Is it really better to have your brain lesion early? A revision of the "Kennard principle"

Effects on visual search of lesions of the superior colliculus in infant or adult rats

The superior colliculus was removed from rats at either one or five days of age or in maturity. Four months later they were tested on two versions of a visual search task. Experiment 1 required animals to retrieve food pellets concealed in a depression in the top of identical narrow pillars arranged in an arena. Rats with lesions of the superior colliculus, regardless of the age at operation, showed a large number of 'return' errors compared with sham-operated controls. Return errors were defined as occasions on which the animal returned to pillars that had previously been visited on that trial, before every pillar had been visited at least once. Experiment 2 compared the ability of infant- and adult-operated animals to detect and locate a single, baited white pillar in an array of black ones. There were no group differences in response latencies to targets presented in the rostral visual field (within 40 degrees of the midline). However, animals operated on in adulthood or at 5 days of age were slower than both sham-operated animals and animals operated on at one day of age in their responses to more peripheral targets. The latter two groups were indistinguishable.

Behavioral states of premature infants: implications for neural and behavioral development

Nine premature and 28 full-term infants were observed in their homes for 7 hr when they were 2, 3, 4, and 5 weeks post-term, and the sleeping and waking states displayed by these infants were compared. For these comparisons, the observation day was divided into two mutually exclusive contexts: times when the baby was alone and times when the baby was with the mother. The premature infants spent more time alone (mean of 5.4 hr a day) than the full-terms (4.6 hr). Over the total 7-hr day, the premature infants spent more time in alert, nonalert waking activity, and sleep-wake transition than the full-terms, and they spent less time in drowse and total sleep. These results clearly indicate that, at the same post-term ages, the sleep-wake states of premature infants differ markedly from those of full-terms. Four states showed significant Group X Context interactions indicating that state differences between premature and full-term infants were also a function of the context in which the infants were observed. For example, the prematures exhibited more fuss or cry and more drowse when alone; whereas the full-terms exhibited more of these states when with their mothers. This finding of context-related differences between prematures and full-terms has implications for the conflicting reports in the literature, as heretofore the states of prematures and full-terms have been compared from observations made in a single situation. The results indicate that prematures exhibit significant commonality in their neurobehavioral development through the early post-term period despite heterogeneity among them in their exposure to prenatal, perinatal, and early postnatal stresses.

Brain development in the neonatally decorticated rat

The brain weights, cross-sectional areas of subcortical structures, gross morphology, and retrograde degeneration in the thalamus and basal ganglia were compared in rats with neonatal (1 or 5 days of age) or adult ablation of all of the neocortex. Neonatal decorticate and control rats also were given injections of fluorescent retrograde tracers. True blue or Nuclear yellow into the striatum, hippocampus, or subiculum in order to identify afferent pathways to these structures. Neonatal decortication resulted in morphological changes that included a dramatically shrunken and misshapen thalamus, a shrunken brainstem, distortion of the hippocampal structure and abnormal growth of the subiculum, none of which were observed following adult decortication. There was a complete absence of gliosis and calcification in the 1-day decorticates that was present in 5-day and adult decorticates. Although most striatal and hippocampal afferents from subcortical structures were present, label was not found in the ventral tegmental area or thalamus after True blue injections into the striatum and hippocampus, respectively. Acetylcholinesterase staining showed the normal striatal patchy organization. The absence of evidence for the development of new pathways or the reorganization of existing pathways is consistent with previous behavioral studies showing little sparing of function after neonatal decortication. The results suggest that the cortex plays little role in programming pattern formation and connections in subcortical structures and that cortical and subcortical structures provide unique contributions to the control of behavior.

Topography of corticopontine remodelling after cortical lesions in newborn rats

Autoradiographic and axonal degeneration staining techniques were combined in individual animals to study the distribution of corticopontine fibers. In normal animals, forelimb and hindlimb motor cortical projections terminated somatotopically within the ipsilateral pontine nuclei. Sparse crossed projections also displayed a somatotopic pattern. After unilateral sensorimotor cortical lesions in newborn rats, an increase in the crossed corticopontine fibers arising from the opposite unablated motor cortex was observed at maturity. These fibers distributed in a topographic pattern similar to the normal ipsilateral corticopontine pattern; forelimb motor cortical projections terminated rostral to hindlimb motor cortical fibers. The specific distribution of the anomalous fibers suggests that they constitute a functional pathway.

Hemi-inattention resulting from left hemisphere brain damage during infancy

A case of right visual hemi-inattention is reported in a right-handed 14-year-old male who, as an infant, suffered a left hemisphere subdural hemorrhage and subsequently developed a large left posterior porencephalic cyst and right homonomous hemianopsia. Neuropsychological assessment reveals a significantly higher Verbal than Performance IQ, and evidence of hemi-inattention on tests of line bisection, picture matching from memory, and drawing figures from memory. Eye movement testing reveals poorer accuracy and latency of saccades for left-to-right eye movements than for right-to-left eye movements, but no appreciable difference in searching time for the left vs. the right visual spatial field. The findings are discussed in the context of developmental, educational, and rehabilitative implications with respect to the paucity of reports of visual hemi-inattention in children, and with consideration for effects unique to suffering a lesion in early infancy.

Recovery of function after neonatal or adult hemispherectomy in cats. II. Limb bias and development, paw usage, locomotion and rehabilitative effects of exercise

To investigate the relationship of age-related processes in the recovery of complex motor functions of the limbs, a variety of neurobehavioral assessments were applied to cats with either neonatal (n = 9) or adult (n = 11) removal of the entire left hemitelencephalon (hemispherectomy). Neonatal-lesioned kittens showed no paw preference in reaching for a manipulandum between 5 and 8 weeks of age; thereafter they developed a preference (5.6%) for the unimpaired left limb which persisted throughout adulthood. Adult-lesioned cats showed a significantly greater left bias (13.9%) than neonatal-lesioned cats and they exhibited more abnormal movements and postures when reaching with the impaired limb. Exercising the impaired limb, was effective in reversing the paw preference bias in all lesioned cats. To master a food retrieval task with the impaired limb, adult-lesioned cats required more trials than the neonatal-lesioned group, extensive food deprivation and at least 1 month of postsurgical recovery. However, after mastering this task all cats could continue to perform it indefinitely in their home cages. In a paw print analysis of locomotion, only adult-lesioned cats showed abnormalities including splayed paws, decreased stride length, and adduction of the right hind limb. The results support the 'Kennard Principle' of enhanced recovery following neonatal vs late brain lesions for the present complex motor patterns and are interpreted in the context of neural plasticity and anatomical reorganization during development.

Recovery of function after neonatal or adult hemispherectomy in cats: I. Time course, movement, posture and sensorimotor tests

Cats with removal of the left hemitelencephalon (hemispherectomy) as neonates (n = 12) or in adulthood (n = 14), were compared using a battery of 16 neurological and behavioral tests given when they were young adults (kittens) or at least 5 months after the lesion (adults). The neonatal-lesioned subjects grew normally and performed markedly and significantly better than adult-lesioned cats in 13 tests covering the wide range of movement, posture and sensory functions which were assessed. None of the animals recovered tactile placing of the right forelimb or a normal vision in the right visual field. However, the overall recovery was outstanding for all cats such that the neonatal-lesioned were hard to differentiate from intact controls in their spontaneous, daily activities. Because the lesions were similar in the two age-at-lesion groups, and since numerous functions were followed for prolonged, comparable postlesion time, we conclude that, after hemispherectomy in the cat, there definitely is greater functional recovery if the lesion is sustained early in life. We propose that the enhanced recovery of function in neonatal-lesioned cats is largely due to the extensive anatomical reorganization which we have demonstrated in ongoing studies, and which contrasts with a lesser remodeling in adult-lesioned cats.

Denervation-induced collateral sprouting: no case for tracing methods

Sprouting of axonal collaterals is assumed to represent a major feature of the regenerative capacity of the CNS. Following the denervation of a brain region, synaptic contact sites become free and are replaced by sprouted collaterals of intact afferents of this area. During the last three years, numerous studies have been published which have used anatomical tracing methods to demonstrate this morphological consequence of lesions. This paper criticizes the use of tracing methods in this research field because of the striking problems in quantifying the altered connectivity of a denervated structure. This critique is illustrated by summarizing the studies on the lesion-induced morphological effects within the nigro-striatal system which has become a paradigmatic neuronal circuit for the study of neuronal and functional reorganization. It is concluded that anatomical tracing methods seem to be inappropriate in studying denervation-induced collateral sprouting.

Neonatal hemidecortication and bilateral cutaneous stimulation in rats

In humans, a dominant somatosensory consequence of extensive unilateral neocortex damage is "simultaneous extinction," which is an interhemispheric perceptual interaction that is operationally distinguishable from neglect. A tactile stimulus presented on the contralateral side of the body is detected when presented singly, but is completely masked during bilateral stimulation. Analogous tests designed to calibrate somatosensory asymmetries in rats were used to determine the long-term effects of hemidecortication sustained on postnatal Day 1. These data were compared with that observed in adult operated rats at a comparable postoperative period. In one respect the neonatal brain was more vulnerable than the adult brain. That is, unlike adult operated rats which were tested at 3 postoperative months, a sensory asymmetry appeared to be permanent in the neonatally operated rats, at least for the duration of testing (3-9 months). Further analysis suggested that in another way the neonates were more resistant to the effects of hemidecortication than were the adults. Neonatally operated rats appeared to be capable of processing input from both sides of the body simultaneously, even during markedly asymmetrical input. In other words, the early occurrence of brain damage may have spared them from a condition reminiscent of "simultaneous extinction." Finally, the adult operated and neonatally operated rats both displayed a subtle motor abnormality. Thus, depending on the test used, the neonatal operation yielded more severe, less severe, or comparable behavior deficits.

Induction of functional retinal projections to the somatosensory system

Optic axons can be induced to form permanent, retinotopic connections in the auditory (medial geniculate, MG) and somatosensory (ventrobasal, VB) nuclei of the Syrian hamster thalamus; this occurs when the principal targets of retinofugal axons are ablated in newborn hamsters and alternative terminal space is created by partial deafferentation of MG or VB. The experimentally induced retinal projection to the somatosensory nucleus occurs by the stabilization of an early, normally transient projection. The present study was undertaken to determine whether the anomalous, stabilized retino-VB projection is functional. Newborn hamsters were operated on to produce permanent retino-VB projections and when the animals were mature, neurophysiological recordings were made in the cortical targets of VB, the first and second somatosensory cortices (SI and SII, respectively). Visual stimulation within well-defined receptive fields reliably evoked multi-unit responses in SI and SII of operated, but not normal hamsters. The representations of the visual field in SI and SII showed a partially retinotopic organization. These results demonstrate that optic tract axons can form functional synapses in the thalamic somatosensory nucleus, and suggest that neural structures which normally process information specific to one sensory modality have the potential to mediate function for other modalities.

Electrophysiological analysis of motor cortical plasticity after cortical lesions in newborn rats

Intracortical microstimulation of the motor cortex in normal adult rats evoked low threshold contralateral forelimb movements and high threshold ipsilateral movements. Ablation of the opposite sensorimotor cortex in adult animals did not alter these thresholds. However, stimulation of the unablated hemisphere in adult rats that sustained unilateral sensorimotor cortical lesions as neonates elicited low threshold ipsilateral forelimb movements that were similar to contralateral movements. These low threshold ipsilateral movements may be mediated via aberrant corticofugal pathways which are known to develop following neonatal cortical lesions.

Promoting functional plasticity in the damaged nervous system

Damage to the central and peripheral nervous system often produces lasting functional deficits. A major focus of neuroscience research has been to enhance functional restitution of the damaged nervous system and thereby produce recovery of behavioral or physiological processes. Promising procedures include surgical, physical, and chemical manipulations to reduce scar formation and minimize the disruption of support elements, administration of growth-stimulating substances, tissue grafts to bridge gaps in fiber pathways, and embryonic brain tissue grafts to provide new cells with the potential to generate fiber systems. Two elements are required for functional nervous system repair: (i) neurons with the capacity to extend processes must be present, and (ii) the regenerating neurites must find a continuous, unbroken pathway to appropriate targets through a supportive milieu.

An anterograde HRP-WGA study of aberrant corticorubral projections following neonatal lesions of the rat sensorimotor cortex

Anterograde transport of horseradish peroxidase - wheat germ agglutinin (HRP-WGA) was used to examine the effect of unilateral neonatal ablation of the sensorimotor cortex on the remaining corticofugal projections to the midbrain in the rat. In unlesioned animals, the sensorimotor cortical efferents to the midbrain were entirely ipsilateral, terminal labeling being evident in the red nucleus, the midbrain reticular formation, the periaqueductal gray, the intermediate gray layer of the superior colliculus, the nucleus parafascicularis prerubralis and the perilemniscal area. Corticorubral fibers were seen to reach the midbrain through the thalamus or the cerebral peduncle. In the red nucleus, terminal labeling was essentially restricted to the parvocellular region. In neonatally lesioned adults, aberrant corticofugal fibers crossed the midline to terminate in the contralateral red nucleus, the midbrain reticular formation, the periaqueductal gray, the nucleus parafascicularis prerubralis and the intermediate gray layer of the superior colliculus. The aberrant projections maintained the topographic specificity of the normal ipsilateral projections. This was most evident in the corticorubral projection, where the aberrant contralateral fibers terminated in the parvocellular area of the red nucleus.

The superior colliculus and visual neglect in rat and hamster. I. Behavioural evidence

Lesions of the superior colliculus in rats and hamsters produce a severe visual neglect. Three questions are asked concerning the nature of this impairment. Is the neglect a specific deficit, or part of a general disorder? It appears that the impairment is a relatively specific one, because, for example, collicular animals learn many visual discriminations as fast as controls. This pattern of behaviour leads to the second question. What is tested in neglect tasks but not in discrimination problems? Two answers have been proposed. (a) Orienting responses are required in tests of neglect but not in conventional visual-discrimination tasks. Accordingly, it has been suggested that damage to the superior colliculus interferes specifically with the orienting response. However, analysis of recent evidence indicates that rats and hamsters with collicular damage usually make no detectable response of any kind in tests of neglect, and that in some situations they do not respond to visual stimuli that produce a variety of behaviours in normal animals, such as freezing or fleeing, or activation of the EEG unaccompanied by any gross movement. Collicular neglect cannot therefore be explained solely as a response-specific impairment. (b) The stimuli used on tests of neglect are usually small, moving and presented in the peripheral visual field. In contrast, visual discriminanda are typically large, stationary and can be viewed with the central field. Recent experiments provide direct demonstrations that rats with lesions of the superior colliculus can orient to small flashing lights in central regions of the visual field, but unlike control animals may fail to respond if the lights are made dimmer, or are moved into the periphery. It appears that rats and hamsters with collicular damage fail to register particular kinds of visual stimulus. The final question concerns the nature of this stimulus-specific impairment: Do rats and hamsters with lesions of the superior colliculus neglect certain stimuli because, as has been proposed, they have difficulty in attending to the stimuli, or because they actually are incapable of detecting them? The fragmentary evidence currently available suggests that attentional factors are not important for stimuli that are very small, or that are presented in the far periphery of the visual field: such factors may be more important for large transient stimuli in the central field.(ABSTRACT TRUNCATED AT 400 WORDS)

The superior colliculus and visual neglect in rat and hamster. II. Possible mechanisms

The observations that removal of the superior colliculus in rats and hamsters produces a striking visual neglect, whereas damage to striate cortex does not, require explanation in terms of the anatomy and physiology of visual pathways in these animals. One proposal is that neglect is produced because the superior colliculus is the only visual structure directly concerned with the production of orienting movements. However, recent behavioural evidence indicates that this mechanism on its own is insufficient: collicular neglect is not confined to orienting movements, but is in part an inability to register particular kinds of visual stimulus. Two additional mechanisms are considered. The superior colliculus receives visual information that the geniculostriate pathway does not. A variety of anatomical and electrophysiological evidence suggests that this is the case. Although the exact nature of the information is unclear, there is some suggestion that the superior colliculus has a stronger representation of the peripheral field than the geniculostriate pathway, and may be more concerned with small transient stimuli throughout the visual field. An intact superior colliculus is essential for normal functioning of the geniculostriate system. Anatomical evidence indicates that there are pathways whereby: (a) visual cortex could use the superior colliculus as an output station; and (b) the superior colliculus could control signals entering or leaving the geniculostriate system, although the nature of the information carried by the pathways is not yet understood. The precise contribution of these two additional mechanisms to collicular neglect remains to be determined. However, it may be conjectured that the first would underly the deficit in stimulus detection that appears to be one component of collicular neglect, whereas the second mechanism might underly an attentional component.

Extrinsic visual and auditory cortical connections in the 4-day-old kitten

The major extrinsic projections to and from the visual and auditory cortical areas were examined in 4-day-old kittens using axonal transport of horseradish peroxidase (HRP) and/or tritiated proline. Six different afferent and seven different efferent systems were studied; all 13 were present by postnatal day 4 as revealed by either HRP, or autoradiography alone, or these two techniques combined. Topographical projections were found for the corticopetal pathways from the thalamus and claustrum and for the corticofugal pathways to the thalamus, claustrum, striatum, and tectum, as well as for the inter- and intrahemispheric pathways. No topographical relations were seen in projections to the cortex from the basal ganglia or the lower brainstem. The results of the present study indicate that most or all of the major extrinsic connections of the kitten's visual and auditory cortical areas are present neonatally, and that both the cells of origin and the axonal targets are arranged topographically much like those of adult cats. However, the origins of callosal projections from visual cortex are more widespread in newborn kittens than in adult cats. In addition, the laminar arrangements of the kitten's corticocortical connections differ from those of adult cats in a number of details. The results suggest that the sparing of some visual and auditory functions after neonatal lesions occurs despite the fact that the cortical areas removed have formed extrinsic connections.

Forebrain lesions disrupt development but not maintenance of song in passerine birds

The magnocellular nucleus of the anterior neostriatum is a forebrain nucleus of passerine birds that accumulates testosterone and makes monosynaptic connections with other telencephalic nuclei that control song production in adult birds. Lesions in the magnocellular nucleus disrupted song development in juvenile male zebra finches but did not affect maintenance of stable song patterns by adult birds. These results represent an instance in which lesions of a discrete brain region during only a restricted phase in the development of a learned behavior cause permanent impairment. Because cells of the magnocellular nucleus accumulate androgens these findings raise the possibility that this learning is mediated by hormones.

Dichotic listening ear preference after childhood cerebral lesions

Patients with unilateral (right or left) nonprogressive cerebral lesions acquired in infancy (before age one) or childhood (ages one to fifteen) were given a dichotic listening test. The two groups of patients with the childhood lesions showed the pattern of ear preference typically seen after hemispheric lesions in adults; loss of right ear preference after left hemisphere (LH) lesions, and enhanced right ear preference after right hemisphere (RH) lesions. The two groups of patients with the very early lesions failed to show any consistent ear preference or to differ from one another in ear preference. It is postulated that this lack of a consistent lesion effect following very early lateralized brain lesions is due to the greater degree of functional reorganization that takes place after such lesions.

Plasticity of the corticospinal tract following midthoracic spinal injury in the postnatal rat

Rats received a midthoracic spinal cord "overhemisection" including right hemicord and left dorsal funiculus at birth (neonatal operates, N = 15) or 21 days of age (weanling operates, N = 14). In a second experiment neonatal (N = 6), 6-day (N = 3), and 12-day (N = 7) rats sustained a right sensorimotor cortex (SmI) ablation to destroy the left corticospinal tract (CST) at the same time as the spinal injury (double lesion operates). Later (3-12 months) injections of 3H-proline and autoradiography were used to label the left or right CST. The results of the first experiment showed that most right CST axons failed to grow around the spinal lesion in neonatal operates (N = 9). There was an increase in the density of label, mainly to CST projection areas, in a 1-mm zone rostral to the lesion. However, left CST axons bypassed the lesion by growing through the intact tissue in neonatal operates (N = 6). These displaced axons were consistently located within the dorsal portion of the lateral funiculus (dLF) and remained within that location caudal to the lesion, an area normally containing only a few CST axons. In spite of this abnormal position, these axons terminated bilaterally throughout the remainder of the cord in normal CST sites. In weanling operates, CST axons severed by the lesion did not regenerate around the lesion site. An increased density of label over the few spared axons within the left dLF and in CST projection zones immediately caudal to the lesion site suggested axonal sprouting by these axons. The results of the second experiment showed that the lack of growth of right CST axons around this injury in neonatal operates was, at least partially, due to an interaction with left CST axons. In neonatal double lesion operates, right CST axons grew around the spinal injury for a varying distance within the left dLF and distributed bilaterally to normal CST sites. The number of right CST axons bypassing the lesion was related to the configuration of the lesion site. A smaller number of right CST axons bypassed the lesion in 6-day double lesion operates and most terminated within 2-3 mm of the lesion site. Right CST axons failed to grow around this injury in 12-day double lesion operates.(ABSTRACT TRUNCATED AT 400 WORDS)

Cortico-cortical connections reorganize in hamsters after neonatal transection of the callosal bridge

Neonatal hamsters were subjected to transection of the callosal bridge. Later examination of the brains showed complete or partial absence of the corpus callosum and an anomalous bilateral longitudinal bundle of fibers. In addition, aberrant commissural fibers were seen to connect heterotopically the parietal cortex with the contralateral frontal cortex through a callosal remnant over the septum, and the olfactory cortex with the opposite frontal and parietal cortices through the anterior commissure.

Rate of synaptic replacement in denervated rat hippocampus declines precipitously from the juvenile period to adulthood

Synaptic contacts per unit area in the rat dentate gyrus reach adult numbers by the end of the first month after birth and remain constant thereafter. This experiment demonstrated that the rate at which synapses were replaced by sprouting after a lesion declined dramatically from 35 to 90 days of age. Thus, the juvenile period of the rat's life is marked by a considerable change in neuronal plasticity. This may be related to age-dependent effects in recovery from brain damage.

Early recovery of function after olfactory tract section correlated with reinnervation of olfactory tubercle

Behavioral recovery and cortical reinnervation after early olfactory tract section were assessed in the infant golden hamster (Mesocricetus auratus). Hamster pups show strong thermotaxis at birth which declines abruptly after postnatal day (P) 8 in normal pups. Unilateral olfactory bulbectomy on P5 causes persistent thermotaxis through the second postnatal week. In this study, the bulb's output pathway, the lateral olfactory tract, was unilaterally severed on P5 and pup thermotaxis was tested through P15. Complete tract section, like bulbectomy, prolonged thermal responding beyond P8. In contrast to bulbectomy, however, some tract-sectioned pups showed recovery before P15 while others continued to show persistent thermotaxis throughout testing. The olfactory bulb projection was examined 10 days after tract section in order to determine whether recovery and persistent thermotaxis were associated with different patterns of cortical innervation. Eleven pups with complete transections showed recovery during the second week. In 10 of these pups, olfactory bulb fibers had penetrated the damaged region after surgery to reinnervate the olfactory tubercle. Three of these pups also exhibited some reinnervation of piriform cortex. The lesions of pups showing persistent thermotaxis were more severe, extending bilaterally or into deep cortical layers, and olfactory fibers had failed to reinnervate caudal terminal fields. All pups with olfactory tract sections showed extensive sprouting rostral to the cut, regardless of their behavioral profile. In no case had postlesion growth innervated the entorhinal or amygdaloid areas. Inhibition of thermotaxis was associated with reinnervation of the olfactory tubercle rather than more rostral, lateral or caudal olfactory cortex. We conclude that regrowth of olfactory tract fibers caudal to early transection is rapid and has functional consequences for early behavioral development.

Functional role of regrowing pyramidal tract fibers

When pyramidal tract axons are severed in the infant hamster, the damaged fibers regrow via a new pathway to their normal terminal sites in the medulla and spinal cord and there form synaptic connections (Kalil and Reh, '79, '82). We studied the behavior of animals with infant and adult lesions of the medullary pyramid to determine the functional significance of the new pathway in maintaining normal motor behavior. Examination of behaviors normally mediated by the pyramidal tract, particularly the manipulation of sunflower seeds during feeding, revealed a correlation between the presence of the new tract and the preservation of function. Furthermore, in the adult animal with an infant lesion, the spared behaviors were lost when the new pathway was destroyed.

The effects of cerebellar hemispherectomy in the young rat. I. Behavioral sequelae

The effects of unilateral cerebellar hemispherectomy in rats operated on the 5th, 10th and 30th days were investigated. Their locomotor behaviour was studied longitudinally until the age of 360 days. Locomotion remained impaired to a variable extent in all experimental groups. Rats operated on the 30th day showed fewer deviations in locomotion than rats operated on the 5th or 10th day.

Developmental neuropsychiatry: concepts, issues and prospects

The empirical findings on the cognitive and behavioral sequelae of head injury in childhood are integrated with others in the literature in order to consider some of the outstanding issues and dilemmas that remain in the field of developmental neuropsychiatry. Particular attention is paid to the apparent nonspecificity of the cognitive sequelae of localized brain lesions in childhood, possible specificities in brain-behavior relationships, the probable importance of age effects in altering the impact of brain lesions, and the threshold above which brain injury may have persistent psychological sequelae.

Alterations in Clarke's column secondary to granule cell degeneration in the neurological mutant mice, weaver and staggerer

In the neurological mutants weaver and staggerer, the granule cells either do not successfully migrate into the granular (weaver) or, having migrated, die during the second postnatal week (staggerer). We wished to determine if the resulting agranular cortex might produce retrograde transneuronal changes in the spinal cord similar to the changes observed after neonatal hemicerebellectomy. Golgi analysis of Clarke's column indicates that neurons in weaver are affected; neurons in staggerer are not. These observations support the view that neuronal maturation requires the presence of a target nucleus during "critical' postnatal growth periods.

Some observations on the functional organization of the golden hamster's visual system

A selective review is provided of recent studies dealing with the functional organization of the golden hamster's visual system. Three main topics are considered: (a) the normal organization of the superior colliculus and visual cortex: (b) functional correlates of neonatal damage to the visual system; and (c) the influence of environmental manipulations upon collicular and cortical physiology.

Short- and long-term effects of neonatal and adult visual cortex lesions on the retinal projection to the pulvinar in cats

An increased retinal projection to th pulvinar occurred in cats following neonatal ablation of visual cortical areas 17, 18, and 19. After unilateral lesions, the retinopulvinar projection visualized with autoradiography was larger and denser on the lesioned side than on the intact side. This increased projection was first recognized when the lesioned kittens were 1 week old, when increased labeling in the pulvinar was first detected. The retinopulvinar projection on the intact side of the brain decreased in size with age over the first 3 months of life. This decrease in size was not observed on the lesioned side of the brain. The absence of this decrease on the lesioned side may represent a failure of retraction of exuberant connections, but because of the active invasion by retinal fibers immediately after the lesion, observations of the normal retraction may simply be obscured. Severe cellular degeneration in the dorsal lateral geniculate occurred a few days before the changes in the retinopulvinar projection began; this loss of a postsynaptic target for many retinal axons may trigger the reorganization of input to the pulvinar retino-recipient zone (pulivnar-RRZ). After adult visual cortex lesions, no reorganization of the retinopulvinar projection took place. The modification of the projection from the retina to the pulvinar that occurs after ablation of visual cortex in the newborn cat may contribute to the sparing of visual abilities seen after these lesions.