On the Evolution of Handedness: Evidence for Feeding Biases

Citation: Flindall JW, Gonzalez CLR ( On the Evolution of Handedness: Evidence for Feeding Biases Jason W. Flindall 0 Claudia L. R. Gonzalez 0 David Frayer, University of Kansas, United States of America 0 The Brain in Action Laboratory, Department of Kinesiology and Physical Education, University of Lethbridge , Lethbridge, Alberta , Canada Many theories have been put forward to explain the origins of right-handedness in humans. Here we present evidence that this preference may stem in part from a right hand advantage in grasping for feeding. Thirteen participants were asked to reach-to-grasp food items of 3 different sizes: SMALL (CheeriosH), MEDIUM (Froot LoopsH), and LARGE (Oatmeal SquaresH). Participants used both their right- and left-hands in separate blocks (50 trials each, starting order counterbalanced) to grasp the items. After each grasp, participants either a) ate the food item, or b) placed it inside a bib worn beneath his/her chin (25 trials each, blocked design, counterbalanced). The conditions were designed such that the outward and inward movement trajectories were similar, differing only in the final step of placing it in the mouth or bib. Participants wore Plato liquid crystal goggles that blocked vision between trials. All trials were conducted in closed-loop with 5000 ms of vision. Hand kinematics were recorded by an Optotrak Certus, which tracked the position of three infrared diodes attached separately to the index finger, thumb, and wrist. We found a task (EAT/PLACE) by hand (LEFT/RIGHT) interaction on maximum grip aperture (MGA; the maximum distance between the index finger and thumb achieved during grasp pre-shaping). MGAs were smaller during right-handed movements, but only when grasping with intent to eat. Follow-up tests show that the RIGHT-HAND/EAT MGA was significantly smaller than all other hand/task conditions. Because smaller grip apertures are typically associated with greater precision, our results demonstrate a right-hand advantage for the grasp-to-eat movement. From an evolutionary perspective, early humans may have preferred the hand that could grasp food with more precision, thereby maximizing the likelihood of retrieval, consumption, and consequently, survival. - Funding: The current study was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC Fund # 14367), the Canadian Foundation for Innovation (CFI Fund # 45561), the Canada Research Chairs program, and the University of Lethbridge. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Previous research has indicated that the defining characteristic of handedness, that is, a lateralized manual hand preference, does not develop until 21 months of age in humans [13]. Many manipulative tasks do not show lateralization until much later in development [47]. With few exceptions, children younger than 4 years of age do not demonstrate a hand preference for manipulative reach-to-grasp actions on non-food objects; instead, they use whichever hand is ipsilateral to the target object [8,9]. Recently, however, it has been shown that when the target is a food-object, children as young as one will demonstrate a robust right-hand preference for the reach-to-grasp action [10]. In the Sacrey et al. study, 3- to 5-year-old children were presented with food items (Froot LoopsH) and non-food items of comparable size and colour (LEGOH construction blocks) and their hand preference for reaching-to-grasp the items was recorded. A righthand preference for grasping the blocks was found in the 4- and 5year old cohorts, but not in the 3-year old group; this finding was consistent with previous research [8,9]. When the target was a food-object, however, 3-year-olds showed a greater than 80% right hand preference for grasping. In fact, when younger groups were tested, this preference was observed in children as young as one year of age [10]. This finding suggests that a right hand preference for reach-to-grasp for food (henceforth referred to as grasp-to-eat) develops earlier, and perhaps is altogether separate from hand preference for reach-to-grasp for objects to manipulate (i.e., graspto-place). This suggestion is further supported by studies which show that infants are able to produce accurate hand-to-mouth movements earlier than accurate reach-to-grasp movements [11]. If hand preference is susceptible to the end goal of an action, it is reasonable to speculate that kinematics may also vary according to the actors intent. Several studies describing the kinematics of prehension have shown that the end goal of an action significantly influences the kinematics of the reach and grasp [1216]. In movements with a similar initial lifting phase, but different consecutive movements (i.e. with differing intent; for example, grasp-to-place versus graspto-throw), peak velocity, peak deceleration, and peak grip aperture of the approach phase of the grasp movement have been shown to vary according to the purpose of the grasp [12]. While food has been used as a target in kinematic analyses [1721] and imaging studies [22], few have investigated whether action intention influences movement kinematics when grasping a food item. In the only such study (of which we are aware), participants were asked to reach and grasp a sugar cube in order to put it in their own mouths (presumably to eat), in the mouth of another person (i.e. a conspecific), or in a fake mouth placed over their own mouths. The results showed greater automaticity for movements directed to the self than either the conspecific or the fake mouth [23]. These studies indicate that the final objective or purpose for which a reach-to-grasp action is executed significantly influences the kinematics of the movement. They also demonstrate the sensitivity of kinematic parameters when detecting differences in seemingly similar actions. Investigations into the kinematics of left- versus right-handed movements have shown, at most, only minor differences between the hands in reach-to-grasp actions [2427]. For example, [24] used a reach-to-grasp task in which a cylindrical object was grasped and placed into a target slot to compare kinematic data between the left and right hands of participants. Other than a minor difference in insertion time (in which the dominant hand was faster than the non-dominant hand), the researchers found no significant differences in movement kinematics between the hands. This is quite puzzling given that, if the right hand is used preferentially for the grasp-to-place action, and this preference is to be driven by a kinematic advantage, one would expect to find kinematic differences between the hands. The studies cited above, however, have used grasp-to-place tasks in their search for manual asymmetries. Where hand differences are absent in the gras (...truncated)