Reactive Postural Response to Lateral Perturbation in Patients with the Posterior Parietal Cortex Lesion: eficits and Effects of Precues
Date Issued
2008
Date
2008
Author(s)
Lin, Ying-Hui
Abstract
Background and Purpose─ Reactive postural control ability depends upon the integration of all sensory inputs and descending motor drives. Recent studies suggest that all sensory inputs are integrated and attended, and then are transformed into motor commands (sensory-motor transformation) in the posterior parietal cortex (PPC) for planning and executing movements. Therefore, it is expected that lesions over the PPC would cause deficits in attention and motor control. However, little empirical evidence has been provided regarding standing balance deficits and effects of precues in patients with the PPC lesion. ethods─ Nine patients with stroke (mean age= 60.8±13.9yrs) with lesions involving the PPC region (PPC+ group), nine patients with stroke (mean age= 69.9±10.3yrs) with lesions not involving the PPC region (PPC- group) and nine healthy, age-matched adults (mean age= 67.9±13.7yrs) participated in this study (healthy group).ll participants were asked to maintain standing balance without stepping, if possible, after experiencing lateral pulling perturbation in the no-cue and cue conditions. There were 6 trials each in the no-cue and cue conditions. An auditory precue regarding the pulling direction was given to the participants in the cue condition. Postural muscle activation patterns, the earliest muscle onset latency (OLEarliest), reaction time (RT) of lateral force, time-to-peak phase duration (PDTTP) and total force impulse (IMPTotal) were analyzed to investigate the performance of each participant.esults─ In the no-cue condition, the gluteus medius (GM) and peroneal longus (PL) muscles of the leg on the side ipsilateral to the pulling perturbation direction (GMIpsi and PLIpsi) were the predominant muscles generating postural responses in the healthy group (firing rate: 88.5% and 73.1%, respectively). Similar results were also found when both stroke groups responded to perturbation towards the unaffected side. When responding to perturbation towards the affected side, the GM of the leg contralateral to the perturbation direction (GMContra) became one of the predominant muscles. Half subjects of the PPC- relied on co-contraction of GMIpsi and GMContra for reactive responses. And one-third of the subjects in the PPC+ groups relied on GMContra in response to perturbation. Compared to the healthy (OLEarliest = 129.1 ms) and PPC- (OLEarliest = 136.3 ms) groups, the PPC+ group showed longer OLEarliest (167.0 ms) while responding to perturbation towards the affected side (healthy vs. PPC+: p = .008; PPC- vs. PPC+: p = .039), but there was no difference in OLEarliest between the healthy and PPC- groups. In addition, the PPC+ group, but not the PPC- group, showed significantly longer OLEarliest when responding to perturbation towards the affected side (167.0 ms) than unaffected side (124.8 ms) (p = .001). For the RT and PDTTP, there was no group, neither direction, main effect. For the IMPTotal, both stroke groups generated smaller IMPTotal than the healthy group after a pull to the affected side (healthy vs. PPC-: p = .004; healthy vs. PPC+: p = .028). After cuing, the muscle activation patterns of the PPC+ group in response to a pull to the affected side became similar to those of the healthy group, and the healthy group showed shorter RT (p = .035). There were no significant effects of the auditory cue on OLEarliest, PDTTP and IMPTotal (p > .05) in this study.iscussion and Conclusions─ The PPC+ group showed the poorest reactive postural responses to lateral pulls among these three groups, which may be due to their deficits of sensory-motor transformation. Provision of a prior auditory cue seems to improve the construction or selection of proper postural strategy, but not the time needed to generate sufficient muscle or force responses, in patients with the PPC lesion. These temporal parameters may be determined by how fast the patients could process and integrate all sensory information, how fast they could perform the sensory-motor transformation and how fast they could generate force necessary for postural responses. Our auditory cue did not seem to improve the rate of information processing, the sensory-motor transformation and force generation of the PPC+ subjects. Further studies are required to investigate whether precue of other forms, such as visual or tactile cues, would effectively improve reactive postural responses of these patients.
Subjects
stroke
motor control
reactive postural response
posterior parietal cortex
cue
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