Motion perception in the ipsilateral visual field of a hemispherectomized patient

Probing Human Visual Deficits with Functional Magnetic Resonance Imaging

Much remains to be understood about visual system malfunction following injury. The resulting deficits range from dense, visual field scotomas to mild dysfunction of visual perception. Despite the predictive value of anatomical localization studies, much patient-to-patient variability remains regarding (a) perceptual abilities following injury and (b) the capacity of individual patients for visual rehabilitation. Visual field perimetry is used to characterize the visual field deficits that result from visual system injury. However, standard perimetry mapping does not always precisely correspond to underlying anatomical or functional deficits. Functional magnetic resonance imaging can be used to probe the function of surviving visual circuits, allowing us to classify better how the pattern of injury relates to residual visual perception. Identifying pathways that are potentially modifiable by training may guide the development of improved strategies for visual rehabilitation. This review discusses primary visual cortex lesions, which cause dense contralateral scotomas.

Functional reorganization after hemispherectomy in humans and animal models: What can we learn about the brain's resilience to extensive unilateral lesions?

Hemispherectomy (HS) is an effective surgical procedure aimed at managing otherwise intractable epilepsy in cases of diffuse unihemispheric pathologies. Neurological recovery in subjects treated with HS is not limited to seizure reduction, rather, sensory-motor and behavioral improvement is often observed. This outcome highlights the considerable capability of the brain to react to such an extensive lesion, by functionally reorganizing and rewiring the cerebral cortex, especially early in life. In this narrative review, we summarize the animal studies as well as the human neurophysiological and neuroimaging studies dealing with the reorganizational processes that occur after HS. These topics are of particular interest in understanding mechanisms of functional recovery after brain injury. HS offers the chance to investigate contralesional hemisphere activity in controlling ipsilateral limb movements, and the role of transcallosal interactions, before and after the surgical procedure. These post-injury neuroplastic phenomena actually differ from those observed after less extensive brain damage. Therefore, they illustrate how different lesions could lead the contralesional hemisphere to play the "good" or "bad" role in functional recovery. These issues may have clinical implications and could inform rehabilitation strategies aiming to improve functional recovery following unilateral hemispheric lesions. Future studies, involving large cohorts of hemispherectomized patients, will be necessary in order to obtain a greater understanding of how cerebral reorganization can contribute to residual sensorimotor, visual and auditory functions.

Adaptive neuroplastic responses in early and late hemispherectomized monkeys

Behavioural recovery in children who undergo medically required hemispherectomy showcase the remarkable ability of the cerebral cortex to adapt and reorganize following insult early in life. Case study data suggest that lesions sustained early in childhood lead to better recovery compared to those that occur later in life. In these children, it is possible that neural reorganization had begun prior to surgery but was masked by the dysfunctional hemisphere. The degree of neural reorganization has been difficult to study systematically in human infants. Here we present a 20-year culmination of data on our nonhuman primate model (Chlorocebus sabeus) of early-life hemispherectomy in which behavioral recovery is interpreted in light of plastic processes that lead to the anatomical reorganization of the early-damaged brain. The model presented here suggests that significant functional recovery occurs after the removal of one hemisphere in monkeys with no preexisting neurological dysfunctions. Human and primate studies suggest a critical role for subcortical and brainstem structures as well as corticospinal tracts in the neuroanatomical reorganization which result in the remarkable behavioral recovery following hemispherectomy. The non-human primate model presented here offers a unique opportunity for studying the behavioral and functional neuroanatomical reorganization that underlies developmental plasticity.

Partial recovery of hemiparesis following hemispherectomy in infant monkeys

Hemiparesis, unilateral weakness or partial paralysis, is a common outcome following hemispherectomy in humans. We use the non-human primate as an invaluable translational model for our understanding of developmental plasticity in response to hemispherectomy. Three infant vervet monkeys (Chlorocebus sabeus) underwent hemispherectomy at a median age of 9 weeks and two additional monkeys at 48 months. Gross motor assessment was conducted in a large open field that contained a horizontal bar spanning the width of the cage. Subjects were assessed yearly following surgery in infantile lesions for a period of 3 years. Adult-lesioned subjects were assessed 40 months following surgery. Shortly after surgery both infant and adult-lesioned subjects were unable to move the contralateral side of their body, but all subjects were able to walk within 6 months following surgery. At each time point the lower limb gait was normal in infant-lesioned subjects with no apparent limp or dragging, however the upper limb demonstrated significant impairment. Horizontal bar crossing was significantly impaired during the first 24 months following surgery. Adult-lesioned subjects also displayed upper limb movement impairments similar to infant-lesioned subjects. In addition the adult-lesioned subjects displayed a noticeable lower limb limp, which was not observed in the infant-lesioned group. Both groups at each time point showed a propensity for ipsiversive turning. The upper limb gait impairment and horizontal bar crossing of lesioned subjects are reminiscent of hemiparesis seen in hemisperectomized humans with the young-lesioned subjects showing a greater propensity for recovery.

Training-induced cortical representation of a hemianopic hemifield

BACKGROUND

Patients with homonymous hemianopia often have some residual sensitivity for visual stimuli in their blind hemifield. Previous imaging studies suggest an important role for extrastriate cortical areas in such residual vision, but results of training to improve vision in patients with hemianopia are conflicting.

OBJECTIVE

To show that intensive training with flicker stimulation in the chronic stage of stroke can reorganise visual cortices of an adult patient.

METHODS

A 61-year-old patient with homonymous hemianopia was trained with flicker stimulation, starting 22 months after stroke. Changes in functioning during training were documented with magnetoencephalography, and the cortical organisation after training was examined with functional magnetic resonance imaging (fMRI).

RESULTS

Both imaging methods showed that, after training, visual information from both hemifields was processed mainly in the intact hemisphere. The fMRI mapping results showed the representations of both the blind and the normal hemifield in the same set of cortical areas in the intact hemisphere, more specifically in the visual motion-sensitive area V5, in a region around the superior temporal sulcus and in retinotopic visual areas V1 (primary visual cortex), V2, V3 and V3a.

CONCLUSIONS

Intensive training of a blind hemifield can induce cortical reorganisation in an adult patient, and this case shows an ipsilateral representation of the trained visual hemifield in several cortical areas, including the primary visual cortex.

Areas MT/V5 and their transcallosal connectivity in cortical dysplasia by fMRI

A patient with unilateral focal cortical dysplasia with a significant impairment of visual motion perception within the contralateral hemifield was examined with fMRI. During hemifield visual motion stimulation primary visual cortex areas were activated contralaterally and deactivated ipsilaterally to the stimulated hemifield. Transcallosal visuo-visual interaction was further evident as bilateral activation in temporo-occipital areas that best correspond to the motion sensitive areas MT/V5. MT/V5 was displaced anteriorly, superiorly, and medially within the dysplastic hemisphere and separated into two distinct activation clusters. During visual motion stimulation the parieto-insular vestibular cortex showed signal decreases that agree with the concept of inhibitory visuo-vestibular interaction. Thus, fMRI is a suitable tool for detecting preserved function and transcallosal connections in patients with focal cortical dysplasia.