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

Organization of the crossed tecto-reticulo-spinal projection in rat--II. Electrophysiological evidence for separate output channels to the periabducens area and caudal medulla

The previous paper (Redgrave et al., Neuroscience 37, 571-584, 1990) presented anatomical evidence indicating there are at least two largely segregated components of the crossed tecto-reticulo-spinal pathway which project to the periabducens area and caudal medulla. An immediate question arising from this finding is whether tectal cells which project either to the periabducens area or to the caudal medulla have different electrophysiological response properties. An answer to this question would be relevant to the issue of whether different components of the tecto-reticulo-spinal system are specialized for the production of different classes of orienting movement. Accordingly, extracellularly recorded units in the superior colliculus of urethane anaesthetized rats were tested for antidromic activity following electrical stimulation of the periabducens area or the caudal medulla. When antidromic potentials were successfully recorded the sensory properties of the units were tested with a range of unimodal visual, somatosensory and auditory stimuli. The following results were obtained. (i) Tectal cells antidromically activated by stimulation of the caudal medulla were preferentially sensitive to somatosensory stimuli from the perioral region, while cells activated from the periabducens area were more frequently responsive to auditory stimuli. (ii) Tectal fibres activated by stimulation of the caudal medulla had significantly higher conduction velocities than the fibres activated by electrodes in the periabducens region. (iii) More than 90% of antidromically activated cells were located in stratum album intermediale or dorsal stratum profundum. These electrophysiological findings confirm and extend previous anatomical observations which indicate that components of the crossed descending projection of the colliculus may be functionally specialized for the production of different classes of orienting movements.

Organization of the crossed tecto-reticulo-spinal projection in rat--I. Anatomical evidence for separate output channels to the periabducens area and caudal medulla

The superior colliculus has been used to study principles of sensorimotor transformation underlying the guidance of orienting movements by multimodal sensory stimuli. We have previously suggested that there may be two different classes of mechanism which can produce orienting-like movements towards a novel event; one that locates a stimulus on the basis of remembered position, and another which uses continuous feedback relating to target velocity. The crossed descending pathway of the superior colliculus is widely considered the projection most likely to relay signals associated with the production of orienting movements. However, if different neural mechanisms are used to produce functionally distinct types of orienting, we might expect this pathway to have separate anatomical components related to function. The purpose of the present experiment was to see if collicular fibres innervating two important pre-motor targets of the crossed descending pathway, the periabducens area and the caudal medulla-spinal cord, come from the same population of tectal cells. One of the retrogradely transported fluorescent tracers (Diamidino Yellow) was injected into the periabducens area, and another (True Blue or Fast Blue) was injected into tectospinal fibres at the level of the ventromedial caudal medulla. Under these conditions we found: (i) less than 10% of labelled cells within the superior colliculus contained both tracers; (ii) the bulk of singly labelled cells projecting to the periabducens area or the caudal medulla were concentrated at different locations within the colliculus, (iii) in regions of the superior colliculus where there was overlap of singly labelled cells, neurons projecting to the periabducens area or the caudal medulla could be distinguished morphologically. These data provide three classes of evidence which indicate that the crossed descending projection in rat can be subdivided into at least two relatively independent anatomical components. This conclusion may, in part, provide an anatomical substrate for the functional dissociations proposed for orienting movements.

Neuroanatomical studies of the nigrotectal projection in the cat

We have used retrograde and anterograde transport methods to analyze the nigrotectal projection in the cat. This projection arises from both pars reticulata (SNr) and pars lateralis (SNl) and distributes to all cellular laminae of the superior colliculus. This extensive nigrotectal innervation is not a simple, single circuit. Rather it appears to consist of several parallel channels, with each taking origin from a particular zone of the substantia nigra and terminating within specific collicular laminae and/or sublaminae. For instance, only neurons within the SNl project to the stratum griseum superficiale; such neurons also project diffusely to all other tectal laminae. Cells in the most lateral portion of the SNr project to a horizontal, patchy tier in the interface region between the stratum opticum and the stratum griseum intermediate (SGI). Finally, more medially placed neurons within the SNr project to a horizontal patchy tier within the middle of the SGI and to a wedge-shaped locus in the stratum griseum profundum. Our findings provide an anatomical substrate for electrophysiological data (Karabelas and Moschovakis: J. Comp. Neurol. 239: 309-329, '85) showing a widespread distribution of nigrorecipient tectal neurons in the cat.

Dorsal raphe serotoninergic branching neurons projecting both to the lateral geniculate body and superior colliculus: a combined retrograde tracing-immunohistochemical study in the rat

Injections of HRP into the superior colliculus labelled cells in the lateral cell groups of the dorsal raphe nucleus. The cytoarchitectural features and location of these cells showed remarkable similarities with those known to project to the lateral geniculate body, and, therefore, the possible existence of branching neurons in the dorsal raphe nucleus projecting to these two visual structures was tested. Injections into the lateral geniculate body and the superior colliculus of several fluorescent tracers--namely, Fast Blue, Fluoro-Gold, propidium iodide, rhodamine-B-isothiocyanate, and Diamidino Yellow, used in different combinations, showed single- and double-labelled neurons in the lateral wings of the dorsal raphe nucleus. In order to verify the chemical nature of these cells, the tissue was processed for immunofluorescence with serotonin antibodies. The results obtained showed several triple-labelled cells exhibiting two fluorescent tracers as well as 5-hydroxytryptamine-like immunoreactivity. Some immunonegative tracer-positive cells were also observed, suggesting their nonserotoninergic nature. Finally, electrolytic lesions of the lateral wings of the dorsal raphe nucleus caused a gradual disappearance of serotonin-immunoreactive fibers in these visual areas following different survival times. This correlated well with a decrease in the serotonin content studied by high-pressure liquid chromatography. These results support a role of the serotoninergic dorsal raphe projection to the lateral geniculate body and to the superior colliculus in the processing of visual information, and they suggest that serotonin may have a coordinating influence on primary visual centers.

Plasticity of behavioural response to repeated injection of glutamate in cuneiform area of rat

Whereas a single microinjection of L-glutamate (10 nmol) into the cuneiform area of rats gives freezing, a second or third injection (delivered at 4-min intervals to the same site)can produce fast running. To examine whether this plasticity of response was caused by a simple increase in the amount of glutamate present, 30 nmol of glutamate were given in a single injection. In 93% of sites in the cuneiform area this procedure gave only freezing, although subsequent testing with repeated injections produced fast running in 53% of these sites. Thus, response potentiation to glutamate appears to require repeated stimulation, and may therefore be related to processes underlying the natural conditioning of defensive responses.

Responses resembling defensive behaviour produced by microinjection of glutamate into superior colliculus of rats

Electrical stimulation of the superior colliculus in rats elicits not only orienting movements, as it does in other mammals, but also behaviours resembling such natural defensive responses as prolonged freezing, cringing, shying, and fast running and jumping. To investigate the location of the cells mediating these behaviours, the superior colliculus was systematically mapped with microinjections of sodium L-glutamate (50 mM, 200 nl), and the resultant behavioural changes as assessed in an open field were analysed for defence-like responses. The main regions that gave defensive behaviour were (i) rostromedial superior colliculus (all layers), and (ii) both medial and lateral parts of the caudal deep layers. Cells in these areas project into the ipsilateral descending pathway. However, the cells of origin of this pathway are also found in collicular regions, such as rostral intermediate gray and parts of far caudal colliculus, that did not give defensive movements in response to glutamate stimulation. It is unclear whether this is because only parts of the ipsilateral pathway mediate defensive behaviours, or because glutamate is a relatively inefficient stimulating agent for these systems. An unexpected feature of the results was that at a number of collicular sites the nature of the defensive response changed with successive (up to three) injections of glutamate, often appearing to become more intense. Whether the mechanism underlying this potentiation is related to the conditioning of natural defensive behaviour is unknown.

Cardiovascular and respiratory changes elicited by stimulation of rat superior colliculus

Stimulation of the rat superior colliculus can produce either orienting or defensive movements, which if elicited by natural stimuli would be accompanied by cardiovascular changes. To assess whether cardiovascular changes might also be mediated by the superior colliculus, blood pressure and heart rate were measured in Saffan-anaesthetised rats while the dorsal midbrain was systematically explored with electrical and chemical stimulation. Electrical stimulation (10 sec trains of 0.3 msec 100 Hz cathodal pulses, 50 microA) within the superficial and intermediate layers of the rostral superior colliculus transiently lowered blood pressure without affecting heart rate. In contrast sites within the deep layers, and in adjacent periaqueductal grey and midbrain tegmentum, gave pressor responses accompanied by a variety of heart-rate changes, that usually included a period of bradycardia. A roughly similar distribution was obtained with the cell-stimulant bicuculline (200 or 500 nl, 490 microM), though sodium L-glutamate (200 nl, 0.05 or 1.0 M) was ineffective. These results suggest that (a) cardiovascular responses can be produced by stimulation of the rat superior colliculus; (b) their nature depends on the location of the stimulation; and (c) they may be mediated in part by cells differentially sensitive to glutamate and to bicuculline. In addition, in some animals respiratory responses were measured stethographically. Short-latency increases in thoracic girth, often accompanied by increases in respiratory rate and depth, were elicited by electrical stimulation from 61% of the collicular sites examined, and by microinjection of glutamate from 56% of collicular sites. These data suggest that (a) cells within the superior colliculus are capable of influencing respiration; (b) given the widespread distribution of responsive sites within the superior colliculus, the respiratory changes may be preparatory for both approach and defensive movements; (c) the collicular cells that affect respiration may be different from those that influence blood pressure, because the latter are relatively insensitive to microinjection of glutamate.

A functional analysis of the collicular output pathways: a dissociation of deficits following lesions of the dorsal tegmental decussation and the ipsilateral collicular efferent bundle in the Mongolian gerbil

Mongolian gerbils received either lesions of the superior colliculus, small lesions of the uncrossed efferents of the superior colliculus, knifecuts of the dorsal tegmental decussation, or sham operations. The animals were subsequently tested for avoidance of overhead visual threat, orientation and locomotion toward small targets, and negotiation of a large barrier in order to reach a small target. Animals with collicular lesions showed no responses to overhead threat and had severe deficits in orienting to small targets. Animals with lesions of the uncrossed tectal efferents showed diminished responses to overhead threat but had no deficits in orienting to targets. Animals with lesions of the dorsal tegmental decussation showed only slight reductions in responses to overhead threats but clear impairments in the orientation tasks. The impairments in orientation, however, were less severe than those seen in collicular animals. Animals in all groups were able to negotiate barriers efficiently. These results suggest that separate output pathways of the superior colliculus mediate different types of visuomotor behaviours. The results further suggest that visual orientation to small targets does not depend completely on output through the predorsal bundle, but must also involve other collicular outputs.

Visual discrimination in the absence of visual cortex

Normal rats and rats with devascularization lesions ranging from subtotal removals of striate cortex (Area 17) to complete removal of neocortex were trained in a horizontal/vertical stripe discrimination for a liquid reinforcer. Subgroups of animals were identified on the basis of size and location of lesion (with particular reference to striate cortex) as Subtotal, Striate, Posterior and Decorticate. Some animals in all of the lesion groups were able to acquire the discrimination, but there was a direct relationship between lesion size and number of training trials. Those animals which reached criterion on the original discrimination were trained on a second horizontal/vertical discrimination under either transfer or reversal conditions using 'rotated obliques' stimuli. Performance on this second discrimination indicated that animals from all lesion groups had been using visual stimuli based on stripe orientation in the original problem. Members of all lesion groups solved the rotated obliques problem under the transfer condition, though the speed and completeness with which they did so was again inversely related to lesion size. These data show high levels of visual competence in the absence of visual cortex even when stimuli thought to detect form discrimination are used and thus reinforce the view that superior colliculus may be a more significant visual area for the rat than was previously assumed. They also support other observations that animals do not use residual visual capacities without extensive experience and appropriate motivation.

The role of the predorsal bundle in head and body movements elicited by electrical stimulation of the superior colliculus in the Mongolian gerbil

Thirty-two Mongolian gerbils received bilateral chronic implants of stainless steel electrodes in the superior colliculus. The movements elicited by electrical stimulation were recorded on videotape and measured by means of a computer-assisted image analyzing procedure. Ipsiversive body movements were elicited by stimulation of the anterior part of the superior colliculus. Contraversive head and body movements could be elicited by stimulation over the entire superior colliculus. Amplitudes of head and body movements were dependent upon both stimulation parameters (current and train duration) and the animal's posture at stimulus onset. In a second experiment, the predorsal bundle was cut at its decussation by means of a stereotaxic microknife. After such cuts, contraversive turns were either abolished or were replaced by ipsiversive movements. Ipsiversive movements were unaffected by the knife cuts. This experiment provides evidence that the distinct types of movements that can be elicited by collicular stimulation are subserved by anatomically separate output pathways.

Head and body movements produced by electrical stimulation of superior colliculus in rats: effects of interruption of crossed tectoreticulospinal pathway

Stimulation of the superior colliculus in rats produces movements of the head and body that resemble either orientation and approach towards a contralateral stimulus, or avoidance of, or escape from, such a stimulus. A variety of evidence indicates that the crossed descending pathway, which runs in the contralateral predorsal bundle to the pontomedullary reticular formation and the spinal cord, is involved in orienting movements. The nature of this involvement was investigated, by assessing the effects on tectally-elicited movements of midbrain knife-cuts intended to section the pathway as it crosses midline in the dorsal tegmental decussation. As expected, ipsilateral movements resembling avoidance or escape were little affected by dorsal tegmental decussation section, whereas contralateral circling movements of the body were almost abolished. However, contralateral movements of the head in response to electrical stimulation were not eliminated, nor were orienting head movements to visual or tactile stimuli. There was some suggestion that section of the dorsal tegmental decussation increased the latency of head movements from electrical stimulation at lateral sites, and decreased the accuracy of orienting movements to sensory stimuli. These results support the view that the crossed tectoreticulospinal system is concerned with approach rather than avoidance movements. However, it appears that other, as yet unidentified, tectal efferent systems are also involved in orienting head movements. It is possible that this division of labour may reflect functional differences between various kinds of apparently similar orienting responses. One suggestion is that the tectoreticulospinal system is concerned less in open-loop orienting responses (that are initiated but not subsequently guided by sensory stimuli), than in following or pursuit movements.

Hypothalamic and ventral thalamic projections to the superior colliculus in the cat

The present report describes the organization of collicular afferents that arise within either the hypothalamus or the ventral thalamus. Following the placement of large injections of WGA-HRP into the superior colliculus of the cat, retrogradely labeled neurons are located within the reticular nucleus of the thalamus, the zona incerta, the fields of Forel, and throughout the hypothalamus. Although the dorsal hypothalamic area contains the largest number of labeled hypothalamic neurons, labeled cells are also found within the periventricular, paraventricular, dorsomedial, ventromedial, posterior, lateral, and anterior hypothalamic nuclei. A strikingly similar pattern of distribution of labeled neurons is also observed following placement of small injections of WGA-HRP that are restricted within the stratum griseum intermedium (SGI). In contrast, hypothalamic and ventral thalamic labeling is not seen after placement of injections within the stratum griseum superficiale. Following the placement of injections of tritiated anterograde tracers within the dorsal hypothalamic area, transported label is organized in two bands of clusters over the SGI. When injections of tritiated tracers are placed within the zona incerta, terminal label is also located over the SGI; however, the distribution of silver grains does not appear as clusters or distinct puffs. On the basis of the comparison of the cellular types that give rise to these projections and the differences in terminal distribution, we suggest that the hypothalamic and ventral thalamic projections to the superior colliculus are totally separate and unrelated pathways. The functional implications of the hypothalamotectal pathway are also discussed.

Functional reconfiguration of midbrain neurons by ovarian steroids in behaving hamsters

Previous studies have demonstrated the importance of the midbrain, especially the tectum, in the sensorimotor control of the sexually-receptive posture, lordosis, in female golden hamsters. In the present study, midbrain single unit activity was recorded during hormonal induction of lordosis, to identify neuronal activity mediating the sensorimotor control of the response and to observe functional changes in these neurons associated with the hormones' behavioral effect. Progesterone administration to estrogen-primed hamsters initiated pronounced changes in neuronal activity levels, somatosensory responsiveness and movement-related firing. These changes began in some neurons within 10 min of subcutaneous hormone injection and affected progressively more neurons until lordosis was elicitable 2-4 hr later. The pattern of these changes, including increased neuronal responsiveness to lumbosacral stimuli, appearance of lordosis-related firing (especially in the tectum) and reduced incidence of firing associated with lordosis-incompatible behaviors, constituted a transformation, or reconfiguration of midbrain sensorimotor function. It is proposed that this reconfiguration enables the elicitation and maintenance of lordosis by lumbosacral stimuli. Neural effects of comparable magnitude didn't result from control progesterone injections which failed to induce lordosis.

Tonic desynchronisation of cortical electroencephalogram by electrical and chemical stimulation of superior colliculus and surrounding structures in urethane-anaesthetised rats

Damage to the superior colliculus in rats impairs desynchronisation of the cortical electroencephalogram in response to light flashes. However, it is unclear which elements within the superior colliculus, and which efferent collicular pathways, might be involved in alerting cerebral cortex to visual stimuli. To investigate this problem, the superior colliculus and surrounding structures were stimulated either electrically (3 s trains of 0.2 ms 100 Hz cathodal pulses), or chemically (200 nl of 5 mM sodium L-glutamate), in rats anaesthetised with urethane. The cortical electroencephalogram was recorded bilaterally from frontal cortex. At each site tested with electrical stimulation the threshold current (up to 60 microA) required to produce tonic desynchronisation (outlasting stimulation-offset by at least 10 s) was determined. Comparison of the effects of electrical and chemical stimulation suggested the following: (1) stimulation of cells in the deep layers of the superior colliculus can desynchronise the cortical electroencephalogram. There may also be an additional effective area in the rostral part of the superficial layers, but this needs to be confirmed in unanaesthetised animals. (2) Stimulation of fibres in the deep white layers of caudal superior colliculus, and of cells in a wide area of caudal midbrain reticular formation, are also effective at desynchronising the cortical electroencephalogram. It is therefore possible that the ipsilateral descending pathway, that runs from the superior colliculus to terminate in the parabigeminal and cuneiform nuclei and surrounding reticular formation, is involved in mediating cortical desynchronisation initiated by the superior colliculus. Evidence from other studies indicates that some sites in this pathway may be part of a "defence arousal system". (3) Sites on the ascending pathways from the superior colliculus, to structures including dorsal thalamus, pretectum, zona incerta and rostral midbrain reticular formation, were relatively ineffective at tonically desynchronising the cortex. However, some of these pathways might mediate phasic, movement-related arousal of collicular origin.

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)