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.     

Exploring motor and visual imagery in Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a motor neuron disease characterized by the progressive atrophy of both the first and the second motor neurons. Although the cognitive profile of ALS patients has already been defined by the occurrence of language dysfunctions and frontal deficit symptoms, it is less clear whether the degeneration of upper and lower motor neurons affects motor imagery abilities. Here, we directly investigated motor imagery in ALS patients by means of an established task that allows to examine the presence of the effects of the biomechanical constraints. Twenty-three ALS patients and 23 neurologically unimpaired participants have been administered with the (1) hand laterality task (HLT) in which participants were asked to judge the laterality of a rotated hand and the (2) mirror letter discrimination task (MLD) in which participants were asked to judge whether a rotated alphanumeric character was in its canonical or mirror-reversed form (i.e. control task). Results show that patients present the same pattern of performance as unimpaired participants at the MLD, while at the HLT, they present only partially with the effects of biomechanical constraints. Taken together, our findings provide evidences that motor imagery abilities, related to the mental simulation of an action, are affected by this progressive disease.     

Action observation improves motor imagery: specific interactions between simulative processes

In the present study, we demonstrated that observation of hand rotation had specific facilitation effects on a classical motor imagery task, the hand-laterality judgement. In Experiment 1, we found that action observation improved subjects’ performance on the hand laterality but not on the letter rotation task (stimulus specificity). In Experiment 2, we demonstrated that this facilitation was not due to mere observation of a moving hand, because it was triggered by observation of manual rotation but not of manual prehension movements (motion specificity). In Experiment 3, this stimulus- and motion-specific effect was found to be right hand-specific, compatible with left-hemispheric specialization in motor imagery but not in action observation. These data provided direct support to the idea that different simulation states, such as action observation and motor imagery, share some common mechanisms but also show specific functional differences.     

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.     

Developmental changes of the biomechanical effect in motor imagery

Motor imagery has been investigated in childhood and early adolescence, but not across adolescence stages; moreover, available evidence did not clarify whether the involvement of motor information in mental rotation of body parts becomes stronger or weaker during development. In the present study, we employed the hand laterality task to assess motor imagery in ninety-seven typically developing adolescents divided into three age groups (i.e., 11–12, 14–15, and 17–18 years); mental rotation of objects and letters were also assessed. As a specific marker of the motor involvement in mental rotation of body parts, we assessed the so-called biomechanical effect, that is, the advantage for judging hand pictures showing physically comfortable positions with respect to hand pictures showing physically impossible or awkward positions. Results demonstrated that the biomechanical effect did not significantly affect early adolescents’ performance, whereas it became significant in 14- to 15-year-old participants and even more stronger in 17- to 18-year-old participants; this pattern did not depend on an increase in processing speed to mentally rotate both corporeal and non-corporeal (objects and letters) stimuli. The present findings demonstrated that: (1) motor imagery undergoes a continuous and progressive refinement throughout adolescence, and (2) full exploitation of motor information to mentally transform corporeal stimuli can be attained in late adolescence only.     

On the link between action planning and motor imagery: a developmental study

We examined the link between action planning and motor imagery in 6- and 8-year-old children. Action planning efficiency was assessed with a bar transport task. Motor imagery and visual imagery abilities were measured using a hand mental rotation task and a number (i.e., non-body stimuli) mental rotation task, respectively. Overall, results showed that performance varied with age in all tasks, performance being progressively refined with development. Importantly, action planning performance was correlated with motor imagery, whereas no relationship was evident between action planning and visual imagery at any age. The results showed that the ability to engage sensorimotor mechanisms when solving a motor imagery task was concomitant with action planning efficiency. The present work is the first demonstration that evaluating the consequences of the upcoming action in grasping depends on children’s abilities to mentally simulate the response options to choose the most efficient grasp.     

Balancing bistable perception during self-motion

In two experiments we investigated whether bistable visual perception is influenced by passive own body displacements due to vestibular stimulation. For this we passively rotated our participants around the vertical (yaw) axis while observing different rotating bistable stimuli (bodily or non-bodily) with different ambiguous motion directions. Based on previous work on multimodal effects on bistable perception, we hypothesized that vestibular stimulation should alter bistable perception and that the effects should differ for bodily versus non-bodily stimuli. In the first experiment, it was found that the rotation bias (i.e., the difference between the percentage of time that a CW or CCW rotation was perceived) was selectively modulated by vestibular stimulation: the perceived duration of the bodily stimuli was longer for the rotation direction congruent with the subject’s own body rotation, whereas the opposite was true for the non-bodily stimulus (Necker cube). The results found in the second experiment extend the findings from the first experiment and show that these vestibular effects on bistable perception only occur when the axis of rotation of the bodily stimulus matches the axis of passive own body rotation. These findings indicate that the effect of vestibular stimulation on the rotation bias depends on the stimulus that is presented and the rotation axis of the stimulus. Although most studies on vestibular processing have traditionally focused on multisensory signal integration for posture, balance, and heading direction, the present data show that vestibular self-motion influences the perception of bistable bodily stimuli revealing the importance of vestibular mechanisms for visual consciousness.     

Does negative priming occur by rotated characters?

We conducted three experiments in order to investigate the effect of stimulus orientation on negative priming (NP). Using the picture naming task, Murray (1995) reported the occurrence of semantic NP by rotated distractors. As the rotation of picture stimuli seems to have little effect to reduce distractor interference, in the present study we used the character (katakana) identification task to ensure the effect of stimulus rotation. When the distractors were rotated (180 degrees), no NP was observed whether the targets were upright (Experiment 1) or rotated (Experiment 2). On the other hand, significant NP was observed when the distractors were upright and the targets were rotated (Experiment 3). These results suggest that the inhibitory mechanism of attention may not operate on the rotated distractor characters.     

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.     

Saccades to mentally rotated targets

 In order to investigate the role of mental rotation in the directional control of eye movements, we instructed subjects to make saccades in directions different from that of a visual stimulus (rotated saccades). Saccadic latency increased linearly with the amount of directional transformation imposed between the stimulus and the response. This supports the hypothesis that reorienting a saccade is accomplished through a mental rotation process. No differences were found in amplitude, duration, velocity, and curvature between rotated and visually guided saccades. Analogous to mental rotation tasks involving reaching arm movements, it is surmised that frontal/prefrontal cortical structures participate in rotated saccades by reorienting the intended saccadic direction. A linear increase in response time with the imposed directional transformation was also found in an analogous mental task not requiring a directed motor response, namely, mentally localizing a point in space at a certain angle from a stimulus direction. However, the speed of mental rotation was systematically lower than in the rotated saccade task. These findings indicate that mental rotation is a rather general mechanism through which directional transformations are achieved. Received: 11 May 1998 / Accepted: 27 February 1999     

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.     

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.     

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.     

Functional properties of brain areas associated with motor execution and imagery

Imagining motor acts is a cognitive task that engages parts of the executive motor system. While motor imagery has been intensively studied using neuroimaging techniques, most studies lack behavioral observations. Here, we used functional MRI to compare the functional neuroanatomy of motor execution and imagery using a task that objectively assesses imagery performance. With surface electromyographic monitoring within a scanner, 10 healthy subjects performed sequential finger-tapping movements according to visually presented number stimuli in either a movement or an imagery mode of performance. We also examined effects of varied and fixed stimulus types that differ in stimulus dependency of the task. Statistical parametric mapping revealed movement-predominant activity, imagery-predominant activity, and activity common to both movement and imagery modes of performance (movement-and-imagery activity). The movement-predominant activity included the primary sensory and motor areas, parietal operculum, and anterior cerebellum that had little imagery-related activity (-0.1 ~ 0.1%), and the caudal premotor areas and area 5 that had mild-to-moderate imagery-related activity (0.2 ~ 0.7%). Many frontoparietal areas and posterior cerebellum demonstrated movement-and-imagery activity. Imagery-predominant areas included the precentral sulcus at the level of middle frontal gyrus and the posterior superior parietal cortex/precuneus. Moreover, activity of the superior precentral sulcus and intraparietal sulcus areas, predominantly on the left, was associated with accuracy of the imagery task performance. Activity of the inferior precentral sulcus (area 6/44) showed stimulus-type effect particularly for the imagery mode. A time-course analysis of activity suggested a functional gradient, which was characterized by a more "executive" or more "imaginative" property in many areas related to movement and/or imagery. The results from the present study provide new insights into the functional neuroanatomy of motor imagery, including the effects of imagery performance and stimulus-dependency on brain activity.     

Differing roles for the dominant and non-dominant hands in the hand laterality task

Determining the handedness of visually presented stimuli is thought to involve two separate stages--a rapid, implicit recognition of laterality followed by a confirmatory mental rotation of the matching hand. In two studies, we explore the role of the dominant and non-dominant hands in this process. In Experiment 1, participants judged stimulus laterality with either their left or right hand held behind their back or with both hands resting in the lap. The variation in reactions times across these conditions reveals that both hands play a role in hand laterality judgments, with the hand which is not involved in the mental rotation stage causing some interference, slowing down mental rotations and making them more accurate. While this interference occurs for both lateralities in right-handed people, it occurs for the dominant hand only in left-handers. This is likely due to left-handers' greater reliance on the initial, visual recognition stage than on the later, mental rotation stage, particularly when judging hands from the non-dominant laterality. Participants' own judgments of whether the stimuli were 'self' and 'other' hands in Experiment 2 suggest a difference in strategy for hands seen from an egocentric and allocentric perspective, with a combined visuo-sensorimotor strategy for the former and a visual only strategy for the latter. This result is discussed with reference to recent brain imaging research showing that the extrastriate body area distinguishes between bodies and body parts in egocentric and allocentric perspective.     

Action‐associated modulation of visual event‐related potentials evoked by abstract and ecological stimuli

This study investigated the influence of action‐associated predictive processes on visual ERPs. In two experiments, we sought evidence for sensory attenuation (SA) indexed by ERP amplitude reductions for self‐induced stimuli when compared to passive viewing of the same images. We assessed if SA is (a) present for both ecological and abstract stimuli (pictures depicting hands or checkerboards), (b) modulated by the degree of stimulus predictability (certain or uncertain action‐effect contingencies), and (c) sensitive to laterality of hand movements (dominant or subdominant hand actions). We found reduced occipital responses in the early 77–90 ms time interval (C1 component), irrespective of stimulus type, predictability, or the laterality of hand movements. However, the subsequent P1 component was increased (rather than reduced) for all action‐associated stimuli. In addition, this P1 effect was influenced by the degree of stimulus predictability for ecological stimuli only. Finally, the posterior N1 component was not modulated by self‐initiated actions. Overall, our findings indicate that movement‐related predictive processes attenuate early visual responses. Moreover, we propose that amplitude modulations in the P1 time range reflect the interaction between expectation‐based SA and attention‐associated amplitude enhancements. These results can have implications for assessing the influence of action‐associated predictions on visual processing in psychiatric disorders characterized by aberrant sensory predictions and alterations in hemispheric asymmetry, such as schizophrenia.     

What drives children’s limb selection for reaching in hemispace?

Arguably, the act of reaching constitutes one of the most devoted lines of contemporary developmental research. In addition to the underlying dynamical characteristics of motor coordination, a key element in programming is limb selection, a phenomenon (handedness) that has so far resisted any reasonable unified explanation. From a more contemporary view, two factors appear to have the most influence on hand selection for a given task: motor dominance and attentional information related to task demands. This study was designed to determine what factor(s) influence choice of limb for reaching in hemispace in reference to motor dominance, object proximity, and a hemispheric bias favoring use of the hand on the same side as the stimulus. Strong right-handed children were asked to reach and retrieve a small object across right and left hemispace locations beginning with the arms uncrossed and arms-crossed. With the arms-crossed condition, an imagined and actual movement execution was administered. Results from the uncrossed condition supported previous reported findings for adults and children. That is, participants responded ipsilaterally using the hand on the same side as the stimulus, thus supporting the case for object proximity and hemispheric bias. However, in the arms-crossed condition the vast majority of participants preferred keeping the limbs crossed in response to right and left hemispace stimuli, which leads to the suggestion that object proximity rather than hemispheric bias was the driving factor in this context. The behavioral pattern for imagined and actual movement was not significantly different. Overall, the findings add to the growing acceptance that limb selection is task and context dependent, rather than a biologically based invariant feature of motor behavior.     

Vibrotactile discriminative capacity is impacted in a digit-specific manner with concurrent unattended hand stimulation

A number of perceptual and neurophysiological studies have investigated the effects of delivering unilateral versus bilateral tactile sensory stimulation. While a number of studies indicate that perceptual discrimination degrades with opposite-hand stimulation, there have been no reports that examined the digit specificity of cross-hemispheric interactions to discriminative capabilities. The purpose of this study was to determine whether unattended hand (UH) stimulation significantly degraded or improved amplitude discriminative capacity on the attended hand (AH) in a digit-specific manner. The methods are based on a sensory perceptual task (vibrotactile amplitude discriminative capacity on the tips of the fingers D2 and D3 of the left hand) in the absence and presence of conditioning stimuli delivered to D2 and D3 of the right hand. Non-specific equal-amplitude stimulation to D2 and D3 of the UH significantly worsened amplitude discrimination (AD) performance, while delivering unequal-amplitude stimuli to D2 and D3 of the UH worsened task performance only under the condition in which the unattended stimuli failed to appropriately match the stimulus parameters on the AH. Additionally, delivering single-site stimuli to D2 or D3 of the UH resulted in degraded performance on the AD task when the stimulus amplitude did not match the amplitude of the stimulus applied to homologous digits of the AH. The findings demonstrate that there is a reduction in performance under conditions where UH stimulation least matched stimulation applied to the AH, while there was little or no change in performance when stimulus conditions on the homologous digit(s) of the contralateral sites were similar. Results suggest that bilateral interactions influence perception in a context-dependent manner that is digit specific.