Abstract
Limb apraxia is a higher brain dysfunction that typically occurs after left hemispheric stroke and its cause cannot be explained by sensory disturbance or motor paralysis. The comparison of motor signals and visual feedback to generate errors, i.e., visuo-motor integration, is important in motor control and motor learning, which may be impaired in apraxia. However, in apraxia after stroke, it is unknown whether there is a specific deficit in visuo-motor temporal integration compared to visuo-tactile and visuo-proprioceptive temporal integration. We examined the precision of visuo-motor temporal integration and sensory-sensory (visuo-tactile and visuo-proprioception) temporal integration in apraxia after stroke by using a delayed visual feedback detection task with three different conditions (tactile, passive movement, and active movement). The delay detection threshold and the probability curve for delay detection obtained in this task were quantitative indicators of the respective temporal integration functions. In addition, we performed subtraction and voxel-based lesion-symptom mapping to identify the brain lesions responsible for apraxia and deficits in visuo-motor temporal integration. The behavioral experiments showed that the delay detection threshold was extended and that the probability curve for delay detection was less steep in apraxic patients compared to controls (pseudo-apraxic patients and unaffected patients), only for the active movement condition, and not for the tactile and passive movement conditions. Furthermore, the severity of apraxia was significantly correlated with the delay detection threshold and the steepness of the probability curve in the active movement condition. These results indicated that multisensory (i.e., visual, tactile, and proprioception) feedback was normally temporally integrated, but motor prediction and visual feedback were not correctly temporally integrated in apraxic patients. That is, apraxic patients had difficulties with visuo-motor temporal integration. Lesion analyses revealed that both apraxia and the distortion of visuo-motor temporal integration were associated with lesions in the fronto-parietal motor network, including the left inferior parietal lobule and left inferior frontal gyrus. We suppose that damage to the left inferior fronto-parietal network could cause deficits in motor prediction for visuo-motor temporal integration, but not for sensory-sensory (visuo-tactile and visuo-proprioception) temporal integration, leading to the distortion of visuo-motor temporal integration in patients with apraxia.
Highlights
In apraxia, no experimental study has examined whether the time window for visuo-motor integration in the brain is distorted
In consideration of the frequent occurrence of apraxia and aphasia [69], we evaluated the presence of aphasia in the patients using the standard language test of aphasia (SLTA) [70] and the supplementary tests for the SLTA (SLTA-ST) [71] to be certain that the patients could understand and respond to the experimental task
The voxel-based lesion-symptom mapping (VLSM) results in the current study showed that damage to the insula is involved in both apraxia and the distortion of visuo-motor temporal integration
Summary
In apraxia, no experimental study has examined whether the time window for visuo-motor integration in the brain is distorted. It is suggested that the sense of agency caused by the temporal integration of visual feedback and motor signals is lost [53], but no study has examined this issue. As apraxia involves an impairment of visuo-motor integration, the time window may be distorted. It may be difficult for subjects with apraxia to detect a temporal error between motor signals and visual feedback. We believe that by quantitatively evaluating the time window for visuo-motor integration, which operates downstream to mechanisms involved in stored motor representation and technical reasoning, we can understand apraxia better
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