Non-technical summary Even when the hand is stationary we know its position. This information is needed by the brain to plan movements. If the sensory input from a limb is removed through an accident, or an experiment with local anaesthesia, then a 'phantom' limb commonly develops. We used ischaemic anaesthesia of one arm to study the mechanisms which define the phantom hand. Surprisingly, if the wrist and fingers are held straight during anaesthesia, the perceived phantom hand becomes bent at the wrist and fingers, but if they are bent during anaesthesia, the final phantom is extended at the wrist and fingers. There is no 'default' posture for the phantom hand. Further, the hand appears to increase gradually in size as anaesthesia develops. The start of these perceptual changes occurs when input from large-diameter sensory nerve fibres is declining. These results provide new information about how the brain generates phantom limbs.Abstract Contorted 'phantom' limbs often form when sensory inputs are removed, but the neural mechanisms underlying their formation are poorly understood. We tracked the evolution of an experimental phantom hand during ischaemic anaesthesia of the arm. In the first study subjects showed the perceived posture of their hand and fingers using a model hand. Surprisingly, if the wrist and fingers were held straight before and during anaesthesia, the final phantom hand was bent at the wrist and fingers, but if the wrist and fingers were flexed before and during anaesthesia, the final phantom was extended at wrist and fingers. Hence, no 'default' posture existed for the phantom hand. The final perceived posture may depend on the initial and evolving sensory input during the block rather than the final sensory input (which should not differ for the two postures). In the second study subjects selected templates to indicate the perceived size of their hand. Perceived hand size increased by 34 ± 4% (mean ± 95% CI) during the block. Sensory changes were monitored. In all subjects, impairment of large-fibre cutaneous sensation began distally with von Frey thresholds increasing before cold detection thresholds (Aδ fibres) increased. Some C fibres subserving heat pain still conducted at the end of cuff inflation. These data suggest that changes in both perceived hand size and perceived position of the finger joints develop early when large-fibre cutaneous sensation is beginning to degrade. Hence it is unlikely that block of small-fibre afferents is critical for phantom formation in an ischaemic block.

The purpose of this study was to examine the serial information processing in adolescents with mental retardation, autism, and Down syndrome by using a serially patterned tracking task. Analyses indicated that 7 adolescents with mental retardation, 8 with autism, and 3 with Down syndrome had significantly slower and more variable simple reaction times than did 10 college students. Also, the autistic adolescents had significantly faster mean simple reaction time than those with Down syndrome. On a task of tracking serial light stimulation, mentally retarded adolescents had significantly faster reaction time than college students. The autistic subjects excessively had faster anticipatory reaction time than did the subjects in the other three groups. On the other hand, adolescents with Down syndrome had markedly slower and more variable reaction time than did adolescents with non-Down-syndrome mental retardation. As for motor organization of keystrokes on the tracking task, mentally retarded adolescents responded with six movements, in which these individuals pressed a series of keys 1, 2, 3, 4, 5, and 6, as a chunk, as exhibited by college students. Adolescents with autism and Down syndrome, however, did not produce this movement-output chunking.

If two people lift and carry an object, they not only produce complementary forces on the object but also walk in synchrony. Previous studies have not examined how two types of coordination strategy are adopted simultaneously. The present study thus tested the hypothesis that complementary and synchronous strategies simultaneously facilitate the action coordination performed by two people. Ten pairs of participants produced periodic isometric forces such that the sum of forces they produced was the target force cycling between 5% and 10% of maximum voluntary contraction with an interval of 1,000 ms (joint action), while individuals alone produced the same target forces with the right hand (individual action). The correlation between forces produced by two participants was highly negative when the total force was visible, indicating that the two participants produced complementary forces. When the image of the total or partner force was presented, the coherence between force-time series produced by two participants was highest at 1 Hz. The relative phase angles were also distributed at the 0-20° phase region. These innovative findings indicate that two participants simultaneously adopted both complementary and temporal synchronous strategies exclusively when the total force was visible. With the vision of total force, surprisingly, while the joint action exhibited a less variable force than the individual action, the joint action exhibited a smaller absolute error of forces than the individual action. These new findings indicated that the joint action controlled force more accurately than the individual action.

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