Kinesthetic perceptions of intrinsic anterior-posterior axes

Testing the limits of optimal integration of visual and proprioceptive information of path trajectory

Many studies provide evidence that information from different modalities is integrated following the maximum likelihood estimation model (MLE). For instance, we recently found that visual and proprioceptive path trajectories are optimally combined (Reuschel et al. in Exp Brain Res 201:853-862, 2010). However, other studies have failed to reveal optimal integration of such dynamic information. In the present study, we aim to generalize our previous findings to different parts of the workspace (central, ipsilateral, or contralateral) and to different types of judgments (relative vs. absolute). Participants made relative judgments by judging whether an angular path was acute or obtuse, or they made absolute judgments by judging whether a one-segmented straight path was directed to left or right. Trajectories were presented in the visual, proprioceptive, or combined visual-proprioceptive modality. We measured the bias and the variance of these estimates and predicted both parameters using the MLE. In accordance with the MLE model, participants linearly combined and weighted the unimodal angular path information by their reliabilities irrespective of the side of workspace. However, the precision of bimodal estimates was not greater than that for unimodal estimates, which is inconsistent with the MLE. For the absolute judgment task, participants' estimates were highly accurate and did not differ across modalities. Thus, we were unable to test whether the bimodal percept resulted as a weighted average of the visual and proprioceptive input. Additionally, participants were not more precise in the bimodal compared with the unimodal conditions, which is inconsistent with the MLE. Current findings suggest that optimal integration of visual and proprioceptive information of path trajectory only applies in some conditions.

Visual cortex activation in kinesthetic guidance of reaching

The purpose of this research was to determine the cortical circuit involved in encoding and controlling kinesthetically guided reaching movements. We used (15)O-butanol positron emission tomography in ten blindfolded able-bodied volunteers in a factorial experiment in which arm (left/right) used to encode target location and to reach back to the remembered location and hemispace of target location (left/right side of midsagittal plane) varied systematically. During encoding of a target the experimenter guided the hand to touch the index fingertip to an external target and then returned the hand to the start location. After a short delay the subject voluntarily moved the same hand back to the remembered target location. SPM99 analysis of the PET data contrasting left versus right hand reaching showed increased (P < 0.05, corrected) neural activity in the sensorimotor cortex, premotor cortex and posterior parietal lobule (PPL) contralateral to the moving hand. Additional neural activation was observed in prefrontal cortex and visual association areas of occipital and parietal lobes contralateral and ipsilateral to the reaching hand. There was no statistically significant effect of target location in left versus right hemispace nor was there an interaction of hand and hemispace effects. Structural equation modeling showed that parietal lobe visual association areas contributed to kinesthetic processing by both hands but occipital lobe visual areas contributed only during dominant hand kinesthetic processing. This visual processing may also involve visualization of kinesthetically guided target location and use of the same network employed to guide reaches to visual targets when reaching to kinesthetic targets. The present work clearly demonstrates a network for kinesthetic processing that includes higher visual processing areas in the PPL for both upper limbs and processing in occipital lobe visual areas for the dominant limb.

Unilateral posterior parietal lobe lesions disrupt kinaesthetic representation of forearm orientation

OBJECTIVE

To apply the lesion method to assess neuroanatomical substrates for judgments of forearm orientation from proprioceptive cues in humans.

METHODS

Participants were 15 subjects with chronic unilateral brain lesions and stable behavioural deficits, and 14 neurologically normal controls. Subjects aligned the forearm to earth fixed vertical and trunk fixed anterior-posterior (A-P) axes ("straight ahead"), with the head aligned to the trunk and with head and shoulder orientations varied on each trial.

RESULTS

Most subjects with posterior parietal lobe lesions made larger variable errors than controls in aligning the forearm to the earth fixed vertical axis and the trunk A-P axes, whether the head was held upright or oriented in different positions. Lesion subjects and controls made similar constant errors for aligning the forearm to gravitational vertical. Variable error magnitude correlated positively with greater lesion volume of right and left superior parietal lobules (SPL), but not with lesions in other brain areas. Larger variable errors for aligning the forearm to the trunk fixed A-P axis were also correlated with the volume of SPL lesions, but constant error magnitude correlated with larger volume lesions in premotor areas, inferior parietal lobules, and posterior regions of the superior temporal gyri, but not with SPL lesion volume.

CONCLUSIONS

The findings suggest that the right and left superior and inferior parietal lobules, posterior superior temporal gyri, and premotor areas play a role in defining higher level coordinate systems for specifying orientation of the right and left forearm.

Kinesthetic perception of visually specified axes

The purpose of this research was to determine whether human subjects could align the forearm more accurately to the orientation of an external object than to earth-fixed vertical and trunk-fixed anterior-posterior (a-p) axes. Ten young adults aligned the unseen forearm to earth-fixed vertical and trunk-fixed a-p axes, and to a visually presented rod (external visual axis) held by an experimenter in various oblique vertical and horizontal orientations. The head and trunk orientations were varied by left/right lateral flexion when aligning the forearm to vertical plane axes and by rotation about the vertical axis when aligning the forearm to horizontal plane axes. Perceptual errors for aligning the forearm to vertical plane axes were much lower when aligning the forearm to earth-fixed vertical than to an external visual axis positioned in a vertical plane. Furthermore, the perceptual errors for aligning the forearm to the visually presented rod were correlated with rod orientation while errors for aligning the forearm to vertical while viewing the rod were unaffected by rod orientations. Clearly, human subjects cannot use an oblique external visually presented axis to provide a frame of reference for accurate perception of forearm orientation in vertical planes. Perceptual errors were similar for aligning the forearm to the horizontal trunk-fixed a-p axis and external visual axis when head and trunk orientation were varied. These perceptual errors were not correlated with rod orientation in the horizontal plane, giving no evidence of bias toward the trunk or external visual axis in horizontal plane perception of forearm orientation. Thus, humans can use either the trunk-fixed a-p axis or the visually specified orientation of an external object as a frame of reference for the kinesthetic system to specify forearm orientation in the horizontal plane.

Lateralized effects on reaching by children

This study examined the limb selection profiles of children for a reach-to-grasp task presented in various positions of hemispace. Underlying questions focused on the use of attentional information and lateralized effects in motor programming for reaching movements. As expected, both right- and left-handed groups used their dominant limb more frequently at the midline and in their own ipsilateral hemispace. However, in response to stimuli presented in contralateral (to the dominant limb) hemispace, both groups switched to using their nondominant limbs at significant levels. As a general comparison, right-handers exhibited greater use of their dominant limb, but arguably, motor dominance in this context may have intervened with the participant's ability to use attentional information to produce a more efficient response. Overall, these findings address the phenomenon associated with motor dominance and use of attentional information in programming.

Neural adaptation in the generation of rhythmic behavior

Motor systems can adapt rapidly to changes in external conditions and to switching of internal goals. They can also adapt slowly in response to training, alterations in the mechanics of the system, and any changes in the system resulting from injury. This article reviews the mechanisms underlying short- and long-term adaptation in rhythmic motor systems. The neuronal networks underlying the generation of rhythmic motor patterns (central pattern generators; CPGs) are extremely flexible. Neuromodulators, central commands, and afferent signals all influence the pattern produced by a CPG by altering the cellular and synaptic properties of individual neurons and the coupling between different populations of neurons. This flexibility allows the generation of a variety of motor patterns appropriate for the mechanical requirements of different forms of a behavior. The matching of motor output to mechanical requirements depends on the capacity of pattern-generating networks to adapt to slow changes in body mechanics and persistent errors in performance. Afferent feedback from body and limb proprioceptors likely plays an important role in driving these long-term adaptive processes.