Telling right from right: the influence of handedness in the mental rotation of hands

Handedness

This chapter offers an overview of the literature on human handedness and its assessment in clinical neurologic practice and research. There are two major forms of handedness: hand preference, which describes a subjective preference to use one hand over the other for skilled motor activities like writing, and hand skill, which describes objectively measured mother skill. This chapter gives an overview of widely used questionnaires and tests to assess hand preference and hand skill, as well as suggestions on how to determine handedness categories such as left-handed, right-handed, and mixed-handed based on the results of these questionnaires and tests. Handedness is just one form of hemispheric asymmetry in the human motor system, and the chapter also provides an overview of its association with other motor asymmetries such as footedness. Moreover, the associations of handedness with functional brain activation as well as with structural markers on the cortical, subcortical, cerebellar, and spinal levels are discussed. Furthermore, the potential relevance of handedness retraining for clinical neurologic research and the association of handedness and cognitive abilities are discussed. The chapter concludes with an outlook on the critical importance of including handedness in clinical neurologic research and practice.

No Incidental Memory Advantage for Mixed Handed vs. Consistent Right Handed Participants: Conflicting Results From Earlier Research

Individuals who vary their preferred hand when performing different types of manual activities, so-called mixed handers (MH), have been frequently reported to outperform individuals with a consistent (right) hand preference (cRH) on tasks assessing declarative-memory functions. For example, in one influential study, this MH advantage extended to incidental learning from presumed "deep" semantic processing of verbal stimuli but not from "shallow" phonemic or structural processing. In the present study, we aimed to replicate this research finding in two separate participant samples. First, in a pre-registered and sample-size planned experiment we confronted 49 participants (23 MH; 26 cRH) with "phonemic" and "semantic" word evaluation tasks (using a within design), followed by a surprise delayed recognition test. In a second experiment, we repeated the same procedure with 65 other participants (31 MH, 34 cRH). A mixed-effect analyses of variance found a significant main effect of Encoding Condition (phonemic vs. semantic tasks) in both experiments (effect size: ηp2 = .81 to .85), indicating the classical level-of processing effect with higher recognition hits and sensitivity (d') for words that followed semantic versus phonemic encoding. However, the predicted interaction effect of Encoding Condition with Handedness Group was not statistically significant for either sample (all ηp2 < .03), nor was the main effect of Handedness Group. Thus, our findings conflicted with those of the original study in two independent samples. As we had sufficient statistical power to be confident in our failure to detect a genuine group difference, we cannot confirm the previously reported MH over cRH advantage in incidental learning of verbal material. We discuss possible reasons for these contradictory results and the theoretical implications of this discovery.

Implicit motor imagery: examining motor vs. visual strategies in laterality judgments among older adults

Cognitive states like motor imagery (MI; simulating actions without overtly executing them) share a close correspondence with action execution, and hence, activate the motor system in a similar way. However, as people age, reduction in specific cognitive abilities like motor action simulation and action planning/prediction are commonly experienced. The present study examined the effect of visual-spatial processing for both typical and challenging upper-limb movements using the Hand Laterality Judgment Task (HLJT), in which participants were asked to judge whether the depicted hand is a left or right hand. Several main findings emerged: (1) Compared to younger adults, older adults exhibited slower responses and greater error rates in both Experiment 1 and 2. This suggests that visual-spatial transformations undergo alterations with age; (2) Older adults displayed higher error rates with realistic hands at both back and palm viewpoints of the hands compared to younger adults. However, this pattern did not hold for response times; (3) Participants responded faster to medial hand orientations (i.e., closer to the midline of the body) compared to lateral hand orientations (i.e., farther from the midline of the body) for palm-views in both Experiment 1 and Experiment 2. Given that we observed better performance on medial orientations compared to lateral orientations, this suggests that participants follow the same motor rules and biomechanical constraints of the represented movement. Novel information is provided about differences in individuals' use of strategies (visual vs. motor imagery) to solve the HLJT for both mannequin and real hands.

Degraded Visibility Body-Specifically Affects Mental Rotation

The way we perceive our own body is shaped by our perception. Changes in sensory input, such as visual degradation, can lead to visual-to-motor shifts in the reference frame used to mentally represent the body. While this effect has been demonstrated in mental representation of hands, it is still unknown whether it also affects mental representation of other body parts. To fill this gap, we asked 35 neurotypical participants to perform mental rotation (laterality judgement) of hand, foot, and full-body images, while the images' visibility (figure/background contrast) was manipulated. Visibility deteriorations increased the steepness of the response time (RT) slopes for mental rotation of hand images shown from a less common view (palm) and of foot images from a more common view (dorsum), but not of full-body images from either the common or uncommon views. Suggesting that steeper and flatter RT slopes evoke the activation of a motor- or vision-based cognitive strategy for mental rotation, respectively, we propose that visual deterioration induces body-specific visual-to-motor shifts in mental processing. These findings show that the reliance on visual or motor aspects to mentally represent the body can be modulated by a reduction in sensory input, which changes the employed cognitive strategy.

Phenotyping in clinical laterality research: a comparison of commonly used methods to determine mixed-handedness and ambidexterity

An increased prevalence of mixed-handedness has been reported in several neurodevelopmental and psychiatric disorders. Unfortunately, there is high between-study variability in the definition of mixed-handedness, leading to a major methodological problem in clinical laterality research and endangering replicability and comparability of research findings. Adding to this challenge is the fact that sometimes researchers use the concepts of mixed-handedness and ambidexterity interchangeably. Therefore, having a consensus on how to determine mixed-handedness and how to distinguish it from ambidexterity is crucial for clinical laterality research. To this end, hand preference and hand performance data from more than 600 participants from the Dortmund Vital Study (Trial registration: ClinicalTrials.gov NCT05155397), a population-based study in Germany, was analyzed to ascertain an optimal classification to determine mixed-handedness and ambidexterity. Using a combination of latent class analyses, effect size determination, and comparisons with the existing literature, we establish that an LQ cut-off criterion of +/-60 for mixed-handedness is optimal for future clinical laterality studies. Moreover, we show that mixed-handedness and ambidexterity are not identical and that the terms should not be used interchangeably. We further highlight the need for a consensus on how to mathematically determine ambidexterity as results of existing categorization schemes largely differ.Trial registration: ClinicalTrials.gov NCT05155397; https://clinicaltrials.gov/ct2/show/NCT05155397.

Does mental rotation emulate motor processes? An electrophysiological study of objects and body parts

Several arguments suggest that motor planning may share embodied neural mechanisms with mental rotation (MR). However, it is not well established whether this overlap occurs regardless of the type of stimulus that is manipulated, in particular manipulable or non-manipulable objects and body parts. We here used high-density electroencephalography (EEG) to examine the cognitive similarity between MR of objects that do not afford specific hand actions (chairs) and bodily stimuli (hands). Participants had identical response options for both types of stimuli, and they gave responses orally in order to prevent possible interference with motor imagery. MR of hands and chairs generated very similar behavioral responses, time-courses and neural sources of evoked-response potentials (ERPs). ERP segmentation analysis revealed distinct time windows during which differential effects of stimulus type and angular disparity were observed. An early period (90-160 ms) differentiated only between stimulus types, and was associated with occipito-temporal activity. A later period (290-330 ms) revealed strong effects of angular disparity, associated with electrical sources in the right angular gyrus and primary motor/somatosensory cortex. These data suggest that spatial transformation processes and motor planning are recruited simultaneously, supporting the involvement of motor emulation processes in MR.