Published papers

In the semantic variant of verbal fluency tests (VFTs), clustering analysis has become popular for examining the semantic structure. While the computational psycholinguistics approach has recently drawn attention to increasing the reproducibility of clustering analysis, such an approach is not available in all languages. To make the computational approach available in the Japanese language, we constructed a Japanese latent Dirichlet allocation (LDA) model. Our LDA model enables researchers and clinicians to objectively quantify the associative relationships of words, thereby making it possible to automatically detect semantic clusters. We conducted the semantic VFT with healthy young Japanese adults to examine the validity of our LDA model. We performed clustering analyses using the computational approach with our LDA model and the conventional manual approach with human coders. The results showed that the LDA model identified semantic clusters, as did the human coders. In addition, we demonstrated for the first time that response intervals within a cluster were significantly shorter than those outside of clusters, regardless of the clustering approaches. This indicates that both approaches reflect a broadly accepted assumption that closer semantic relations require less processing time. However, LDA-based clustering produced, on average, larger clusters than human-based clustering did, indicating that the LDA model captured semantic relationships between words that human coders would not recognize. Taken together, the present results demonstrated the validity of our LDA model. We hope that our LDA model fosters the use of the computational linguistic approach in semantic VFTs with Japanese participants.

Bilateral movement has been reported to enhance actual motor performance more than unilateral movement and increase motor cortex activation more than unilateral movement. We hypothesized that bilateral motor imagery would result in greater improvements in event-related desynchronization (ERD) amplitude and ball rotation performance than unilateral motor imagery. Forty healthy young adults participated in this study. The mental practice session was performed in the unilateral hand condition (right and left hands) and bilateral hand condition for four blocks. ERD in mental practice was calculated using electroencephalography (EEG). The ball rotations were compared between the pre-and post-tests of mental practice. The results showed a greater improvement in actual motor performance with unilateral motor imagery than with bilateral motor imagery. ERD analysis showed no significant differences in multiple comparisons. These results did not support this hypothesis. Nonetheless, the present study showed that unilateral movements resulted in greater performance gains than bilateral movements, suggesting that bilateral motor imagery may have additional control processes similar to actual movements.

Verbal fluency (VF) represents an important aspect of intelligence, in which oral word generation is demanded following semantic or phonemic cues. Two reliable phenomena of VF execution have been reported: A decay in performance across 1-minute trial and a discrepancy score between the semantic and phonemic VF tests (VFTs). Although, these characteristics have been explained from various cognitive standpoints, the fundamental role of speech breathing has not yet been considered. Therefore, the present study aims to evaluate the role of respiratory function for word generation in VFTs in healthy individuals. Thirty healthy young adults performed VFTs during definite periods of 1 minute while wearing a pneumotachograph mask. Duration, peak and volume of airflow were acquired during inspirations and expirations. Also, respiratory rate and acoustic data of verbal responses were registered, and accuracy scores were calculated. Each 1-minute trial was divided into four intervals of 15-seconds where parameters were calculated. Repeated measures ANOVAs and repeated measures correlations were used in the statistical analyses. Data revealed that respiratory function was significantly coupled to VF performance mostly during inhalations. Small but constant increments of inhale airflow occurred in phonemic VFT as well as higher peak airflow in both tasks, being higher for semantic VFT. High respiratory rate characterized performance of both VFTs across intervals. Airflow adjustments corresponded to better VF accuracy, while increments in respiratory rate did not. The present study shows a complex interplay of breathing needs during VF performance that varies along the performance period and that notably connects to inspirations.

In robot-assisted rehabilitation, it is unclear which type of haptic guidance is effective for regaining motor function because of the lack of direct comparisons among multiple types of haptic guidance. The objective of this study was to investigate the effects of different types of haptic guidance on upper limb motor learning in a spiral drawing task. Healthy young participants performed two experiments in which they practiced the drawing movement using a robotic manipulandum with a virtual wall (Path guidance), running direction pushing and virtual wall (Path & Push guidance), restriction to the target movement (Target guidance), or without haptic guidance (Free guidance). Experiment 1 compared the learning effects of the four types of guidance. Experiment 2 investigated the effects of pre-learning with Path, Path & Push, or Target guidance on post-learning with Free guidance. In Experiment 1, Free guidance demonstrated the greatest learning effect, followed by Path guidance, which showed a significantly greater improvement in task performance than the other two types of guidance. In Experiment 2, the type of pre-learning did not influence post-learning with Free guidance. The results suggested that learning with Path guidance showed a slightly slower but comparable effect to Free guidance and was the most effective among the three types of haptic guidance. The superiority of Path guidance over other haptic guidance was interpreted within the framework of error-based learning, in which the intensity of sensory feedback and voluntary motor control play important roles.

This study investigated the effects of the time interval between virtual reality (VR) sessions on visually induced motion sickness (VIMS) reduction to better understand adaptation to and recovery from a nauseating VR experience. The participants experienced two 6-min VR sessions of a first-person motorcycle ride through a head-mounted display with (1) a 6-min interval, (2) an interval until the VIMS score reached zero, and (3) a 60-min interval. The results showed that for each condition, VIMS in the second session was aggravated, unchanged, or attenuated, respectively, indicating that additional resting time was necessary for VIMS adaptation. This study suggests that a certain type of multisensory learning attenuates VIMS symptoms within a relatively short time, requiring at least 20 min of additional resting time after subjective recovery from VIMS symptoms. This finding has important implications for reducing the time interval between repeated challenges when adapting to nauseating stimuli during VR experiences.

Semantic verbal fluency (VF), assessed by animal category, is a task widely used for early detection of dementia. A feature not regularly assessed is the occurrence of errors such as perseverations and intrusions. So far, no investigation has analyzed the how and when of error occurrence during semantic VF in aging populations, together with their possible neural correlates. The present study aims to address the issue using a combined methodology based on latent Dirichlet allocation (LDA) analysis for word classification together with a time-course analysis identifying exact time of errors’ occurrence. LDA is a modeling technique that discloses hidden semantic structures based on a given corpus of documents. We evaluated a sample of 66 participants divided into a healthy young group (n = 24), healthy older adult group (n = 23), and group of patients with mild Alzheimer’s disease (AD) (n = 19). We performed DTI analyses to evaluate the white matter integrity of three frontal tracts purportedly underlying error commission: anterior thalamic radiation, frontal aslant tract, and uncinate fasciculus. Contrasts of DTI metrics were performed on the older groups who were further classified into high-error rate and low-error rate subgroups. Results demonstrated a unique deployment of error commission in the patient group characterized by high incidence of intrusions in the first 15 s and higher rate of perseverations toward the end of the trial. Healthy groups predominantly showed very low incidence of perseverations. The DTI analyses revealed that the patients with AD committing high-error rate presented significantly more degenerated frontal tracts in the left hemisphere. Thus, our findings demonstrated that the appearance of intrusions, together with left hemisphere degeneration of frontal tracts, is a pathognomic trait of mild AD. Furthermore, our data suggest that the error commission of patients with AD arises from executive and working memory impairments related partly to deteriorated left frontal tracts.

Robots are used by humans not only as tools but also to interactively assist and cooperate with humans, thereby forming physical human-robot interactions. In these interactions, there is a risk that a feedback loop causes unstable force interaction, in which force escalation exposes a human to danger. Previous studies have analyzed the stability of voluntary interaction but have neglected involuntary behavior in the interaction. In contrast to the previous studies, this study considered the involuntary behavior: a human's force reproduction bias for discrete-event human-robot force interaction. We derived an asymptotic stability condition based on a mathematical bias model and found that the bias asymptotically stabilizes a human's implicit equilibrium point far from the implicit equilibrium point and destabilizes the point near the point. The bias model, convergence of the interaction toward the implicit equilibrium point, and divergence around the point were consistently verified via behavioral experiments under three kinds of interactions using three different body parts: 1) a hand finger; 2) wrist; and 3) foot. Our results imply that humans implicitly secure a stable and close relationship between themselves and robots with their involuntary behavior.

While studies have increasingly used virtual hands and objects in virtual environments to investigate various processes of psychological phenomena, conflicting findings have been reported even at the most basic level of perception and action. To reconcile this situation, the present study aimed 1) to assess biases in size perception of a virtual hand using a strict psychophysical method and 2) to provide firm and conclusive evidence of the kinematic characteristics of reach-to-grasp movements with various virtual effectors (whole hand or fingertips only, with or without tactile feedback of a target object). Experiments were conducted using a consumer immersive virtual reality device. In a size judgment task, participants judged whether a presented virtual hand or an everyday object was larger than the remembered size. The results showed the same amplitude of underestimation (approximately 5%) for the virtual hand and the object, and no influence of object location, visuo-proprioceptive congruency, or short-term experience of controlling the virtual hand. Furthermore, there was a moderate positive correlation between actual hand size and perception bias. Analyses of reach-to-grasp movements revealed longer movement times and larger maximum grip aperture (MGA) for a virtual, as opposed to a physical, environment, but the MGA did not change when grasping was performed without tactile feedback. The MGA appeared earlier in the time course of grasping movements in all virtual reality conditions, regardless of the type of virtual effector. These findings confirm and corroborate previous evidence and may contribute to the field of virtual hand interfaces for interactions with virtual worlds.

Usually, our bodily movements are performed against gravity. Most studies using a force field have focused on adaptation processes to force applied in the horizontal plane, which is novel to us, but not to force in the gravitational direction. The present study investigated the immediate effects (aftereffects) of a force toward the gravitational direction on the kinematics of reach-to-grasp movements as well as short-term adaptation to the force, simply by adding a weight to participants’ forearm. Healthy young adults performed blocks of 10 reach-to-grasp movements under three weight conditions; as the weights were changed between blocks, the participants experienced weight changes ranging from − 200 to + 200 g. We obtained three main results; first, the height of movement trajectory (trajectory height) was remarkably higher immediately after the forearm weight changed to lighter than after the weight changed to heavier, suggesting that participants planed the trajectory height with the same muscle efforts as in the previous trial. Second, the trajectory height at the end of the block became higher only in 200 g condition, indicating that the participants could not achieve same trajectory height as that without any weight load, at least in ten trials of adaptation period to the 200 g weight load. Third, the coordination between reach and grasp components was preserved immediately after forearm-weight changes. These findings may contribute to further understand how we perform adaptive reach-to-grasp movements with frequent weight changes that are inevitable in everyday life.

Studies have shown that explicit strategies make a significant contribution to visuomotor adaptation. However, little attention has been given to potential unconscious cognitive biases in these strategies, despite that they involve a sequence of cognitive decision-making processes. To reveal the possible cultural biases involved in motor learning, we compared Norwegian and Japanese participants in a visuomotor adaptation task using a verbal report paradigm. The results showed that Japanese participants aimed at locations more deviant from the target to account for rotated visual feedback. Additionally, a greater proportion of Japanese participants changed their aiming direction more frequently than Norwegian participants, even after successfully hitting the target. However, both groups showed similar behavioral performance, with comparable reaching accuracy and aftereffect amplitudes. These results suggest that the explicit component, which is estimated based on verbal reports, includes cognitive biases. The present study challenges the assumption of universality of motor learning among cultures.

Deterioration of semantic memory represents an important feature of Alzheimer's disease (AD), which has been widely addressed in neuropsychological research. A way to understand semantic integrity in AD patients is through a detailed analysis of verbal fluency (VF) performance. In the present study, we used an innovative methodology that combines measures of between-words latencies together with automatized identification of semantic clusters via Latent Dirichlet Analysis (LDA) to acquire a more precise understanding of the dynamics and semantic organization of VF in patients at early stages of AD. Importantly, and diverging from customary procedures, we included VF errors (i.e., repetitions and intrusions) across analyses. For comparison, a group of healthy older adults and young individuals were also examined. Standard parameters including total correct answers, number of clusters, mean cluster size (MCS), cluster duration, and within and out-of-cluster intervals were calculated. These parameters were expressed as mean values in 1-min VF trials and by calculating mean values in four 15-s time windows. Results for the 1-min trial demonstrated significantly larger mean cluster sizes (MCS) and fewer generated answers in AD patients compared to the healthy groups. No additional group differences were found neither on time intervals (both within and out-of-clusters), nor on the 15-s time windows analyses. These data suggest that the clustering ability of mild AD patients might be affected by executive impairments promoting larger MCS. At the same time, we found similar semantic processes and timings in patients and healthy participants. The main difference resides in the structure of the patients' clusters, which encompassed erroneous answers. We advance the idea that production of errors might not only be a consequence of executive dysfunction or working memory deterioration, but also a sign that associative semantic mechanisms are still active early in the disease, despite an evident loss of information.

Through cognitive task performance, we examined the functional role of finger writing (kūsho) in a Japanese patient with moderate sensory aphasia and reading difficulties. We hypothesized that the visual feedback of kūsho would improve visual language processing, which we tested with a “kanji construction task” using character subparts. Results showed a higher number of correct responses 1) when the patient used kūsho and 2) when visual feedback of finger movements was available. The results suggest that kūsho may not improve the retrieval of phonological information but does aid the visual processing necessary to assemble character subparts.