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

Pathways from the Superior Colliculus to the Basal Ganglia

The present work aims to review the structural organization of the mammalian superior colliculus (SC), the putative pathways connecting the SC and the basal ganglia, and their role in organizing complex behavioral output. First, we review how the complex intrinsic connections between the SC's laminae projections allow for the construction of spatially aligned, visual-multisensory maps of the surrounding environment. Moreover, we present a summary of the sensory-motor inputs of the SC, including a description of the integration of multi-sensory inputs relevant to behavioral control. We further examine the major descending outputs toward the brainstem and spinal cord. As the central piece of this review, we provide a thorough analysis covering the putative interactions between the SC and the basal ganglia. To this end, we explore the diverse thalamic routes by which information from the SC may reach the striatum, including the pathways through the lateral posterior, parafascicular, and rostral intralaminar thalamic nuclei. We also examine the interactions between the SC and subthalamic nucleus, representing an additional pathway for the tectal modulation of the basal ganglia. Moreover, we discuss how information from the SC might also be relayed to the basal ganglia through midbrain tectonigral and tectotegmental projections directed at the substantia nigra compacta and ventrotegmental area, respectively, influencing the dopaminergic outflow to the dorsal and ventral striatum. We highlight the vast interplay between the SC and the basal ganglia and raise several missing points that warrant being addressed in future studies.

Context dependence of head bobs in gerbils and potential neural contributions

Headbobs are up-down movements of the cranium associated with the use of motion parallax for depth perception. Mongolian gerbils (aka jirds; Meriones unguiculatus) often execute a series of headbobs prior to jumping between surfaces. Gerbils were tested in a jumping stand task and headbobs videotaped under three light levels approximating low daylight, dawn/dusk, and moonlight across a range of distances to target. Headbobs per trial increased linearly with increasing distance to the target platform, whereas headbob frequency (rate of headbobbing pre-jump on the start platform) increased with gap distance up to an intermediate level and then decreased. Overall, gerbils made the most headbobs per trial under the darkest conditions, whereas their headbobbing rate was highest for medium illumination, especially for medium-long gap distances. There was a positive correlation between headbob frequency and volume of the superior colliculus (SC), but no relationship between headbobs and relative size of the temporo-posterior (TP) visual cortex. The results suggest that gerbils employ a specific visuomotor strategy for depth perception differentially under different conditions. We suggest that the deployment of headbobs under specific conditions may be part of an SC-driven vigilant state, of which more rapid sampling of the visual environment using headbobs for depth estimation is one component. Moreover, the findings highlight the importance of considering ecological factors in designing studies of visual behavior and its underpinnings in rodents.

Reticular Formation Connections Underlying Horizontal Gaze: The Central Mesencephalic Reticular Formation (cMRF) as a Conduit for the Collicular Saccade Signal

The central mesencephalic reticular formation (cMRF) occupies much of the core of the midbrain tegmentum. Physiological studies indicate that it is involved in controlling gaze changes, particularly horizontal saccades. Anatomically, it receives input from the ipsilateral superior colliculus (SC) and it has downstream projections to the brainstem, including the horizontal gaze center located in the paramedian pontine reticular formation (PPRF). Consequently, it has been hypothesized that the cMRF plays a role in the spatiotemporal transformation needed to convert spatially coded collicular saccade signals into the temporally coded signals utilized by the premotor neurons of the horizontal gaze center. In this study, we used neuroanatomical tracers to examine the patterns of connectivity of the cMRF in macaque monkeys in order to determine whether the circuit organization supports this hypothesis. Since stimulation of the cMRF produces contraversive horizontal saccades and stimulation of the horizontal gaze center produces ipsiversive saccades, this would require an excitatory cMRF projection to the contralateral PPRF. Injections of anterograde tracers into the cMRF did produce labeled terminals within the PPRF. However, the terminations were denser ipsilaterally. Since the PPRF located contralateral to the movement direction is generally considered to be silent during a horizontal saccade, we then tested the hypothesis that this ipsilateral reticuloreticular pathway might be inhibitory. The ultrastructure of ipsilateral terminals was heterogeneous, with some displaying more extensive postsynaptic densities than others. Postembedding immunohistochemistry for gamma-aminobutyric acid (GABA) indicated that only a portion (35%) of these cMRF terminals are GABAergic. Dual tracer experiments were undertaken to determine whether the SC provides input to cMRF reticuloreticular neurons projecting to the ipsilateral pons. Retrogradely labeled reticuloreticular neurons were predominantly distributed in the ipsilateral cMRF. Anterogradely labeled tectal terminals were observed in close association with a portion of these retrogradely labeled reticuloreticular neurons. Taken together, these results suggest that the SC does have connections with reticuloreticular neurons in the cMRF. However, the predominantly excitatory nature of the ipsilateral reticuloreticular projection argues against the hypothesis that this cMRF pathway is solely responsible for producing a spatiotemporal transformation of the collicular saccade signal.

The role of the ventrolateral caudoputamen in predatory hunting

The ventrolateral caudoputamen (VLCP) is well known to participate in the control of orofacial movements and forepaw usage accompanying feeding behavior. Previous studies from our laboratory have shown that insect hunting is associated with a distinct Fos up-regulation in the VLCP at intermediate rostro-caudal levels. Moreover, using the reversible blockade with lidocaine, we have previously suggested that the VLCP implements the stereotyped actions seen during prey capture and handling, and may influence the motivational drive to start attacking the roaches, as well. However, considering that (1) lidocaine suppresses action potentials not only in neurons, but also in fibers-of-passage, rendering the observed behavioral effect not specific to the ventrolateral caudoputamen; (2) the short lidocaine-induced inactivation period had left a relatively narrow window to observe the behavioral changes; and (3) that the restriction stress to inject the drug could have also disturbed hunting behavior, in the present study, we have examined the role of the VLCP in predatory hunting by placing bilateral NMDA lesions three weeks previous to the behavior testing. We were able to confirm that the VLCP serves to implement the stereotyped sequence of actions seen during prey capture and handling, but the study did not confirm its role in influencing the motivational drive to hunt. Together with other studies from our group, the present work serves as an important piece of information that helps to reveal the neural systems underlying predatory hunting.

Effects of superior colliculus ablation on the air-righting reflex in the rat

To examine how the superior colliculus, the motor center of orientation and avoidance, could interact with postural reflexes, we investigated effects of unilateral and bilateral ablations on air-righting reflex movements in otherwise intact rats. Superior colliculus ablations variously modified righting movements: After falling from the supine position, the rats sometimes showed dorsiflexion instead of normal ventriflexion; the motor sequence of rotation from the fore- to the hindquarter was often modified to simultaneous rotation; lateral turn from supine to prone position was occasionally insufficient; body direction that was normally kept constant during falling was often changed; final posture sometimes deviated from the horizontal position. The first three abnormalities occurred almost twice in frequency as lesions increased from unilateral to bilateral ablation, and in unilaterally ablated rats, did so in righting contraversive to the lesions. Multiple influences of tectoreticular input to the air-righting reflex center are discussed.

The laminar distribution of macaque tectobulbar and tectospinal neurons

The superior colliculus exerts its most direct influence over orienting movements, and saccades in particular, via its descending projections to the brain stem and spinal cord. However, while there is detailed physiological data concerning the generation of saccade-related activity in the primate superior colliculus, there is relatively little data on the detailed connectivity of this structure in primates. Consequently, retrograde transport techniques were utilized to determine the locations of the cells of origin of these descending pathways in macaque monkeys. Tectal cells that projected to the ipsilateral pontine reticular formation were mainly found in the deep gray layer and occasionally in the intermediate gray layer. Tectal cells that projected to the contralateral pontine reticular formation were predominantly located in the intermediate gray layer. The contralaterally projecting population could be subdivided into two groups. The cells in upper sublamina of the intermediate gray layer project primarily to the saccade-related regions of the paramedian reticular formation. Cells in the lower sublamina project primarily to more lateral regions of the pontine reticular formation and to the spinal cord. We conclude that the primate colliculus is provided with at least three descending output channels, which are likely to differ in their connections and functions. Specifically, it seems likely that the lower portion of the intermediate gray layer may be specialized to subserve combined head and eye orienting movements, while the upper sublamina subserves saccades.

Anatomical evidence for interconnections between the central mesencephalic reticular formation and cervical spinal cord in the cat and macaque

A gaze-related region in the caudal midbrain tegementum, termed the central mesencephalic reticular formation (cMRF), has been designated on electrophysiological grounds in monkeys. In macaques, the cMRF correlates with an area in which reticulotectal neurons overlap with tectoreticular terminals. We examined whether a region with the same anatomical characteristics exists in cats by injecting biotinylated dextran amine into their superior colliculi. These injections showed that a cat cMRF is present. Not only do labeled tectoreticular axons overlap the distribution of labeled reticulotectal neurons, these elements also show numerous close boutonal associations, suggestive of synaptic contact. Thus, the presence of a cMRF that supplies gaze-related feedback to the superior colliculus may be a common vertebrate feature. We then investigated whether cMRF connections indicate a role in the head movement component of gaze changes. Cervical spinal cord injections in both the cat and monkey retrogradely labeled neurons in the ipsilateral, medial cMRF. In addition, they provided evidence for a spinoreticular projection that terminates in this same portion of the cMRF, and in some cases contributes boutons that are closely associated with reticulospinal neurons. Injection of the physiologically defined, macaque cMRF demonstrated that this spinoreticular projection originates in the cervical ventral horn, indicating it may provide the cMRF with an efference copy signal. Thus, the cat and monkey cMRFs have a subregion that is reciprocally connected with the ipsilateral spinal cord. This pattern suggests the medial cMRF may play a role in modulating the activity of antagonist neck muscles during horizontal gaze changes.

Connectivity of the goldfish optic tectum with the mesencephalic and rhombencephalic reticular formation

The optic tectum of goldfish, as in other vertebrates, plays a major role in the generation of orienting movements, including eye saccades. To perform these movements, the optic tectum sends a motor command through the mesencephalic and rhombencephalic reticular formation, to the extraocular motoneurons. Furthermore, the tectal command is adjusted by a feedback signal arising from the reticular targets. Since the features of the motor command change with respect to the tectal site, the present work was devoted to determining, quantitatively, the particular reciprocal connectivity between the reticular regions and tectal sites having different motor properties. With this aim, the bidirectional tracer, biotin dextran amine, was injected into anteromedial tectal sites, where eye movements with small horizontal and large vertical components were evoked, or into posteromedial tectal sites, where eye movements with large horizontal and small vertical components were evoked. Labeled boutons and somas were then located and counted in the reticular formation. Both were more numerous in the mesencephalon than in the rhombencephalon, and ipsilaterally than contralaterally, with respect to the injection site. Furthermore, the somas showed a tendency to be located in the area containing the most dense labeling of synaptic endings. In addition, labeled boutons were often observed in close association with retrogradely stained neurons, suggesting the presence of a tectoreticular feedback circuit. Following the injection in the anteromedial tectum, most of the boutons and labeled neurons were found in the reticular formation rostral to the oculomotor nucleus. Conversely, following the injection in the posteromedial tectum, most of the boutons and neurons were also located in the caudal mesencephalic reticular formation. Finally, boutons and neurons were found in the rhombencephalic reticular formation surrounding the abducens nucleus. They were more numerous following the injection in the posteromedial tectum. These results demonstrate characteristic patterns of reciprocal connectivity between physiologically different tectal sites and the mesencephalic and rhombencephalic reticular formation. These patterns are discussed in the framework of the neural substratum that underlies the codification of orienting movements in goldfish.

Bypassing the Saccadic Pulse Generator: Possible Control of Head Movement Trajectory by Rat Superior Colliculus

Saccades produced by electrical stimulation of the superior colliculus in primates are influenced primarily by the location of the stimulating electrode, with the suprathreshold intensity or frequency of the stimulating pulse train having little effect. Any given collicular site produces a characteristic movement of relatively fixed amplitude and velocity. In accordance with this finding, in models of the saccadic eye movement system the superior colliculus specifies the change of eye position: the velocity of movement components are determined by 'pulse generators' located between the superior colliculus and the oculomotor neurons. Previous findings in rodents, however, have suggested that eye and head movements induced by stimulation at some collicular sites may be critically dependent on stimulation parameters, implying that in these animals the superior colliculus has access to a non-saccadic control system. To investigate this possibility, rats with electrodes implanted into the lateral intermediate layers were stimulated with pulse trains of varying frequency and duration, and the resultant head movements analysed from video tape. At seven of the nine sites studied, amplitude of the horizontal component of the head movement was linearly related to stimulating frequency for fixed-duration trains, in some cases over a ten-fold range. Subsequent variation of train duration showed that amplitude was affected not by frequency as such, but by the number of pulses in the train; frequency was related to the mean velocity of the movement. By appropriate setting of these parameters, independent control of head movement amplitude and velocity could be achieved. These results suggest that the rodent superior colliculus may be able to control head movement without recourse to a pulse generator, and thus influence the trajectory of the movement directly. If so, it may prove to be a useful preparation for testing theories of trajectory formation.

Organization of the neurons of origin of the descending pathways from the ferret superior colliculus

The superior colliculus (SC), through its descending projections to the brainstem and spinal cord, is involved in initiating sensory-driven orienting behaviors. Ferrets are carnivores that hunt both above and below ground using visual (and auditory) cues in the daylight but non-visual cues in darkness and in subterranean environments. The present investigation sought to determine whether the ferret SC shows organizational features similar to those found in other visually dominant animals (e.g. cats), or whether characteristics of colliculi from non-visually dominant animals (e.g. rodents) prevail. Injection of retrograde tracer into the identified targets of the colliculus (cervical spinal cord, the contralateral pontomedullary reticular formation, or the ipsilateral pontine reticular formation) labeled tectospinal, crossed tectoreticular, and ipsilateral tectoreticular neurons, respectively, within the adult ferret SC. Labeled tectospinal and crossed tectoreticular neurons were far outnumbered by neurons with ipsilateral reticular projections. Like those of their visually dominant relatives, ferret tectospinal neurons were well represented throughout the anterior-posterior extent of the SC and crossed tectoreticular neurons tended to be distributed more broadly across the intermediate gray layer than those of rodents. Thus, even though ferrets perform well as subterranean predators where non-visual cues initiate orienting behaviors, these anatomical characteristics indicate that their colliculi are organized similar to that of their visually dominant, carnivorous relatives.

Motor effects and mapping of cerebral alterations in animal models of Parkinson's and Huntington's diseases

Changes in stimulant-induced behavioral effects and subcortical c-Fos expression were compared between rodent models of Parkinson's disease (PD) and Huntington's disease (HD). Rats received either a unilateral 6-hydroxydopamine (6-OHDA)-induced lesion of the nigrostriatal dopamine pathway (PD model) or a unilateral infusion of antisense oligodeoxynucleotides targeting c-fos into the striatum (HD model). Dopamine-lesioned animals received intraperitoneal injections of either d-amphetamine (6-OHDAamp group) or apomorphine (6-OHDAapo group), whereas all animals that received antisense infusions received d-amphetamine (ASF group). All groups exhibited robust circling behavior upon stimulant challenge. Changes in subcortical activation, as assessed by the induction of Fos-like immunoreactivity (Fos-LI), were examined in several brain regions. The 6-OHDAamp and ASF groups exhibited robust, ipsiversive circling behavior, with similar changes in Fos-LI in the striatum, entopeduncular nucleus, superior colliculus, and ventromedial thalamus. The 6-OHDAapo group exhibited contraversive rotation and had reciprocal patterns of Fos-LI in these regions. Despite exhibiting the same direction of rotation, the 6-OHDAamp and ASF groups had markedly different patterns of Fos-LI in the globus pallidus and the pontine reticular formation. These results suggest that the globus pallidus may undergo distinct alterations in PD and HD and that the pontine reticular formation is particularly susceptible to changes in mesencephalic dopamine sources.

The objects of action and perception

Two major functions of the visual system are discussed and contrasted. One function of vision is the creation of an internal model or percept of the external world. Most research in object perception has concentrated on this aspect of vision. Vision also guides the control of object-directed action. In the latter case, vision directs our actions with respect to the world by transforming visual inputs into appropriate motor outputs. We argue that separate, but interactive, visual systems have evolved for the perception of objects on the one hand and the control of actions directed at those objects on the other. This 'duplex' approach to high-level vision suggests that Marrian or 'reconstructive' approaches and Gibsonian or 'purposive-animate-behaviorist' approaches need not be seen as mutually exclusive, but rather as complementary in their emphases on different aspects of visual function.

Disinhibition of the superior colliculus restores orienting to visual stimuli in the hemianopic field of the cat

Following unilateral removal of all known visual cortical areas, a cat is rendered hemianopic in the contralateral visual field. Visual orientation can be restored to the blind hemifield by transection of the commissure of the superior colliculus or by destruction of the superior colliculus (SC) or the substantia nigra pars reticulata (SNpr) contralateral to the cortical lesion. It is hypothesized that a mechanism mediating recovery is disinhibition of the SC ipsilateral to the cortical lesion. The ipsilateral nigrotectal projection exerts a robust inhibitory tone onto cells in the SC. However, ibotenic acid destruction of SNpr neurons, which should decrease inhibition onto the SC, does not result in recovery. The failure of ipsilateral SNpr lesions to produce recovery puts into question the validity of SC disinhibition as a mechanism of recovery. We directly tested the disinhibition hypothesis by reversibly disinhibiting the SC ipsilateral to a visual cortical lesion with a gamma-aminobutyric acid (GABA)A antagonist, bicuculline methiodide. In accordance with the hypothesis, transient disinhibition of the SC restored visual orienting for several hours in three of eight animals. Recovery was not a volume or pH effect and was distinct from the release of irrepressible motor effects (i.e., approach and avoidance behaviors) seen within the first hour after injection. Thus, in the absence of all visual cortical areas unilaterally, disinhibition of the SC can transiently restore the ability of the cat to orient to visual stimuli in the previously "blind" hemifield.

Activity of cells in the deeper layers of the superior colliculus of the rhesus monkey: evidence for a gaze displacement command

When the head is free to move, microstimulation of the primate superior colliculus (SC) evokes coordinated movements of the eyes and head. The similarity between these stimulation-induced movements and visually guided movements indicates that the SC of the primate is involved in redirecting the line of sight (gaze). To determine how movement commands are represented by individual collicular neurons, we recorded the activity of single cells in the deeper layers of the superior colliculus of the rhesus monkey during coordinated eye-head gaze shifts. Two alternative hypotheses were tested. The "separate channel" hypothesis states that two displacement commands are generated by the SC: one signal specifying the amplitude and direction of eye movements and a second signal specifying the amplitude and direction of head movements. Alternatively, a single gaze displacement command could be generated by the SC ("gaze displacement" hypothesis). The activity of collicular neurons was examined during three behavioral dissociations of gaze, eye, and head movement amplitude and direction (metrics). Subsets of trials were selected in which the amplitude and direction of either gaze shifts or eye movements or head movements were relatively constant but the metrics of the other two varied over wide ranges. Under these conditions, the separate channel and gaze displacement hypotheses make differential predictions about the patterns of SC activity. We tested these differential predictions by comparing observed patterns with predicted patterns of neuronal activity. We obtained data consistent with the predictions of the gaze displacement hypothesis. The predictions of the separate channel hypothesis were not confirmed. Thus microstimulation data, single-unit recording data, and behavioral data are all consistent with the gaze displacement hypothesis of collicular function--the hypothesis that a gaze displacement signal is derived from the locus of activity within the motor map of the SC and subsequently is decomposed into separate eye and head displacement signals downstream from the colliculus.

Differential expression of fos-like immunoreactivity in the descending projections of superior colliculus after electrical stimulation in the rat

In rodent, there is evidence that the orienting behaviour elicited by direct stimulation of the superior colliculus (SC) is partly mediated by contralateral descending projections, while avoidance-type behaviour is associated with ipsilateral descending projections. However, the identity of target structures in the brainstem which mediate these different behavioural responses is unknown. The c-fos immediate early gene is expressed polysynaptically in neurons in response to a wide range of extracellular stimuli, and hence has been proposed as a technique for mapping functional pathways. The purpose of this study was, therefore, to use the c-fos technique to investigate the functional specificity of brainstem regions which are innervated by the two main descending projections of the SC. Patterns of fos-like immunoreactivity (FLI) were observed throughout the brainstem following electrical stimulation of the SC in Urethane-anaesthetized rats. Previously, the electrical stimulation had been shown to elicit either approach-like or avoidance-like movement. The main results of this experiment were; (i) animals in which the stimulation elicited defensive behaviour had elevated levels of immunostaining in specific terminal areas of the ipsilateral descending projections, e.g. the ventrolateral midbrain/pontine reticular formation, the cuneiform area and rostral periaqueductal grey; (ii) there was no FLI expression in any of the terminal areas of the crossed descending projection, even in animals where the electrical stimulation elicited approach. Control experiments showed that the lack of expression in the crossed descending pathway was not due to the restricted range of stimulation parameters used in the main study, or to the effects of the anaesthetic. In conclusion, this experiment was able to identify likely substrates for the mediation of defensive reactions elicited by tectal stimulation. However, given the total lack of expression in a pathway which is known to be activated, it also provides further evidence that c-fos cannot simply be used as a high resolution neuronal activity marker for mapping functional pathways.

Nociceptive neurones in rat superior colliculus. I. Antidromic activation from the contralateral predorsal bundle

Accumulating evidence suggests that the rodent superior colliculus (SC) plays as important a role in avoidance and defensive behaviours as it does in orientation and approach. These two complementary behaviours are associated with two anatomically segregated tectofugal output pathways, such that orientation and approach are mediated by the crossed descending projection, whereas avoidance and defence are subserved via the uncrossed projection. Because nociceptive neurones in the SC have been presumed to participate in withdrawal or defensive behaviours, it has been proposed that they have direct access only to the uncrossed efferent pathway. However, in certain behavioural situations, the most adaptive response to injury, or to a painful object in prolonged contact with the skin, is to orient towards the source of discomfort so that the skin can be licked and/or the offending object removed. Presumably then, nociceptive as well as low-threshold neurones would have access to the crossed descending pathway in order to initiate such behaviours. Determining whether or not this is the case was the objective of the present study. Both nociceptive-specific (82%) and wide-dynamic-range (18%) SC neurones were identified using long-duration (up to 6 s), frankly noxious mechanical and thermal stimuli in urethane-anaesthetised Long-Evans hooded rats. The majority (85.7%) of the nociceptive neurones encountered were located within the intermediate layers, which corresponds with the location of the cells-of-origin of the crossed descending projection. Nearly half (44.9%) were activated antidromically from electrical stimulation of the crossed descending pathway at a site in the brainstem below its decussation. The mean conduction velocity of these nociceptive output neurones was 9.02 m/s, which corresponds well to previous estimates of conduction velocity in the crossed tecto-reticulo-spinal tract. These data demonstrate that a significant proportion of nociceptive neurones in the rat SC have axons that project to the contralateral brainstem via the crossed descending projection. Nociceptive neurones could, therefore, effect orientation responses to noxious stimuli via similar output pathways that low-threshold neurones utilize to initiate orientation to innocuous stimuli.

Nociceptive neurones in rat superior colliculus. II. Effects of lesions to the contralateral descending output pathway on nocifensive behaviours

A wealth of evidence implicates the crossed descending projection from the superior colliculus (SC) in orientation and approach behaviours directed towards novel, non-noxious stimuli. In our preceding paper, we identified a population of nociceptive neurones in the rat SC that have axons that project to the contralateral brainstem via this output pathway. The purpose of the present study was, therefore, to evaluate the prediction that the crossed descending projection of the SC is also involved in the control of orientation and approach movements of the head and mouth made during the localisation of persistent noxious stimuli. An independent-groups design was used to test the effects of interrupting the contralateral descending projection from the SC on the behavioural reactions elicited by noxious mechanical stimuli presented to the tail and hindpaws. In different groups of animals, a microwire knife was used to cut the contralateral descending fibres at two different locations: (1) a sagittal cut at the level of the dorsal tegmental decussation; (2) a bilateral coronal cut of the predorsal bundle at the level of the medial pontine reticular formation. Retrograde anatomical tracing techniques were then used to evaluate the effectiveness of the cuts and to assess possible involvement of non-collicular fibre systems in both lesioned and control animals. Additional behavioural procedures were performed to test for general neurological status and responsiveness of animals to non-noxious stimuli. Anatomical tracing data indicated that the largest population of neurones with fibres severed by both cuts were the cells-of-origin of the contralateral descending projection in the intermediate white layer of the SC. Behavioural results showed that significantly more animals in both lesion groups failed to locate and bite a mechanical clip placed on the tail. Instead of switching to motor behaviours to localise and remove noxious stimuli, they persisted with defensive reactions, which included freezing, vocalisation or forward and backward escape. In contrast, when the clip was placed on the hindpaws, it was successfully localised by most lesioned and control animals; however, lesioned animals had reliably longer latencies and spent less time in close contact with the clip. Consistent with the established role of the contralateral descending projection in non-noxious orientation, lesioned animals also showed orienting deficits to a range of non-noxious sensory stimuli. These data suggest that, under certain behavioural circumstances, nociceptive information from the SC is integral to the elaboration of orienting and approach movements of the head and mouth elicited by persistent noxious stimuli.

Electrical stimulation of neural tissue to evoke behavioral responses

This review yields numerous conclusions. (1) Both unit recording and behavioral studies find that current activates neurons (i.e., cell bodies and axons) directly according to the square of the distance between the electrode and the neuron, and that the excitability of neurons can vary between 100 and 4000 microA/mm2 using a 0.2-ms cathodal pulse duration. (2) Currents as low as 10 microA, which is considered within the range of currents typically used during micro-stimulation, activate from a few tenths to several thousands of cell bodies in the cat motor cortex directly depending on their excitability; this indicates that even low currents activate more than a few neurons. (3) Electrode tip size has no effect on the current density--or effect current spread--at far field, but tip size limits the current-density generated at near field. (4) To minimize neuronal damage, the electrode should be discharged after each pulse and the pulse duration should not exceed the chronaxie of the stimulated tissue. (5) The amount of current needed to evoke behavioral responses depends not only on the excitability of the stimulated substrate but also on the type of behavior being studied.

Physiological properties of the output neurons in the deep layers of the superior colliculus of the rabbit

Using antidromic and orthodromic stimulation techniques, we studied physiological properties of the output neurons in the deep layers of the superior colliculus (SC) of 34 Now Zealand rabbits. SC cells antidromically activated from the contralateral predorsal bundle (PDB) could also be activated by stimulation of the contralateral SC and ipsilateral central lateral nucleus of the thalamus (CL). The majority of these output neurons responded predominantly to the stimulation of the optic nerve, and only a small proportion of the output neurons were responsive to the stimulation of somatosensory and auditory (and/or vestibular) nerves. These results suggest that the orienting reflex might be elicited mainly by visual afferents in the rabbit. The output SC neurons were subject to a 70 ms inhibition after antidromic stimulation of the PDB and a 40 ms inhibition after transsynaptic (orthodromic) stimulation of the optic chiasm (OX), indicating that the output neurons in the deep layers of the SC might be subject to at least two inhibitory circuits. These results are discussed in the context of a putative saccadic suppression circuitry model.

Opposing excitatory and inhibitory influences from the cerebellum and basal ganglia converge on the superior colliculus: an electrophysiological investigation in the rat

We recently showed (Westby et al., Eur. J. Neurosci., 5, 1378-1388, 1993) that the cerebellar interpositus nucleus is a source of excitatory drive for a population of spontaneously active neurons in the lateral intermediate layers of the contralateral superior colliculus. Anatomical and physiological studies have shown that this region of the colliculus contains cells of origin of the crossed descending tectoreticulospinal tract and receives GABAergic input from the ipsilateral basal ganglia. In the present study we tested the hypothesis that the same neurons receiving excitatory drive from the cerebellum also receive tonic inhibitory input from the substantia nigra pars reticulata. From a sample of 73 spontaneously active collicular cells we found that in 53% the firing rate was suppressed by GABA microinjection into the contralateral deep cerebellar nuclei; a further 15% showed a frequency increase. Of the collicular cells identified as receiving excitatory cerebellar input, 85% were found to be disinhibited by nigral GABA microinjection. The remainder were all inhibited by nigral GABA. These data show that the main excitatory influence from the cerebellum and the main inhibitory influence from the substantia nigra converge on at least one population of spontaneously active cells in the lateral intermediate layers of the superior colliculus. This finding is discussed in relation to the possible function of these spontaneous cells in movement control and nociception.

Excitatory drive from deep cerebellar neurons to the superior colliculus in the rat: an electrophysiological mapping study

The cerebello-tectal projection arising from the interpositus nucleus was investigated electrophysiologically to test the hypothesis that the deep cerebellar nuclei constitute a source of tonic excitation in the superior colliculus. A total of 117 spontaneously active collicular neurons were recorded during GABA microinjection into 26 interpositus sites, where tonic single-cell deep cerebellar activity was also simultaneously recorded. GABA injection always led to suppression of interpositus activity, while in the colliculus a clear pattern of results emerged. 58% of superior colliculus cells showed no response to suppression of interpositus activity, 35% showed a frequency decrease and 7% showed a frequency increase. The majority of these responsive cells were found in a laterally located sheet of cells mainly restricted to the intermediate white layer, in close register with the known cells of origin of the predorsal bundle and completely overlapping the terminals of the nigrotectal pathway originating in dorsolateral substantia nigra pars reticulata. The implications of these results for cooperative theories of head movement control involving the superior colliculus, cerebellum and precerebellar nuclei are discussed.

An uncrossed tectopontine pathway mediates ipsiversive circling

Stimulation of the superior colliculus (SC) of rodents, following knife cuts to the predorsal bundle decussation, evokes ipsiversive circling along with "cringing" or avoidance responses. A major uncut SC output is the uncrossed tectopontine pathway that projects heavily to the ventrolateral pons (VLP). Stimulation of this pathway in the VLP also evokes ipsiversive circling, but the circling is smoother, lacks the avoidance components, and begins with a shorter latency than SC circling. To determine whether continuous tectopontine axons mediate ipsiversive circling in both sites, the collision method of Shizgal et al. was used. Pairs of stimulating pulses were presented to the two sites, conditioning (C) pulses to one site and testing (T) pulses to the other site. Collision was evidenced when the frequencies required to evoke circling were higher at short conditioning-testing (C-T) intervals than at long C-T intervals. Between SC and VLP, collision varied from 25 to 64%. Refractory periods ranged from 0.4 to 1.0 ms in most VLP sites, and from 0.45 to roughly 3 ms in SC sites. Conduction velocities ranged from 1.2 to 19 m/s, but most were concentrated in two ranges, 1.2 to 2.7 m/s and 10 to 19 m/s. The contribution of the slower population was higher in electrode pairs where the percent collision was higher. Therefore, continuous axons from colliculus to ventrolateral pons mediate most of the ipsiversive circling produced by collicular stimulation. Slight asymmetries in the collision were observed between 3 pairs with high threshold colliculus electrodes, suggesting transsynaptic collisions across colliculus synapses transmitting from dorsal to ventral.

Topographical organization of the nigrotectal projection in rat: evidence for segregated channels

Recent evidence suggests that projections from the superior colliculus to the brainstem in rat are organized into a series of anatomically segregated output channels. To understand how collicular function may be modified by the basal ganglia it is important to know whether particular output modules of the superior colliculus can be selectively influenced by input from substantia nigra. The purpose of the present study was, therefore, to examine in more detail topography within the nigrotectal system in the rat. Small injections (10-50 nl) of a 1% solution of wheatgerm agglutinin conjugated with horseradish peroxidase were made at different locations within substantia nigra and surrounding structures. A discontinuous puff-like pattern of anterogradely transported label was found in medial and caudal parts of the ipsilateral intermediate layers of the superior colliculus. In contrast, the rostrolateral enlargement of the intermediate layers contained a greater density of more evenly distributed terminal label. Injection sites associated with this dense pattern of laterally located label were concentrated in lateral pars reticulata, while the puff-like pattern was produced by injections into ventromedial pars reticulata. Retrograde tracing experiments with the fluorescent dyes True Blue and Fast Blue revealed that injections involving the rostrolateral intermediate layers were consistently associated with a restricted column of labelled cells in the dorsolateral part of ipsilateral pars reticulata. Comparable injections into medial and caudal regions of the superior colliculus produced retrograde labelling in ventral and medial parts of the rostral two-thirds of pars reticulata. Both anterograde and retrograde tracing data indicated that contralateral nigrotectal projections arise from cells located in ventral and medial pars reticulata. The present results suggest that the main ipsilateral projection from substantia nigra pars reticulata to the superior colliculus comprises two main components characterized by regionally segregated populations of output cells and spatially separated zones of termination. Of particular interest is the apparent close alignment between terminal zones of the nigrotectal channels and previously defined populations of crossed descending output cells in the superior colliculus. Thus, the rostrolateral intermediate layers contain a concentration of terminals specifically from dorsolateral pars reticulata and output cells which project to the contralateral caudal medulla and spinal cord. Conversely, the medial and caudal intermediate layers receive terminals from ventral and medial pars reticulata and contain cells which project specifically to contralateral regions of the paramedian pontine and medullary reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of superior colliculus lesions on sensory unit responses in the intralaminar thalamus of the rat

The effects of kainic acid lesions of the intermediate and deep layers of the superior colliculus on the sensory input to the intralaminar thalamus of the rat were determined. Ipsiversive circling and contralateral sensory neglect were consistently seen after lesion placement. Two to 7 days later, the intralaminar thalamus was systematically explored for extracellular mechanoreceptive unit responses to high threshold and low threshold stimuli. On the side ipsilateral to the lesion the number of responsive units was reduced by 51%. The loss was particularly marked for nociceptive units (80%), and low threshold and complex units with orofacial receptive fields (73%). This effect may involve a partial deafferentation of the intralaminar thalamus as well as altered excitatory thresholds of thalamic neurons. It is suggested that the functionally distinct direct tectothalamic projection as well as the indirect tecto-reticulo-thalamic pathway are implicated.

Sensory responses of intralaminar thalamic neurons activated by the superior colliculus

The intralaminar thalamus of anesthetized rats was explored for neurons activated by stimulation of the superior colliculus and responsive to sensory inputs. Neurons activated by stimulation of the intermediate and deep collicular layers were distributed throughout the intralaminar thalamus. Approximately one half of them responded to tectal as well as sensory inputs. The majority were nociceptive or had a more complex response pattern including responses to auditory stimulation. A small population of low threshold units had contralateral orofacial receptive fields and responded to light taps; these units were preferentially localized anteriorly in the central lateral and paracentral nuclei. Neurons responsive to tectal and sensory stimulation were randomly intermingled with other neurons which had no detectable sensory input. The results indicate that ascending projection neurons of the intermediate and deep layers of the superior colliculus provide an input to functionally diverse subpopulations of intralaminar thalamic neurons. In view of its projections to motor cortex and basal ganglia, the intralaminar thalamus appears directly implicated in basal ganglia and superior colliculus related mechanisms of attention, arousal and postural orienting.

The effects of posterior cortical lesions on responses to visual threats in the Mongolian gerbil (Meriones unguiculatus)

Mongolian gerbils received aspiration lesions of either primary visual cortex (PVC), medial extrastriate visual cortex, retrosplenial cortex (RSC), or sham operations. The responses of gerbils to the presentation of an overhead visual stimulus were recorded in an open field. In all groups, presentation of the stimulus produced an increase in rearing. This suggests that the stimulus was detected by all animals. Gerbils with RSC or PVC lesions showed reduced levels of response to the stimulus. We suggest that some of the observed deficits can be explained as failures to produce responses to threat that are appropriate to the context in which the the threat was presented.

Tectal induction of cortical arousal: evidence implicating multiple output pathways

The rodent superior colliculus mediates a wide range of physiological and behavioural responses to sudden stimuli, including desynchronisation of the cortical electroencephalogram (EEG). To investigate how this desynchronisation is produced, one of two powerful excitatory agents, sodium L-glutamate (200 nl, 10 nmol) or bicuculline methiodide (200 nl, 40 pmol), was injected into the dorsal midbrain of sleeping rats. Microinjections at sites widely distributed throughout all layers of the superior colliculus were able to desynchronise the cortical EEG. i) In the superficial layers, bicuculline was effective at more sites than glutamate, whereas the reverse was true for the deep layers. ii) At some sites EEG desynchronisation occurred together with the defensive or orienting movements that are obtained from collicular stimulation in awake animals. At other sites cortical arousal occurred without such movements. iii) Comparison with a previous study suggested that urethane selectively blocks cortical arousal to glutamate injections in the superficial and intermediate grey layers. This evidence suggests that multiple collicular output pathways can desynchronise the cortical EEG, perhaps reflecting multiple functions for EEG desynchronisation.

Output pathways from the rat superior colliculus mediating approach and avoidance have different sensory properties

Neuroanatomical studies have demonstrated that the two major descending pathways from the superior colliculus arise from regionally segregated, distinct, cells of origin. Stimulation and lesion studies have implicated the crossed descending tecto-reticulo-spinal projection in approach movements towards novel stimuli whereas the ipsilateral pathway appears to be involved in the control of avoidance and escape-like behaviours. The present electrophysiological study attempted to characterise the sensory properties of antidromically identified cells of origin of these pathways in anaesthetised rats. We found that the contralaterally projecting predorsal bundle (PDB) efferents were primarily somatosensory while the ipsilateral cuneiform (CNF) projection was primarily visual. PDB cells, mainly found in the intermediate layers, responded principally to vibrissal stimulation with their overlying visual fields optimally stimulated by small dark moving objects in the lower rostral and lateral field. In contrast, most CNF cells were located rostromedially, with the greatest contribution from visual cells responsive to stimuli in the upper rostral field. A significant proportion of these showed no response to small moving dark discs but fired vigorously to 'looming' stimuli. Ethological considerations suggest that these are appropriate stimulus characteristics for a system controlling approach and avoidance behaviour in an animal such as the rat where predators generally appear from above and prey is found on the ground.

Somatosensory neurons projecting from the superior colliculus to the intralaminar thalamus in the rat

Neurons of the rat superior colliculus projecting to the intralaminar thalamus were tested for their responses to somatosensory stimulation. They were identified by antidromic stimulation of the parafascicular nucleus and central lateral nucleus. To establish the existence of descending as well as ascending axon collaterals antidromic stimulation was applied to the upper cervical spinal cord in some cases. Somatosensory receptive fields were delineated and their laminar location in the superior colliculus was noted. Units were distributed throughout the intermediate and deep tectal layers, none were located in the superficial layers. Units with somatosensory receptive fields could be classified as low threshold, high threshold, wide dynamic range or complex. The majority of the peripherally responsive units (52%) were low threshold somatosensory units with contralateral receptive fields. All units were distributed throughout the intermediate and deep layers. Their distribution reflected the typical somatotopic organization of the superior colliculus. These results indicate that the intralaminar thalamus receives some sensory information by way of the tectum. In turn, the basal ganglia may gain direct access to this information by way of the thalamoneostriatal projection.

Neuronal activity related to head and eye movements in cat superior colliculus

1. Movement-related discharges were recorded from single cells in the superior colliculus of alert cats while they made eye saccades (with head fixed) or gaze saccades (with head free). 2. Visual and auditory stimuli were used as saccade targets. In addition, saccades were made to the remembered location of targets and spontaneously, in the absence of targets, during intertrial intervals. 3. When the head was still and the cat performed either spontaneous saccades or saccades to remembered targets, only one class of tectal neurone, the saccade-related burst neurone, inevitably discharged prior to all saccades of appropriate amplitude and direction. 4. Neurones with longer lead times and less intense presaccadic discharges were obligately linked only to visually elicited saccades. The discharge of some of these long-lead neurones was also influenced by the spatial position of the visual target (that is, by craniotopic motor error), while that of the saccade-related burst neurones was a function of retinocentric motor error. 5. Most neurones which discharged before head movements also discharged before eye movements and had large, contralateral movement fields.

Head and body movements evoked electrically from the caudal superior colliculus of rats: pulse frequency effects

The effects of pulse frequency and current intensity on circling elicited from the caudal superior colliculus (SC) of rats were studied. The displacement of the head with respect to the body were measured for different levels of frequency (20, 29, and 50 Hz) and current (200 or 500 microA) at a pulse duration of 0.1 ms. The rate of circling increased monotonically with frequency and current. The rate at which the head was displaced laterally varied as a function of frequency. It is postulated that lateral head and body movements are affected by the firing frequency of SC output neurons.

Involvement of nigrotecto-reticulospinal pathways in the iminodipropionitrile (IDPN) model of spasmodic dyskinesias: a 2-deoxy-D-[1-14C]glucose study in the rat

Chronic administration of iminodipropionitrile (IDPN) to rats causes a persistent behavioral syndrome characterized by lateral and vertical head twitches, random circling, and increased tactile and acoustic startle responses. In order to identify brain areas which are affected in rats manifesting this syndrome, we used the autoradiographic 2-deoxy-D-[1-14C]glucose ([14C]DG) method to map cerebral glucose utilization in IDPN-treated rats. One day after the development of the dyskinetic syndrome, there were significant decreases in local glucose utilization in the substantia nigra pars reticulata (SNr) and compacta (SNc), the dorsal raphe, the superficial and deep layers of the superior colliculus, the inferior colliculi, the interpeduncular nucleus, the medial and dorsolateral geniculate nuclei, and the superior and lateral vestibular nuclei. There were also significant decreases in layer 2 of the cingulate cortex and in the temporal and occipital cortices. In contrast, there were no changes in the motor cortex, the caudate-putamen, the nucleus accumbens, or the median raphe. These findings suggest that deleterious effects of IDPN on the nigrotectal pathways which affect head and neck movements and circling behaviors via the brainstem reticulospinal tracts may play an important role in the IDPN-induced persistent spasmodic dyskinetic syndrome in rats.

Event or emergency? Two response systems in the mammalian superior colliculus

Recent studies of the effects of stimulating the superior colliculus (SC) in rodents suggest that this structure mediates at least two classes of response to novel sensory stimuli. One class contains the familiar orienting response, together with movements resembling tracking or pursuit, and appears appropriate for undefined sensory 'events'. The second class contains defensive movements such as avoidance or flight, together with cardiovascular changes, that would be appropriate for a sudden emergency such as the appearance of a predator, or of an object on collision course. The two response systems appear to depend on separate output projections, and are probably subject to different sensory and forebrain influences. These findings (1) suggest an explanation for the complex anatomical organization of the SC, with multiple output pathways differentially accessed by a very wide variety of inputs, (2) emphasize the similarities between the SC and the optic tectum in non-mammalian species, and (3) suggest that the SC may be useful as a model for studying both the sensory control of defensive responses, and how intelligent decisions can be taken about relatively simple sensory inputs.

The protein synthesis inhibitor, anisomycin, causes exacerbation of the iminodipropionitrile-induced spasmodic dyskinetic syndrome in rats

The effects of anisomycin on dyskinetic head movements, circling, and locomotor activity were investigated in the IDPN-induced syndrome. Intracerebroventricular (ICV) injections of anisomycin in conjunction with IDPN caused exacerbation of all aspects of the syndrome, although circling and vertical head dyskinesias (retrocollis) were the most affected. Animals treated with only anisomycin showed persistent retrocollis but not laterocollis or circling. Biochemical studies confirmed the increases in the concentration of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) previously observed in the striata of IDPN-treated rats two weeks after stopping administration of the drug. Rats treated with anisomycin alone also showed significant increases in striatal 5-HT and 5-HIAA concentrations which were somewhat higher on the side of the ICV infusions. Coadministration of IDPN and anisomycin did not cause any further increases in 5-HT or 5-HIAA. These results suggest that inhibition of protein synthesis by IDPN may be one of the processes involved in the development of the persistent dyskinetic syndrome.

Behavior evoked by electrical stimulation of the hamster superior colliculus

Syrian golden hamsters were implanted with fixed or moveable stimulating electrodes aimed at the superior colliculus (SC). Behavior was observed in response to trains of 0.1 ms pulses at 200 Hz while the animals were moving freely in an open arena or in their home cages. At threshold stimulating currents, the responses consisted almost entirely of freezing or contraversive turning, which occurred in two forms: fast turns, resembling orienting movements to sunflower seeds, and slow turns that were smooth and continuous. Other responses, including head raising and lowering, ipsiversive turning and backing movements were seen occasionally. Increasing the stimulating current usually gave a variety of responses, including circling movements, prolonged freezing, ipsilateral movements and running escape behavior. The sites in SC giving freezes at threshold tended to be located superficially (SO and above), or deep (SGP and below), while sites giving turns were in the intermediate layers. Most freeze sites occurred in the rostro-medial SC that represents the upper visual field, while turn sites occurred predominantly in caudo-lateral SC. Apart from the turns, most of the stimulated responses resembled natural defensive behavior, supporting the view that SC in rodents plays a role in organizing responses to predators, as well as in orienting behavior.

Turning responses evoked by stimulation of visuomotor pathways

Lateral eye, head, and body movements are produced by electrical stimulation of many brain regions from frontal cortex to pons. A new collision method shows that at least 5 separate axon bundles mediate stimulation-elicited lateral head and body movements in rats. One bundle passes between the rostromedial tegmentum and medial pons, with conduction velocities of 0.8-18 m/s. A second bundle passes between the superior colliculus and contralateral medial pons, with conduction velocities of 1.7-13 m/s. A third bundle passes between the superior colliculus and ventrolateral pons, with conduction velocities of 1.3-20 m/s. A fourth bundle passes between the internal capsule and medial substantia nigra, with conduction velocities of 0.9-4.4 m/s. A fifth bundle passes between the anteromedial cortex and rostral striatum, with conduction velocities of 2.4-36 m/s. Collision effects have not been observed between the anteromedial cortex and the internal capsule, medial substantia nigra, superior colliculus, rostromedial tegmentum, or medial pons, which suggests that these sites are not connected by axons mediating turning. Possible synaptic linkages between the 5 bundles and possible transmitters are discussed.

Food wrenching and dodging: use of action patterns for the analysis of sensorimotor and social behavior in the rat

Developments of a procedure to study two movements, food wrenching (stealing food from a conspecific) and dodging (escaping with food from a conspecific), used in the competition for food by rats is described. These include, (A) procedures for adaptation, (B) procedures for filming and scoring, and (C) procedures for measuring dimensions of movements. The character of the movements have features of action patterns in the sense that the term is used by ethologists. It is suggested that they can be used to study the neural basis of complex sequencing of behavior as well as to study the neural basis of sensorimotor behavior and sensorimotor asymmetries.

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.

Circling elicited from the anteromedial cortex and medial pons: refractory periods and summation

Contraversive circling is evoked by stimulating the anteromedial cortex (AMC) of rats, and ipsiversive circling is evoked by stimulating the medial pons (PONS). During AMC circling, lateral and vertical head movements and vibrissae movements were exhibited. During PONS circling, although lateral head movements were exhibited, vertical head movements and vibrissae movements were not exhibited. Refractory periods were estimated by delivering trains of paired pulses and measuring the frequency thresholds for circling at various intrapair intervals. Refractory periods at AMC circling sites were much longer (range 1.4-3.3 ms) than at PONS circling sites (range 0.5-1.0 ms). To determine the degree of summation between the AMC and contralateral PONS, the two sites were stimulated concurrently. Summation of 95-100% was observed for AMC and PONS circling. No collision was observed at short intrapair intervals of paired pulses. Thus, the AMC and PONS are not connected axonally but are related, perhaps serially, for the production of circling.