Spatial deficits on radial maze after large tectal lesions in rats: possible role of impaired scanning

Relationships between the superior colliculus and hippocampus: Neural and behavioral considerations

Theories of superior collicular and hippocampal function have remarkable similarities. Both structures have been repeatedly implicated in spatial and attentional behaviour and in inhibitory control of locomotion. Moreover, they share certain electrophysiological properties in their single unit responses and in the synchronous appearance and disappearance of slow wave activity. Both are phylogenetically old and the colliculus projects strongly to brainstem nuclei instrumental in the generation of theta rhythm in the hippocampal EEC

On the other hand, close inspection of behavioural and electrophysiological data reveals disparities. In particular, hippocampal processing mainly concerns stimulus ambiguity, contextual significance, and spatial relations or other subtle, higher order characteristics. This requires the use of largely preprocessed sensory information and mediation of poststimulus investigation. Although collicular activity must also be integrated with that of “higher” centres (probably to a varying degree, depending on the nature of stimuli being processed and the task requirements), its primary role in attention is more “peripheral” and specific in controlling orienting/localisation via eye and body movements toward egocentrically labelled spatial positions. In addition, the colliculus may exert a nonspecific influence in alerting higher centres to the imminence of information potentially worthy of focal attention. Nevertheless, it is noteworthy that collicular and hippocampal lesions produce deficits on similar tasks, although the type of deficit is usually different (often opposite) in each case. Functional overlap between hippocampus and colliculus (i.e., strategically synchronised or mutually interdependent activity) is virtually certain vis-à-vis stimulus sampling, for example in the acquisition of information via vibrissal movements and visual scanning. In addition, insofar as stimulus significance is a factor in collicular orienting mechanisms, the hippocampus — cingulate – cortex — colliculus pathway may play a significant role, modulating collicular responsiveness and thus ensuring an attentional strategy appropriate to current requirements (stimulus familiarity, stage of learning). A tentative “reciprocal loop” model is proposed which bridges physiological and behavioural levels of analysis and which would account for the observed degree and nature of functional overlap between the superior colliculus and hippocampus.

Landmark control and updating of self-movement cues are largely maintained in head direction cells after lesions of the posterior parietal cortex

Head direction (HD) cells discharge as a function of the rat's directional orientation with respect to its environment. Because animals with posterior parietal cortex (PPC) lesions exhibit spatial and navigational deficits, and the PPC is indirectly connected to areas containing HD cells, we determined the effects of bilateral PPC lesions on HD cells recorded in the anterodorsal thalamus. HD cells from lesioned animals had similar firing properties compared to controls and their preferred firing directions shifted a corresponding amount following rotation of the major visual landmark. Because animals were not exposed to the visual landmark until after surgical recovery, these results provide evidence that the PPC is not necessary for visual landmark control or the establishment of landmark stability. Further, cells from lesioned animals maintained a stable preferred firing direction when they foraged in the dark and were only slightly less stable than controls when they self-locomoted into a novel enclosure. These findings suggest that PPC does not play a major role in the use of landmark and self-movement cues in updating the HD cell signal, or in its generation.

Locomotion and head scanning initiated by hypothalamic stimulation are inversely related

Stimulation in the hypothalamus elicits locomotor stepping. Before stepping is initiated, head scanning movements occur. We determined the relationships between the latency of locomotor initiation and the number, extent and direction of the head scanning movements. Chronic stimulation electrodes were stereotaxically implanted in and around the hypothalamus of 29 rats. Under awake conditions, 38 locomotor sites were tested in a runway apparatus. Behaviors occurring between the onset of stimulation and the first step were recorded on videotape. Points on the rat were digitized at sampling rate of 6 Hz to produce measures of head angles in the vertical, horizontal, and sagittal planes. The priming paradigm was used with a current selected for each site that was minimally sufficient to produce reliable stepping. In trials at approximately 1-min intervals, a 5-s train of stimulation (the control) was followed by a second train (the test) delivered 5-20 s later. Initiation latency on control trains was strongly correlated with head movement measures. Vertical and lateral head movements were independent of one another. Together, their frequency and extent accounted for 85% of the variance in locomotor initiation latencies. In effective priming trials, when locomotor initiation latencies were reduced on the test train, the frequency and extent of vertical and lateral head movements were also reduced. In non-effective priming trials, when latencies were not reduced, head movements were not reduced. Head scanning and locomotor initiation reflect reciprocal processes.

Impaired maze learning and cerebral glucose utilization in aged hypertensive rats

To elucidate the effects of prolonged hypertension on brain function during aging, we examined learning of an eight-arm radial maze task and local cerebral glucose utilization in young-adult (3 to 4 months old) and aged (16 to 17 months old) spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Young-adult SHR learned the task more slowly than young-adult WKY, but cerebral glucose utilization, measured by the [14C]2-deoxyglucose method in 24 brain structures, was not significantly different in the two groups. The aged SHR and WKY exhibited impaired learning ability. Cerebral glucose utilization was reduced (13% to 23%) in six regions in aged WKY and in 12 regions in aged SHR compared with values in the respective young-adult groups. Furthermore, the aged SHR showed a greater disturbance of learning acquisition and more profound reduction of cerebral glucose utilization in five regions than the aged WKY. In SHR, hypometabolism, indicated by a decrease in glucose utilization in 15 brain structures including the cerebral cortex, hippocampus, and visual system, was significantly correlated with impaired learning acquisition, indicated by an increase in total error choices. These findings show that (1) hypertension per se does not impair maze learning or cerebral glucose utilization in young-adult rats, and (2) brain function is impaired during aging and prolonged hypertension is an additional factor facilitating brain dysfunction associated with neuronal hypoactivities, resulting in behavioral deterioration including learning disability. Thus, early control of hypertension seems important for preventing or reducing brain dysfunction in senescence.

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.

The colliculus superior modulates ACTH-induced excessive grooming

In previous studies the involvement of nigrostriatal dopaminergic activity in ACTH(1-24)-induced grooming has been established. It was suggested that the dopaminergic modulation of ACTH(1-24)-induced excessive grooming is exerted through the striato-nigro-collicular pathway. To obtain further evidence it was investigated, whether local application of GABAergic agents into the colliculus superior modulates excessive grooming occurring after an intraventricular injection with ACTH(1-24). It appeared that intra-collicular picrotoxin (a GABAergic antagonist) suppressed ACTH-induced grooming, whereas muscimol (a GABAergic agonist) enhanced the grooming response. The picrotoxin-induced R(unning) F(it) B(ehavior), elicited from the colliculus superior was also seen after intraventricular administration of picrotoxin. A detailed comparison of this behavioral response seen after both routes of administration of picrotoxin suggests that intraventricularly injected picrotoxin may well induce the RFB via a direct effect on the colliculus superior. Lesions placed in the colliculus superior completely abolished picrotoxin-induced RFB, exploration and orientation behavior. Yet, these lesions did not reduce excessive grooming suggesting that although this region may be involved in the modulation of ACTH-induced grooming it is not the primary site of peptide action.

The nature of the visual discrimination impairment after neonatal or adult ablation of superior colliculi in rats

The superior colliculi were removed in rats at either one or five days of age or in adulthood. Seven months later they were tested on four successively presented two-choice intensity discriminations. The intensity difference between the discriminanda was reduced across the four problems to encourage choice by comparison. The purpose was to establish whether impoverished scanning is a feature of rats with collicular lesions and whether the age at which the lesion is incurred is important. The number of door-push and approach errors made in reaching criterion were used as measures of performance and the number of head-scans during acquisition was counted. The results provide no evidence that either one- or five-day operated rats exhibit sparing or recovery of the ability to scan discriminanda since all operated animals were impaired. Furthermore, novel retinal projections, present in one-day operated animals, fail to mediate such sparing. Finally, the results did not demonstrate a selective increase in approach errors following collicular lesions and were therefore inconsistent with the view that the impairment is one of visually-guided locomotion. It is concluded that visual discrimination learning is impaired following collicular lesions in circumstances where scanning of discriminanda is required for efficient performance.

Spatial memory and visual evoked potentials in young and old rats after housing in an enriched environment

The effects of aging and of housing in an enriched environment on performance in an 8-arm radial maze were evaluated in young adult (7-8 months) and old (30-33 months) male Brown-Norway rats, using a procedure in which the rats were confined for 8 s to the central platform of the maze between consecutive choices. Although the old rats attained a level of performance which was clearly above change, they were shown to perform worse than the young rats. No performance differences were found between differentially housed rats of the same age group. In a second experiment recovery cycles of visual evoked potentials were determined in the same rats by using paired flashes with an interstimulus time of 400, 300, 200, or 100 ms. Recovery was consistently smaller in the old rats as compared to the young ones. No correlation could be demonstrated, however, between radial maze performance or housing condition and recovery functions of the visual evoked potentials. This finding indicates that a decline in visual sensitivity cannot readily explain the impaired radial maze performance of old rats. Evidence which suggests that age-related hippocampal changes play a major role in the radial maze performance deficit is discussed.

Effects of collicular lesions in the hamster during visual discrimination. An analysis from computer-video actograms

Recent reports have suggested that lesions of the superior colliculus may impair the orienting or scanning movements of the head that are thought to serve a key function in visual discrimination. In the present investigation computer-video “actograms” were used to quantify the head movements of freely moving hamsters performing a simultaneous visual discrimination. Hamsters with collicular lesions did not differ from the controls in their head movement spectra, but there was a significant reduction in the incidence of the pauses in locomotion during which these movements are made. Intact animals showed slower and irregular progression interspersed with pauses during which they made scanning movements, whereas collicular hamsters made straighter runs. Although the rate of learning of lesioned hamsters was not impaired, our data strongly suggest that they used different orienting and learning strategies. When tested for the effects of novel stimuli, unrelated to the task, normal hamsters reacted strongly with active exploratory scanning movements, but collicular animals did not, although qualitative changes in their behaviour suggested that the novel stimuli were not being ignored.

Superior colliculus and visual neglect in rat and hamster. III. Functional implications

In comparison with the geniculostriate pathway, the retinotectal projection in rat and hamsters appears to emphasize information concerning localized transient stimuli, particularly in the periphery of the visual field. An important question is whether the superior colliculus merely relays this information elsewhere, or instead takes part in its analysis. This question is broken down into two parts. First, what decisions do rats and hamsters have to take concerning localized transient visual stimuli in the periphery? It is suggested that the following decisions are taken: (a) does the stimulus require any response? If the transient is self-produced, or is known on the basis of past experience to predict no important consequence, then it may be ignored; and (b) does the stimulus convey enough information to determine a response, either unlearnt (e.g. attack, flee, freeze) or learnt? If the stimulus appears to warrant some response, but it is not clear which, then it requires investigation. Second, what evidence is there that the superior colliculus participates in any of these decisions? It is argued on general grounds that the involvement of the superior colliculus in investigative orienting necessitates its knowing about the other decisions, since a useful orienting device cannot respond promiscuously to uninteresting or dangerous stimuli. This argument is supported by evidence from stimulation and recording studies, which in addition suggest that the superior colliculus is directly involved in producing a number of responses appropriate to peripheral transients, besides orienting. Thus, one function of the superior colliculus may be to help analyze and take decisions about localized transients in the periphery of the field.(ABSTRACT TRUNCATED AT 250 WORDS)

Pre- and postweaning indices of neurotoxicity in rats: effects of triethyltin (TET)

Pre- and postweaning measures of learning and locomotor activity were used as indices of CNS function after early perinatal neurotoxic insult. Triethyltin (0.0, 3.0, or 6.0 mg/kg, ip) administered on Postnatal Day 5 (PND5) was used as the neurotoxicant. Learning deficits and alterations in locomotor activity were observed during both the pre- and postweaning periods. Preweaning learning ability was evaluated with an appetitive alleyway paradigm, while an automated radial-arm maze (RAM) was used to assess juvenile learning. Preweaning open-field locomotion was evaluated in the presence and absence of homecage litter while postweaning activity was measured in an automated device, the Motron, or as a component of performance in the RAM. The 6.0-mg/kg TET-exposed animals required more trials to acquire the alleyway task. RAM subjects receiving 6.0 mg/kg of TET were less accurate than those given 3.0 mg/kg or vehicle control. TET did not affect open-field activity on PND10; low levels of spontaneous locomotion occurred regardless of treatment. On PND13, there was a dose-related decrease in locomotion over home-cage litter while all groups exhibited equivalent low rates of locomotion in the absence of home-cage cues. All treatment groups were more active when tested over litter than over no litter. In contrast, on PND21 and throughout the RAM testing period, TET-exposed subjects exhibited dose-related increases in activity. TET produced treatment-related decreases in wet weight of whole brain, hippocampus, and cerebellum in both developing (PND23) and adult (PND200) animals with the hippocampus being most affected on a percentage basis. In contrast, no treatment-related body weight differences were observed at these times. The neurotoxicity of TET can be demonstrated during the preweaning period whether a commonly used endpoint in assessing CNS function (locomotor activity) is monitored or a more complex endpoint (learning) is evaluated provied (1) the method used to monitor locomotor activity incorporates both the normally low levels of preweaning locomotion and the more elevated levels induced by home-cage cues and (2) the learning paradigms utilized are appropriate for the capabilities of the animal.

Locomotor activity of rats in open field after microinjection of procaine into superior colliculus or underlying reticular formation

Whereas large lesions of the superior colliculus in rats increase locomotor activity in the open field, bilateral collicular microinjections of muscimol (an agonist of the inhibitory neurotransmitter GABA) have been reported to reduce open-field activity. This difference might be due to muscimol's acting on a subpopulation of collicular neurones, or to some feature of the microinjection technique. The issue was investigated by observing open-field behavior after reversible lesions produced by bilateral microinjections of the local anaesthetic procaine (10-300 micrograms in 0.5 microliter) into midbrain sites. Injections of procaine into the superior colliculus produced effects similar to those reported after muscimol injections: both locomotor activity and other exploratory responses were suppressed, with the rats spending much of their time motionless in an alert posture. In contrast, animals with injections of procaine into the mesencephalic reticular formation (MRF) ventral to the superior colliculus resembled rats given large collicular lesions: they showed very striking increases in locomotor activity, while their rearing and exploratory head movements were reduced. It is suggested that in some experiments large collicular lesions may have increased locomotor activity in the open field because they invaded underlying MRF. However, it is also possible that in rodents the acute effects of collicular inactivation, as assessed by microinjection of muscimol or procaine, are different from the chronic effects that are observed in experiments with electrolytic or radiofrequency lesions.

Visual pathways and acuity hooded rats

Three experiments on the effects of lesions of the visual system on contrast-detection in hooded rats are described, in which the ability of rats to detect stationary high-contrast square-wave gratings of various fundamental frequencies presented in the central visual field was measured before and after operation. The results suggested the following conclusions: (i) The pathways from retina to striate cortex via dorsal lateral geniculate nucleus (dLGN) conveys information about high spatial frequencies sufficient for normal detection of these gratings, that is up to about 1 cycles/deg. It may be the only pathway to carry this information, and may thus play a unique role in the analysis of fine detail. The high-frequency information is probably relayed from striate cortex to extrastriate cortex, rather than to subcortical sites. (ii) The superior colliculus receives information from the retina up to at least 0.7 cycles/deg, which it then relays to extrastriate visual cortex, probably via the lateral posterior nucleus of the thalamus. (iii) Neither the projections from superior colliculus to other, non-thalamic sites nor the remaining pathways from the retina (e.g. to ventral LGN) appear to carry contrast information higher than 0.3 cycles/deg. These sets of projections therefore do not appear to be used for precise analysis of stationary scenes. These findings suggest that there are considerable similarities between the visual systems of rats and other mammals with respect to the routing of information about stationary spatial contrast, and may help to explain the results of some experiments that have used tasks besides contrast-detection to assess the visual capacities of rats after lesions.