Grasping time does not influence the early adherence of aperture shaping to Weber's law

Abstract

The “just noticeable difference” (JND) represents the minimum amount by which a stimulus must change to produce a noticeable variation in one's perceptual experience (i.e., Weber's law). Recent work has shown that within-participant standard deviations of grip aperture (i.e., JNDs) increase linearly with increasing object size during the early, but not the late, stages of goal-directed grasping. A visually based explanation for this finding is that the early and late stages of grasping are respectively mediated by relative and absolute visual information and therefore render a time-dependent adherence to Weber's law. Alternatively, a motor-based explanation contends that the larger aperture shaping impulses required for larger objects gives rise to a stochastic increase in the variability of motor output (i.e., impulse-variability hypothesis). To test the second explanation, we had participants grasp differently sized objects in grasping time criteria of 400 and 800 ms. Thus, the 400 ms condition required larger aperture shaping impulses than the 800 ms condition. In line with previous work, JNDs during early aperture shaping (i.e., at the time of peak aperture acceleration and peak aperture velocity) for both the 400 and 800 ms conditions scaled linearly with object size, whereas JNDs later in the response (i.e., at the time of peak grip aperture) did not. Moreover, the 400 and 800 ms conditions produced comparable slopes relating JNDs to object size. In other words, larger aperture shaping impulses did not give rise to a stochastic increase in aperture variability at each object size. As such, the theoretical tenets of the impulse-variability hypothesis do not provide a viable framework for the time-dependent scaling of JNDs to object size. Instead, we propose that a dynamic interplay between relative and absolute visual information gives rise to grasp trajectories that exhibit an early adherence and late violation to Weber's law.

Semantic grasping escapes Weber's law

“…2b). Thus, we replicated the finding of previous studies showing that visually guided grasping violates Weber's law (Ganel et, 2008a(Ganel et, , 2008bHadad et al, 2012;Heath et al, 2012Heath et al, , 2011Holmes et al, , 2011. Although not statistically significant, the scaling of the standard deviation of MGA was slightly negative.…”

“…Thus, visually guided grasping does not follow Weber's law. While this result seems like a violation of a very fundamental principle in psychological science, it has been replicated in many studies (Ganel et al, 2008b;Hadad et al, 2012;Heath et al, 2012Heath et al, , 2011Holmes et al, , 2011.…”

Distinct Visual Cues Mediate Aperture Shaping for Grasping and Pantomime-Grasping Tasks

“…In the present investigation, participants grasped and pantomime-grasped differently sized objects (20, 30, 40, and 50 mm) and we evaluated participants sensitivity to detecting changes in object size by computing the within-participant standard deviations of grip aperture (i.e., the JNDs) at normalized deciles of grasping time (see . In accord with previous work (Heath, Holmes, Mulla, & Binsted, 2012;Heath, Mulla, Holmes, & Smuskowitz, 2011;Holmes & Heath, 2013;Holmes, Mulla, Binsted, & Heath, 2011), it was predicted that JNDs during the later stages of the grasping condition would elicit a null scaling to object size. More directly, we predicted that the trial-to-trial stability (i.e., visuomotor certainty) of participants grip aperture (i.e., the comparator) during the late stage of the response would be refractory to the size of the to-be-grasped target object (i.e., the initial stimulus).…”

“…Importantly, however, a statistical criterion for the computation of a JND is not available for a grasping task. Instead, the JNDs computed here and in other grasping studies Heath, Holmes, et al, 2012;Heath et al, 2011;Holmes & Heath, 2013;Holmes et al, 2011) represent the within-participant standard deviations of grip aperture. To illustrate our computation of JNDs, Figure 1 presents trial-to-trial grip apertures at time-normalized deciles of grasping time for an exemplar participant when grasping and pantomime-grasping the 20 and 50 mm target objects.…”

Visual control of action directed toward two-dimensional objects relies on holistic processing of object shape

“…On the other hand, a more recent study that measured the within-subject variability of the response rather than looking only at the average response, found an adherence to Weber's law for 2D but not for 3D objects. Particularly, JNDs during MGAs linearly increased with object size when actions were directed toward 2D objects (Holmes & Heath, 2013), but not when actions were directed toward real objects (Ganel et al, 2008Heath, Holmes, Mulla, & Binsted, 2012;Heath, Mulla, Holmes, & Smuskowitz, 2011;Holmes, Mulla, Binsted, & Heath, 2011).…”