Judging hand laterality from my or your point of view: Interactions between motor imagery and visual perspective

Motor imagery tasks (hand laterality judgment) are usually performed with respect to a self-body (egocentric) representation, but manipulations of stimulus features (hand orientation) can induce a shift to other's body (allocentric) reference frame. Visual perspective taking tasks are also performed in self-body perspective but a shift to an allocentric frame can be triggered by manipulations of context features (e.g., another person present in the to-be-judged scene). Combining hand laterality task and visual perspective taking, we demonstrated that both stimulus and context features can modulate motor imagery performance. In Experiment 1, participants judged laterality of a hand embedded in a human or non-human silhouette. Results showed that observing a human silhouette interfered with judgments on “egocentric hand stimuli” (right hand, fingers up). In Experiment 2, participants were explicitly required to judge laterality of a hand embedded in a human silhouette from their own (egocentric group) or from the silhouette's perspective (allocentric group). Consistent with previous results, the egocentric group was significantly faster than the allocentric group in judging fingers-up right hand stimuli. These findings showed that concurrent activation of egocentric and allocentric frames during mental transformation of body parts impairs participants’ performance due to a conflict between motor and visual mechanisms.     

Numbers are represented in egocentric space: Effects of numerical cues and spatial reference frames on hand laterality judgements

Convergent findings demonstrate that numbers can be represented according to a spatially oriented mental number line. However, it is not established whether a default organization of the mental number line exists (i.e., a left-to-right orientation) or whether its spatial arrangement is only the epiphenomenon of specific task requirements. To address this issue we performed two experiments in which subjects were required to judge laterality of hand stimuli preceded by small, medium or large numerical cues; hand stimuli were compatible with egocentric or allocentric perspectives. We found evidence of a left-to-right number–hand association in processing stimuli compatible with an egocentric perspective, whereas the reverse mapping was found with hands compatible with an allocentric perspective. These findings demonstrate that the basic left-to-right arrangement of the mental number line is defined with respect to the body-centred egocentric reference frame.     

Whose hand is this? Handedness and visual perspective modulate self/other discrimination

We required healthy subjects to recognize visually presented one’s own or others’ hands in egocentric or allocentric perspective. Both right- and left-handers were faster in recognizing dominant hands in egocentric perspective and others’ non-dominant hand in allocentric perspective. These findings demonstrated that body-specific information contributes to sense of ownership, and that the “peri-dominant-hand space” is the preferred reference frame to distinguish self from not-self body parts.     

Brain activity associated with recognition of appropriate action selection based on allocentric perspectives

We investigated brain activity associated with recognition of appropriate action selection based on allocentric perspectives using functional magnetic resonance imaging. The participants observed video clips in which one person (responder) passed one of three objects after a request by a second person (requester). The requester was unable to see one of the three objects because it was occluded by another object. Participants were asked to judge the appropriateness of the responder's action selection based on the visual information from the requester's perspective (i.e., allocentric perspective), not the responder's perspective (i.e., egocentric perspective). The experimental factors included the congruency of request interpretation and the appropriateness of action selection. The results showed that brain regions including the right temporo-parieto-occipital (TPO) junction and the left inferior parietal lobule (IPL) were more activated when the interpretation of the requested object differed between the egocentric and allocentric perspectives than when it was the same (the effect of incongruency for consistency). On the other hand, greater activation was found in the right dorsolateral prefrontal cortex (DLPFC) when the incongruency effect was compared only between the conditions of appropriate action selection (the interaction effect). These results suggest that both the TPO junction and IPL are involved in obtaining visual information from the allocentric perspective when visual information based on only the egocentric perspective is insufficient to interpret another person's request. The right DLPFC is likely related to this process to override the interference of action selection based on the egocentric perspective.     

Implementation of specific motor expertise during a mental rotation task of hands

Mental rotation of the hands classically induces kinesthetic effects according to the direction of the rotation, with faster response times to the hands’ medial rotations compared with lateral rotations, and is thus commonly used to induce engagement in motor imagery (MI). In the present study, we compared the performances of table tennis players (experts on hand movements), who commonly execute and observe fast hand movements, to those of soccer players (non-experts on hand movements) on a mental rotation task of hands. Our results showed a significant effect of the direction of rotation (DOR) confirming the engagement of the participants in MI. In addition, only hand movement experts were faster when the task figures corresponded to their dominant hand compared with the non-dominant hand, revealing a selective effect of motor expertise. Interestingly, the effect of the DOR collapsed in hand movement experts only when the task figures corresponded to their dominant hand, but it is noteworthy that lateral and medial rotations of the right-hand stimuli were not faster than medial rotations of the left-hand stimuli. These results are discussed in relation to possible strategies during the task. Overall, the present study highlights the embodied nature of the mental rotation task of hands by revealing selective effects of motor expertise.     

Differential influence of hands posture on mental rotation of hands and feet in left and right handers

The representation of the body in the brain is continuously updated with regard to peripheral factors such as position or movement of body parts. In the present study, we investigated the effects of arm posture on the mental rotation of hands and feet. Sixteen right-handed and ten left-handed participants verbally judged the laterality of visually presented pictures of hands and feet in two different postural conditions. In one condition they placed their right hand on their right knee and their left hand behind the back, in the other condition the hand position was reversed. For right-handed participants response times for the laterality judgment of right hands increased when participants kept their right hand behind the back. This was not found for images of the left hand nor for images of the feet. For the left-handed participants, there was no effect of arm posture on hand or feet stimulus judgments. Thus, the body-part posture effect on mental rotation was found to be specific for the side and the body part for which the posture was modified only in right-handed participants, but it was absent for left-handed participants. For both samples, we also found a progressive disruption of the mental rotation function depending on the view from which the body parts were seen (i.e. dorsal, thumb/big toe, palm/plantar, little finger/toe). Posture and view effects on body parts representations are discussed with respect to proprioception, handedness, visual familiarity and the influence of anatomical joint constraints on motor imagery.     

Giving a helping hand: effects of joint attention on mental rotation of body parts

Research on joint attention has addressed both the effects of gaze following and the ability to share representations. It is largely unknown, however, whether sharing attention also affects the perceptual processing of jointly attended objects. This study tested whether attending to stimuli with another person from opposite perspectives induces a tendency to adopt an allocentric rather than an egocentric reference frame. Pairs of participants performed a handedness task while individually or jointly attending to rotated hand stimuli from opposite sides. Results revealed a significant flattening of the performance rotation curve when participants attended jointly (experiment 1). The effect of joint attention was robust to manipulations of social interaction (cooperation versus competition, experiment 2), but was modulated by the extent to which an allocentric reference frame was primed (experiment 3). Thus, attending to objects together from opposite perspectives makes people adopt an allocentric rather than the default egocentric reference frame.     

Egocentric and allocentric localization during induced motion

This research examined motor measures of the apparent egocentric location and perceptual measures of the apparent allocentric location of a target that was being seen to undergo induced motion (IM). In Experiments 1 and 3, subjects fixated a stationary dot (IM target) while a rectangular surround stimulus (inducing stimulus) oscillated horizontally. The inducing stimulus motion caused the IM target to appear to move in the opposite direction. In Experiment 1, two dots (flashed targets) were flashed above and below the IM target when the surround had reached its leftmost or rightmost displacement from the subject’s midline. Subjects pointed open-loop at either the apparent egocentric location of the IM target or at the bottom of the two flashed targets. On separate trials, subjects made judgments of the Vernier alignment of the IM target with the flashed targets at the endpoints of the surround’s oscillation. The pointing responses were displaced in the direction of the previously seen IM for the IM target and to a lesser degree for the bottom flashed target. However, the allocentric Vernier judgments demonstrated no perceptual displacement of the IM target relative to the flashed targets. Thus, IM results in a dissociation of egocentric location measures from allocentric location measures. In Experiment 2, pointing and Vernier measures were obtained with stationary horizontally displaced surrounds and there was no dissociation of egocentric location measures from allocentric location measures. These results indicate that the Roelofs effect did not produce the pattern of results in Experiment 1. In Experiment 3, pointing and Vernier measures were obtained when the surround was at the midpoint of an oscillation. In this case, egocentric pointing responses were displaced in the direction of surround motion (opposite IM) for the IM target and to a greater degree for the bottom flashed target. However, there was no apparent displacement of the IM target relative to the flashed targets in the allocentric Vernier judgments. Therefore, in Experiment 3 egocentric location measures were again dissociated from allocentric location measures. The results of this experiment also demonstrate that IM does not generate an allocentric displacement illusion analogous to the “flash-lag” effect.

Assessing the effect of posture change on tactile inhibition-of-return

If a peripheral target follows an ipsilateral cue with a stimulus-onset-asynchrony (SOA) of 300 ms or more, its detection is delayed compared to a contralateral-cue condition. This phenomena, known as inhibition-of-return (IOR), affects responses to visual, auditory, and tactile stimuli, and is thought to provide an index of exogenous shifts of spatial attention. The present study investigated whether tactile IOR occurs in a somatotopic vs an allocentric frame of reference. In experiment 1, tactile cue and target stimuli were presented to the index and middle fingers of either hand, with the hands positioned in an uncrossed posture (SOA 500 or 1,000 ms). Speeded target detection responses were slowest for targets presented from the cued finger, and were also slower for targets presented to the adjacent finger on the cued hand than to either finger on the uncued hand. The same pattern of results was also reported when the index and middle fingers of the two hands were interleaved on the midline (experiment 2), suggesting that the gradient of tactile IOR surrounding a cued body site is modulated by the somatotopic rather than by the allocentric distance between cue and target.     

Mental rotation of kinesthetic hand images and modes of stimulus presentation

Three experiments on mental rotation were carried out to investigate conditions under which hand images are operated kinesthetically. In Experiment 1 a, either a left or right hand was presented in a photographic slide, and subjects' task was left-right identification. In Exp. 2, each slide consisted of two hands, identical hands or mirror-imaged hands, and same-different judgment was required. In Exp. 3, two hands were presented successively, requiring same-different (mirror-reversed) judgement. On the other hand, subjects in Exp. 1 b were asked to rate physical difficulty of actual hand movements to imitate stimuli. Six to 12 undergraduate students served as subjects in each experiment. The results suggested that subjects' mental operations of hand images were kinesthetic in Exp. 1 a but visual in Exp. 2 and 3, on the basis of comparison between reaction times in the three experiments and the ratings in Exp. 1 b. Conditions which give rise to kinesthetic image processes were argued in relation with task structures.     

Mental rotation task of hands: differential influence number of rotational axes

Various studies on the hand laterality judgment task, using complex sets of stimuli, have shown that the judgments during this task are dependent on bodily constraints. More specific, these studies showed that reaction times are dependent on the participant’s posture or differ for hand pictures rotated away or toward the mid-sagittal plane (i.e., lateral or medial rotation, respectively). These findings point to the use of a cognitive embodied process referred to as motor imagery. We hypothesize that the number of axes of rotation of the displayed stimuli during the task is a critical factor for showing engagement in a mental rotation task, with an increased number of rotational axes leading to a facilitation of motor imagery. To test this hypothesis, we used a hand laterality judgment paradigm in which we manipulated the difficulty of the task via the manipulation of the number of rotational axes of the shown stimuli. Our results showed increased influence of bodily constraints for increasing number of axes of rotation. More specifically, for the stimulus set containing stimuli rotated over a single axis, no influence of biomechanical constraints was present. The stimulus sets containing stimuli rotated over more than one axes of rotation did induce the use of motor imagery, as a clear influence of bodily constraints on the reaction times was found. These findings extend and refine previous findings on motor imagery as our results show that engagement in motor imagery critically depends on the used number of axes of rotation of the stimulus set.     

Cerebral Dynamics of SEPs to Non-Painful and Painful Cutaneous Electrical Stimulation of the Thenar and Hypothenar

Early, middle and late latency somatosensory evoked potentials (SEPs) elicited by cutaneous electrical stimulation (painful vs. non-painful) of right and left hands were recorded. The aims were to study (1) if lifelong use of dominant right hand would result in different SEP topographies compared to non-dominant left hand stimulation, (2) if painful and non-painful stimuli resulted in different SEP activation patterns for the different latency components and (3) if these results were consistent between two areas of the hand. Electrical stimuli were applied cutaneously above the thenar and hypothenar muscles of the left and right hand. A two-way repeated measures ANOVA was used to test the effects of laterality and intensity for a given peak amplitude and latency. Statistical results yielded no significant difference in peak amplitude for either thenar and hypothenar between the two hands. In contrast, a significant difference in amplitude was observed for 6 components for each stimulus location when the two intensities were compared. These components were found at early, middle and late latencies. No significant latency shift was observed between the two hands. Only the P30 component showed a significant latency shift for both locations with the painful condition having the shorter latency. Thus, life-long use of the dominant hand does not generate detectable changes in cortical evoked activity to sensory input from the skin above thenar and hypothenar muscles. Several SEP components across the time course (0-400 ms) showed increased amplitude when the stimulus was increased from non-painful to painful intensity.     

Self-touch affects motor imagery: a study on posture interference effect

Several studies showed that mental rotation of body parts is interfered with by manipulation of the subjects’ posture. However, the experimental manipulations in such studies, e.g., to hold one arm flexed on one’s own chest, activated not only proprioceptive but also self-tactile information. Here, we tested the hypothesis that the combination of self-touch and proprioception is more effective than proprioception alone in interfering with motor imagery. In Experiment 1 right- and left-handers were required to perform the hand laterality task, while holding one arm (right or left) flexed with the hand in direct contact with their chest (self-touch condition, STC) or with the hand placed on a wooden smooth surface in correspondence with their chest (no self-touch condition, NoSTC); in a third neutral condition, subjects kept both arms extended (neutral posture condition, NPC). Right-handers were slower when judging hand laterality in STC with respect to NoSTC and NPC, particularly when the sensory manipulation involved their dominant arm. No posture-related effect was observed in left-handers. In Experiment 2, by applying the same sensory manipulations as above to both arms, we verified that previous results were not due to a conflict between perceived position of the two hands. These data highlighted a complex interaction between body schema and motor imagery, and underlined the role of hand dominance in shaping such interaction.     

Motor imagery and visual event recognition

In order to investigate the influence of covert motor processes in the recognition of visual events, we compared the response times (RT) in two similar tasks, one involving a to-be-grasped object and the other involving a to-be-observed object. In one task, we asked right-handed subjects to tell whether an observed screwdriver presented in different orientations and rotating on its main axis was screwing or unscrewing (screwdriver task). In the other task the visual stimuli were precisely the same, but subjects had to think of the screwdriver as being the pivot pin of an imagined clock, turning its hands from the back (clock task). They had to tell whether the imagined clock hands were moving clockwise or counterclockwise. In the screwdriver task, a prominent right-left asymmetry consisting of higher RTs for stimulus orientations awkward for a right-hand grip was present, suggesting that subjects adopted a strategy based upon mentally simulating the grabbing of the screwdriver handle with the dominant hand. Consistent with the hypothesis that the crucial factor that triggers these motor imagery processes is the "graspability" of the relevant object in the scene, in the clock task the right-left asymmetry disappeared in most subjects, RTs mirroring the symmetry of the visual stimuli. These findings indicate that, when interpreting a scene involving a to-be-grasped object, a strategy based upon motor imagery (mental grasping), probably unfolding procedural knowledge, is activated. When the scene involves a to-be-observed object, the recognition task can be accomplished through other, possibly visual, strategies.     

When the left hand does not know what the left hand is doing: response mode affects mental rotation of hands

Mentally simulating a movement is known to share temporal and kinematic characteristics with the execution of the same movement, and this is thought to be reflected in the sharing of neural resources between the two activities. A powerful method of implicitly facilitating such mental simulation (or motor imagery) in individuals is to present them with a picture of a hand and ask them to identify its laterality (i.e. left or right). The mental rotation undertaken in order to complete this hand laterality recognition task (HLRT) provides an effective form of motor imagery, and the task has become an influential tool in clinical and experimental studies. However, performance on the task is modified by numerous factors, and there is a suggestion that the method of response demanded by different versions of the task may have a modulating effect. Here, we compared performance on the HLRT when responding verbally or manually in a group of unimpaired right-handed participants. For manual responses, we also compared performance when participants responded unimanually, using the index and middle fingers of their dominant or non-dominant hand. Performance was poorer for the manual compared to the verbal condition both in terms of accuracy and response time. Furthermore, for manual responses, the requirement to make a response with a specific limb selectively disrupted the ability to recognise an image of the corresponding limb. The disruption is considered to reflect difficulty in concurrently planning two actions with the same limb (manual response and mental rotation). Implications for the interpretation of existing and future studies are discussed.     

Restriction of early exploratory forays effects specific aspects of spatial processing in weanling hamsters

Normally reared hamsters, but not hamsters reared on a liquid diet, demonstrated spatial memory for the location of odor cues in an allocentric task (Experiment 1). In Experiment 2, an egocentric task, liquid-reared hamsters detected a change in the spatial location of odor cues. In Experiment 3 liquidreared hamsters detected a change in the spatial location of two visual cues under allocentric task conditions. Female hamsters on a liquid diet retrieved their pups more often than dams on solid food, resulting in reduced exploratory opportunities for their pups during the period when olfaction mediates behavior. Hamsters in Experiment 4 experienced a direct restriction of early forays. The restricted-rearing group failed to detect a change in the spatial location of odor cues in an allocentric task. These findings suggest that restriction of early exploratory experience during a narrow period of development results in specific spatial processing deficits.     

Induction of plasticity in the dominant and non-dominant motor cortices of humans

There are clear hemispheric differences in the human motor system. Studies using magnetic resonance morphometry have shown that representation of hand muscles is larger in the dominant hemisphere than the non-dominant hemisphere. There is some limited evidence of electrophysiological differences between hemispheres. For example, it has been reported recently that there is less intracortical inhibition in the dominant hemisphere than the non-dominant hemisphere, and it has been hypothesised that this reduction in inhibition may facilitate use-dependent plasticity in the dominant motor cortex. In the present study we examined this hypothesis in human subjects by examining plasticity induction in both dominant and non-dominant hemispheres using an experimental paradigm known to induce motor cortical plasticity, namely paired associative stimulation (PAS). Additionally, we investigated changes in dominant and non-dominant hand performance on a simple ballistic training task. Short-interval intracortical inhibition (SICI) was also measured for both dominant and non-dominant hands at a range of conditioning intensities. There was significantly less SICI in the dominant motor cortical hand area than in the non-dominant hand area. PAS induced a significant, and similar, increase in motor cortical excitability in both the dominant and non-dominant hemispheres. Motor training resulted in significant performance improvement in both dominant and non-dominant hands. However, there was significantly more improvement in the non-dominant hand. The results from these studies provide some further evidence of electrophysiological differences between the motor cortices of the two hemispheres. Additionally, these findings offer no support for the hypothesis that the dominant hemisphere is positioned more favourably, due to decreased inhibitory tone, than the non-dominant hemisphere for use-dependent plasticity.     

Mental rotation of primate hands: human-likeness and thumb saliency

Mental rotation of human hands has been found to differ essentially from mental rotation of objects in such a way that reaction times and error rates of handedness judgements are influenced by the comfort and familiarity of the presented hand postures. To investigate the role of the similarity of the presented hands to the participant’s own hand, we used different primates’ hands as stimuli in a mental rotation task. Five out of 24 primate hands were chosen for their ratings in human-likeness and saliency of the thumb according to a questionnaire study and presented in two mental rotation experiments; in the second experiment, they were modified in such a way that all hands appeared thumbless. Results of both experiments revealed effects of species and orientation on reaction times, and an interaction between species and hand side occurred in the second experiment. In the first experiment, the thumbless Colobus hand differed from all other hands, showing the highest reaction times and error rates and failing to show the expected medial-over-lateral advantage. In the second experiment, the eccentricity of the Colobus hand was decreased and the facilitating effect of human-likeness was slightly increased. We conclude that motor strategies were applied that relied less on the asymmetry of the stimuli but rather on their similarity to the human hand. We argue that motor simulation might facilitate the processing of incomplete stimuli by mentally completing them, especially if all stimuli can be processed in a consistent manner.     

The role of the primary motor cortex in mental rotation: a TMS study

Mental rotation (MR) is sustained by a network of brain regions, including parietal, pre-motor and primary motor (M1) cortices. However it is still not clear whether M1 is recruited only when individuals mentally rotate hands or whether it is also enhanced by MR of non-body parts. Here we report two experiments in which the involvement of M1 in MR of hands and letters was tested using TMS. In Experiments 1a and 1b participants were asked to judge whether two line drawings, depicting either hands or letters, were the same or mirror images of each other (N = 112). Subjects were presented with pairs of stimuli with the same orientation (baseline condition) in half of the trials, while in the other half the stimulus in the right visual field was rotated (rotation condition). They performed the same-different task in three experimental situations: TMS of the primary motor hand area delivered at 400?ms after stimulus onset, sham TMS, and no-TMS. We stimulated the left M1 in Experiment 1a, and the right in Experiment 1b. Results showed that in Experiment 1a participants were slower after TMS when they performed MR of hands but not of letters. In Experiment 1b we failed to find an effect of TMS on MR of hands and letters. While in Experiment 1 the stimulus to be rotated was always presented in the right visual field, in Experiment 2 it was presented either in the left or in the right visual field. Results showed that only when TMS was delivered to the left M1, participants' ability to mentally rotate right and left hands slowed down. Taken together, these findings suggest that the left but not the right M1 plays a critical role in MR of hands.