Graded sparing of visually-guided orienting following primary visual cortex ablations within the first postnatal month

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.

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.

Greater Sparing of Visual Search Abilities in Children After Congenital Rather Than Acquired Focal Brain Damage

Background. Visual search refers to the capacity of an individual to find a target among simultaneously presented distracters and is based on visual abilities such as a fast visual processing and an accurate control of ballistic eye movements (saccades) that guide the fovea to the target location. Objective. In adults, visual field defects caused by brain damage are often associated with visual search disorders; in children, little is known about the effects of early brain lesions on visual search abilities. Methods. To test the presence of visual search defects and to investigate the role of cortical plasticity after early brain lesions, 29 children with congenital or acquired cerebral lesions, with and without visual field defects, underwent a visual search test battery. Results. The children with acquired lesions and visual field defects had longer reaction times (RTs) in the contralesional visual field compared with the ipsilesional, whereas those with congenital lesions and visual field defects did not have differences in RTs between the contralateral and ipsilateral visual fields and had a visual search pattern similar to children without a visual field defect. Conclusions. These findings support the hypothesis of more effective mechanisms of functional compensation and reorganization of the visual system in children with very early brain lesions, as opposed to those with later damage.

Plasticity of the visual system after early brain damage

The aim of this review is to discuss the existing evidence supporting different processes of visual brain plasticity after early damage, as opposed to damage that occurs during adulthood. There is initial evidence that some of the neuroplastic mechanisms adopted by the brain after early damage to the visual system are unavailable at a later stage. These are, for example, the ability to differentiate functional tissue within a larger dysplastic cortex during its formation, or to develop new thalamo-cortical connections able to bypass the lesion and reach their cortical destination in the occipital cortex. The young brain also uses the same mechanisms available at later stages of development but in a more efficient way. For example, in people with visual field defects of central origin, the anatomical expansion of the extrastriatal visual network is greater after an early lesion than after a later one, which results in more efficient mechanisms of visual exploration of the blind field. A similar mechanism is likely to support some of the differences found in people with blindsight, the phenomenon of unconscious visual perception in the blind field. In particular, compared with people with late lesions, those with early brain damage appear to have stronger subjective awareness of stimuli hitting the blind visual field, reported as a conscious feeling that something is present in the visual field. Expanding our knowledge of these mechanisms could help the development of early therapeutic interventions aimed at supporting and enhancing visual reorganization at a time of greatest potential brain plasticity.

Retinal projections to the lateral posterior-pulvinar complex in intact and early visual cortex lesioned cats

In intact cats, it is generally considered that the lateral posterior-pulvinar complex (LP-pulvinar) does not receive direct retinal terminals, with the exception of the retino-recipient zone known as the geniculate wing. There is, however, some evidence that early lesions of the visual cortex can occasionally induce the formation of novel retinal projections to the LP nucleus. Given the importance of knowing the connectivity pattern of the LP-pulvinar complex in intact and lesioned animals, we used the B fragment of cholera toxin, a sensitive anterograde tracer, to reinvestigate the retinal projections to the LP-pulvinar in normal cats and in cats with early unilateral lesions of the visual cortex (areas 17 and 18). Immunohistochemical localization of the toxin was performed to show the distribution and morphology of retinofugal terminals. A direct bilateral but predominantly contralateral retinal projection reached the caudal portion of LPl and LPm in the form of patches located mainly along its dorsomedial surface and many scattered terminals. The distribution of retinal projections to LP-pulvinar in intact and operated cats did not differ. Contrary to what had been previously reported, we found no evidence for lesion-induced sprouting of retinal axons in these higher-order thalamic nuclei. Retinal input to the LP-pulvinar might modulate visual responses driven by primary visual cortex or superior colliculus.

Greater sparing of visually guided orienting behavior after early unilateral occipital lesions: insights from a comparison with the impact of bilateral lesions

We know that cats with bilateral lesions of occipital visual cortical areas 17, 18 and 19 sustained during the first postnatal week exhibit a modest level of sparing of the ability to re-orient head and eyes to new stimuli relative to cats that incurred equivalent lesions in adulthood. We now report that cats with equivalent unilateral lesions sustained during the first postnatal week (P1-4), or at the end of the first postnatal month (P27-30), orient to stimuli presented in the contralesional field as proficiently as to stimuli introduced into the ipsilesional field. Moreover, levels of proficiency are indistinguishable from those exhibited by intact cats. Thus, the sparing is greater following unilateral lesions than following bilateral lesions, and the level of sparing approaches completeness. The difference between the bilateral and unilateral lesion results suggests types of pathway reorganizations that may emerge as a result of unilateral occipital lesions. We postulate that the greater sparing is based on modifications in both excitatory and inhibitory circuitry linked to the intact hemisphere, and we provide a framework for future investigations that should be relevant to the comprehension of the repercussions of early unilateral and bilateral lesions sustained by monkeys and humans, which also show more robust residual vision following early relative to later damage of occipital cortex.

Functional circuitry underlying natural and interventional cancellation of visual neglect

A large body of work demonstrates that lesions at multiple levels of the visual system induce neglect of stimuli in the contralesional visual field and that the neglect dissipates as neural compensations naturally emerge. Other studies show that interventional manipulations of cerebral cortex, superior colliculus or deep-lying midbrain structures have the power to attenuate, or cancel, the neglect and reinstate orienting into a neglected hemifield, and even into a profound cortically blind field. These results, and those derived from experiments on the behavioral impacts of unilateral and bilateral lesions, lead us to evaluate the repercussions of unilateral and bilateral deactivations, neural compensations and cancellations of attentional deficits in terms of an overarching hypothesis of neglect. The cancellations can be both striking and enduring, and they suggest that therapeutic strategies can be developed to reverse or ameliorate neglect in human patients. Animal studies show that in many instances of neglect adequate representations and the accompanying motor mechanisms are present despite the lesion and they simply need to be unmasked and brought into use to effect a remedy.

Limit of spared pattern vision following lesions of the immature visual cortex

Lesions of primary visual cortex sustained early in life spare certain aspects of visual processing that can be linked to expansions of bypass pathways to extrastriate cortex. They also trigger, in an age-dependent way, partial or complete transneuronal retrograde degeneration of beta (X) retinal ganglion cells, which are implicated in visual processing under conditions of low contrast. We used two-dimensional geometric patterns whose saliency was reduced by gradually increasing levels of superimposed masking lines, and by reductions in spatial contrast. Normative data were collected from intact cats, and baseline lesion data were collected from cats with lesions sustained as young adults (postnatal day 180, P180). Experimental data were collected from cats that sustained lesions on P1-3 or P26-30. For high contrast patterns, the adult group was impaired at both acquisition (sequential progressive levels of masking) and concurrent (parallel high and low levels of masking) performance, whereas the early-lesioned groups were impaired only at concurrent performance. All lesion groups were equally impaired when contrast was reduced to modest or lower levels. These results show that sparing of masked-pattern learning is limited to the high end of the spatial contrast domain.

Plasticity of the visual cortex after injury: what's different about the young brain?

The repercussions of localized injury of the cerebral cortex in young brains differ from the repercussions triggered by equivalent damage of the mature brain. In the young brain, some distant neurons are more vulnerable to the lesion, whereas others survive and expand their projections to bypass damaged and degenerated structures. The net result is sparing of neural processing and behaviors. This article summarizes both the modifications in visual pathways resulting from visual cortex lesions sustained early in life and the neural and behavioral processes that are spared or permanently impaired. Experiments using reversible deactivation show that at least two highly localizable functions of normal cerebral cortex are remapped across the cortical surface as a result of an early lesion of the primary visual cortex. Moreover, the redistributions have spread the essential neural operations underlying orienting behavior from the visual parietal cortex to a normally functionally distinct type of cortex in the visual temporal system, and in the opposite direction for complex-pattern recognition. Similar functional reorganizations may underlie sparing of neural processes and behavior following early lesions in other cerebral systems, and these other systems may respond well to emerging therapeutic strategies designed to enhance the sparing of functions.

Role of the superior colliculus in analyses of space: superficial and intermediate layer contributions to visual orienting, auditory orienting, and visuospatial discriminations during unilateral and bilateral deactivations

The superior colliculus (SC) has been implicated in spatial analyses of the environment, although few behavioral studies have explicitly tested this role. To test its imputed role in spatial analyses, we used a battery of four spatial tasks combined with unilateral and bilateral cooling deactivation of the upper and intermediate layers of the superior colliculus. We tested the abilities of cats to orient to three different stimuli: (1) moving visual, (2) stationary visual, (3) stationary white-noise aural. Furthermore, we tested the ability of the cats to discriminate the relative spatial position of a landmark. Unilateral cooling deactivation of the superficial layers of the SC induced a profound neglect of both moving and stationary visual stimuli presented in, and landmark objects located within, the contralateral hemifield. However, responses to auditory stimuli were unimpaired. Unilateral cooling deactivation of both the superficial and intermediate layers induced a profound contralateral neglect of the auditory stimulus. Additional and equivalent deactivation of the opposite SC largely restored orienting to either moving visual or auditory stimuli, and restored landmark position reporting to normal levels. However, during bilateral SC deactivation, orienting to the static visual stimulus was abolished throughout the entire visual field. Overall, unilateral SC deactivation results show that the upper and intermediate layers of the SC contribute in different ways to guiding behavioral responses to visual and auditory stimuli cues. Finally, bilateral superior colliculus deactivations reveal that other structures are sufficient to support spatial analyses and guide visual behaviors in the absence of neural operations in the superior colliculus, but only under certain circumstances.