Kayla D. Stone

Hand use for grasping in a bimanual task: evidence for different roles?

It has been proposed that the two hands play different roles during bimanual object interaction. The right hand takes on an explorative, highly precise, manipulative role while the left hand supports and stabilizes the object. Does this division of labour influence hand use during visually guided grasping? Three experiments were designed to address this question: right-handed individuals put together 3D models using big or small building blocks scattered across a tabletop. Participants were free to build the models; however, it felt comfortable (Experiment 1) or they were required to build on a large (Experiment 2) or small (Experiment 3) base plate. In Experiment 1, the right hand was preferred for grasping while the left hand stabilized the building model. When participants used the large base plate (Experiment 2), right hand use for grasping decreased and left hand use increased. The plate provided freedom to the left hand from having to stabilize the building model, but it also interfered with right/left hand movements directed towards the opposite side of the grasping hand (contralateral movements). To investigate which of these two factors would explain the change in hand use for grasping, a very small base plate was used in the last experiment. Results showed similar right hand use values to those seen in the first experiment (without the use of a plate), even though the left hand was ‘released from its stabilizing duties.’ The results predict a left-hemisphere right hand advantage in the control of grasping.

Evidence for right-hand feeding biases in a left-handed population

We have recently shown that actions with similar kinematic requirements, but different end-state goals may be supported by distinct neural networks. Specifically, we demonstrated that when right-handed individuals reach-to-grasp food items with intent to eat, they produce smaller maximum grip apertures (MGAs) than when they grasp the same item with intent to place it in a location near the mouth. This effect was restricted to right-handed movements; left-handed movements showed no difference between tasks. The current study investigates whether (and to which side) the effect may be lateralized in left-handed individuals. Twenty-one self-identified left-handed participants grasped food items of three different sizes while grasp kinematics were captured via an Optotrak Certus motion capture array. A main effect of task was identified wherein the grasp-to-eat action generated significantly smaller MGAs than did the grasp-to-place action. Further analysis revealed that similar to the findings in right-handed individuals, this effect was significant only during right-handed movements. Upon further inspection however, we found individual differences in the magnitude and direction of the observed lateralization. These results underscore the evolutionary significance of the grasp-to-eat movement in producing population-level right-handedness in humans as well as highlighting the heterogeneity of the left-handed population.

Hand preference across the lifespan: effects of end-goal, task nature, and object location

In the present study we investigate age-related changes in hand preference for grasping and the influence of task demands on such preference. Children (2–11), young-adults (17–28) and older-adults (57–90) were examined in a grasp-to-eat and a grasp-to-construct task. The end-goal of these tasks was different (eat vs. construct) as was the nature of the task (unimanual vs. bimanual). In both tasks, ipsilateral and contralateral grasps were analyzed. Results showed a right-hand preference that did not change with age. Across the three age groups, a more robust right-hand preference was observed for the unimanual, grasp-to-eat task. To disentangle if the nature (unimanual) or the end-goal (grasp-to-eat) was the driver of the robust right-hand preference, a follow up experiment was conducted. Young-adult participants completed a unimanual grasp-to-place task. This was contrasted with the unimanual grasp-to-eat task and the bimanual grasp-to-construct task. Rates of hand preference for the grasp-to-eat task remained the highest when compared to the other two grasping tasks. Together, the results demonstrate that hand preference remains stable from childhood to older adulthood, and they suggest that a left hemisphere specialization exists for grasping, particularly when bringing food to the mouth.

The contributions of vision and haptics to reaching and grasping

This review aims to provide a comprehensive outlook on the sensory (visual and haptic) contributions to reaching and grasping. The focus is on studies in developing children, normal, and neuropsychological populations, and in sensory-deprived individuals. Studies have suggested a right-hand/left-hemisphere specialization for visually guided grasping and a left-hand/right-hemisphere specialization for haptically guided object recognition. This poses the interesting possibility that when vision is not available and grasping relies heavily on the haptic system, there is an advantage to use the left hand. We review the evidence for this possibility and dissect the unique contributions of the visual and haptic systems to grasping. We ultimately discuss how the integration of these two sensory modalities shape hand preference.

Manual preferences for visually- and haptically-guided grasping.

Studies have shown that individuals exhibit a right-hand preference for grasping during visually-guided tasks. Recently, we have found that when vision is occluded right-hand preference decreases dramatically. It remains unknown however, if this decrease is a result of visual occlusion or the effects of relying only on haptic feedback. Therefore, in the present study, we sought to explore the contributions of vision and haptics (separately and in conjunction) to hand preference for grasping. Right- and left-handed individuals were tested on a block building task under four different visual and haptic conditions: 1) vision/normal haptic feedback (V/H), 2) no vision/normal haptic feedback (NV/H), 3) vision/constrained haptic feedback (V/Constrained-H), and 4) no vision/constrained haptic feedback (NV/Constrained-H). Vision was occluded using a blindfold and haptic feedback was constrained by asking participants to wear textured gloves. Right-handed individuals displayed a right-hand preference when vision was available (V/H and V/Constrained-H groups), but this preference was much greater when haptic feedback was constrained (V/Constrained-H group). When vision was occluded and haptic feedback was used to complete the task (NV/H) no hand preference was found. Finally hand preference was similar between the V/H and the NV/Constrained-H groups. For left-handed individuals, no differences in hand use were found between the different sensory groups, but the NV/H group showed a clear left-hand preference for haptically-guided grasping. The results suggest that haptics plays an important role in hand preference for grasping. Furthermore, they support a left-hand/right-hemisphere specialization for haptically-guided grasping (regardless of handedness) and a right-hand/left-hemisphere specialization for visually-guided grasping (at least in right-handed individuals).

Grasping without Sight: Insights from the Congenitally Blind

We reach for and grasp different sized objects numerous times per day. Most of these movements are visually-guided, but some are guided by the sense of touch (i.e. haptically-guided), such as reaching for your keys in a bag, or for an object in a dark room. A marked right-hand preference has been reported during visually-guided grasping, particularly for small objects. However, little is known about hand preference for haptically-guided grasping. Recently, a study has shown a reduction in right-hand use in blindfolded individuals, and an absence of hand preference if grasping was preceded by a short haptic experience. These results suggest that vision plays a major role in hand preference for grasping. If this were the case, then one might expect congenitally blind (CB) individuals, who have never had a visual experience, to exhibit no hand preference. Two novel findings emerge from the current study: first, the results showed that contrary to our expectation, CB individuals used their right hand during haptically-guided grasping to the same extent as visually-unimpaired (VU) individuals did during visually-guided grasping. And second, object size affected hand use in an opposite manner for haptically- versus visually-guided grasping. Big objects were more often picked up with the right hand during haptically-guided, but less often during visually-guided grasping. This result highlights the different demands that object features pose on the two sensory systems. Overall the results demonstrate that hand preference for grasping is independent of visual experience, and they suggest a left-hemisphere specialization for the control of grasping that goes beyond sensory modality.