Brain damage and behavioral recovery: serial lesion phenomena

Implications of age at lesion onset for neuropsychological outcomes: A systematic review focusing on focal brain lesions

Theories of the relation between age at lesion onset and outcomes posit different views of the young brain: resilient and plastic (i.e., the so-called "Kennard Principle"), or vulnerable (i.e., the Early Vulnerability Hypothesis). There is support for both perspectives in previous research and questions about the "best" or "worst" times to sustain brain injury remain. Here, we present a systematic review investigating the influence of age at focal brain lesion onset on cognitive functioning. This systematic review identifies and qualitatively synthesizes empirical studies from 1985 to 2021 that investigated age at lesion onset as a variable of interest associated with neuropsychological outcomes. A total of 45 studies were identified from PubMed, PsycINFO, and CINAHL databases. Almost all studies indicated that brain injury earlier in the developmental period predicts worse cognitive outcomes when compared to onset either later in the developmental period or in adulthood. More specifically, the overwhelming majority of studies support an "earlier is worse" model for domains of intellect, processing speed, attention and working memory, visuospatial and perceptual skills, and learning and memory. Relatively more variability in outcomes exists for domains of language and executive functioning. Outcomes for all domains are influenced by various other age and injury variables (e.g., lesion size, lesion laterality, chronicity, a history of epilepsy). Continued interdisciplinary understanding and communication about the influence of age at lesion onset on neuropsychological outcomes will aid in promoting the best possible outcomes for patients.

Lesion Studies in Contemporary Neuroscience

Studies of humans with focal brain damage and non-human animals with experimentally induced brain lesions have provided pivotal insights into the neural basis of behavior. As the repertoire of neural manipulation and recording techniques expands, the utility of studying permanent brain lesions bears re-examination. Studies on the effects of permanent lesions provide vital data about brain function that are distinct from those of reversible manipulations. Focusing on work carried out in humans and nonhuman primates, we address the inferential strengths and limitations of lesion studies, recent methodological developments, the integration of this approach with other methods, and the clinical and ecological relevance of this research. We argue that lesion studies are essential to the rigorous assessment of neuroscience theories.

Predictors of cognitive function in the acute phase after aneurysmal subarachnoid hemorrhage

BACKGROUND

Cognitive dysfunction is the most common form of neurological impairment after aneurysmal subarachnoid hemorrhage (aSAH) in the chronic phase. Cognitive deficits in the acute phase after aSAH, however, remain scarcely investigated. The aim of the present study was to test cognitive function and to identify medical predictors of cognitive deficits in the acute phase of aSAH.

METHODS

Prospective study including 51 patients treated for aSAH. Patients were treated in accordance with a standardized institutional protocol and subjected to neuropsychological evaluation around discharge from neurosurgical care. The neuropsychological test results were transformed into a global cognitive impairment index where an index value of 0.00 is considered normal and 1.00 is considered maximally pathological. Patients with an index score of less than 0.75 were considered having good global cognitive function while those with an index score equal to or above 0.75 were considered having poor global cognitive function. Univariate and multiple regression analysis were used to identify medical predictors of cognitive function.

RESULTS

Fifty-seven percent of the patients had poor cognitive function. They showed severe cognitive deficits, with most tests falling well below two standard deviations from the expected normal mean. Poor cognitive function was not reflected in a poor modified Rankin score in almost half of the cases. Patients with good cognitive function showed only mild cognitive deficits with most tests falling only slightly below the normal mean. Delayed memory was the most affected function in both groups. Univariate analysis identified acute hydrocephalus and aSAH-acquired cerebral infarction to be predictors of poor cognitive function. Cerebrospinal fluid drainage in excess of 2000 ml six-folded the risk of poor cognitive function, whereas a new cerebral infarction 11-folded the respective risk of poor cognitive function.

CONCLUSION

More than half of aSAH patients have severe cognitive deficits in the acute phase. The modified Rankin Score should be combined with neuropsychological screening in the acute phase after aSAH to get a more accurate description of the patients' disabilities. Acute hydrocephalus and aSAH-acquired cerebral infarction are the strongest predictors of poor cognitive function in the acute phase.

The need for theory to guide concussion research

Although research into concussion has greatly expanded over the past decade, progress in identifying the mechanisms and consequences of head injury and recovery are largely absent. Instead, data are accumulated without the guidance of a systematic theory to direct research questions or generate testable hypotheses. As part of this special issue on sports concussion, I advance a theory that emphasizes changes in spatial and temporal distributions of the brain's neural networks during normal learning and the disruptions of these networks following injury. Specific predictions are made regarding both the development of the network as well as its breakdown following injury.

Kaolin-induced ventriculomegaly at weaning produces long-term learning, memory, and motor deficits in rats

Ventriculomegaly occurs when there is imbalance between creation and absorption of cerebrospinal fluid (CSF); even when treated, long-term behavioral changes occur. Kaolin injection in the cisterna magna of rats produces an obstruction of CSF outflow and models one type of hydrocephalus. Previous research with this model shows that neonatal onset has mixed effects on Morris water maze (MWM) and motoric performance; we hypothesized that this might be because the severity of ventricular enlargement was not taken into consideration. In the present experiment, rats were injected with kaolin or saline on postnatal day (P)21 and analyzed in subgroups based on Evan's ratios (ERs) of the severity of ventricular enlargement at the end of testing to create 4 subgroups from least to most severe: ER0.4-0.5, ER0.51-0.6, ER0.61-0.7, and ER0.71-0.82, respectively. Locomotor activity (dry land and swimming), acoustic startle with prepulse inhibition (PPI), and MWM performance were tested starting on P28 (122cm maze) and again on P42 (244cm maze). Kaolin-treated animals weighed significantly less than controls at all times. Differences in locomotor activity were seen at P42 but not P28. On P28 there was an increase in PPI for all but the least severe kaolin-treated group, but no difference at P42 compared with controls. In the MWM at P28, all kaolin-treated groups had longer path lengths than controls, but comparable swim speeds. With the exception of the least severe group, probe trial performance was worse in the kaolin-treated animals. On P42, only the most severely affected kaolin-treated group showed deficits compared with control animals. This group showed no MWM learning and no memory for the platform position during probe trial testing. Swim speed was unaffected, indicating motor deficits were not responsible for impaired learning and memory. These findings indicate that kaolin-induced ventriculomegaly in rats interferes with cognition regardless of the final enlargement of the cerebral ventricles, but final size critically determines whether lasting locomotor, learning, and memory impairments occur.

Growth defects in the dorsal pallium after genetically targeted ablation of principal preplate neurons and neuroblasts: a morphometric analysis

The present study delineates the large-scale, organic responses of growth in the dorsal pallium to targeted genetic ablations of the principal PP (preplate) neurons of the neocortex. Ganciclovir treatment during prenatal development [from E11 (embryonic age 11) to E13] of mice selectively killed cells with shared S-phase vulnerability and targeted expression of a GPT [golli promoter transgene; GPT linked to HSV-TK (herpes simplex virus-thymidine kinase), τ-eGFP and lacZ reporters] localized in PP neurons and their intermediate progenitor neuroblasts. The volume, area and thickness of the pallium were measured in an E12-P4 (postnatal age 4) longitudinal study with comparisons between ablated (HSV-TK(+/0)) and control (HSV-TK(0/0)) littermates. The extent of ablations was also systematically varied, and the effect on physical growth was assessed in an E18 cross-sectional study. The morphological evidence obtained in the present study supports the conclusion that genetically targeted ablations delay the settlement of the principal PP neurons of the dorsal pallium. This leads to progressive and substantial reductions of growth, despite compensatory responses that rapidly replace the ablated cells. These growth defects originate from inductive cellular interactions in the proliferative matrix of the ventricular zone of the pallium, but are amplified by subsequent morphogenic and trophic cellular interactions. The defects persist during the course of prenatal and postnatal development to demonstrate a constrained dose-response relationship with the extent of specific killing of GPT neurons. The defects propagate simultaneously in both the horizontal and vertical cytoarchitectural dimensions of the developing pallium, an outcome that produces a localized shortfall of volume in the telencephalic vesicles.

Acute neuroprotection to pilocarpine-induced seizures is not sustained after traumatic brain injury in the developing rat

Following CNS injury there is a period of vulnerability when cells will not easily tolerate a secondary insult. However recent studies have shown that following traumatic brain injury (TBI), as well as hypoxic-ischemic injuries, the CNS may experience a period of protection termed "preconditioning." While there is literature characterizing the properties of vulnerability and preconditioning in the adult rodent, there is an absence of comparable literature in the developing rat. To determine if there is a window of vulnerability in the developing rat, post-natal day 19 animals were subjected to a severe lateral fluid percussion injury followed by pilocarpine (Pc)-induced status epilepticus at 1, 6 or 24 h post TBI. During the first 24 h after TBI, the dorsal hippocampus exhibited less status epilepticus-induced cell death than that normally seen following Pc administration alone. Instead of producing a state of hippocampal vulnerability to activation, TBI produced a state of neuroprotection. However, in a second group of animals evaluated 20 weeks post injury, double-injured animals were statistically indistinguishable in terms of seizure threshold, mossy fiber sprouting and cell survival when compared to those treated with Pc alone. TBI, therefore, produced a temporary state of neuroprotection from seizure-induced cell death in the developing rat; however, this ultimately conferred no long-term protection from altered hippocampal circuit rearrangements, enhanced excitability or later convulsive seizures.

Effects of combined dorsolateral and dorsal funicular lesions on sensorimotor behaviour in rats

The purpose of this research was to investigate the compensatory role of undamaged spinal pathways after partial spinal injury in rats. We have previously shown that bilateral lesions of the dorsal funiculus (DF) at the cervical level caused changes in overground and skilled locomotion that affected the forelimbs more than the hindlimbs. The same lesions also caused fore-paw deficits during a skilled pellet retrieval task (Kanagal and Muir, 2007). In contrast, bilateral cervical lesions of the dorsolateral funiculus (DLF) caused alterations in overground and skilled locomotion that were most marked in the hindlimbs rather than the forelimbs, but also caused fore-paw deficits during skilled pellet retrieval (Muir et al., 2007). We hypothesized that the relative lack of forelimb deficits during locomotion after DLF lesions was due to compensatory input arising from intact pathways in the DF. We tested this hypothesis in the present study by performing bilateral DF lesions in animals in which both DLFs had been transected 6 weeks previously. These secondary DF lesions involved either only ascending sensory pathways (DLF+ASP group) in the DF, i.e. sparing the corticospinal tract (CST), or involved both the ASP and the CST (DLF+DF group). All animals were assessed during overground locomotion, while crossing a horizontal ladder and during a pellet retrieval task. During overground locomotion, both groups moved with slightly altered forces and timing in both forelimbs and hindlimbs. During both ladder crossing and reaching, secondary lesions to DF (with or without CST) exacerbated the deficits seen after initial DLF lesions and additionally caused changes in the manner in which the rats used their forelimbs during reaching. Nevertheless, the relative magnitude of the deficits indicates that DF pathways in rats likely do not compensate for loss of DLF pathways during the execution of locomotor tasks, though there is indirect evidence that DLF-lesioned rats might rely more on ascending sensory pathways in the DF during skilled forelimb movements. The plastic changes mediating recovery are therefore necessarily occurring in other regions of the CNS, and, importantly, need time to develop, because animals with DLF+DF lesions performed simultaneously displayed marked functional deficits and were unable to use their forelimbs for skilled locomotion or reaching.

Incomplete nigrostriatal dopaminergic cell loss and partial reductions in striatal dopamine produce akinesia, rigidity, tremor and cognitive deficits in middle-aged rats

The present study was conducted to determine if the full array of parkinsonian symptoms could be detected in rats with nigrostriatal cell loss and striatal dopamine depletions similar to levels reported in the clinical setting, and to determine if older rats exhibit more robust parkinsonian deficits than younger rats. Young (2 months old) and middle-aged (12 months old) rats received bilateral striatal infusions of 6-OHDA, over the next 3 months they were assessed with a battery of behavioral tests, and then dopaminergic nigrostriatal cells and striatal dopamine and DOPAC levels were quantified. The results of the present study suggest that: (1) the full array of parkinsonian symptoms (i.e. akinesia, rigidity, tremor and visuospatial cognitive deficits) can be quantified in rats with incomplete nigrostriatal dopaminergic cell loss and partial reductions in striatal dopamine levels (2) parkinsonian symptoms were more evident in middle-aged rats with 6-OHDA infusions, and (3) there was evidence of substantial neuroplasticity in the older rats, but regardless of the age of the animal, endogenous compensatory mechanisms were unable to maintain striatal dopamine levels after rapid, lesion-induced nigrostriatal cell loss. These results suggest that using older rats with nigrostriatal dopaminergic cell loss and reductions in striatal dopamine levels similar to those in the clinical condition, and measuring behavioral deficits analogous to parkinsonian symptoms, might increase the predictive validity of pre-clinical rodent models.

Orphan neurones and amine excess: the functional neuropathology of Parkinsonism and neuropsychiatric disease

The aetiology and treatment of Parkinsonism is currently conceptualised within a dopamine (DA) deficiency-repletion framework. Loss of striatal DA is thought to cause motor impairment of which tremor, bradykinaesia and rigidity are prominent features. Repletion of deficient DA should at least minimise parkinsonian signs and symptoms. In Section 2, based on extensive pre-clinical and clinical findings, the instability of this approach to Parkinsonism is scrutinised as the existing negative findings challenging the DA deficiency hypothesis are reviewed and reinterpreted. In Section 3 it is suggested that Parkinsonism is due to a DA excess far from the striatum in the area of the posterior lateral hypothalamus (PLH) and the substantia nigra (SN). This unique area, around the diencephalon/mesencephalon border (DCMCB), is packed with many ascending and descending fibres which undergo functional transformation during degeneration, collectively labelled 'orphan neurones'. These malformed cells remain functional resulting in pathological release of transmitter and perpetual neurotoxicity. Orphan neurone formation is commonly observed in the PLH of animals and in man exhibiting Parkinsonism. The mechanism by which orphan neurones impair motor function is analogous to that seen in the diseased human heart. From this perspective, to conceptualise orphan neurones at the DCMCB as 'Time bombs in the brain' is neither fanciful nor unrealistic [E.M. Stricker, M.J. Zigmond, Comments on effects of nigro-striatal dopamine lesions, Appetite 5 (1984) 266-267] as the DA excess phenomenon demands a different therapeutic approach for the management of Parkinsonism. In Section 4 the focus is on this novel concept of treatment strategies by concentrating on non-invasive, pharmacological and surgical modification of functional orphan neurones as they affect adjacent systems. The Orphan neurone/DA excess hypothesis permits a more comprehensive and defendable interpretation of the interrelationship between Parkinsonism and schizophrenia and other related disorders.

Ineffectiveness of GM1 and ORG2766 on behavioural recovery after prefrontal cortical lesions in adult rats

This study examines whether treatment with GM1 ganglioside or the corticotropin (ACTH)(4-9) analogue ORG2766 can facilitate the behavioural recovery of adult rats with medial prefrontal cortex (mPFC) lesions, as animals are impaired in their food hoarding and spatial delayed alternation performance following mPFC lesions. No ameliorating effects of GM1 treatment on performance of these behaviours were observed. Although treatment with ORG2766 somewhat improved the hoarding performance of lesioned animals, the intermediate amount of pellets hoarded was not significantly different from that of either sham-operated or vehicle-treated lesioned rats. No effect of ORG2766 treatment was observed in the spatial delayed alternation test. Further, no changes were detected in the mesocortical dopamine innervation, presumed to be involved in the neural mechanism of behavioural sparing, in response to either treatment.

Activation of mesocortical dopaminergic system in the rat in response to neonatal medial prefrontal cortex lesions. Concurrence with functional sparing

Neonatal lesions of the medial part of the rat prefrontal cortex (mPFC) (performed at the age of 6 days) resulted in a sparing in the performance of spatial delayed alternation (SDA) and an increase in dopaminergic (DA) innervation. The increased DA innervation was primarily observed in the remaining part of the mPFC. The DA fibre density was considerably higher in the non-ablated part of the mPFC, and the fibres were thicker with more large varicosities compared with sham-operated controls. Biochemical measurements showed a 3.5-fold increase in DA concentration in the remaining part of the mPFC of the animals with neonatal lesions when compared with the mPFC of sham-operated animals. In addition the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were increased. The metabolite/transmitter ratios, indicating DA utilisation, did not significantly differ from controls. The increased DA innervation and the increased concentration of DA and its metabolites in the animals with neonatal lesions further support our hypothesis that the mesocortical DA system is involved in the neural mechanism of sparing of function observed after neonatal mPFC lesions. However, sparing of function in animals with no discernable mPFC forces us to conclude that this DA response cannot be the only factor involved in the mechanism of sparing of function.

Neurological correlates of unilateral and bilateral "strokes" of the middle cerebral artery in the rat

Rats with unilateral lesions of the middle cerebral artery (MCA) were tested for the ability to detect (touch) and remove a square of adhesive tape from each forepaw, and for performance on a number of neurological tests (e.g., placing and hopping reflexes, activity). Rats with MCA damage showed deficits in both touching and removing the tape from the paw contralateral to the damage, but not ipsilateral to the damage, while performing within normal limits on the other tests. After scores of the MCA rats dropped into the control group range on the adhesive tape test, they sustained damage to the opposite MCA. This did not reinstate the original deficit, suggesting that the recovery seen after the unilateral lesion was not mediated by the opposite cortex. The second lesion, however, caused a deficit in removing the adhesive tape from the limb opposite the new stroke. Some of the rats that originally had sham operations received bilateral MCA lesions at this time. These animals showed much more severe deficits on the adhesive tape test than the rats with sequential strokes. Rats with bilateral MCA damage (simultaneous or sequential) also slipped on a long narrow plank more often than control animals. Nimodipine did not enhance recovery on any of the behavioral measures.

Neurotransmitter amino acid levels in rat thalamus and cerebral cortex after cerebellectomy

Glutamate, aspartate, GABA, glycine and taurine levels have been measured in rat thalamus and in cerebral cortex at different time intervals (3rd, 7th, 15th, 30th day) after cerebellectomy. A decrease in glutamate, aspartate and GABA was detected at the 7th day after cerebellectomy in the thalamus and at the 15th day in the cerebral cortex; at the 30th day after cerebellectomy the levels of these amino acids in the thalamus and in the cerebral cortex were observed to have recovered to control values. No statistically significant difference in glycine and taurine levels in the thalamus and in the cerebral cortex after cerebellectomy could be seen. These results show that the functional recovery process after cerebellar injury is associated with a complex modification of amino acid levels in thalamus and in cerebral cortex.

Functional subdivisions of the rat somatic sensorimotor cortex

The behavioural impairments and subsequent recovery were studied in rats with circumscribed unilateral lesions in the somatic sensorimotor cortex (SMC). Lesions were made in the caudal forelimb region (CFL), the rostral forelimb region (RFL), the anteromedial cortex (AMC) or the hindlimb area. Rats with damage in the CFL produced a deficit in placing the forelimb contralateral to the lesion during exploratory locomotion on a grid surface. Rats with AMC damage circled in the direction ipsilateral to the lesion. Lesions in the CFL or AMC produced an ipsilateral somatosensorimotor asymmetry on the bilateral-stimulation test (responding to adhesive patches placed on the contralateral forelimb was slower) that recovered in 7 days following AMC lesions or 28 days following CFL lesions. Finally, RFL lesions produced an ipsilateral asymmetry on the bilateral-stimulation task that was more severe and enduring (recovery in 60 days). After behavioral recovery, the effects of an additional lesion placed in the homotopic contralateral cortex were examined (two-stage bilateral lesion). Rats receiving two-stage bilateral lesions in the RFL or CFL responded slower to tactile stimulation of the forelimb contralateral to the second lesion. In the case of CFL-damaged rats, placing deficits also appeared contralateral to the most recent injury. In contrast, rats receiving two-stage bilateral AMC lesions did not exhibit behavioral asymmetries following the second lesion. These results provide evidence to suggest that subdivisions of the rat SMC can be distinguished with lesion/behavioral experiments. Moreover, a comparison of the effects of unilateral and two-stage bilateral lesions may help in the parcellation of the rat SMC into functionally distinct subareas and provide a basis for studying the processes of recovery and maintenance of function following brain damage.

Motor recovery after serial spinal cord lesions of defined descending pathways in cats

The food-taking movement by which a cat uses its forepaw to take a piece of food and bring it to its mouth normally depends on the cortico- (CS) and rubrospinal (RS) tracts and disappears when they are transected in C5; a slow reappearance over months is due to bulbospinal (BS) take-over. After complete CS transection but minimal RS transection, food-taking remains. If, one month later, the RS tract is completely transected, food-taking is not abolished as it is when transection is made in one session. It is permanently abolished after a third transection of the ventral quadrant in C2. It is suggested that the food-taking remaining after the first lesion is due to combined RS and BS activity and that the RS tract induces the BS neurones to contribute to the extent that they can take over when the RS tract is completely transected.

Lesion size and recovery of function: some new perspectives

The present article discusses the possibility that functional recovery following brain damage may be to a large degree dependent on the amount of nervous tissue destroyed, such that more neuronal destruction may lead to more and not (as commonly suggested) to less recovery. This assumption may derive from the neuropsychological and neurological literature: many cases with circumscribed brain lesions are implicated with severe functional losses. However, patients with dramatic and severe brain destructions often show astonishingly normal behavior regarding cognition, speech, visuospatial, motor and sensory functions. Animal experimentation as well shows that an extensive lesion of a brain area may be associated with equal or less functional detriment than a small lesion of the same area. Along with the well-known variables of age, lesion growth, or personality and environmental factors, the amount of tissue destroyed should be considered as a potent mediator of functional recovery. At least for some functions and brain regions, the likeliness of recovery may increase with the extent of the lesion and thus the necessity of the brain to fulfill plastic changes.

Homologous cholinergic efferents spared by partial fimbrial lesions contribute to the recovery of hippocampal cholinergic enzymes in adult rats

Cholinergic fibers spared by partial lesions of the fimbrial bundle contribute to the recovery of enzyme markers for hippocampal cholinergic terminals. This recovery, most likely representing structural restoration of cholinergic terminals lost to lesion-induced degeneration, is amplified by a 'conditioning' lesion. This model of functionally relevant postlesion reconstruction, by and of structures within the CNS, is suitable to search for means to enhance homotypic collateral sprouting.

Simple reaction-time changes in patients with unilateral brain damage

Changes of simple visual reaction time were analysed in two groups of unilateral brain-damaged patients in order to evaluate to what extent properties of lesions, clinical parameters and experimental variables might influence speed of motor response. The results confirmed that brain damage, independent of its side, produces a retardation of speed. However, the two hemispheric groups differed in so far as volume of damage had a different bearing depending on side of lesion. In spite of such a difference the presence of a general interaction between size of damage and rate of progression of lesion was noted in both the hemispheres, reminiscent of Jackson's concept of 'lesion momentum'. Aphasia was related to a significant retardation of speed in left-hemisphere-diseased patients, although a specific effect of the disturbance of language could not be demonstrated. Experimental variables such as warned vs unwarned stimulation did not affect significantly the performance of brain-damaged patients.

Effects of two-stage lesions of the medial preoptic area on sexual behavior of male rats

The effects of lesions of the medial preoptic area (MPOA) on sexual behavior of adult male rats were examined. Bilateral lesions were performed in either one or two stages. Bilateral, but not unilateral, destruction of the MPOA virtually eliminated mounts, intromissions, and ejaculation. No sparing of the male rats' sexual behavior was observed after two-stage MPOA lesions.

Effects of lesions of prefrontal cortex and dorsomedial thalamus on delayed go/no-go alternation in rats

The effect of prefrontal cortex (PFC) and/or dorsomedial thalamus (DMT) lesions on a delayed go/no-go alternation task was studied in the rat. The lesion gave rise to the impairment of smooth alternation of one response to another, resulting in the generation of some successive repetitions of either response. However, either type of response was generated in virtually equal frequencies during pre- and post-surgery sessions. These results suggest that the DMT and PFC are involved in the memory and response control process for generating the alternation response, and they do not mediate either one type of response. Furthermore, DMT or DMT/PFC lesions induced a larger behavioral impairment than PFC lesions indicating that the DMT plays a major role in generating the alternation behavior based on the memory for the previous response.

Amine accumulation in behavioural pathology

When nigro-striatal and meso-cortical neurons degenerate there is a loss of dopamine in the terminal fields and an accumulation of amines in the axons of these systems as they traverse the hypothalamus through the medial forebrain bundle. Traditional lines of thought have attributed the occurrence of motor and consummatory deficits which occur after dopamine neuron degeneration to the loss of functional dopamine neurotransmitter in the terminal fields. However, we have hypothesized that hypothalamic amine accumulation represents an area of brain tissue where processes such as neurotransmitter release, ephaptic transmission or local axon swelling may be affecting adjacent neurons and may thereby participate in the production of behavioural deficits. There is a considerable amount of evidence from studies on both peripheral and central catecholamine-containing neurons indicating that when their axons degenerate a release of functional neurotransmitter can occur. Information from neuropharmacological studies indicates that several drugs which facilitate behavioural recovery from dopamine-depleting lesions may do so by affecting amine release or receptor sensitivity near areas of accumulation rather than depleted terminal fields. We conclude that amine accumulation is a component of dopamine neuron degeneration which should be considered when assessing the role of the central catecholamine systems in the control of various behavioural and physiological processes.

A role for amine accumulation in the syndrome of ingestive deficits following lateral hypothalamic lesions

Lesions of the lateral hypothalamus produce ascending catecholamine neuron degeneration which results in terminal depletion and proximal accumulation above the lesions. The occurrence of deficits in ingestive behaviour has been attributed traditionally to the loss of functional dopamine neurotransmitter in the terminal fields. However, release of functional amines may occur in the lateral hypothalamus at areas of accumulation, to produce at least some of the behavioural symptoms characterizing the lateral hypothalamic syndrome. Recovery from behavioural deficits as a result of various pharmacological treatments, after dopamine-depleting lesions, may be mediated by changes in amine release or modified sensitivity of receptors affected by released amines. We conclude that amine accumulation should be considered when interpreting experiments implicating central catecholamine systems in the control of consumatory behaviour and the regulation of body weight.

Decortication abolishes place but not cue learning in rats

The experiments examined whether decorticate rats are able to acquire a place learning strategy, as compared with a cue learning strategy, to successfully navigate from one place to another and whether the hippocampus, in the absence of the neocortex, contributes to successful performance. Decorticate rats, with or without hippocampectomy, were unable to locate an "invisible" platform submerged at a fixed place in a tank of cool water (made opaque by milk), rather they scrabbled at the edges of the tank and failed even to initiate search strategies. They were able to learn to swim directly to the platform if it was visible. Their ability to find the hidden platform was not enhanced by presurgical experience or two-stage ablations with training before and after ablations. When pretrained on the cue task and tested on the place task, they learned to inhibit scrabbling at the tank edges and "search" in a haphazard fashion for the hidden platform, but they never learned to swim directly toward it. When decorticate rats, trained on the cue task, received superior colliculus or basal ganglia removal in a second operation, cue learning was abolished. Hippocampal removal after decortication left performance on the cue task unaffected. The results demonstrate: (1) the integrity of the neocortex is essential for place learning; (2) the brainstem, including superior colliculus and basal ganglia, is sufficient for cue learning; and (3) in the absence of the neocortex the hippocampus plays no role in guiding either type of navigation. It is concluded that sensorimotor subsystems of the forebrain play a special role as detector-response systems for guiding behaviour in response to constellations of distal stimuli, whereas subcortical structures are sufficient for navigation to a single stimulus.

Brain systems and long-term memory

This paper focuses mainly on those findings derived from lesion studies on the rat which help to identify ensembles of neural structures concerned with the expression of previously learned responses. At the outset, the use of the lesion method in the search for those neurological circuits underlying memory is defended. This is followed by an evaluation of neocortical and subcortical systems in long-term memory. Subsequently, a modest list of tentative functional neural "complexes" involved in the maintenance of certain classes of learned responses is given, based largely upon the author's own research. It is concluded that the key to the understanding of the neurological substrates of long-term memory lies in the identification of those subcortical sites which interact with neocortical sites in the performance of complex learned tasks. The most likely subcortical sites involved in this interaction appear to inhabit the regions of the basal ganglia, limbic midbrain area, and ventral portions of the brainstem reticular formation.

Serial lesion effect in rat medial frontal cortex as a function of age

This experiment examined the potential for behavioral recovery in juvenile, adult, and senescent rats following serial lesions of medial frontal cortex. The subjects were trained on spatial delayed alternation in a T-maze under conditions designed to enhance the probability of a serial lesion effect. All subjects were given extensive handling and adaptation to the maze, interoperative training, and long interoperative and postoperative intervals. There were several major behavioral findings: (a) the aged intact subjects were not impaired in their ability to learn spatial delayed alternation, (b) one-stage bilateral lesions of frontal cortex produced equivalent deficits on spatial delayed alternation at all ages, (c) subjects in all of the age categories demonstrated a serial lesion effect, but (d) the 150 day and 570 day serial lesions groups demonstrated significantly better performances than the 35 day serial lesions group on several measures of performance.

Effects of one- and two-stage lesions of the posterior hypothalamus on temperature regulation in the rat

Rats received 1-stage bilateral and sequential unilateral (serial) lesions of the posterior hypothalamus and were tested for the ability to regulate body temperature after a lengthy recovery period. The groups with lesion differed from the sham-operated groups in the cold, although not under ambient or warm conditions. The fact that the serial lesion group performed the same as the 1-stage lesion groups in the cold is significant because earlier tests on these same animals revealed much better recovery after serial lesions in swimming, and a partial serial lesion effect in open field performance.

Tumor induced brain damage in rats: implications for behavioral and anatomical studies with aging animals

In rats, spontaneously occurring tumors and infarcts induce damage which destroys or distorts regions of the brainstem and cerebral hemispheres. Despite such destruction, there may be no gross evidence of behavioral or metabolic pathology.