Receptive field properties of somatosensory neurons in the cat superior colliculus

Depth relationships and measures of tissue thickness in dorsal midbrain

Dorsal human midbrain contains two nuclei with clear laminar organization, the superior and inferior colliculi. These nuclei extend in depth between the superficial dorsal surface of midbrain and a deep midbrain nucleus, the periaqueductal gray matter (PAG). The PAG, in turn, surrounds the cerebral aqueduct (CA). This study examined the use of two depth metrics to characterize depth and thickness relationships within dorsal midbrain using the superficial surface of midbrain and CA as references. The first utilized nearest-neighbor Euclidean distance from one reference surface, while the second used a level-set approach that combines signed distances from both reference surfaces. Both depth methods provided similar functional depth profiles generated by saccadic eye movements in a functional MRI task, confirming their efficacy for delineating depth for superficial functional activity. Next, the boundaries of the PAG were estimated using Euclidean distance together with elliptical fitting, indicating that the PAG can be readily characterized by a smooth surface surrounding PAG. Finally, we used the level-set approach to measure tissue depth between the superficial surface and the PAG, thus characterizing the variable thickness of the colliculi. Overall, this study demonstrates depth-mapping schemes for human midbrain that enables accurate segmentation of the PAG and consistent depth and thickness estimates of the superior and inferior colliculi.

Increased functional connectivity between superior colliculus and brain regions implicated in bodily self-consciousness during the rubber hand illusion

Bodily self-consciousness refers to bodily processes operating at personal, peripersonal, and extrapersonal spatial dimensions. Although the neural underpinnings of representations of personal and peripersonal space associated with bodily self-consciousness were thoroughly investigated, relatively few is known about the neural underpinnings of representations of extrapersonal space relevant for bodily self-consciousness. In the search to unravel brain structures generating a representation of the extrapersonal space relevant for bodily self-consciousness, we developed a functional magnetic resonance imaging (fMRI) study to investigate the implication of the superior colliculus (SC) in bodily illusions, and more specifically in the rubber hand illusion (RHi), which constitutes an established paradigm to study the neural underpinnings of bodily self-consciousness. We observed activation of the colliculus ipsilateral to the manipulated hand associated with eliciting of RHi. A generalized form of context-dependent psychophysiological interaction analysis unravelled increased illusion-dependent functional connectivity between the SC and some of the main brain areas previously involved in bodily self-consciousness: right temporoparietal junction (rTPJ), bilateral ventral premotor cortex (vPM), and bilateral postcentral gyrus. We hypothesize that the collicular map of the extrapersonal space interacts with maps of the peripersonal and personal space generated at rTPJ, vPM and the postcentral gyrus, producing a unified representation of space that is relevant for bodily self-consciousness. We suggest that processes of multisensory integration of bodily-related sensory inputs located in this unified representation of space constitute one main factor underpinning emergence of bodily self-consciousness.

Cerebrovascular responses of the rat brain to noxious stimuli as examined by functional near-infrared whole brain imaging

While near-infrared (NIR) spectroscopy has been increasingly used to detect stimulated brain activities with an advantage of dissociating regional oxy- and deoxyhemoglobin concentrations simultaneously, it has not been utilized much in pain research. Here, we investigated and demonstrated the feasibility of using this technique to obtain whole brain hemodynamics in rats and speculated on the functional relevance of the NIR-based hemodynamic signals during pain processing. NIR signals were emitted and collected using a 26-optodes array on rat's dorsal skull surface after the removal of skin. Following the subcutaneous injection of formalin (50 μl, 3%) into a hindpaw, several isolable brain regions showed hemodynamic changes, including the anterior cingulate cortex, primary/secondary somatosensory cortexes, thalamus, and periaqueductal gray (n = 6). Time courses of hemodynamic changes in respective regions matched with the well-documented biphasic excitatory response. Surprisingly, an atypical pattern (i.e., a decrease in oxyhemoglobin concentration with a concomitant increase in deoxyhemoglobin concentration) was seen in phase II. In a separate group of rats with innocuous brush and noxious pinch of the same area (n = 11), results confirmed that the atypical pattern occurred more likely in the presence of nociception than nonpainful stimulation, suggesting it as a physiological substrate when the brain processes pain. In conclusion, the NIR whole brain imaging provides a useful alternative to study pain in vivo using small-animal models. Our results support the notion that neurovascular response patterns depend on stimuli, bringing attention to the interpretation of vascular-based neuroimaging data in studies of pain.

Projections of somatosensory cortex and frontal eye fields onto incertotectal neurons in the cat

The goal of this study was to determine whether the input-output characteristics of the zona incerta (ZI) are appropriate for it to serve as a conduit for cortical control over saccade-related activity in the superior colliculus. The study utilized the neuronal tracers wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and biotinylated dextran amine (BDA) in the cat. Injections of WGA-HRP into primary somatosensory cortex (SI) revealed sparse, widespread nontopographic projections throughout ZI. In addition, region-specific areas of more intense termination were present in ventral ZI, although strict topography was not observed. In comparison, the frontal eye fields (FEF) also projected sparsely throughout ZI, but terminated more heavily, medially, along the border between the two sublaminae. Furthermore, retrogradely labeled incertocortical neurons were observed in both experiments. The relationship of these two cortical projections to incertotectal cells was also directly examined by retrogradely labeling incertotectal cells with WGA-HRP in animals that had also received cortical BDA injections. Labeled axonal arbors from both SI and FEF had thin, sparsely branched axons with numerous en passant boutons. They formed numerous close associations with the somata and dendrites of WGA-HRP-labeled incertotectal cells. In summary, these results indicate that both sensory and motor cortical inputs to ZI display similar morphologies and distributions. In addition, both display close associations with incertotectal cells, suggesting direct synaptic contact. From these data, we conclude that inputs from somatosensory and FEF cortex both play a role in controlling gaze-related activity in the superior colliculus by way of the inhibitory incertotectal projection.

Somatosensory-motor neuronal activity in the superior colliculus of the primate

The superior colliculus (SC) in primates plays an important role in orienting gaze and arms toward novel stimuli. Here we ask whether neurons in the intermediate and deep layers of the SC are also involved in the interaction with objects. In two trained monkeys we found a large number of SC units that were specifically activated when the monkeys contacted and pushed a target that had been reached with either hand. These neurons, however, were silent when the monkeys simply looked at or reached for the target but did not touch it. The activity related to interacting with objects was spatially tuned and increased with push strength. Neurons in the SC with this type of activity may be involved in a somatosensory-motor feedback loop that monitors the force of the active muscles together with the spatial position of the limb required for proper interaction with an object.

The mammalian superior colliculus: laminar structure and connections

The superior colliculus is a laminated midbrain structure that acts as one of the centers organizing gaze movements. This review will concentrate on sensory and motor inputs to the superior colliculus, on its internal circuitry, and on its connections with other brainstem gaze centers, as well as its extensive outputs to those structures with which it is reciprocally connected. This will be done in the context of its laminar arrangement. Specifically, the superficial layers receive direct retinal input, and are primarily visual sensory in nature. They project upon the visual thalamus and pretectum to influence visual perception. These visual layers also project upon the deeper layers, which are both multimodal, and premotor in nature. Thus, the deep layers receive input from both somatosensory and auditory sources, as well as from the basal ganglia and cerebellum. Sensory, association, and motor areas of cerebral cortex provide another major source of collicular input, particularly in more encephalized species. For example, visual sensory cortex terminates superficially, while the eye fields target the deeper layers. The deeper layers are themselves the source of a major projection by way of the predorsal bundle which contributes collicular target information to the brainstem structures containing gaze-related burst neurons, and the spinal cord and medullary reticular formation regions that produce head turning.

Modeling cross-modal enhancement and modality-specific suppression in multisensory neurons

Cross-modal enhancement (CME) occurs when the neural response to a stimulus of one modality is augmented by another stimulus of a different modality. Paired stimuli of the same modality never produce supra-additive enhancement but may produce modality-specific suppression (MSS), in which the response to a stimulus of one modality is diminished by another stimulus of the same modality. Both CME and MSS have been described for neurons in the deep layers of the superior colliculus (DSC), but their neural mechanisms remain unknown. Previous investigators have suggested that CME involves a multiplicative amplifier, perhaps mediated by N-methyl D-aspartate (NMDA) receptors, which is engaged by cross-modal but not modality-specific input. We previously postulated that DSC neurons use multisensory input to compute the posterior probability of a target using Bayes' rule. The Bayes' rule model reproduces the major features of CME. Here we use simple neural implementations of our model to simulate both CME and MSS and to argue that multiplicative processes are not needed for CME, but may be needed to represent input variance and covariance. Producing CME requires only weighted summation of inputs and the threshold and saturation properties of simple models of biological neurons. Multiplicative nodes allow accurate computation of posterior target probabilities when the spontaneous and driven inputs have unequal variances and covariances. Neural implementations of the Bayes' rule model account better than the multiplicative amplifier hypothesis for the effects of pharmacological blockade of NMDA receptors on the multisensory responses of DSC neurons. The neural implementations also account for MSS, given only the added hypothesis that input channels of the same modality have more spontaneous covariance than those of different modalities.

Mechanisms of within- and cross-modality suppression in the superior colliculus

The present studies were initiated to explore the basis for the response suppression that occurs in cat superior colliculus (SC) neurons when two spatially disparate stimuli are presented simultaneously or in close temporal proximity to one another. Of specific interest was examining the possibility that suppressive regions border the receptive fields (RFs) of unimodal and multisensory SC neurons and, when activated, degrade the neuron's responses to excitatory stimuli. Both within- and cross-modality effects were examined. An example of the former is when a response to a visual stimulus within its RF is suppressed by a second visual stimulus outside the RF. An example of the latter is when the response to a visual stimulus within the visual RF is suppressed when a stimulus from a different modality (e. g., auditory) is presented outside its (i.e., auditory) RF. Suppressive regions were found bordering visual, auditory, and somatosensory RFs. Despite significant modality-specific differences in the incidence and effectiveness of these regions, they were generally quite potent regardless of the modality. In the vast majority (85%) of cases, responses to the excitatory stimulus were degraded by >/=50% by simultaneously stimulating the suppressive region. Contrary to expectations and previous speculations, the effects of activating these suppressive regions often were quite specific. Thus powerful within-modality suppression could be demonstrated in many multisensory neurons in which cross-modality suppression could not be generated. However, the converse was not true. If an extra-RF stimulus inhibited center responses to stimuli of a different modality, it also would suppress center responses to stimuli of its own modality. Thus when cross-modality suppression was demonstrated, it was always accompanied by within-modality suppression. These observations suggest that separate mechanisms underlie within- and cross-modality suppression in the SC. Because some modality-specific tectopetal structures contain neurons with suppressive regions bordering their RFs, the within-modality suppression observed in the SC simply may reflect interactions taking place at the level of one input channel. However, the presence of modality-specific suppression at the level of one input channel would have no effect on the excitation initiated via another input channel. Given the modality-specificity of tectopetal inputs, it appears that cross-modality interactions require the convergence of two or more modality-specific inputs onto the same SC neuron and that the expression of these interactions depends on the internal circuitry of the SC. This allows a cross-modality suppressive signal to be nonspecific and to degrade any and all of the neuron's excitatory inputs.

Reciprocal connections between the zona incerta and the pretectum and superior colliculus of the cat

The goal of the present experiments was to examine the relationships of the zona incerta with two structures associated with visuomotor behavior, the superior colliculus and pretectum. The experiments were carried out in the cat, a species commonly used in studies of visuomotor integration, and utilized wheat germ agglutinin horseradish peroxidase and biocytin as retrograde and anterograde neuronal tracers. Retrograde axonal transport demonstrated that most cells in the ventral subdivision of the zona incerta project to the superior colliculus. Anterograde tracers demonstrated that the incertotectal terminal field is most dense in the intermediate gray layer, which is the primary source of the descending pathway from the superior colliculus to brainstem gaze centers. Further experiments showed that scattered cells within the intermediate gray layer give rise to a reciprocal pathway that terminates in both the dorsal and ventral subdivisions of the zona incerta. The distribution of both labeled incertotectal cells and tectoincertal terminals extends dorsolateral to the zona incerta proper, between the reticular thalamic nucleus and the external medullary lamina. Electron microscopic examination of labeled tectoincertal terminals demonstrated that they contain mainly spherical vesicles and have slightly asymmetric to symmetric synaptic densities. Labeled terminals were observed contacting labeled cells in the zona incerta, suggesting that the reciprocal pathway may be monosynaptic. The zona incerta is also reciprocally interconnected with the pretectum. The anterior pretectal nucleus provides a dense projection to the ventral part of the zona incerta and receives a sparse reciprocal projection. The posterior pretectal nucleus and nucleus of the optic tract may also project to the zona incerta. The pretectoincertal fibers form terminals that contain primarily spherical vesicles and make distinctly asymmetric synaptic contacts. In summary, these results indicate that the deep layers of the superior colliculus, which are important for controlling saccades, are the target of a projection from the ventral subdivision of the zona incerta. Like the substantia nigra, the zona incerta may play a permissive role in the tectal initiation of saccadic eye movements. The incertotectal terminal field in the cat is less dense than that observed previously in the rat, suggesting species differences in the development of this pathway. An additional finding of this study is that one of the main sources of input to these incertotectal cells is the anterior pretectal nucleus. This pretectal incertal tectal pathway is likely to play a role in the guidance of tectally initiated saccades by somatosensory stimuli.

A projection linking motor cortex with the LM-suprageniculate nuclear complex through the periaqueductal gray area which surrounds the nucleus of Darkschewitsch in the cat

Whereas a previous study by one of us (Hicks et al., 1986) suggested that periaqueductal gray (PAG) neurons projecting to the lateralis medialis-suprageniculate (LM-SG) complex might mediate transmission of affective-related nociceptive information, our present work suggests instead, a function in processes related to movement. Cells of the nucleus of Darkschewitsch (ND) are known to have reciprocal projections with the motor cortex (MX), in particular with the hand area of MX, and also to project to the rostral medial accessory olivary (MAO) nucleus (Onodera and Hicks, 1995a). That the ND might be related to saccadic oculomotor function, as well as to the control of hand movements through its connections via the olivo-cerebellar circuit, is indicated by the fact that ND receives a strong projection from the substantia nigra pars reticulata and zona incerta (SNR/ZI) and projects directly and/or indirectly to eye movement nuclei (Onodera and Hicks, 1995b). Thus, ND may function in permitting integration of eye-hand motor coordination. This study focussed on the area of PAG surrounding ND. WGA-HRP was injected into MX and many labelled terminals and large neurones were in ND, with lesser numbers being observed in the area of the PAG surrounding ND. After injections into ND and closely adjacent areas, labelled terminals were observed sparsely distributed with a restricted area of the LM-SG complex. After injections into LM-SG area, small neuronal somata were seen in the area of the PAG surrounding ND, but no labelled somata were detected in ND. Thus if the cells of this PAG area, like those of ND, have similar functions owing to their common reciprocal connections with MX, then the small neurones in PAG projecting to LM-SG may constitute an important link in the circuitry subserving visual processing and/or the regulation of orienting movements of the hand, head and eye.