H.C. Dijkerman

A long-lasting improvement of somatosensory function after prism adaptation, a case study

Previous studies have observed a reduction of visual and representational neglect symptoms after visuo-manual adaptation to rightward displacing prisms. Recently, improvements have also been observed on somatosensory tasks, such as locating the centre of a haptically explored circle and tactile double simultaneous stimulation. In the current single case study we assessed whether prism adaptation with the ipsilesional hand improved two aspects of contralesional somatosensory function, pressure sensitivity and proprioception. After the first application of prism adaptation improvements in pressure sensitivity and proprioception were observed. A second prism adaptation confirmed the improvements in contralesional somatosensory function. The effects of prism adaptation on position sense were longer lasting than have been reported previously, but consistent with reductions of visual neglect symptoms after prism adaptation. The current findings suggest that prism adaptation can have a non-spatial effect on neglect-related supra-modal deficits.

Preserved obstacle avoidance during reaching in patients with left visual neglect.

We asked 12 patients with left visual neglect to bisect the gap between two cylinders or to reach rapidly between them to a more distal target zone. Both tasks demanded a motor response but these responses were quite different in nature. The bisection response was a communicative act whereby the patient indicated the perceived midpoint. The reaching task carried no imperative to bisect the gap, only to maintain a safe distance from either cylinder while steering to the target zone. Optimal performance on either task could only be achieved by reference to the location of both cylinders. Our analysis focused upon the relative influence of the left and right cylinders on the lateral location of the response. In the bisection task, all neglect patients showed qualitatively the same asymmetry, with the left cylinder exerting less influence than the right. In the reaching task, the neglect group behaved like normal subjects, being influenced approximately equally by the two cylinders. This was true for all bar two of the patients, who showed clear neglect in both tasks. We conclude that the visuomotor processing underlying obstacle avoidance during reaching is preserved in most patients with left visual neglect.

Reaching errors in optic ataxia are linked to eye position rather than head or body position.

When reaching towards a visual stimulus, spatial information about the target must be transformed into an appropriate motor command. Visual information is coded initially in retinotopic coordinates, while the reaching movement ultimately requires the specification of the target position in limb-centred coordinates. It is well established that the posterior parietal cortex (PPC) plays an important role in transforming visual target information into motor commands. Lesions in the PPC can result in optic ataxia, a condition in which the visual guidance of goal-directed movements is impaired. Here, we present evidence from two patients with unilateral optic ataxia following right PPC lesions, that the pattern of reaching errors is linked to an eye-centred frame of reference. Both patients made large errors when reaching to visual targets on the left side of space, while facing and fixating straight ahead. By varying the location of fixation and the orientation of the head and body, we were able to establish that these large errors were made specifically to targets to the left of eye-fixation, rather than to the left of head-, body-, or limb-relative space. These data support the idea that visual targets for reaching movements are coded in eye-centred coordinates within the posterior parietal cortex.

Ipsilesional and contralesional sensorimotor function after hemispherectomy: Differences between distal and proximal function

Previous studies have reported mainly on contralesional somatosensory and motor function after hemispherectomy. So far, ipsilesional impairments have received little attention even though these have been reported in patients with less extensive lesions. In the current study we assessed ipsilesional and contralesional sensorimotor function in a group of 12 patients with hemispherectomy. In addition, we focused on differences between distal and proximal function and investigated several factors that may have contributed to individual differences between patients. The tests included tapping, force production, tactile double simultaneous stimulation, pressure sensitivity, passive joint movement sense and sensitivity to hot and cold. Ipsilesional impairments were found on all tests, except passive joint movement sense. Unexpectedly, no significant ipsilateral distal-proximal gradient was found for any of the measures. Both the removal of the diseased cerebral hemisphere and possible changes to the remaining brain structures may have affected ipsilesional sensorimotor function. Contralesional performance was impaired on all tests except for passive joint movement in the shoulder. The contralesional impairments were characterized by a distal-proximal gradient measured on all tests, except that of sensitivity to hot and cold. Distal function was always most impaired. The difference between distal and proximal motor function is in agreement with the established concepts of the motor pathways, with the motoneurons innervating proximal muscles receiving bilateral cortical and subcortical input. Age at onset of original brain damage correlated significantly with passive joint movement sense. Patients with known abnormalities to the remaining brain structures performed inferior on the tapping test only. No effect was found of the hemispheric side of removal.

Differences in finger localisation performance of patients with finger agnosia

Several neuropsychological studies have suggested parallel processing of somatosensory input when localising a tactile stimulus on ones own by pointing towards it (body schema) and when localising this touched location by pointing to it on a map of a hand (body image). Usually these reports describe patients with impaired detection, but intact sensorimotor localisation. This study examined three patients with a lesion of the angular gyrus with intact somatosensory processing, but with selectively disturbed finger identification (finger agnosia). These patients performed normally when pointing towards the touched finger on their own hand but failed to indicate this finger on a drawing of a hand or to name it. Similar defects in the perception of other body parts were not observed. The findings provide converging evidence for the dissociation between body image and body schema and, more importantly, reveal for the first time that this distinction is also present in higher-order cognitive processes selectively for the fingers.

Stimulation of the parietal cortex affects reaching in a patient with epilepsy

A 15-year-old girl with complex partial epilepsy had subdural electrodes implanted. She was tested on reaching tasks during electrocortical stimulation prior to neurosurgery. MRI showed left mesial parietal dysplasia. Stimulation of the superior parietal cortex (electrode pair 37–45, figure, A) during line bisection and visually guided reaching resulted in movement arrest followed by the arm drifting upward (figure, B, …