Hands are unique in mental rotation
The mental rotation of hands is a specific subtype of MRT in which the participants must determine whether two-dimensional hand pictures are the same (e.g., both are left hands or both are right hands) or different hands (a left hand and a right hand). In the mental rotation of hands, response time is much faster and more invariant to changes in orientation than for the mental rotation of other objects (Cooper, 1975; Cooper & Shepard, 1975; Folk & Luce, 1987; Shepard & Metzler, 1971; Stieff, 2007). The goal of the current study is to explore the cognitive mechanisms underlying this unusual effect using palm-up hand stimuli. Previous researchers have explained this unique effect in terms of people’s familiarity with hands (e.g., Parsons, 1987). However, two possible types of familiarity could exist for right or left hands: embodied experience and world knowledge. These two types of familiarity, by extension, lead to two different theories that could help explain the mental rotation of hands.
Embodied experience theory
Hands, as parts of the human body, provide us with embodied experience through our interaction with the world. The hand that is used more often in daily life—the dominant hand—typically provides more embodied experience. Therefore, people are likely to have greater embodied experience with their dominant hand than with their nondominant hand. Consistent with this theory, one study showed that people who had lost their dominant hand responded more slowly and less accurately in a hand MRT than people who had lost their nondominant hand (Nico, Daprati, Rigal, Parsons, & Sirigu, 2004). This finding suggests the importance of embodied experience, especially from dominant hands, in the mental rotation of hands.
World knowledge theory
Approximately 90% of human beings are right-handed, defined either by skill or by preference in spite of culture or ethnicity (Coren & Porac, 1977; Previc, 1991). Almost all tools and facilities (e.g., scissors, notebooks, spiral staircases) in daily life are designed for right-handed people. In other words, we live in a right-handed world and have more experience seeing people use their right hands (e.g., for writing) than their left hands. Therefore, the possibility exists that everyone is more familiar with right hands than left hands, regardless of their own handedness. In support of this theory, a study of split-brain patients revealed that the right hand has an advantage in representing acquired tool use, regardless of whether the person is right- or left-handed (Frey, Funnell, Gerry, & Gazzaniga, 2005). This result suggests that experience in a right-handed world could affect the abilities of even left-handers. Perhaps more direct evidence comes from a study in which researchers put left-handed mice in a right-handed world (their food was put in the right corner of an environment such that it was much easier to access with the right paw), and some of the left-handed mice changed to be right-handed (Collins, 1975). This finding indicates that the design of the world for right-handers is so powerful that it can potentially overcome natural proclivities. Other research has described how left-handers struggle living in this right-handed world (e.g., Masud & Ajmal, 2012; Suitner, Maass, Bettinsoli, Carraro, & Kumar, 2017; Zaghloul, Saquib, Al-Mazrou, & Saquib, 2018).
The present study tested the extent to which people’s spatial thinking is influenced by world knowledge and embodied experience. A good way to test these two theories is by contrasting performance between left-handers and right-handers. One theory is that mental rotation of hands is supported by world knowledge, which provides more familiarity with right hands than with left hands for all people. Under this theory, we predict that all people will perform better on right-hand stimuli than on left-hand stimuli, independent of their handedness. In contrast, if the mental rotation of hands is supported by embodied experience, an advantage for the dominant hand is expected. Thus, left-handers are predicted to have better performance for left-hand stimuli, and right-handers are predicted to have better performance for right-hand stimuli.
Two primary tasks have been developed to study the mental rotation of hands. One task is the hand laterality task (HLT), meaning the person must determine whether a single hand shown on the screen is a left hand or a right hand. This task is simple but is prone to verbal labeling errors. The second type of task is a modified Shepard and Metzler task (SMT) in which subjects are presented with two hands simultaneously, one on each side of the screen. The task is to decide whether the two hands are the same (both left hands or both right hands) or different (one is a left hand; one is a right hand). The HLT is used more often to study motor behavior (e.g., Parsons, 1994), while the hand version of the SMT is more commonly tested together with SMT of other stimuli (e.g., tools, letters, cubes) to illustrate the striking unique reaction time pattern in the mental rotation of hands. In addition, the laterality task is classified as an egocentric perspective transformation because spatial information is formed with respect to oneself, whereas the SMT is an object-based spatial transformation because spatial information is formed independent of the observer’s view (Zacks, Mires, Tversky, & Hazeltine, 2002). Although an egocentric perspective could be advantageous, since participants can imagine rotating their own hand to complete the task, there are also a number of biomechanical limitations to this perspective (Parsons, 1987), described in detail below. Accordingly, we used the SMT for this study.
Handedness
Strength of handedness
In addition to studying the direction of handedness (i.e., left or right), another thread of research focuses on the strength of handedness. The strength of handedness varies from mixed (inconsistent hand preference for activities) to extreme (very consistent in using either the left or the right hand). There is some evidence that extreme-handed individuals—whether right- or left-handed—have less cognitive flexibility than mixed-handed individuals (Badzakova-Trajkov, Häberling, & Corballis, 2011; Barnett & Corballis, 2002; Nicholls, Orr, & Lindell, 2005). In contrast, mixed-handed individuals perform better on memory tasks that require hemispheric interaction (e.g., paired associate recall) (Lyle, McCabe, & Roediger, 2008; Lyle & Orsborn, 2011; Propper, Christman, & Phaneuf, 2005). Mixed-handed individuals also have better memory for the frequency of using one hand or the other in everyday unimanual tasks (e.g., brushing one’s teeth) (Edlin, Carris, & Lyle, 2013). Because extreme-handers have more embodied experience with their dominant hands than mixed-handers, we expect subjects’ performance for right-hand stimuli will increase from extreme left-handers having the worst performance, to mixed left-handers, to mixed right-handers, to extreme right-handers having the best performance; performance for left-hand stimuli is expected to show the reverse pattern among handedness groups. Thus, we included people with both mixed and extreme handedness in the sample (although due to their relative rarity, the sample size of extreme left-handers is fairly small), aiming to take a closer look at the influence of handedness strength.
Handedness effects on the mental rotation of hands
To our knowledge, only one previous study has tested the influence of handedness on the mental rotation of hands. The researchers used six hand gestures in the HLT, including one palm-up and five palm-down gestures. They found a reaction time advantage for right-hand stimuli in right-handers, but they also showed a speed–accuracy trade-off (Ní Choisdealbha, Brady, & Maguinness, 2011). No difference in performance between left and right-hand stimuli was found in left-handers. These results indicate that left-handers and right-handers might have different mechanisms for responding to left-hand stimuli and right-hand stimuli in the mental rotation of hands. They also found that reaction time for all five palm-down gestures showed a standard pattern across rotation angles, while the palm-up gesture peaked at a larger rotation angle, indicating that the palm-up gesture might be treated differently from other gestures.
Prior to Ní Choisdealbha’s work, Sekiyama (1982) studied mental rotation of hands with five different hand gestures (three in a palm-up position, two in a palm-down position) in right-handed people by using a hand laterality paradigm. Like Ní Choisdealbha’s research, one big difference was found between palm-up left-hand stimuli and palm-down right-hand stimuli. Specifically, the reaction time pattern of the degree of clockwise rotations for the palm-up left-hand stimuli was similar to that of the counterclockwise rotations for the palm-down right-hand stimuli, indicating a “wrong-hand” effect that will be explained in more detail later. We next turn to how sensory and motor systems could explain these effects.
Sensorimotor theories of the mental rotation of hands
Motor simulation theory
The traditional view of the mental rotation of hands is based on motor simulation theory. Under this theory, the motor system that guides the intended action is automatically activated during the mental rotation of hands, which could cause a feeling of moving (Parsons, 1987, 1994; Parsons, Gabrieli, Phelps, & Gazzaniga, 1998). This theory suggests an alignment between the spatial representation of the subject’s own hand and the image of a hand on the screen: An image of a left hand, for example, will always align with the subject’s left hand (regardless of whether it is palm up or palm down) because the motor system requires a consistent internal representation of body position.
In the 1980s, Lawrence M. Parsons carried out a series of studies to test the motor simulation theory for the mental rotation of hands by using HLTs. In his tasks, participants viewed hand stimuli from different perspectives, with the orientation varying from the normal physical range of motion to an awkward range that is difficult to produce biomechanically (Parsons, 1987). For example, a palm-down left hand turned in a counterclockwise direction would be considered an “awkward” orientation, whereas a palm-down left hand turned in a clockwise direction would be considered “normal” range. Parsons found that across all hand views, awkward orientations took longer than normal orientations for both right and left hands. When a palm-down hand stimulus was viewed, reaction time increased slightly with each increasing angle of orientation for both normal and awkward orientations. When a palm-up hand stimulus was viewed, however, Parsons found a flat reaction time pattern for normal orientations and a pattern with a peak for awkward orientations. These results indicate that mental rotation of palm-up hands is relatively more invariant to changes in orientation than mental rotation of palm-down hands.
It is possible that handedness could contribute to some of those findings. However, all the participants recruited in Parsons’ study (1987) were right-handed. Those right-handed participants were slower overall in responding to left-hand stimuli than to right-hand stimuli for both the palm-up and the palm-down gestures, although this effect was more robust for palm-down hands. This finding suggests that right-handers have an advantage in responding to right-hand stimuli compared with left-hand stimuli, supporting the embodied experience hypothesis.
In order to explore the influence of the HLT itself, Parsons (1987) asked subjects to complete the same experiment by imagining transforming their own hands to the position of the presented hand stimuli. Subjects only needed to verbally report “now” to indicate that they had completed the mental spatial transformation. In normal orientations, reaction time was faster for right hands than for left hands when the stimuli were palm-down hands, but this advantage switched to be faster for left hands than for right hands when the stimuli were palm-up hands. This result suggests some kind of confusion about the shape of the hand, or a “wrong-hand effect.” As these studies specifically required participants to imagine transforming their own hands, the similar results for both studies suggest that the preferred strategy in a HLT is to imagine moving one’s hand to simulate the orientation of the stimulus.
Visual-proprioceptive integration theory and the wrong-hand effect
Viswanathan, Fritz, and Grafton (2012) challenged the conventional view of motor simulation processes underlying the mental rotation of hands by proposing a visual-proprioceptive integration theory (Grafton & Viswanathan, 2014; Viswanathan et al., 2012). Under the visual-proprioceptive integration theory, information from different sensory modalities is integrated to enable a coherent experience of an object. In the case of the mental rotation of hands, the hand stimuli on the screen and the subject’s own hand share a spatial feature (e.g., outline shape or digit ratio). Visual-proprioceptive integration is the processing of that shared spatial information. In this case, it is the multisensory integration of the visual input of the spatial configuration (the outline or “shape”) of the image of a hand on the screen and the proprioceptive input (information about where each body part is) of the response hand. Note that in this theory, visual details indicating whether the hand is palm up or palm down are ignored, and only the outline shape of the hand is considered. In their task, the subject’s response hands—both left and right hands—were in a palm-down position to make their responses on the keyboard. This setup created a shape match: The shape of a right hand in a palm-up gesture on the screen matches the shape of the palm-down left hand of the subject making the response, and vice versa for a left-hand palm-up.
The researchers found that people’s hand laterality judgments can be easily manipulated by the sequence of perceptual processing of the shape and view of a hand (Viswanathan et al., 2012). The stimuli presentation was manipulated so that participants preferentially processed either shape information or view information—the information about whether the person is looking at the palm or back of the hand. The hand stimulus consisted of a visual outline of a hand (a black silhouette), with a colored dot as the only indication of whether the hand was palm up or palm down. When the researchers cued the trials such that view information was preferentially processed, a left palm-up gesture on the screen was recognized as a left hand and vice versa for a right palm-up gesture. When the shape information was cued to be preferentially processed; however, a left palm-up gesture on the screen was recognized as a right hand and vice versa for a right palm-up gesture, suggesting that people were biased toward processing an ambiguous shape as the back of the hand. This “wrong-hand effect” could be due to the premature binding of the observer’s felt hand, which was palm down, with the ambiguous hand on the screen.
However, it is unknown whether shape information is processed separately when both shape and details showing whether it is the palm or the back of the hand are presented simultaneously. In order to answer this question, in the present study, we tested stimuli with details clearly showing that it is the palm of the hand.
Hypotheses and predictions
The goal of the present study was to explore the cognitive mechanisms underlying the mental rotation of hands. In this experiment, left-handed and right-handed subjects were recruited to complete a modified SMT with hand stimuli. We started with the goal of purely exploring the influence of world knowledge and embodied experience, but we also needed to address the additional contrasting hypotheses regarding the information-processing mechanisms of hand mental rotation (motor imagery and visual-proprioceptive integration). Therefore, we crossed two orthogonal hypothesis axes to yield four competing hypotheses. One axis of the hypothesis space contrasted (i) world knowledge of a right-handed world versus (ii) embodied experience with one’s own hands; the other hypothesis axis contrasted (a) motor imagery (i.e., motor simulation) versus (b) visual-proprioceptive integration based on shape information alone (i.e., the wrong-hand effect). A brief overview of the predictions of each of the theories is stated below.
- i.
World knowledge. Because left-handers and right-handers share the same knowledge of a right-handed world, this theory predicts better performance for right-hand stimuli than for left-hand stimuli for the mental rotation of hands for all individuals. If all subjects respond faster or more accurately to right-hand stimuli, then this hypothesis would be supported.
- ii.
Embodied experience. An alternative theory is that people respond better to a hand stimulus that matches their dominant hand. If left-handers respond faster or more accurately for left-hand stimuli and right-handers respond faster or more accurately for right-hand stimuli, then the embodied experience hypothesis would be supported.
- a.
Motor imagery. Under the motor imagery theory (i.e., motor simulation theory), the match between one’s own hand and the hand seen on screen is based on the hand’s details (e.g., shape, visual details). The view of a right hand in any orientation automatically activates a motor representation of a person’s right hand, and the view of a left hand activates the motor representation of a person’s left hand. While this hypothesis cannot on its own indicate how handedness influences performance, it makes predictions in combination with world knowledge or embodied experience. Specifically, when combined with either world knowledge or embodied experience it predicts better performance by right-handers for right-hand stimuli. However, for left-handers, it predicts better performance for right hands under world knowledge and better performance for left hands under embodied experience.
- b.
Visual-proprioceptive integration. Under this theory, a “wrong-hand effect” will be expected, whereby the match of spatial configuration (“shape”) of the hand stimuli on the screen and the proprioceptive information from the hand making the response is preferentially processed. We tested only palm-up stimuli in our experiment, and the hand making the response in this task was in a palm-down position on the keyboard. Thus, this theory predicts that right-handed subjects will perform better for left-hand stimuli than for right-hand stimuli, regardless of embodiment or world knowledge, which is contrary to the prediction of motor imagery theory. Left-handed people will perform better for right hands if combined with embodiment, and better for left hands if combined with world knowledge.
These two sets of theories represent orthogonal features in the mental rotation of hands. In order to fully test the interaction of these two sets of theories, we crossed these two pairs of theories to yield four specific hypotheses. Figure 1 illustrates these four hypotheses, by showing predictions both based on the handedness of the individual (Fig. 1a) and based on the hand stimulus (Fig. 1b).
Hypothesis 1: motor imagery and world knowledge
Prediction: First, based on motor imagery theory, a left palm-up hand stimulus will be recognized as a left hand, and a right palm-up hand stimulus will be recognized as a right hand. Second, based on world knowledge theory, everyone will be more familiar with right hands than with left hands. Therefore, all subjects’ performance for right-hand stimuli will be better than for left-hand stimuli. No differences are expected between mixed- and extreme-handed people in this hypothesis.
Hypothesis 2: motor imagery and embodied experience
Prediction: First, based on motor imagery theory, a left palm-up hand stimulus will be recognized as a left hand and vice versa for right hands. Second, based on embodied experience theory, people will perform better on stimuli that match their dominant hands than on stimuli that match their nondominant hands. Therefore, left-handers will perform better for left-hand stimuli, and right-handers will perform better for right-hand stimuli. Third, extreme-handers will have more embodied experience with their dominant hands than mixed-handers. Thus, extreme-handers will perform better on stimuli that match their dominant hands than mixed-handers and will perform worse on stimuli that match their nondominant hands than mixed-handers.
Hypothesis 3: visual-proprioceptive integration and world knowledge
Prediction: First, based on visual-proprioceptive integration theory, a left palm-up hand stimulus will be recognized as a right hand and vice versa for right hands. Second, based on world knowledge theory, everyone will be more familiar with right hands than with left hands. Therefore, under this hypothesis, all subjects’ performance for left-hand stimuli will be better than for right-hand stimuli. No differences are expected between mixed- and extreme-handed people in this hypothesis.
Hypothesis 4: visual-proprioceptive integration and embodied experience
Prediction: First, based on visual-proprioceptive integration theory, a left palm-up hand stimulus will be recognized as a right hand and vice versa for right hands. Second, based on embodied experience theory, people will perform better on stimuli that match their dominant hands than on stimuli that match their nondominant hands. Therefore, for left-handers, performance for right-hand stimuli will be better than for left-hand stimuli. For right-handers, performance for left-hand stimuli will be better than for right-hand stimuli. Third, extreme-handers will have more embodied experience with their dominant hands than mixed-handers. Thus, extreme-handers will perform better than mixed-handers on palm-up stimuli that match the shape of their dominant hands and will perform worse on palm-up stimuli that match the shape of their nondominant hands. To distinguish between these four hypotheses, we tested both left-handed and right-handed subjects and incorporated left- and right-hand stimuli on the screen.
Other possible factors
Besides handedness direction, some other factors might influence subjects’ performance. Although we tried to control the influence of these factors in our experimental design, it is still possible that they could influence the outcomes. Thus, we will still consider them at a later point in our data analysis in order to have a more thorough understanding of the results. Here we introduce some of the main additional possible factors and how we tried to control them.
Hand gestures
Most previous studies on the mental rotation of hands only used one gesture as a stimulus, usually an open-palm gesture, but these studies also tended to have a ceiling effect (e.g., de Lange, Helmich, & Toni, 2006; Parsons, 1987; Zapparoli et al., 2014). The use of a single gesture could be one factor leading to this ceiling effect, so we added a hand in a pointer gesture to increase the difficulty of the test. To make the task even more challenging, we also included a condition in which the two hand stimuli were different gestures (one pointer, one palm). Because of these modifications, we expected that response accuracy could become another performance indicator in the study, in addition to reaction time.
Response pattern
Here, response pattern refers to which hand pressed the “same” response and which hand pressed the “different” response. For this study, there were two response patterns: left hand pressed “same” and right hand pressed “different”, or left hand pressed “different” and right hand pressed “same.” We counterbalanced this factor by randomly assigning half of the subjects in each handedness group to complete the task with each response pattern; however, we did not analyze this factor specifically.
Strategy
Two primary strategies could be used in this hand MRT: mental rotation and thumb strategies. Mental rotation means solving the problem purely by mentally rotating one hand stimulus to match the other one. The thumb strategy is a trick, comparing whether the thumb is on the same side of each hand stimulus. For example, if there are two left hands on the screen, both thumbs are on the left side of each hand, regardless of their rotation angles, because all hand stimuli in this experiment were palm-up. As mentioned above, extreme-handers tend to be less cognitively flexible than mixed-handers. Thus, it is possible that mixed-handers would have a higher frequency of applying the thumb strategy, while extreme-handers would tend to rely on mental rotation. Therefore, we analyzed strategy related both to handedness direction and to handedness strength.