Effects of Hand and Hemispace on Multisensory Integration of Hand Position and Visual Feedback

Persistence of metric biases in body representation during the body ownership illusion

Our perception of the body’s metric is influenced by bias according to the axis, called the systematic metric bias in body representation. Systematic metric bias was first reported as Weber’s illusion and observed in several parts of the body in various patterns. However, the systematic metric bias was not observed with a fake hand under the influence of the body ownership illusion during the line length judgment task. The lack of metric bias observed during the line length judgment task with a fake hand implies that the tactile modality occupies a relatively less dominant position than perception occurring through the real body. The change in weight between visual and tactile modalities during the body ownership illusion has not been adequately investigated yet, despite being a factor that influences the perception through body ownership illusion. Therefore, this study aimed to investigate whether the dominance of vision over tactile modality is prominent, regardless of the task type. To investigate whether visual dominance persists during the process of inducing body ownership illusion regardless of task type, we introduced spatial visuotactile incongruence (2 cm, 3 cm) in the longitudinal and transverse axes during the visuotactile localization tasks and measured the intensity of the body ownership illusion using a questionnaire. The results indicated that participants perceived smaller visuotactile incongruence when the discrepancy occurred in the transverse axis rather than in the longitudinal axis. The anisotropy in the tolerance of visuotactile incongruence implies the persistence of metric biases in body representation. The results suggest the need for further research regarding the factors influencing the weight of visual and tactile modalities.

Delay of gaze fixation during reaching movement with the non-dominant hand to a distant target

The present study examined the effects of hand and task difficulty on eye-hand coordination related to gaze fixation behavior (i.e., fixating a gaze to the target until reach completion) in single reaching movements. Twenty right-handed young adults made reaches on a digitizer, while looking at a visual target and feedback of hand movements on a computer monitor. Task difficulty was altered by having three target distances. In a small portion of trials, visual feedback was randomly removed at the target presentation. The effect of a moderate amount of practice was also examined using a randomized trial schedule across target-distance and visual-feedback conditions in each hand. The results showed that the gaze distances covered during the early reaching phase were reduced, and the gaze fixation to the target was delayed when reaches were performed with the left hand and when the target distance increased. These results suggest that when the use of the non-dominant hand or an increased task difficulty reduces the predictability of hand movements and its sensory consequences, eye-hand coordination is modified to enhance visual monitoring of the reach progress prior to gaze fixation. The randomized practice facilitated this process. Nevertheless, variability of reach trajectory was more increased without visual feedback for right-hand reaches, indicating that control of the dominant arm integrates more visual feedback information during reaches. These results together suggest that the earlier gaze fixation and greater integration of visual feedback during right-hand reaches contribute to the faster and more accurate performance in the final reaching phase.

A condition that produces sensory recalibration and abolishes multisensory integration

We examined the influence of extended exposure to a visuomotor rotation, which induces both motor adaptation and sensory recalibration, on (partial) multisensory integration in a cursor-control task. Participants adapted to a 30° (adaptation condition) or 0° (control condition) visuomotor rotation by making center-out movements to remembered targets. In subsequent test trials of sensory integration, they made center-out movements with variable visuomotor rotations and judged the position of hand or cursor at the end of these movements. Test trials were randomly embedded among twice the number of maintenance trials with 30° or 0° rotation. The biases of perceived hand (or cursor) position toward the cursor (or hand) position were measured. We found motor adaptation together with proprioceptive and visual recalibrations in the adaptation condition. Unexpectedly, multisensory integration was absent in both the adaptation and control condition. The absence stemmed from the extensive experience of constant visuomotor rotations of 30° or 0°, which probably produced highly precise predictions of the visual consequences of hand movements. The frequently confirmed predictions then dominated the estimate of the visual movement consequences, leaving no influence of the actual visuomotor rotations in the minority of test trials. Conversely, multisensory integration was present for sensed hand positions when these were indirectly assessed from movement characteristics, indicating that the relative weighting of discrepant estimates of hand position was different for motor control. The existence of a condition that abolishes multisensory integration while keeping sensory recalibration suggests that mechanisms that reduce sensory discrepancies (partly) differ between integration and recalibration.

Hand posture affects brain-function measures associated with listening to speech

A major difficulty in studies of the brain, from the molecular to large-scale network level, is ensuring the accuracy and reliability of results, since repeatability has been a problem in studies utilizing functional magnetic resonance imaging (f-MRI) near-infrared spectroscopy (NIRS), and positron-emission tomography (PET). More generally, an effort to replicate psychological studies has shown that the original results were unambiguously reobtained only 39% of the time. It has been suggested that researchers must undertake studies to identify factors that reduce reliability and conduct more carefully controlled studies to improve reliability. In our previous work, we examined whether changes in hand/arm posture can have a confounding effect on task-related brain activity. Here we show a solution to enhance reproducibility in a NIRS study in a hearing task. The results showed that crossed posture can lead to different results than parallel posture with respect to asymmetric functional connectivity, especially during non-resting state. Even when the only task is listening to speech stimuli, participants should be asked to place their hands on a surface and feet on the floor and keep the same stable posture to increase reproducibility of results. To achieve accurate reliability and reproductively of results, stable hand posture through the experiment is important.