Capacity of the retinogeniculate pathway to reorganize following ablation of visual cortical areas in developing and mature cats

An examination of linking hypotheses drawn from the perceptual consequences of experimentally induced changes in neural circuitry

Because targeted early experiential manipulations alter both perception and the response properties of particular cells in the striate cortex, they have been used as evidence for linking hypotheses between the two. However, such hypotheses assume that the effects of the early biased visual input are restricted to just the specific cell population and/or visual areas of interest and that the neural populations that contribute to the visual perception itself do not change. To examine this assumption, we measured the consequences for vision of an extended period of early monocular deprivation (MD) on a kitten (from 19 to 219 days of age) that began well before, and extended beyond, bilateral ablation of visual cortical areas 17 and 18 at 132 days of age. In agreement with previous work, the lesion reduced visual acuity by only a factor of two indicating that the neural sites, other than cortical areas 17 and 18, that support vision in their absence have good spatial resolution. However, these sites appear to be affected profoundly by MD as the effects on vision were just as severe as those observed following MD imposed on normal animals. The pervasive effects of selected early visual deprivation across many cortical areas reported here and elsewhere, together with the potential for perception to be mediated at a different neural site following deprivation than after typical rearing, points to a need for caution in the use of data from early experiential manipulations for formulation of linking hypotheses.

Monocular deprivation provokes alteration of the neuronal cytoskeleton in developing cat lateral geniculate nucleus

Monocular deprivation early in development produces considerable change in the organization of connections within the central mammalian visual system. In the dorsal lateral geniculate nucleus, the soma, dendrites, and axon terminal fields of deprived cells become considerably smaller than nondeprived counterparts. We have examined the possibility that subcellular events enabling structural modification of deprived neurons include modification of proteins comprising the cytoskeleton. We examined the integrity of the cytoskeleton by measuring the response of a subset of its proteins to varying durations of monocular deprivation. Loss of all three neurofilament subunits (light, medium, and heavy) within deprived layers was observed to parallel changes in neuron gross structure. Monocular deprivation initiated beyond early life produced neither a change in structure nor a loss of neurofilament labeling.

Neuronal death in the lateral geniculate nucleus of young ferrets following a cortical lesion: time-course, age dependence and involvement of caspases

In humans and many other mammalian species, the behavioural consequences of a cortical lesion tend to be milder when it occurs early in life, and there is evidence that an important factor contributing to the behavioural sparing in the young is the formation of new thalamo-cortical connections by thalamic neurons initially connected with the lesioned area. However, this plasticity may be hindered by the secondary death of many of these neurons owing to the elimination by the primary lesion of their trophic support from the cortex. With the long-term aim of preventing this neuronal death, we have here characterised its timing in the lateral geniculate nucleus of ferrets following lesions of the visual cortex on postnatal days 5, 10, 20 or 35. After the earliest lesions (P5 or P10), this cell death began rapidly and occurred synchronously, being maximal at 48 h and declining to zero over the next few days. Following later lesions the cell death began more slowly and continued for longer. The dying neurons contained activated caspase-3 and fragmented DNA and their number 2 days after a P5 lesion was reduced by the broad-band caspase inhibitor z-VAD.fmk. These experiments open the way for a concerted effort to enhance adaptive plasticity by neuroprotection in the hours or days following a cortical lesion.

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.

Recovery from early cortical damage in rats. IX. Differential behavioral and anatomical effects of temporal cortex lesions at different ages of neural maturation

Rats were given lesions of the temporal association cortex on postnatal day 4 or 10, or in adulthood. Ninety days later they were trained on two visual tasks (visual-spatial navigation; horizontal-vertical stripes discrimination). Lesion animals were compared behaviorally and neuroanatomically to littermate sham control rats. The day 4 lesions produced a larger deficit in the navigation task than day 10 or adult lesions. There were no deficits in the discrimination task. Analysis of the brains showed that the day 4 lesions produced a smaller brain and thinner cortex than day 10 lesions. The day 10 lesions produced hypertrophy in the dendritic arborization of pyramidal cells in parietal cortex. The results are consistent with the general findings that perinatal cortical injury in rats produces more severe behavioral and morphological effects than similar lesions in the second week of life and that cortical lesions around day 10 lead to an increase in cortical synaptogenesis.

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.

Visual pathways following cerebral hemispherectomy

The anatomical consequences of unilateral cerebral hemispherectomy in some animal models are reviewed. We have shown that the retinogenigulate pathway undergoes severe degenerative changes in hemispherectomized monkeys, greater than those shown in cats and we proposed that remaining retinal terminals to the dorsal lateral geniculate nucleus have little potential for conveying visual information any further. All subdivisions of the pulvinar undergo severe degeneration following hemispherectomy showing that the ascending tectofugal pathway is also shut off. On the other hand, the retina subserving the blind field is not depleted of ganglion cells which still send normal appearing terminals to the midbrain pretectum and superior colliculus. Visual information from the blind hemifield can thus gain access to the brain and could potentially reach the contralateral cerebral cortex through the midbrain commissure and possibly through thalamic commissural cells.

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

We compared the abilities of intact cats and cats that incurred lesions of areas 17 and 18 in adulthood, at one month of age (P28), or on the day of birth (P1), to detect and orient towards visual stimuli either moved into or illuminated in the periphery of the visual field, and to detect and orient towards a stationary, broad-band white-noise auditory stimulus. For all groups of cats, movement of a stimulus into the visual field was a more potent stimulus for evoking visually-guided orienting movements than illumination of a static light-emitting diode (LED). The potency of the auditory stimulus was also extremely high. Proficiency on both visual tasks was graded according to the age at which areas 17 and 18 were ablated in the sequence: adult, P1, P28 and intact in the sequence worst-->best performance. The superior performance of the P1- and P28-groups provided evidence for sparing of visually-guided orienting, but the sparing was incomplete because it did not match performance of intact cats. Lesions of areas 17 and 18 incurred in adulthood had no significant impact on orienting to auditory white-noise stimuli. However, orienting performance to auditory stimuli presented in the peripheral quadrants was slightly superior in the P28 group and reduced in the P1 group. Thus, the visual sparing exhibited by the P1 group may be at the expense of highly proficient orienting to auditory cues. Overall, these results extend our knowledge by showing that in addition to P1-cats, cats that incur lesions of areas 17 and 18 at one month-of-age also exhibit sparing of visually-guided orienting, and that the sparing is not confined to a single stimulation paradigm. Finally, the covariation in the magnitude of pathway modifications with the scale of the orienting proficiency in P1- and P28 cats helps to solidify the linkage between rewired brain pathways and spared visually-guided behaviors.

Transneuronal retrograde degeneration of retinal ganglion cells following cerebral hemispherectomy in cats

We have assessed the extent of transneuronal retrograde degeneration of retinal ganglion cells (RGCs) following the removal of a whole cerebral hemisphere at postnatal age 16 and 25 days. In the P16 animal, the nasal retina contralateral to the lesion suffered a 41% cell loss, whereas cell loss in the temporal retina ipsilateral to the lesion was 33%. Cell loss was greater in nasal retina and mainly included medium sized cells (200-600 microns2). In the P25 animal overall there was no evidence for ganglion cell loss.

Age dependent modification of cytochrome oxidase activity in the cat dorsal lateral geniculate nucleus following removal of primary visual cortex

The purpose of the present study was to assess changes in the levels of cytochrome oxidase (CO) activity in the dorsal lateral geniculate nucleus (dLGN) of the adult cat following removal of primary visual cortical areas 17 and 18 on the day of birth (PI), P28, or in adulthood (> or = 6 months). Cytochrome oxidase activity was measured in histological sections 9 or more months after the cortical ablation. Control measures obtained from intact cats show that CO activity is normally highest in the A-laminae of dLGN, and slightly lower in the C-complex. Following visual cortex ablations incurred at any age, CO activity levels are reduced in the A-laminae. This reduction is most profound following ablations incurred on P28 or in adulthood. In contrast, CO activity in the C-complex of dLGN is at nearly normal levels following ablations on P1 or P28, but not in adulthood. These findings contribute to our understanding of the role played by the dLGN in the transfer of visual signals along retino-geniculo-extrastriate pathways that expand following early removal of areas 17 and 18. Moreover, they have implications for our understanding of spared behavioral functions attributed to the extrastriate cortex in cats which incurred early damage of areas 17 and 18.

Evidence for greater sight in blindsight following damage of primary visual cortex early in life

This review compares the behavioral, physiological and anatomical repercussions of lesions of primary visual cortex incurred by developing and mature humans, monkey and cats. Comparison of the data on the repercussions following lesions incurred earlier or later in life suggests that earlier, but not later, damage unmasks a latent flexibility of the brain to compensate partially for functions normally attributed to the damaged cortex. The compensations are best documented in the cat and they can be linked to system-wide repercussions that include selected pathway expansions and neuron degenerations, and functional adjustments in neuronal activity. Even though evidence from humans and monkeys is extremely limited, it is argued on the basis of known repercussions and similarity of visual system organization and developmental sequence, that broadly equivalent repercussions most likely occur in humans and monkeys following early lesions of primary visual cortex. The extant data suggest potentially useful directions for future investigations on functional anatomical aspects of visual capacities spared in human patients and monkeys following early damage of primary visual cortex. Such research is likely to have a substantial impact on increasing our understanding of the repercussions that result from damage elsewhere in the developing cerebral cortex and it is likely to contribute to our understanding of the remarkable ability of the human brain to adapt to insults.

Recovery of function after neonatal ablation of the auditory cortex in rats (Rattus norvegicus)

Functional recovery following neonatal ablation of the auditory cortex was surveyed in 28 rats. Fourteen neonatal-lesioned rats had their temporal cortex lesioned on the date of birth (P1); 7 adult-lesioned rats had their auditory cortex lesioned at P60; and 7 rats served as controls. The training consisted of two phases using task with a Go/No Go procedure. The first task was to detect the presentation of a tone pulse. The second task was to discriminate the pulse rate of a tone pulse. Results showed that the achievements of neonatal-lesioned and control groups were comparable in both tasks. Adult-lesioned rats, however, failed to discriminate temporal patterns, although they could detect tone presentation as efficiently as the other two groups. These findings suggested that discrimination of temporal patterns could be a critical function of the auditory cortex and that brain injury in infancy was more compensated than the comparable damage in adulthood. Neurological plasticity was suggested in the recovery of function in our neonatal-lesioned animals.

System-wide repercussions of damage to the immature visual cortex

Damage of the primary visual cortex in mammals, including humans, severely disrupts vision by disconnecting much of the cognitive-processing machinery of extrastriate cortex from its source of visual signals in the retina. Studies of the anatomical consequences of damage to the immature primary visual cortex in cats reveal system-wide repercussions on neural circuitry that includes the retina, thalamus, midbrain and extrastriate cortex. The repercussions modify circuits that support relatively normal signal processing and the sparing of certain visually guided behaviors such as aspects of complex-pattern recognition and orienting to novel stimuli introduced into the visual field. These studies have implications for understanding the consequences of damage to the visual cortex in infant monkeys and humans, and for devising therapeutic strategies to attenuate defects in vision induced by cortical lesions.

Expansion of suprasylvian cortex projection in the superficial layers of the superior colliculus following damage of areas 17 and 18 in developing cats

Tritiated proline and leucine were injected into areas 17 and 18 of intact cats and into the medial bank of the lateral suprasylvian (LS) cortex of intact cats and cats from which areas 17 and 18 had been removed on postnatal day 1 (P1), P28, or in adulthood (A). The density of label transported to the superior colliculus was quantified using image-analysis equipment. The results from the intact cats confirmed previous reports that areas 17 and 18 project most heavily to stratum zonale (SZ) and stratum griseum superficiale (SGS) and LS cortex projects most heavily to stratum opticum (SO) of the superior colliculus. However, in cats with lesions of areas 17 and 18, the projections from LS cortex showed an age-dependent reorganization. LS projections to SGS and SZ were enhanced following ablation of areas 17 and 18 on P1, and projections to SGS were enhanced following an ablation on P28. The pattern of LS-collicular projection following ablations incurred in adulthood was indistinguishable from the pattern presented by intact cats. This study demonstrates that the LS corticocollicular projection expands in SGS and possibly substitutes for inputs eliminated by the removal of areas 17 and 18 from the immature brain. This enhanced pathway may contribute to compensatory neuronal changes and to spared behaviors that accompany damage of immature cortex.

Visual cortex damage-induced growth of retinal axons into the lateral posterior nucleus of the cat

Ablation of visual cortical areas 17 and 18 in neonatal and young adult cats induces novel retinal projections to terminate bilaterally in the lateral posterior nucleus (LP) at a position ventromedial from the medial interlaminar nucleus. Comparison with the visual-field maps of LP indicate that the terminations are focussed on the representation of the visual-field center.