Kenji Kunita

Changes in the activity of the cerebral cortex relate to postural response modification when warned of a perturbation

OBJECTIVEnTo determine whether the cerebral cortex contributes to modifying upcoming postural responses to external perturbations when provided with prior warning of the perturbation.nnnMETHODSnElectroencephalographic (EEG) potentials were recorded from 12 healthy human subjects (21-32 years of age) before perturbing their balance with backward translations of a platform under their feet. The subjects responded with and without a visual cue that warned them 2s before the perturbation (the Cue and No Cue conditions, respectively).nnnRESULTSnContingent negative variation (CNV) was evident before perturbation onset in only the Cue condition. In the Cue condition, the subjects also produced smaller center of pressure (CoP) displacements than in the No Cue condition. The cue-related difference in the subjects CNV potentials correlated with the cue-related difference in their CoP displacements. No significant associations existed among the CNV potentials and any cue-related postural adjustments made before the perturbation.nnnCONCLUSIONSnCortical activity before an externally triggered perturbation associates with modifications of the ensuing postural response.nnnSIGNIFICANCEnThis is the first study to demonstrate a cortical correlate for changes in central postural set that modify externally triggered postural responses based on anticipation.

Increase in corticospinal excitability of limb and trunk muscles according to maintenance of neck flexion

The effect of maintenance of neck flexion on corticospinal excitability of limb and trunk muscles was investigated using transcranial magnetic stimulation (TMS). Nine healthy young subjects participated in this experiment. Every measurement was performed with subjects sitting on a chair. Target muscles were the first dorsal interosseous (FDI), biceps brachii (BB), triceps brachii (TB), rectus abdominis (RA), erector spinae (ES), rectus femoris (RF), biceps femoris (BF), tibialis anterior (TA), and gastrocnemius (GcM) on the right side. TMS was applied to the left primary motor cortex, and motor evoked potential (MEP) was measured from the muscles listed above. Optimal stimulus location and resting motor threshold (RMT) were identified for each target muscle, and stimulus intensity used was 120% of RMT. MEPs of the target muscle were recorded with the chin resting on a chin support (chin-on condition) with neck in 20 degrees of flexion, and with voluntary maintenance of the neck flexion posture (chin-off condition). Amplitude and latency of MEP and background activity of target muscles were analyzed. For FDI, BB, TB, ES, and RF, amplitude of MEP increased and latency shortened in the chin-off compared with the chin-on condition. No significant difference in background activity of each target muscle was found between the two conditions. Corticospinal excitability of limb and trunk muscles was selectively enhanced while neck flexion was maintained.

Postural Movement Pattern and Muscle Action Sequence Associated with Self-Paced Bilateral Arm Flexion during Standing

Investigated were postural movement pattern and action sequence of postural muscles while subjects rapidly flexed both arms during standing. The arm movement was started at the subjects own pace. Subjects were healthy individuals; 48 men and 53 women. Postural movement pattern was classified based on the movement angles of foot-leg (ankle joint) and leg-trunk (hip joint). Electromyograms were recorded from the anterior deltoid, biceps femoris, and erector spinae. The time difference between action onsets of the latter two muscles and the anterior deltoid was analyzed. Movement angles of the ankle and hip for both sexes were distributed on a similar linear regression line (y = −2.092 × −2.552 (r = −.71). The postural movement pattern was categorized based on the distribution into three types: hip flexion (in the 2nd quadrant), backward leaning (the 3rd), and hip extension (the 4th). The proportion of subjects was 26% in the hip flexion type, 55% in backward leaning type, and 19% in hip extension type. The action of biceps femoris and erector spinae significantly preceded that of anterior deltoid in the backward leaning and hip extension types but did not in the hip flexion type.

EFFECT OF VIBRATION STIMULATION TO NECK EXTENSOR MUSCLES ON REACTION TIME IN VARIOUS SACCADIC EYE MOVEMENTS

We investigated the effect of vibration stimulation to the neck extensors on reaction time in various saccades that are controlled by different higher neural system. Visually guided, memory-guided and anti saccades were performed under no-vibration and vibration (frequency, 100 Hz; amplitude, 0.5 mm) applied to the trapezius muscles. For memory-guided and anti saccades, the reaction time under the vibration was significantly shorter than that under no-vibration. A significant correlation was found between the shortening of reaction time for memory-guided saccade and that for visually guided saccade (r = 0.726, p <.01), but no significant correlation was found between anti-saccade and other saccades. These findings suggested that the higher saccadic neural pathway was strongly activated during the vibration stimulation to the neck extensors, and furthermore the effect of activation on facilitatory and inhibitory functions differed among the three types of saccades.

Saccades and Prefrontal Hemodynamics in Basketball Players

We investigated saccade performance and prefrontal hemodynamics in basketball players with different skill levels. Subjects were 27 undergraduate basketball players and 13 non-athlete undergraduates (control group: CON). The players were divided into two groups: those who had played in the National Athletic Meet during high school or played regularly (n=13, elite group: ELI) and those who were bench warmers (n=14, skilled group: SKI). Horizontal eye movement and oxy-, deoxy-, and total-hemoglobin (Hb) concentration in the prefrontal cortex during pro- and anti-saccade were measured using electro-oculography and near-infrared spectroscopy, respectively. Only error rate in anti-saccade was less in ELI (4.8+/-4.0%) than SKI (13.7+/-12.6%) and CON (13.9+/-8.3%) (p<0.05). In ELI alone, oxy- (-0.15+/-0.18 mmol*mm) and total-Hb (-0.12+/-0.15 mmol*mm) during anti-saccade decreased significantly compared with that during rest (p<0.05), while those in CON significantly increased (oxy-Hb: 0.17+/-0.15 mmol*mm, total-Hb: 0.14+/-0.14 mmol*mm) (p<0.05). These results suggest that inhibition of eye movement to a visual target changes from voluntary to automatic through the motor learning of basketball.

Neck-shortening effect on prosaccade reaction time formed through saccadic training accompanied by maintenance of neck flexion

We investigated the effect of neck-shortening on prosaccade reaction time formed through saccadic training accompanied by maintenance of neck flexion. The subjects were 30 university students who exhibited no significant shortening of prosaccade reaction time during maintenance of neck flexion, assigned to three groups: prosaccade training subjects at rest neck position (rest training group); prosaccade training subjects at 20° neck flexion position (neck training group); and untrained subjects (control group). Saccadic training for 1xa0min was performed ten times per day, and the training period was 14xa0days. For the control group, no significant postural or training effects on reaction time were found. For both training groups, reaction time at the rest position after training was significantly shorter than that before training. For the neck training group, reaction time after training was significantly shorter at the neck flexion position than at the rest position. Conversely, no significant neck effect was found for the rest training group. This indicates that the shortening effect associated with maintenance of neck flexion on prosaccade reaction time is formed through saccadic training accompanied by maintenance of neck flexion.

Effect of maintaining neck flexion on anti-saccade reaction time: an investigation using transcranial magnetic stimulation to the frontal oculomotor field

BackgroundReaction time for anti-saccade, in which the gaze is directed to the position opposite to an illuminated target, shortens during maintenance of neck flexion. The present study applied transcranial magnetic stimulation (TMS) to the frontal oculomotor field, and investigated the effect of maintaining neck flexion on information processing time in the anti-saccade neural pathway before the frontal oculomotor field.MethodsThe reaction time was measured with the chin resting on a stand (‘chin-on’ condition) and with voluntary maintenance of neck flexion (‘chin-off’ condition) at 80% maximal neck flexion angle, with and without TMS. The TMS timing producing the longest prolongation of the reaction time was first roughly identified for 10xa0ms intervals from 0 to 180xa0ms after the target presentation. Thereafter, TMS timing was set finely at 2xa0ms intervals from −20 to +20xa0ms of the 10xa0ms step that produced the longest prolongation.ResultsThe reaction time without TMS was significantly shorter (21.9xa0ms) for the chin-off (235.9u2009±u200914.9xa0ms) than for the chin-on (257.5u2009±u200917.1xa0ms) condition. Furthermore, TMS timing producing maximal prolongation of the reaction time was significantly earlier (18.6xa0ms) for the chin-off than the chin-on condition. The ratio of the forward shift in TMS timing relative to the reduction in reaction time was 87.8%.ConclusionsWe confirmed that information processing time in the anti-saccade neural pathway before the frontal oculomotor field shortened while neck flexion was maintained, and that this reduction time accounted for approximately 88% of the shortening of reaction time.

Developmental changes in shortening of pro-saccade reaction time while maintaining neck flexion position

BackgroundWe investigated developmental changes in shortening of pro-saccade reaction time while maintaining neck flexion.MethodsSubjects comprised 135 children (3–14xa0years) and 29 young adults (19–23xa0years). Children were divided into six groups in 2-year age strata. Pro-saccade reaction tasks for 30xa0s were performed in neck rest and flexion positions. Reaction times under each position were averaged in every 10-s period.ResultsUnder neck rest position, reaction time in the 0–10xa0s period was significantly longer in the 3- to 4-year-old group than in the 5- to 6-year-old group and above. No significant age effect was found for reaction time in the 0–10xa0s period in the 5- to 6-year-old group and above. Although a significant effect of neck flexion was not observed until the 9- to 10-year-old group, significant shortening of reaction time with neck flexion was found in the 11- to 12-year-old group and above. Furthermore, this shortening was maintained until the first 20–s period in the 11- to 12-year-old group and during the entire 30xa0s in the 13- to 14-year-old and above.ConclusionsThese results suggest that brain activation with the maintenance of neck flexion, related to shortening of the pro-saccade reaction time, was found from a later age of approximately 11xa0years and above, compared with the age at which information-processing function in the pro-saccade was enhanced. In addition, brain activation with the maintenance of neck flexion was sustained longer with age.

P29-3 Investigation on shortening of anti-saccade reaction time associated with neck flexion by transcranial magnetic stimulation to the frontal eye field

When a briefly presented target is surrounded by four dots that onset at the same time as the target but remain visible after the target terminates, the four dots reduce a person’s ability to identify the target. This phenomenon called object substitution masking (OSM). In the present study we investigated temporal aspects of OSM using a transcranial magnetic stimulation (TMS) to V1 and hV5/MT+ that might play an important role for OSM. Ten participants volunteered for the study. We employed the paradigm proposed by Di Lollo et al. (2000) which could induce OSM effects. Search items were circles with a gap, similar to Landolt-Cs. In the stimulus array, eight items were regularly spaced on an imaginary circle (3.6o in radius). The mask was composed of four dots (0.11o) presented on the vertices of an imaginary square (1.61o×1.61o) centered on one of eight items. Luminance of stimulus was chosen to produce greatest OSM effects in each subject. In a half number of trials, that were randomly chosen, the screen turned black after the target offset (simultaneous-offset trials). In the remaining trials, the sequence was the same, except that the mask remained after the target offset for 160 ms (delayed-offset trials). There were two TMS conditions with double-pulse TMS applied over V1 at 40 80 ms and hV5/MT+ at 80 120 ms from target stimulus offset. It was found that the amount of OSM (simultaneous-offset accuracy minus delayed-offset accuracy) decreased in both conditions at V1 and at hV5/MT+ for subjects who showed a large OSM amount without TMS. Our results showed that both regions in V1 and hV5/MT+ were significantly influenced by TMS, suggesting that OSM, at least in part, is mediated by these areas. We conclude that TMS of V1 at 40 80 ms and hV5/MT+ at 80 120 ms reduced OSM.