However, all these approaches have been met with counter-evidence and are currently bogged down in controversy [21,22,23,24,25,26,27,28,29].…”

“…While we accept the more general point, namely that grasping and size discrimination do not necessarily use the same sensory inputs and that grasping should not be treated as the motor equivalent of a size-judgement task (for a more detailed discussion of this point, see Hesse et al [32] and Schenk et al [24]), we do think that there is evidence to suggest that visual size information is commonly used for grasping in healthy participants (albeit possibly not, or to a lesser extent, by patients with agnosia [33,34]). For example, the above discussed finding that grasping and perceptual judgements are impacted very similarly by visual size illusions [16] seems to indicate that object size is used also for grasping (for a slightly different view, see de Grave, et al [35]).…”

Grasping and perception are both affected by irrelevant information and secondary tasks: new evidence from the Garner paradigm

“…Over the last decades, this very influential model has been criticized (Schenk, 2006(Schenk, , 2012Schenk, Franz, & Bruno, 2011;Schenk & Hesse, 2018) and many studies originially providing supporting evidence for segregated neurological pathways for perception and action processing (e.g., Aglioti, DeSouza, & Goodale, 1995;Ganel, Chajut, & Algom, 2008;Ganel & Goodale, 2003, 2014Goodale et al, 1994;Goodale & Milner, 1992) have been challenged (e.g., see Kopiske, Bruno, Hesse, Schenk, &Franz, 2016 for illusions, andSchenk et al, 2017;Utz, Hesse, Aschenneller, & Schenk, 2015 for Weber's law). However, there are two lines of evidence that have received comparably little scrutiny until now: The Garner Interference effect (Ganel & Goodale, 2003, 2014 and dual-task studies (Liu, Chua, & Enns, 2008;Singhal, Culham, Chinellato, & Goodale, 2007).…”