Experimental Brain Research

Wang, Yun; Asaka, Tadayoshi; Watanabe, Kazuhiko

doi: 10.1007/s00221-013-3449-9pmid: 23571498

We studied multi-muscle synergies of healthy elderly persons during preparation to making a step (self-paced vs. reaction time). The uncontrolled manifold hypothesis was used to explore the organization of leg and trunk muscles into groups (M-modes) and co-variation of M-mode involvement (M-mode synergies) during stepping tasks. We hypothesized that aging accounts for changes in the structure of M-modes, as well as in M-mode synergies. Subjects performed two tasks: (1) a cyclic COP shift over a range corresponding to the maximal amplitude of voluntary COP shift at 1 Hz, (2) stepping tasks under 3 instructions, “comfortably, self-paced,” “very quick, self-paced,” and “as fast as possible to a visual signal.” Electromyographic signals of 10 postural muscles were recorded and analyzed. Principal component analysis was used to identify M-modes within the space of integrated indices of muscle activity in the cyclic sway task. Variance in the M-mode space across stepping trials was partitioned into two components, one that did not affect the average value of COP shift and the other that did. An index (ΔV) corresponding to the normalized difference between two components of variance was computed. The elderly subjects showed more “co-contraction M-mode” and “mixed M-mode” than that of the young subjects. During stepping tasks, both subject groups showed M-mode synergies stabilizing COP shifts in the stepping and supporting legs. The synergies of elderly subjects showed a smaller and delayed value than that of the young subjects. These results suggest that aging is associated with diminished control in multi-muscle synergies in the anticipatory postural adjustments during gait initiation.

Sacheli, Lucia; Tidoni, Emmanuele; Pavone, Enea; Aglioti, Salvatore; Candidi, Matteo

doi: 10.1007/s00221-013-3459-7pmid: 23503771

Performing online complementary motor adjustments is quintessential to joint actions since it allows interacting people to coordinate efficiently and achieve a common goal. We sought to determine whether, during dyadic interactions, signaling strategies and simulative processes are differentially implemented on the basis of the interactional role played by each partner. To this aim, we recorded the kinematics of the right hand of pairs of individuals who were asked to grasp as synchronously as possible a bottle-shaped object according to an imitative or complementary action schedule. Task requirements implied an asymmetric role assignment so that participants performed the task acting either as (1) Leader (i.e., receiving auditory information regarding the goal of the task with indications about where to grasp the object) or (2) Follower (i.e., receiving instructions to coordinate their movements with their partner’s by performing imitative or complementary actions). Results showed that, when acting as Leader, participants used signaling strategies to enhance the predictability of their movements. In particular, they selectively emphasized kinematic parameters and reduced movement variability to provide the partner with implicit cues regarding the action to be jointly performed. Thus, Leaders make their movements more “communicative” even when not explicitly instructed to do so. Moreover, only when acting in the role of Follower did participants tend to imitate the Leader, even in complementary actions where imitation is detrimental to joint performance. Our results show that mimicking and signaling are implemented in joint actions according to the interactional role of the agent, which in turn is reflected in the kinematics of each partner.

Inui, Nobuyuki; Masumoto, Junya

doi: 10.1007/s00221-013-3460-1pmid: 23455730

Our previous studies showed that a fully extended finger, wrist, and elbow became a flexed phantom hand and arm with ischemic anesthesia, and vice versa (Inui et al. in J Physiol 589:5775–5784, 2011 , Exp Brain Res 221:369–375, 2012a , Exp Brain Res 218:487–494, 2012b ). It was anticipated that if the ankle and knee were fixed in full extension or flexion before and during ischemic anesthesia, the perceived positions would move in the opposite direction. The present study examined what happened when participants looked at their fixed foot and leg at the end of the anesthesia. Using the left ankle and knee, ten healthy participants demonstrated the perceived postures of the right joints during an ischemic block of the right thigh (40 min) and after they looked at the right joints at the end of the block. When the right ankle and knee were fully extended before and during the block, the final joints were perceived as flexed by all participants, and vice versa. Although there was no significant difference between joints for the magnitude of the perceived changes in flexion, the magnitude in the knee was larger than that in the ankle in extension. At the end of the experiment, when participants were allowed to see their foot, its perceived position reverted to that indicated by them earlier, during the first 25 min of cuff inflation. This new finding suggests that the position of limbs is coded by visual input more dominantly than by proprioceptive input in the brain.

Sugawara, Kazuhiro; Onishi, Hideaki; Yamashiro, Koya; Soma, Toshio; Oyama, Mineo; Kirimoto, Hikari; Tamaki, Hiroyuki; Murakami, Hiroatsu; Kameyama, Shigeki

doi: 10.1007/s00221-013-3461-0pmid: 23455731

Dionne, Jennifer; Legon, Wynn; Staines, W.

doi: 10.1007/s00221-013-3462-zpmid: 23455852

Previous research suggests that somatosensory cortex is subject to modulation based on the relevancy of incoming somatosensory stimuli to behavioural goals. Recent fMRI findings provide evidence for modulation of primary somatosensory cortex when simultaneous visual and tactile stimuli were relevant to the performance of a motor task. The present study aimed to (1) determine the temporal characteristics of this modulation using event-related potentials (ERPs) and (2) investigate the role of task-relevance in mediating such a modulation. Electroencephalography was collected from healthy subjects during visual, vibrotactile or bimodal stimulation as they performed a sensory-guided motor task. Experiment 1 tested the hypothesis that simultaneous bimodal stimuli would be associated with modulation of somatosensory ERPs, and Experiment 2 tested the hypothesis that such effects would only be seen when both modalities are relevant. ERPs were time-locked to stimulus onset, and mean ERP amplitudes and latencies were extracted for the P50, P100, and N140. The bimodal condition in the first experiment was associated with larger amplitudes at both early and mid-latency components. The manipulation of task-relevance under bimodal conditions produced more complex results for the mid-latency components. For the P50, this enhancement was observed only when both stimuli were relevant, whereas the P100 was smallest when the tactile stimuli were not relevant to the response. These results provide evidence that crossmodal stimuli can modulate early somatosensory event-related potentials and that these effects are mediated by stimulus relevance.

Wallraven, Christian; Whittingstall, Lisa; Bülthoff, Heinrich

doi: 10.1007/s00221-013-3463-ypmid: 23468160

Human observers are experts at visual face recognition due to specialized visual mechanisms for face processing that evolve with perceptual expertize. Such expertize has long been attributed to the use of configural processing, enabled by fast, parallel information encoding of the visual information in the face. Here we tested whether participants can learn to efficiently recognize faces that are serially encoded—that is, when only partial visual information about the face is available at any given time. For this, ten participants were trained in gaze-restricted face recognition in which face masks were viewed through a small aperture controlled by the participant. Tests comparing trained with untrained performance revealed (1) a marked improvement in terms of speed and accuracy, (2) a gradual development of configural processing strategies, and (3) participants’ ability to rapidly learn and accurately recognize novel exemplars. This performance pattern demonstrates that participants were able to learn new strategies to compensate for the serial nature of information encoding. The results are discussed in terms of expertize acquisition and relevance for other sensory modalities relying on serial encoding.

Leuthold, Arthur; Mahan, Margaret; Stanwyck, John; Georgopoulos, Angeliki; Georgopoulos, Apostolos

doi: 10.1007/s00221-013-3464-xpmid: 23503772

Apolipoprotein E (apoE) is involved in lipid metabolism in the brain, but its effects on brain function are not understood. Three apoE isoforms (E4, E3, and E2) are the result of cysteine–arginine interchanges at two sites: there are zero interchanges in E4, one interchange in E3, and two interchanges in E2. The resulting six apoE genotypes (E4/4, E4/3, E4/2, E3/3, E3/2, E2/2) yield five groups with respect to the number of cysteine residues per mole (CysR/mole), as follows. ApoE4/4 has zero cysteine residues per mole (0-CysR/mole), E4/3 has one (1-CysR/mole), E4/2 and E3/3 each has two (2-CysR/mole), E3/2 has three (3-CysR/mole), and E2/2 has four (4-CysR/mole). The use of the number of CysR/mole to characterize the apoE molecule converts the categorical apoE genotype scale, consisting of 6 distinct genotypes above, to a 5-point continuous scale (0–4 CysR/mole). This allows the use of statistical analyses suitable for continuous variables (e.g. regression) to quantify the relations between various variables and apoE. Using such analyses, here, we show for the first time that apoE affects in a graded and orderly manner neural communication, as assessed by analyzing the relation between the number of CysR/mole and synchronous neural interactions (SNI) measured by magnetoencephalography (MEG) in 130 cognitively healthy women. At the one end of the CysR/mole range, the 4-CysR/mole (E2/2) SNI distribution had the highest mean, lowest variance, lowest range, and lowest coefficient of variation, whereas at the other end, 0-CysR/mole (E4/4) SNI distribution had the lowest mean, highest variance, highest range, and highest coefficient of variation. The special status of the 4-CysR/mole distribution was reinforced by the results of a hierarchical tree analysis where the 4-CysR/mole (E2/2) SNI distribution occupied a separate branch by itself and the remaining CysR/mole SNI distributions were placed at increasing distances from the 4-CysR/mole distribution, according to their number of CysR/mole, with the 0-CysR/mole (E4/4) being farthest away. These findings suggest that the 4-CysR/mole (E2/2) SNI distribution could serve as a reference distribution. When the SNI distributions of individual women were expressed as distances from this reference distribution, there was a substantial overlap among women of various CysR/mole. This refocuses the placement of individual brains along a continuous distance from the 4-CysR/mole SNI distribution, in contrast to the common categorical assignment to a specific apoE genotype. Finally, the orderly variation of SNI with the number of CysR/mole found here is in keeping with recent advances and ideas regarding the molecular mechanisms underlying the differential effects of apoE in the brain which emphasize the healthier stability conferred on the apoE molecule by the increasing number of cysteine–arginine interchanges, with 4-CysR/mole (E2/2) being the best case, as opposed to the instability and increased chance of toxic fragmentation of the apoE molecule with lower number of CysR/mole, with 0-CysR/mole (E4/4) as the worst case (Mahley and Huang in Neuron 76:871–885, 2012a ). However, our results also document the appreciable variation of SNI properties within the various CysR/mole groups and individuals which points to the existence and important role of other factors involved in shaping brain function at the network level.

Fiori, F.; Sedda, A.; Ferrè, E.; Toraldo, A.; Querzola, M.; Pasotti, F.; Ovadia, D.; Piroddi, C.; Dell’Aquila, R.; Lunetta, C.; Corbo, M.; Bottini, G.

doi: 10.1007/s00221-013-3465-9

Pradeep Ambati, V.; Murray, Nicholas; Saucedo, Fabricio; Powell, Douglas; Reed-Jones, Rebecca

doi: 10.1007/s00221-013-3466-8pmid: 23479140

Humans use a specific steering synergy, where the eyes and head lead rotation to the new direction, when executing a turn or change in direction. Increasing evidence suggests that eye movement is critical for turning control and that when the eyes are constrained, or participants have difficulties making eye movements, steering control is disrupted. The purpose of the current study was to extend previous research regarding eye movements and steering control to a functional walking and turning task. This study investigated eye, head, trunk, and pelvis kinematics of healthy young adults during a 90° redirection of walking trajectory under two visual conditions: Free Gaze (the eyes were allowed to move naturally in the environment), and Fixed Gaze (participants were required to fixate the eyes on a target in front). Results revealed significant differences in eye, head, and trunk coordination between Free Gaze and Fixed Gaze conditions ( p < 0.001). During Free Gaze, the eyes led reorientation followed by the head and trunk. Intersegment timings between the eyes, head, and trunk were significantly different ( p < 0.05). In contrast, during Fixed Gaze, the segments moved together with no significant differences between segment onset times. In addition, the sequence of segment rotation during Fixed Gaze suggested a bottom-up postural perturbation control strategy in place of top-down steering control seen in Free Gaze. The results of this study support the hypothesis that eye movement is critical for the release of the steering synergy for turning control.

Zabukovec, J.; Boyd, L.; Linsdell, M.; Lam, T.

doi: 10.1007/s00221-013-3468-6pmid: 23494384

Locomotor adaptations to a novel environment can be measured through changes in muscle activity patterns and lower limb kinematics. The location and mechanisms underlying these adaptive changes are unknown. The purposes of the current study were (1) to determine whether corticospinal tract (CST) excitability is altered by resisted walking and (2) to ascertain whether changes in cortical excitability are muscle specific. Forty healthy participants walked with a robotic gait device (Lokomat) that applied a velocity-dependent resistance against hip and knee movements during walking. CST excitability was assessed by quantifying motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation immediately before and after baseline and resisted walking. MEPs were measured in either the biceps femoris (BF) or the rectus femoris (RF). Recruitment curves were collected by stimulating in 5 % increments from 105 to 145 % of active motor threshold. Results demonstrated a significant increase in MEP amplitude in the BF following baseline walking in the Lokomat. The RF did not demonstrate these changes. There was no further change in MEP size following resisted walking in either muscle group. These results suggest that locomotion increases CST excitability in a muscle-specific fashion. As such, it may be important for determining how to enhance the central nervous system’s ability to integrate adaptive strategies during walking.