A letter on a hand: Rotating together or separately?

Abstract

The motor system plays a role in some object mental rotation tasks, and researchers have reported that people may use a strategy of motor simulation to mentally rotate objects. In this study, we used images of a hand with a letter printed on the palm to directly determine whether a hand image can be automatically rotated during the deliberate mental rotation of an object and whether the hand and object are rotated in the same trajectory. A total of 41 participants were shown the stimuli and asked to decide whether the letters, which were upright or tilted at specific degrees, were normal or mirrored. The hand images in the background showed either a left or a right hand in the palm view, with fingers pointing upwards, medial, downwards, or lateral. Reaction times and error rates were measured to determine the speed and accuracy of mental rotation. A complex interaction between the hand posture and letter orientation revealed that the hand image was mentally rotated automatically, together with the deliberate mental rotation of the letter. The biomechanical constraints of the hand also influenced reaction times, suggesting the involvement of the motor system in the concomitant mental rotation of the hand image. Consistent with the motor simulation theory, the participants seemed to imagine the hand carrying the object in its movement. These behavioural data support the motor simulation theory and elucidate specific processes of mental rotation that have not been addressed by neuroimaging studies.

Keywords

Mental rotation objects body parts hand image hand posture motor simulation motor system

Introduction

Mental rotation refers to the act of imagining a shape or object changing its spatial orientation. This topic has generated empirical interest in the past few decades, since the publication of a seminal study of the three-dimensional cube object by Shepard and Metzler (1971). In one of the most frequently used experimental paradigms of mental rotation, participants were asked to judge whether an alphanumeric character rotated at a certain angle was presented in its normal or mirror-image form. Participants’ reaction times (RTs) in this task increased linearly with the character’s angular departure from the upright and then decreased after reaching a peak at 180^° (Cooper & Shepard, 1973). According to these findings, mental rotation is an analogue process of corresponding physical rotation, and upright is the canonical orientation for letter or numeric characters in an individual’s mind. In the mental rotation task, participants are thought to mentally rotate the character along the shortest path to its upright posture.

Notably, similar but not identical effects have been found in mental rotation of body parts, such as hands. When participants decided whether a picture showed a left or a right hand, RTs increased with the hand’s angular departure from the canonical view of the fingers pointing upwards (Cooper & Shepard, 1975; Parsons, 1994). However, the RTs to mentally rotate body parts did not simply vary with the rotation angle. Rather, the RTs strongly depended on the constraints of the movement, as if the rotation were being physically performed (Parsons, 1994; Sekiyama, 1982; ter Horst, van Lier, & Steenbergen, 2010). When the body parts were in nearly biomechanically impossible orientations, the RTs would increase. For example, participants had more difficulty recognising a hand rotated away from the mid-sagittal plane (a lateral rotation) than that rotated towards the mid-sagittal plane (a medial rotation). These data indicate that participants imagined their own hands rotating with the stimulus.

The aforementioned findings suggest that the mental rotations of objects and body parts have distinct mechanisms. Behavioural data from studies in which researchers asked participants to complete both types of mental rotation tasks support this hypothesis. For example, the mental rotation of body parts is faster and more accurate than that of objects (Amorim, Isableu, & Jarraya, 2006; Dalecki, Hoffmann, & Bock, 2012; Jola & Mast, 2005; Kosslyn, Digirolamo, Thompson, & Alpert, 1998).Moreover, the interference of manual rotation reveals that the pattern of disruption of mental rotation depends on the category of visual stimuli. Mental rotations of both objects and hands are impaired by manual rotation. However, the mental rotation of hand stimuli is affected more severely by the discordant direction of manual rotation, whereas that of object stimuli is equally affected by either direction of manual rotation (Sack, Lindner, & Linden, 2007).

Meanwhile, neuroimaging data have elucidated the neural bases of the two types of mental rotations. While some studies have reported the activation of motor areas during the mental rotation of body parts but not objects (e.g., de Lange, Hagoort, & Toni, 2005; Kosslyn et al., 1998), others have shown this activation in object-based mental rotation processes as well (e.g., Eisenegger, Herwig, & Jäncke, 2007; Lamm, Windischberger, Moser, & Bauer, 2007; Richter et al., 2000; Zacks & Michelon, 2005). To interpret the involvement of the motor system in mental rotation of objects, some researchers have reported that participants may use a strategy of motor simulation in the mental rotation of objects (Zacks, 2008; Zacks & Michelon, 2005). Specifically, participants may simulate rotating the object with their hands (Zacks, 2008, p. 4). Briefly, as the mental rotation of an object is implemented, a mental motor manipulation of handling the object is simultaneously performed.

Kosslyn, Thompson, Wraga, and Alpert (2001) provided clear evidence for this suggestion. They asked participants to either view an object being rotated by an electric motor or rotate the object manually. Subsequently, the two groups of participants were asked to perform the classic mental rotation task, imagining the rotation as they had seen previously. The results showed that the imagination of manual activity initiated activation in motor areas during mental rotation. In contrast, imagining objects rotating as a consequence of an external force did not initiate activation in primary motor cortex. This study provided evidence not only that motor simulation is involved in certain object-based mental rotation tasks but also that the motor simulation of endogenous force is only adopted in part of the situation, not all. Although the authors indicated that people can voluntarily adopt one or the other method, we are inclined to think that motor simulation is more likely to be initiated in certain situations, such as the priming of manual activity. A subsequent study (de Vignemont et al., 2006) provided further evidence for this postulation. In this study, gloves were used as the stimuli to be mentally rotated, and participants were explicitly instructed to place their hands in the gloves. The comparison between the glove condition and hand condition showed no difference, indicating that merely presenting a hand-like stimulus is sufficient to activate motor movement.

Although these findings indicate that motor simulation of hands is possibly involved in the mental rotation of objects, many questions remain about the operational process of motor simulation. Is the object mentally rotated as if it were being carried by the hand? Does the simulated movement of the hand have the same trajectory of mental rotation? Is this trajectory totally decided by the deliberate mental rotation of the object, or will the constraints of hand movement also play a role?

In this study, we used an image of a hand as the background of the object to be rotated to directly determine whether a hand image can be automatically rotated during an object rotation task and whether it is rotated in the same trajectory as the object. Consider a letter written on the hand. We might rotate our hand physically to recognise it, and the letter on the hand will rotate simultaneously in the same direction and with the same speed. We could also keep our hand still while mentally rotating the letter. Based on the aforementioned motor simulation theory of mental rotation, we believe that the first strategy will be selected, even when the hand is a background image and can only be mentally rotated. Briefly, the mental rotation of the hand image will be automatically activated. Several studies have found the automatic encoding of hand or foot (Ottoboni, Tessari, Cubelli, & Umiltà, 2005; Tessari, Ottoboni, Baroni, Symes, & Nicoletti, 2012), so an automatic hand mental rotation would be likely to happen in this study.

Furthermore, we were concerned about the trajectories of the letter and hand. Will the letter and hand be mentally rotated together, as during physical rotation? Or will they rotate separately, following their respective shortest paths and constraints? We aimed to answer these questions to contribute to the development of the motor simulation hypothesis and to help elucidate the detailed processes of motor simulation.

Method

Participants

In total, 41 undergraduate and graduate students participated in the experiment (age: 17-26 years, 18 men and 23 women). Participants were recruited through advertisements placed on the student forum of Beijing Normal University. All participants were right-handed, with normal or corrected-to-normal vision. No participant had previous experience with mental rotation experiments.

Stimuli and design

The stimuli consisted of 128 pictures of a human hand with the letter “R” printed in the centre of the palm (see Figure 1). The hand images were photographs of a left or right human hand in the palm view against a black background. The images were presented on a 17-in monitor using E-prime 1.1 software, subtending a visual angle of approximately 11.0^°.

Figure 1.

Hand images with a letter presented in eight orientations labelled from 0^° to 315^° in the clockwise direction for left-hand stimuli (upper left) and in the anticlockwise direction for right-hand stimuli (upper right). Hand images were presented in four orientations: 0^°, 90^°M, 180^°, or 90^°L for both left (lower left) and right hands (lower right).

Hand images were presented in four orientations: 0^° (fingers pointing upwards), 90^°M (fingers pointing in the medial direction and the thumb pointing upwards), 90^°L (fingers pointing in the lateral direction and the thumb pointing downwards), and 180^° (fingers pointing downwards; Bläsing, Brugger, Weigelt, & Schack, 2013). The letters printed on the palms were normal or mirrored and were in one of eight different orientations (range: 0^°-315^°, at intervals of 45^°). For convenience in data analysis, the rotation of letters on left-hand stimuli was labelled clockwise and that of letters on right-hand stimuli was labelled anticlockwise. Therefore, for both left- and right-hand stimuli, the letters were labelled as 0^° to 315^° according to the direction from normal upright (0^°) to pointing towards the mid-sagittal plane of the body (90^°) and continuing for a complete 360^° rotation. For each participant, each of these 128 stimuli (hand laterality: 2 × hand orientation: 4 × letter type: 2 × letter orientation: 8) was presented four times and in a completely random order.

Procedure

The task was to decide whether the letter was normal or mirrored by pressing one of the two keys on a keyboard as quickly and accurately as possible. For half of the participants, the “F” key was used to signal normal letters and the “J” key to signal mirrored letters. For the other half of the participants, the response keys were reversed.

Each trial began with a fixation cross displayed in the centre of the screen for 800 ms, followed by a stimulus. The stimulus remained on the screen until the participant responded; the inter-stimulus interval was 500 ms. Each participant first performed eight practice trials randomly selected from all the stimuli. After practising, each participant completed 512 test trials (four repetitions of the 128 different stimuli). The entire procedure lasted for approximately 30 min.

Data analysis

Preliminary analyses showed no significant main effect or interaction of hand laterality and letter type. To address the main question of this study of whether the hand was mentally rotated with the object, we examined the effects of hand and letter orientations on data analyses, collapsing the data across hand laterality (left or right) and letter type (normal or mirrored). For each hand and letter orientation combination, we calculated the mean RT of correct responses and the error rates for every participant. The RTs and error rates were then analysed using 4 (hand orientation) × 8 (letter orientation) repeated-measures analyses of variance (ANOVAs). Any interactions found in these ANOVAs were further analysed by separate repeated-measures ANOVAs, with hand orientation as a within-subjects factor for each letter orientation.

To further verify potential anatomical constraints in the task, we examined the symmetry of mental rotation. We conducted paired-sample t-tests to compare the RTs and error rates between letters rotated the same degrees in counter directions, such as letters tilted 45^° and 315^°, for each hand orientation condition.

Results

Data of two participants were eliminated from analyses: one for an extremely high error rate (68.90%) and the other for slow response speed with RTs longer than 3 SDs from the mean of the sample. Thus, data from 39 participants were included in the final analysis. The mean accuracy of the remaining sample was 95.92%.

RTs of correct responses

We calculated the mean RTs of correct responses for every participant in each condition (see Table 1). We then used the RTs as the dependent variable in a repeated-measures ANOVA, with hand and letter orientations as within-subjects factors. First, as expected, a main effect of letter orientation was found, F(1.73, 65.82) = 84.78, p < .001, $η_{p}^{2} = . 69$ . The RTs increased from 0^° to 180^° and decreased from 180^° to 315^° (see Figure 2). The data were consistent with the classic results of mental rotation, suggesting that participants mentally rotated the letter from its orientation by the shortest route to the upright (Cooper & Shepard, 1973; Shepard & Judd, 1976). Second, the effect of letter mental rotation was modulated by hand orientation. The interaction between hand and letter orientations was significant, F(7.49, 284.54) = 3.24, p = .002, $η_{p}^{2} = . 08$ . However, the main effect of hand orientation was not significant, F(2.36, 89.67) = 1.68, p = .19, $η_{p}^{2} = . 04$ .

Table 1.

Mean reaction time (ms) and standard deviation by hand and letter orientations.

	0^°		90^°M		180^°		90^°L
	M	SD	M	SD	M	SD	M	SD
0^°	704.45	107.13	728.54	165.00	749.88	118.98	719.50	134.85
45^°	751.52	146.40	749.86	199.16	743.40	144.84	758.67	151.89
90^°	867.89	201.59	865.50	193.41	884.26	229.57	928.16	227.28
135^°	1,232.23	439.11	1,098.84	350.87	1,105	361.59	1,208.42	376.48
180^°	1,402	476.61	1,485.19	682.83	1,433.25	513.77	1,386.89	431.04
225^°	1,072.84	317.96	1,197.76	342.10	1,152.80	350.92	1,096.84	335.20
270^°	852.50	194.50	914.31	224.57	926.28	211.37	840.20	165.19
315^°	757.79	184.78	790.02	209.10	822.59	265.77	741.38	153.23

Figure 2.

Mental rotation of letters is modulated by hand orientation. When the letter orientation was 135^°, participants reacted slower when the hands were upright or 90^°L than when the hands were upside down or 90^°M. In contrast, when the letter orientation was 225^° or 270^°, participants reacted slower when the hands were upside down or 90^°M than when the hands were upright or 90^°L.

To further analyse the interaction between hand and letter orientations, we tested the effect of hand orientation for each letter orientation. Separate repeated-measures ANOVAs were performed, with hand orientation as the within-subjects factor. When letter orientations were closer to upright (0^°, 45^°, 90^°, and 315^°) and when the letter was upside down (180^°), the effects of hand orientation were not significant, Fs < 2.89, ps >.05. The effect of hand orientation was only significant when involving a longer letter mental rotation route: F(2.04, 77.56) = 4.60, p = .01, $η_{p}^{2} = . 11$ for 135^° letter orientation; F(3, 114) = 4.30, p = .007, $η_{p}^{2} = . 10$ for 225^° letter orientation; F(3, 114) = 6.01, p = .001, $η_{p}^{2} = . 14$ for 270^° letter orientation. Least significant difference (LSD) pair-wise comparisons revealed that for these three letter orientation conditions, the effects of hand orientation were based on the differences between the RTs in upright and 90^°L hands and those in upside down and 90^°M hands, ps < .05. However, the directions of these differences were not consistent. When the letter orientation was 135^°, participants reacted slower when the hands were upright or 90^°L than when the hands were upside down or 90^°M. In contrast, when the letter orientation was 225^° or 270^°, participants reacted slower when the hands were upside down or 90^°M than when the hands were upright or 90^°L.

To determine the symmetry of mental rotation, we compared the RTs between letters rotated the same angle in opposite directions (e.g., 45^° and 315^°, tilted towards and away from the mid-sagittal plane by 45^°). Paired-sample t-tests revealed that when the hands were upright, 90^°L, and 90^°M, participants yielded different RTs for letters tilted 135^° inwards (towards the mid-sagittal plane) and outwards (225^°), ts > 2.73, ps < .01. When the hands were upright or 90^°L, participants showed faster mental rotations for letters tilted 225^°. In contrast, when the hands were 90^°M, participants showed faster mental rotations for letters tilted 135^° (see Figure 3). In addition, when the hands were 90^°L, participants’ mental rotations were faster for letters tilted 270^° than for those tilted 90^°, t(38) = 3.43, p < .01.

Figure 3.

Asymmetry of the RT pattern when the hand is 0^°, 90^°M, and 90^°L. Although letters rotated 135^° and 225^° are symmetric about the vertical axis, participants showed faster mental rotations for letters tilted 225^° when the hand in the background was 0^° or 90^°L and for letters tilted 135^° when the hand was 90^°M. Here, we present left-hand stimuli as examples to clarify these conditions.

Error rates

Table 2 presents the mean values and standard deviations of error rates. We performed a two-way ANOVA of error rates, with hand and letter orientations as within-subjects factors. While the main effect of hand orientation was not significant, F(3, 114) = 0.78, p = .51, $η_{p}^{2} = . 02$ , the main effect of letter orientation was significant, F(1.71, 64.94) = 26.88, p < .001, $η_{p}^{2} = . 41$ . Error rates increased as letter orientation increased from 45^° to 180^° and decreased from letter orientation of 180^° to 270^°, consistent with the pattern of mental rotation. The interaction between letter and hand orientations also reached significance, F(9.45, 359.16) = 2.13, p = .02, $η_{p}^{2} = . 05$ . Separate repeated-measures ANOVAs for each letter orientation condition showed a main effect of hand orientation only when the letter was tilted 135^°, F(2.43, 92.30) = 5.56, p = .003, $η_{p}^{2} = . 13$ . Participants made more errors when the hands were upright or 90^°L than when the hands were upside down or 90^°M.

Table 2.

Mean error rate and standard deviation by hand and letter orientations.

	0^°		90^°M		180^°		90^°L
	M (%)	SD (%)	M (%)	SD (%)	M (%)	SD (%)	M (%)	SD (%)
0^°	0.46	1.62	0.46	2.13	1.38	2.91	0.77	2.03
45^°	0.77	2.03	1.56	3.08	1.69	3.06	0.77	2.45
90^°	2.51	4.19	2.05	4.15	2.33	4.37	1.90	4.55
135^°	7.95	10.37	4.26	5.96	6.03	7.08	10.05	10.90
180^°	10.64	11.72	12.13	12.28	10.54	12.80	11.49	13.15
225^°	6.46	7.54	7.56	9.19	7.28	8.40	6.21	8.62
270^°	1.59	4.01	1.74	3.52	3.03	4.34	1.87	3.76
315^°	0.92	2.19	1.85	3.12	2.33	4.14	1.26	3.23

Concerning the symmetry of mental rotation, paired-sample t-tests revealed that when the hands were 90^°M, participants made more errors when the letter was tilted 225^° than when it was tilted 135^°, t (38) = 2.70, p = .01. When the hands were 90^°L, participants made more errors when the letter was tilted 135^° than when it was tilted 225^°, t (38) = 2.66, p = .01.

Briefly, speed and accuracy showed similar trends in the task.

Discussion

Using stimuli of a letter printed on a palm, this study examined the potential activation of the mental rotation of a body part background image. Our results reveal that the hand was automatically mentally rotated, along with the deliberate mental rotation of the letter. Furthermore, the RTs depended on the biomechanical constraints of the hand, consistent with previous findings in hand laterality tasks (Parsons, 1994; Sekiyama, 1982; ter Horst et al., 2010). Briefly, in this study, the hand was mentally rotated as if it were the participants’ own hand, and the motor system was involved in the concomitant mental rotation of the hand.

The main effect of letter orientation showed that RTs and error rates increased from 0^° to 180^° and decreased from 180^° to 315^° letter orientations. This pattern indicated that participants followed the instructions and mentally rotated the letters on the hand. The RT plots of the mental rotations of the letter were influenced by the existence of the hand as a background image. In this study, the RT plots were not symmetric as in previous findings of object mental rotation (e.g., Cooper & Shepard, 1973; Shepard & Metzler, 1971), indicating that mental rotations inwards and outwards were not of equal speed. In addition, the four RT plots were not identical but showed distinct characteristics in accordance with the specific postures of the hand. Because the task in this study was to decide whether the letter was normal or mirrored, participants had no need to manipulate the hand. Our results provide strong evidence that the hands in the background interfered with the course of letter mental rotation. The hands were automatically mentally rotated along with the letter, consistent with the motor simulation theory of object mental rotation (Zacks, 2008; Zacks & Michelon, 2005). According to this theory, an imagined hand would automatically be mentally manipulated as the object is mentally rotated. The deviation from classic symmetric plots found in our study deserves attention because it indicates that the classic task of the mental rotation of objects might not involve the same mental manipulation of the hand as in this study. Rather, in the classic tasks, the objects might be imagined as rotating because of an external force, as suggested by Kosslyn et al (2001).

How were the hands mentally rotated in this study? We first assume that the hand was mentally rotated separately from the letter, aiming towards its canonical view. If this were true, then the mental rotation of the hand would intervene in the performance of the object mental rotation, just as physically rotating the hand would (Schwartz & Holton, 2000; Wexler, Kosslyn, & Berthoz, 1998; Wohlschläger & Wohlschläger, 1998). Briefly, when the two mental rotations are in the same direction, the RTs would decrease, whereas when the directions conflict, the RTs would increase. However, the present findings do not support the assumption of separate rotation paths of the hand and letter. The 0^° hand, which was upright and did not need to be rotated, did not produce a symmetric RT curve. Inward rotations of the letter were faster than outward rotations, although only under the large degree conditions. Therefore, we speculate that a conjunctive mental rotation occurred. Briefly, the hand and letter were not mentally rotated separately, but in the same direction and with identical speed.

This leads us to the next question: Is the rotation path decided by the deliberate mental rotation of the letter while the image of the hand passively rotates along the same trajectory? Or is the mental rotation of the hand subject to biomechanical constraints in conjunctive mental rotation? Our results indicate the latter. When the letter is mentally rotated 135^° outwards (135^° tilted initially), outward rotation would be comfortable for a hand pointed downwards or in the medial direction but uncomfortable for a hand pointed upwards or in the lateral direction. Participants showed slower speed and higher error rates in the uncomfortable hand orientations than in the comfortable hand orientations. Similarly, when the letter is mentally rotated 135^° inwards (225^° tilted initially), rotating inwards would be uncomfortable for a hand pointing downwards or in the medial direction. We found that these hand orientation conditions induce much slower speed than do hands pointing upwards or in the lateral direction, for which rotating inwards would be comfortable. These findings indicate that hand rotation resulted in constraints in the process of object mental rotation.

Furthermore, the asymmetry of RT plots provides converging evidence for the effect of hand movement constraints. When the hand’s initial orientation points in the medial or lateral direction, it should be easier to mentally rotate the hand through the upper semicircle. Our findings are consistent with such hand movement constraints. When the hand pointed inwards, participants showed shorter RTs and lower error rates for letters mentally rotated outwards (135^° tilted initially) than for letters mentally rotated the same degree inwards (225^° tilted initially). When the hand pointed outwards, participants showed faster RTs and lower error rates for letters mentally rotated inwards (225^° and 270^° tilted initially). These faster and more accurate paths all involved hand rotation in the upper semicircle. In addition, when the hand’s initial orientation is pointing upright, it should be easier to mentally rotate the hand inwards according to hand movement constraints (Parsons, 1994), which is consistent with our results. The letter that was mentally rotated inwards (225^° tilted initially) corresponded with a faster RT than did the letter that was mentally rotated outwards (135^° tilted initially). Therefore, convergent evidence from our study supports the assumption that the hand as a background image is mentally rotated spontaneously along with the letter, but the constraints of hand movement affect the process of object mental rotation.

Our results are consistent with the assumption that motor simulation occurs during mental rotation (Zacks, 2008). Although the hands were presented as the background to the object in our study, they were mentally rotated automatically, possibly activating the motor area. Furthermore, the hands were mentally rotated together with the object in the same trajectory of rotation, as if the hand carries or holds the object in its movement, as described in the motor simulation theory.

One question emerged: When does the hand image impose its constraints on object mental rotation? As in previous studies (e.g., ter Horst et al., 2010) that used a hand image laterality judgement task, a possible scenario is that the hand image automatically prompts participants to mentally rotate their own hands from their current position to the position presented in the stimulus.¹ However, in this study, the main effect of hand orientation was not significant, suggesting that this process did not affect the RT. The biomechanical constraint effects shown in this study primarily stem from the mental rotation of the stimuli, not the “mentally putting on” process. In our experiment, the image of the hand was not relevant to the letter judgement task. Therefore, our participants did not need to mentally rotate their own hands to the initial posture as the stimuli presented at the beginning of each trial. Motor simulation was not activated until the hand in the image was automatically mentally rotated along with the letter. Briefly, the biological constraint effects of hand images occur later in the process of object mental rotation. Therefore, we propose that motor simulation was initiated when the hand image was mentally rotated into a comfortable posture. Such a process explains the null effect of initial hand orientation as well as the observed effect of mental rotation trajectory. In future studies, researchers can manipulate participants’ posture while they are undertaking the task and compare the results with those of previous studies to explore the influence of the actual hand position during conjunctive mental rotation.

In conclusion, this study provides convergent evidence that when a letter is mentally rotated, a hand image in the background will automatically be mentally rotated along with it. Furthermore, this conjunctive mental rotation was influenced by the biomechanical constraints of the hand, altering the classic RT plot of the mental rotation of objects and indicating the involvement of the motor system. Our behavioural data support the motor simulation theory and elucidate specific processes of mental rotation that have not been addressed by neuroimaging studies.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by National Natural Science Foundation of China (31500900) and the Fundamental Research Funds for the Central Universities.

Notes

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