Depth perception in cats after cerebral hemispherectomy: comparisons between neonatal- and adult-lesioned animals

Cortical Visual Impairment in Childhood: 'Blindsight' and the Sprague Effect Revisited

The paper discusses and provides support for diverse processes of brain plasticity in visual function after damage in infancy and childhood in comparison with injury that occurs in the adult brain. We provide support and description of neuroplastic mechanisms in childhood that do not seemingly exist in the same way in the adult brain. Examples include the ability to foster the development of thalamocortical connectivities that can circumvent the lesion and reach their cortical destination in the occipital cortex as the developing brain is more efficient in building new connections. Supporting this claim is the fact that in those with central visual field defects we can note that the extrastriatal visual connectivities are greater when a lesion occurs earlier in life as opposed to in the neurologically mature adult. The result is a significantly more optimized system of visual and spatial exploration within the ‘blind’ field of view. The discussion is provided within the context of “blindsight” and the “Sprague Effect”.

Cerebral metabolic response to traumatic brain injury sustained early in development: a 2-deoxy-D-glucose autoradiographic study

Following fluid percussion (FP) traumatic brain injury (TBI), adult rats exhibit dynamic regional changes in cerebral glucose metabolism characterized by an acute (hours) increase and subsequent chronic (weeks) decrease in metabolic rates. The injury-induced hyperglycolysis is the result of ionic fluxes across cell membranes and the degree and extent of metabolic depression is predictive of neurobehavioral deficits. Given that younger animals appear to exhibit similar physiological responses to injury yet show an improved rate of recovery compared to adults, we wanted to determine if this injury-induced dynamic metabolic response to TBI is different if the injury is sustained early in life. Local cerebral metabolic rates for glucose (ICMRglc: micromol/100 g/min) using [14C]2-deoxy-D-glucose were measured immediately, 30 min, 1 day, and 3 days following a mild to moderate level of lateral FP injury in postnatal day 17 (P17) rats. Even though gross morphological damage was not evident, injured pups exhibited ipsilateral hyperglycolysis immediately after injury, predominantly in cortical regions (ranging from 59.2% to 116.5% above controls). This hyperglycolytic state subsided within 30 min, and by 1 day all cerebral structures, except the ipsilateral cerebellar cortex, showed lower rates of glucose metabolism (ranging from 5.7% to 63.0% below controls). This period of posttraumatic metabolic depression resolved within 3 days for all structures measured. Compared to previous adult studies these results suggest that the young rat pup, although exhibiting acute hyperglycolysis, is not subjected to a prolonged period of metabolic depression, which supports the findings that at this level of injury severity, these young animals show remarkable neurological sparing following TBI.

Developmental neuroplasticity in a model of cerebral hemispherectomy and stroke

Cerebral hemispherectomy, a last resort treatment for childhood epilepsy, is a standard procedure which dramatically illustrates the resilience of the brain to extensive damage. If this operation, also mimicking long-term, extensive unilateral capsular stroke, is performed in postnatal cats of up to 60 days of age, there is a remarkable recovery/sparing of neurological functions that is not seen when the lesion occurs during late fetal life or in adulthood. A long-term effect at all ages is loss of neurons in bilateral brain areas remote from the resection site. This is pronounced in adult cats and shows intriguing, paradoxical features in fetal animals, but is substantially attenuated in neonatal cats. Similarly, large-scale reinnervation of subcortical sites (sprouting) by neurons of the remaining, intact hemisphere is prominent in young cats, but not in fetal or adult animals. These and other restorative processes (described herein) in young postnatal animals are matched by relatively higher rates of local cerebral glucose utilization, supporting the notion that they underlie the improved behavioral outcome. Thus, during a critical, defined stage of maturation, presumably common to higher mammals including humans, the brain entirely remodels itself in response to extensive but focal injury. Perhaps the molecular environment allowing for rescue of neurons and enhanced reinnervation at a specific developmental stage could be recreated in subjects with brain lesions at less favorable ages, thereby helping to restore circuitry and spare neurons. However, replacement via transplantation of neurons eliminated by the damage appears to be crucial in attempts to further preserve cells located remotely but yet destined to die or decrease in size. This article presents abundant evidence to show that there is a surprisingly comprehensive long-term morphological remodeling of the entire brain after extensive unilateral damage and that this occurs preferentially during a discrete period of early life. Additional evidence strongly suggests that the remodeling underlies the outstanding behavioral and functional recovery/sparing following early cerebral hemispherectomy. We argue that this period of reduced brain vulnerability to injury also exists in other higher mammals, including man, and suggest ways to enhance restorative processes after stroke/hemispherectomy occurring at other ages.

Cerebral metabolism following neonatal or adult hemineodecortication in cats: effect on oxidative capacity using cytochrome oxidase histochemistry

In order to determine the degree and extent of changes in cerebral oxidative capacity following cerebral hemineodecortication, adult cats which had undergone surgery early postnatally (mean age: 11.4 days) or during adulthood were studied using cytochrome oxidase histochemistry. A total of 18 animals were employed and 50 brain regions were quantified bilaterally using optical densitometry. Although many subcortical regions exhibiting extensive degenerative features revealed lower levels of cytochrome oxidase (C.O.) activity, this reduction was relatively unremarkable compared to intact controls. Nevertheless, it was interesting that this decrease (down to 66-89%) of normal was more pronounced in neonatal-lesioned cats, reaching significance in a number of ipsilateral thalamic nuclei, compared to adult-lesioned animals (91-100% of normal), suggesting a contribution of glial cells to the density of C.O. staining in the latter cats. Regions of the brain spared from degeneration exhibited a bilateral increase in C.O. activity which may reflect the demands for energy to support the anatomical reorganization which is prevalent in these animals. Surprisingly, such increases were more robust within spared regions of the adult-lesioned brain, reaching significance in four ipsilateral and nine contralateral areas with the density of the reaction attaining levels over 125% of control. This may indicate different demands for oxidative metabolism in the adult-lesioned cats. These results enhance our understanding of the mechanism(s) underlying the greater extent of functional sparing or recovery in cats sustaining injury to the cerebral cortex early vs. late in life. In addition, the findings complement our previous companion report on glucose metabolism supporting the concept of energy compartmentalization, which reflects the dynamic interaction between anatomical and functional changes in this age-at-lesion model of recovery.

Neural bases of residual vision in hemicorticectomized monkeys

In this series of studies, we have attempted to characterize anatomically the organization of the retinofugal pathways in monkeys that underwent the surgical removal in infancy of the entire left cerebral hemisphere. Hemidecordication in baby monkeys produced a transneuronal retrograde degeneration of the retinal ganglion cells (RGCs) that affected mainly the foveal rim. Although the density of RGCs in this region was drastically diminished, the soma sizes of the surviving cells remained normal. The lateral geniculate nucleus (dLGN) ipsilateral to the removed cortex was dramatically reduced in size although it still showed normal layering. There was a marked reduction in the number of neurons in both the parvocellular and magnocellular layers and a heavy gliosis. By contrast, the superior colliculus ipsilateral to the lesion was remarkably well preserved: although slightly reduced in volume, it showed little gliosis and a metabolic activity, as revealed by cytochrome oxidase histochemistry, similar to the superior colliculus contralateral to the lesion. Behavioral perimetry indicated a partial sparing of vision up to 45 degrees in the 'blind' hemifield. We argue that the preservation of the retino-tectal pathway mediates most of the residual visual functions found in the 'blind field' of hemispherectomized human subjects.

Cerebral metabolism following neonatal or adult hemineodecortication in cats: I. Effects on glucose metabolism using [14C]2-deoxy-D-glucose autoradiography

In the cat, cerebral hemispherectomy sustained neonatally results in a remarkable degree of recovery and/or sparing of function as compared with the effects of a similar lesion but sustained in adulthood. We have proposed that this effect is due to a combination of reduced neuronal loss within partially denervated structures and a lesion-induced reorganization of corticofugal projections arising from the remaining intact hemisphere in the neonatally lesioned animal. The current study was designed to assess the physiological consequences of these anatomical changes utilizing [14C]2-deoxy-D-glucose autoradiography. A total of 17 adult cats were studied. Seven animals served as intact controls, five received a left cerebral hemineodecortication as neonates (NH; mean age 11.4 days), and five sustained the same lesion in adulthood (AH). Histological analysis indicated that the lesion was very similar between the two age groups and essentially represented a unilateral hemineodecortication. Local CMRglc (LCMRglc; mumol 100 g-1 min-1) values were calculated for 50 structures bilaterally and indicated that in the remaining intact contralateral (right) cerebral cortex (including all areas measured), AH cats exhibited a significantly (p < 0.05) lower level of LCMRglc (ranging from 20 to 72 mumol 100 g-1 min-1) than NH (ranging from 49 to 81 mumol 100 g-1 min-1). In comparison, the rates of NH cats within the cerebral cortex were very similar to those seen in intact animals (ranging from 48 to 119 mumol 100 g-1 min-1). Ipsilateral to the lesion in AH cats, the structures spared by the resection, including the basal ganglia and thalamus, exhibited LCMRglc rates of between 23 and 69 mumol 100 g-1 min-1, which were significantly lower (p < 0.05) than in NH cats (range 47-72 mumol 100 g-1 min-1). Considering all structures, both age-at-lesion groups exhibited a lower level of metabolism compared with similar measurements for intact control animals (LCMRglc range 45-75 mumol 100 g-1 min-1). However, this depression of glucose metabolism was more pronounced in the AH cats (p < 0.05). These results indicate that following neonatal hemineodecortication, LCMRglc is maintained at a higher level in many regions of the brain than in animals that sustain the same resection in adulthood. This higher level of glucose metabolism in NH animals suggests that the lesion-induced anatomical reorganization of structures not directly injured by the lesion plays a functional role that is probably responsible for the greater degree of recovery and/or sparing of function in these early lesioned cats.

Development of a crossed corticotectal pathway following cerebral hemispherectomy in cats: a quantitative study of the projecting neurons

A hypothetical mechanism for the partial sparing of visual function in the contralateral visual field following cerebral hemispherectomy early in life is the formation of a new corticotectal pathway arising from the remaining primary visual cortex (areas 17 and 18) that projects to the contralateral superior colliculus. To test this hypothesis, the left superior colliculus of intact adult and neonatal (5-15 days old) cats and of adult cats with a left cerebral hemispherectomy sustained neonatally (7-9 days old) or in adulthood, was injected with WGA-HRP and the brains were processed for combined TMB/DAB histochemistry. The primary visual cortex was examined, labelled neurons were counted and the cross sectional area of their somata was measured. The left primary visual cortex of intact adult animals exhibited a mean of 959.68 labelled cells +/- 406.5 (S.E.), with a mean soma size of 366.7 microns2 +/- 131.2. For the neonatal intact cats, there was a mean of 75.31 +/- 21.08 cells within the left primary visual cortex which exhibited a mean soma size of 249.56 microns2 +/- 68.18. The peak cell size distribution for both intact groups was similar at 300 microns2. Virtually no labelled neurons were detected in the right primary visual cortex of intact animals (neonatal or adult). For neonatal-hemispherectomized cats, the remaining right primary visual cortex exhibited a mean cell count of 351.09 +/- 126.3 cells, with a mean soma size of 436.1 microns2 +/- 131.5, and a peak cell size distribution of 400 microns2. Finally, for adult-hemispherectomized animals, the contralateral primary visual cortex exhibited 68.27 +/- 20.13 neurons having a mean soma size of 486.6 microns2 +/- 143.2 with a peak cell size distribution of 500 microns2. These results indicate that reorganization of the corticotectal pathway occurs in both adult- and neonatal-hemispherectomized cats but is more pronounced in neonatal-lesioned animals. In addition, the cells of origin of this reorganized pathway tended to be larger, perhaps in response to a greater axonal arborization.

Neurological and behavioral effects of a unilateral frontal cortical lesion in fetal kittens. II. Visual system tests, and proposing an "optimal developmental period" for lesion effects

Nine fetal kittens sustained removal of the left frontal cortex during the last third of gestation (E 43-55) and were compared to animals sustaining a similar lesion postnatally (P 8-14) as well as to intact littermates. Beginning after 6 months of age, the animals' visual field and depth perception were assessed. In addition, pupil size as well as eye alignment were measured. On two visual field tests the fetal-lesioned cats showed test dependent decrements for some angles of vision. In terms of depth perception, only the prenatal-lesioned animals showed a higher binocular threshold; they also showed ocular misalignment which may have contributed to their depth perception impairment. Moreover, these animals had a larger ipsilateral pupil. The neonatal-lesioned animals were like normal cats for all tests and measurements. We conclude that, as for the tests reported in the preceding paper, the outcome for visual related behaviors of a prenatal frontal cortical lesion in the cat is also worse than that of a similar lesion sustained neonatally. Dysgenetic anatomical changes of the visual system induced indirectly by the frontal lesion are proposed as a possible explanation for these age-at-lesion differences. Based on the present work as well as on the literature, we propose the existence of an "optimal developmental period" for the best behavioral and anatomical outcome of perinatal brain lesions. We argue that this concept fits contemporary data and can better explain the different age-at-lesion effects of brain injury across animals species than the "Kennard Principle" (or "infant-lesion effect").

Hemicerebellectomy and motor behaviour in rats. II. Effects of cerebellar lesion performed at different developmental stages

Rats with a right hemicerebellectomy (HCb) performed in adulthood or at weaning were compared behaviourally to rats with a similar lesion performed on the first postnatal day. The age at which animals received cerebellar lesions made a significant difference with respect to the behavioural outcome in adulthood. Posture, locomotion and motor behaviour were analysed by a battery of sensorimotor tests. Behavioural measurements showed a clear relationship between age at surgery and behavioural effects; rats with neonatal cerebellar lesions showed a slight extensor hypotonia contralateral to the lesion side and efficient locomotor activity, while the adult operated group exhibited a severe extensor hypotonia ipsilateral to the lesion side and hampered locomotion characterized by a wide base and ataxia. Weanling operated rats displayed a symptomatology similar to that observed in adult operates, although less severe. In the postural dynamic adjustments which the sensorimotor tests required, the youngest operated animals obtained higher scores in comparison to the other two experimental groups, except for the lack of hindlimb usage in the suspension on a wire test. These results, which show the importance of the age-at-lesion factor for the recovery of motor function after HCb in the rat, are discussed in the light of the widespread anatomical reorganization already demonstrated following neonatal HCb in rats.

Sparing of visual field perception in neonatal but not adult cerebral hemispherectomized cats. Relationship with oxidative metabolism of the superior colliculus

Following cerebral hemispherectomy and using a lick-suppression test, adult-lesioned cats showed a complete contralateral hemianopsia, while neonatal-lesioned animals reliably responded to stimuli presented out to 45 degrees in the visual field contralateral to the lesion. In adult-lesioned cats oxidative metabolism of the superior colliculus ipsilateral to the hemispherectomy was markedly depressed as compared to the contralateral colliculus. In contrast, in the neonatal-lesioned cats this metabolic imbalance was mild. We propose that this ipsilateral depression of oxidative metabolism reflects the loss of excitatory corticotectal input which has been postulated as being partly responsible for the hemianopsia following a unilateral visual cortex ablation. Here we demonstrate, in addition, that this phenomenon is affected by developmental factors and suggest that differential age-at-lesion anatomical effects may also be involved.