Ali Mulla

The visual coding of grip aperture shows an early but not late adherence to Weber's law

An exemplar metric of goal-directed grasping (i.e., peak grip aperture) has been shown to be refractory to a psychophysical principle governing visuoperceptual estimations of object size (i.e., Webers law). This dissociation suggests that vision for action and vision for perception are mediated by absolute and relative visual information, respectively. The present investigation examined whether aperture shaping elicits a unitary or process-dependent violation of Webers law. Participants grasped differently sized objects (20, 30, 40, 50 and 60 mm of width) and just noticeable difference (JND) scores related to grip aperture were computed at the time of peak grip aperture as well as at normalized deciles of the response (i.e., 10-90% of grasping time). JNDs during the early and middle stages of the trajectory scaled to object size whereas values late in the trajectory (>50% of grasping time and including the time of peak grip aperture) did not. Thus, results show an early, but not late, adherence to Webers law and indicate that movement planning and movement control are supported via relative and absolute visual information, respectively.

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

The “just noticeable difference” (JND) represents the minimum amount by which a stimulus must change to produce a noticeable variation in ones perceptual experience (i.e., Webers 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 Webers 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 Webers law.

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

ABSTRACT The authors examined whether the top-down requirements of dissociating the spatial relations between stimulus and response in a goal-directed grasping task renders the mediation of aperture trajectories via relative visual information. To address that issue, participants grasped differently sized target objects (i.e., grasping condition) and also grasped to a location that was dissociated from the target object (i.e., pantomime-grasping condition). Just noticeable difference (JND) values associated with the early through late stages of aperture shaping were computed to examine the extent to which motor output adhered to, or violated, the psychophysical principles of Webers law. As expected, JNDs during the late stages of the grasping condition violated Webers law: a result evincing the use of absolute visual information. In contrast, JNDs for the pantomime-grasping condition produced a continuous adherence to Webers law. Such a result indicates that dissociating a stimulus from a response is a perception-based task and results in aperture shaping via relative visual information.

Pro- and antisaccades: dissociating stimulus and response influences the online control of saccade trajectories.

ABSTRACT The authors examined whether the diminished online control of antisaccades is related to a trade-off between movement planning and control or the remapping of target properties to a mirror-symmetrical location (i.e., vector inversion). Pro- and antisaccades were examined in a standard no-delay schedule wherein target onset served as the movement imperative and a delay cuing schedule wherein responses were initiated 2,000 ms following target onset. Importantly, the delay cuing schedule was employed to equate pro- and antisaccade reaction times. Online control was evaluated by indexing the strength of trajectory amendments at normalized increments of movement time. Antisaccades exhibited fewer online corrections than prosaccades, and this result was consistent across cuing schedules. Thus, the diminished online control of antisaccades cannot be tied to a trade-off between movement planning and control. Rather, the authors propose that the intentional nature of dissociating stimulus and response (i.e., vector inversion) engenders a slow mode of cognitive control that is not optimized for fast oculomotor corrections.