Visual perceptions of head-fixed and trunk-fixed anterior/posterior axes

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.

Head movement control in visually guided tasks: postural goal and optimality

This work investigates the control of horizontal head movements in the context of unconstrained visually guided head and arm/finger aiming tasks. In a first experiment, the head was free to move while gaze was directed at randomly presented eccentric targets distributed horizontally (0 degrees-120 degrees) at eye level. In a second experiment, the horizontal head orientation was constrained to predetermined positions (0 degrees, 15 degrees, 30 degrees, 45 degrees or 60 degrees rightward) while the right index finger aimed at targets with the arm fully extended. Kinematics of head movements in gaze displacements exhibits an initial component weakly correlated with target position, followed by multiple corrections. Since the eyes are assumed to already be aimed at the target when the corrections occur, it is suggested that one goal of head movement control is to achieve a desired final orientation (posture). This hypothesis is supported by results from the second experiment that reveal an association between eye/head orientation angles and errors exhibited in the visuo-spatial representation of the environment. The minimization of error then underlies the control of head movement as a postural response optimized for a given target and task condition.

Perceived versus actual head-on-trunk orientation during arm movement control

Static roll head tilt induces bias in the trajectory of upper limb voluntary movements. The aim of the experiment was to investigate whether this bias is dependant on the perception of body configuration rather than on its actual configuration. We used the 'return' phenomenon as a method to produce dissociation between perceived and actual head tilt. Static roll head tilt in supine subjects was sustained for 15 min during which subjects were periodically required to estimate verbally the tilt of their head respective to their trunk and draw, with their right index finger, straight lines aligned with their trunk. After 15 min, subjects' head were realigned with the trunk, and subjects continued to give verbal estimate of head position and perform the motor task. Results showed that the initial angular deviation of the lines in the direction opposite to head tilt gradually diminished. The adaptation was noticeable within the first 3-5 min of tilt and subsequently diminished. Verbal estimates confirmed the return phenomenon, i.e. subjects perceived their head as slowly returning towards its neutral position after a few minutes of sustained tilt. When realigned with the trunk, subjects experienced the illusion that their head was tilted in the opposite direction to the initial head tilt and a line deviation in the opposite direction to those made on initial exposure was observed (after-effect). These results indicate that the angular deviation in motor production observed in condition of static head tilt were largely related to the perceived body configuration and therefore favour the hypothesis that the conscious perception of body configuration plays a key role in organising sensorimotor tasks.

The visual perception coordinate system uses axes defined by the earth, trunk, and vision

Eight young adults adjusted a line located on one side of a computer display parallel to internally specified Earth-fixed vertical (display in frontal plane), to the horizontal trunk-fixed anterior-posterior axis (display in horizontal plane), and to an oblique line (display in horizontal and vertical planes). All tasks were completed in a dark room with the head and trunk in both a standard erect posture and varied postures. Errors were lowest when setting the line to internally specified vertical in the frontal plane and to an oblique line in the horizontal plane when head and trunk orientations were varied. Constant errors for setting one line parallel to a second line were in opposite directions when the second line was located on the left versus right side of the display, but did not differ in direction when setting the line parallel to internally specified axes. Also, the oblique effect was preserved when the head and trunk were tilted to various orientations, suggesting that it results from integration of an internally specified gravitational reference with visual input. We conclude that the visual perceptual coordinate system uses internally specified vertical and, when available, a visually specified horizontal reference axis to define object orientation.

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.

Unilateral posterior parietal lobe lesions affect representation of visual space

This study assessed accuracy of visually perceived vertical and trunk median plane orientation in 41 subjects: 17 had unilateral brain lesions including the posterior parietal lobe (PPL), 8 had lesions outside PPL, and 16 were neurologically normal. Vertical perception errors clearly increased with size of unilateral lesions to PPL and posterior superior temporal gyrus (PSTG). Median plane perception errors increased only slightly with size of unilateral lesions to frontal lobe premotor areas and supramarginal gyrus. These results are compatible with the hypothesis that accurate visual vertical perception depends critically on intact PPL and PSTG in both cerebral hemispheres while accurate median plane perception likely involves a bihemispheric network that can compensate for lesions to one hemisphere.

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.

Patterns of transfer of adaptation among body segments

Two experiments were conducted in order to determine the patterns of transfer of visuomotor adaptation between arm and head pointing. An altered gain of display of pointing movements was used to induce a conflict between visual and somatosensory representations. Two subject groups participated in Experiment 1: group 1 adapted shoulder pointing movements, and group 2 adapted wrist pointing movements to a 0.5 gain of display. Following the adaptation regimen, subjects performed a transfer test in which the shoulder group performed wrist movements and the wrist group performed shoulder movements. The results demonstrated that both groups displayed typical adaptation curves, initially undershooting the target followed by a return to baseline performance. Transfer tests revealed that both groups had high transfer of the acquired adaptation to the other joint. Experiment 2 followed a similar design except that group 1 adapted head pointing movements and group 2 adapted arm pointing movements. The arm adaptation had high transfer to head pointing while the head adaptation had very little transfer to arm pointing. These results imply that, while the arm segments may share a common target representation for goal-directed actions, individual but functionally dependent target representations may exist for the control of head and arm movements.

In search of biased egocentric reference frames in neglect

The present study was aimed at assessing, by means of visual as well as proprioceptive-kinaesthetic straight-ahead tasks, the possible causal role of the ipsilesional deviation of the egocentric reference frame in determining neglect syndrome. The hypothesis, originally proposed by Ventre et al. [3], that an alteration of the representation of body-centred space can be a cause of asymmetrical spatial behaviour in humans has been recently revived by Karnath and co-workers [24]. The results of the present study seem to challenge the view that a systematic ipsilesional displacement of the egocentric reference is the crucial mechanism responsible for unilateral visual neglect. Under visual conditions, in which patients were required to stop a moving spot as it crossed their perceived midline, the ipsilesional deviation of the egocentric reference frame was dependent upon the direction of visual scanning. Right to left visual scanning direction produced a rightward displacement of the egocentric reference. In contrast, left to right visual scanning direction allowed neglect patients to correctly locate their perceived egocentre with an accuracy which did not differ from controls. The notion that the effect of a deviation of the egocentric reference frame is actually dependent on a bias in the visual scanning orienting response was also confirmed in the proprioceptive straight-ahead pointing tasks, in which the patients were blindfolded and therefore no visual information was available. In these conditions, in which patients were required to judge the subjective midline by using head, trunk and shoulder co-ordinate systems, the displacement of the subjective egocentric midline was not present.