Activity in the human superior colliculus associated with reaching for tactile targets

The superior colliculus (SC) plays a major role in orienting movements of eyes and the head and in the allocation of attention. Functions of the SC have been mostly investigated in animal models, including non-human primates. Differences in the SC's anatomy and function between different species question extrapolations of these studies to humans without further validation. Few electrophysiological and neuroimaging studies in animal models and humans have reported a role of the SC in visually guided reaching movements. Using BOLD fMRI imaging, we sought to decipher if the SC is also active during reaching movements guided by tactile stimulation. Participants executed reaching movements to visual and tactile target positions. When contrasted against visual and tactile stimulation without reaching, we found increased SC activity with reaching not only for visual but also for tactile targets. We conclude that the SC's involvement in reaching does not rely on visual inputs. It is also independent from a specific sensory modality. Our results indicate a general involvement of the human SC in upper limb reaching movements.

Quantification of volumetric morphometry and optical property in the cortex of human cerebellum at micrometer resolution

The surface of the human cerebellar cortex is much more tightly folded than the cerebral cortex. Volumetric analysis of cerebellar morphometry in magnetic resonance imaging studies suffers from insufficient resolution, and therefore has had limited impact on disease assessment. Automatic serial polarization-sensitive optical coherence tomography (as-PSOCT) is an emerging technique that offers the advantages of microscopic resolution and volumetric reconstruction of large-scale samples. In this study, we reconstructed multiple cubic centimeters of ex vivo human cerebellum tissue using as-PSOCT. The morphometric and optical properties of the cerebellar cortex across five subjects were quantified. While the molecular and granular layers exhibited similar mean thickness in the five subjects, the thickness varied greatly in the granular layer within subjects. Layer-specific optical property remained homogenous within individual subjects but showed higher cross-subject variability than layer thickness. High-resolution volumetric morphometry and optical property maps of human cerebellar cortex revealed by as-PSOCT have great potential to advance our understanding of cerebellar function and diseases.

GABAergic retinal ganglion cells regulate innate defensive responses

Gamma-aminobutyric acid (GABA) is regarded as the most important inhibitory neurotransmitter in the central nervous system, including the retina. However, the roles of GABA-immunolabeled retinal ganglion cells (RGCs) have not been explored. Here, we report the expression of GABAergic RGCs that project to many brain areas in mice, including the superior colliculus. Selective ablation of the superior colliculus-projecting GABAergic RGCs, leaving other GABAergic RGCs intact, reduces the looming stimulus-induced defensive response without affecting image-forming functions; it also significantly enhances glucose metabolism in the superior colliculus, as determined by [18F]-fluorodeoxyglucose PET (FDG PET). Our findings demonstrate that superior colliculus-projecting GABAergic RGCs control the visually active defensive response by regulating superior colliculus neurons.

Optogenetic fUSI for brain-wide mapping of neural activity mediating collicular-dependent behaviors

Neuronal cell types are arranged in brain-wide circuits that guide behavior. In mice, the superior colliculus innervates a set of targets that direct orienting and defensive actions. We combined functional ultrasound imaging (fUSI) with optogenetics to reveal the network of brain regions functionally activated by four collicular cell types. Stimulating each neuronal group triggered different behaviors and activated distinct sets of brain nuclei. This included regions not previously thought to mediate defensive behaviors, for example, the posterior paralaminar nuclei of the thalamus (PPnT), which we show to play a role in suppressing habituation. Neuronal recordings with Neuropixels probes show that (1) patterns of spiking activity and fUSI signals correlate well in space and (2) neurons in downstream nuclei preferentially respond to innately threatening visual stimuli. This work provides insight into the functional organization of the networks governing innate behaviors and demonstrates an experimental approach to explore the whole-brain neuronal activity downstream of targeted cell types.

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.

Differential coupling between subcortical calcium and BOLD signals during evoked and resting state through simultaneous calcium fiber photometry and fMRI

Task based and resting state fMRI has been widely utilized to study brain functions. As the foundation of fMRI, the underlying neural basis of the BOLD signal has been extensively studied, but the detailed mechanism remains elusive, particularly during the resting state. To examine the neurovascular coupling, it is important to simultaneously record neural and vascular signals. Here we developed a novel setup of camera based, scalable simultaneous calcium fiber photometry and fMRI in rats. Using this setup, we recorded calcium signals of superior colliculus (SC) and lateral geniculate nucleus (LGN) and fMRI simultaneously during visual stimulation and the resting state. Our results revealed robust, region-specific coupling between calcium and BOLD signals in the task state and weaker, whole brain correlation in the resting state. Interestingly, the spatial specificity of such correlation in the resting state was improved upon regression of white matter, ventricle signals and global signals in fMRI data. Overall, our results suggest differential coupling of calcium and BOLD signals for subcortical regions between evoked and resting states, and the coupling relationship in the resting state was related with resting state BOLD preprocessing strategies.

Colocalization of Tectal Inputs With Amygdala-Projecting Neurons in the Macaque Pulvinar

Neuropsychological and neuroimaging studies have suggested the presence of a fast, subcortical route for the processing of emotionally-salient visual information in the primate brain. This putative pathway consists of the superior colliculus (SC), pulvinar and amygdala. While the presence of such a pathway has been confirmed in sub-primate species, it has yet to be documented in the primate brain using conventional anatomical methods. We injected retrograde tracers into the amygdala and anterograde tracers into the colliculus, and examined regions of colocalization of these signals within the pulvinar of the macaque. Anterograde tracers injected into the SC labeled axonal projections within the pulvinar, primarily within the oral, lateral and medial subdivisions. These axonal projections from the colliculus colocalized with cell bodies within the pulvinar that were labeled by retrograde tracer injected into the lateral amygdala. This zone of overlap was most notable in the medial portions of the medial (PM), oral (PO) and inferior pulvinar (PI), and was often densely concentrated in the vicinity of the brachium of the SC. These data provide an anatomical basis for the previously suggested pathway mediating fast processing of emotionally salient information.

Bidirectional encoding of motion contrast in the mouse superior colliculus

Detection of salient objects in the visual scene is a vital aspect of an animal's interactions with its environment. Here, we show that neurons in the mouse superior colliculus (SC) encode visual saliency by detecting motion contrast between stimulus center and surround. Excitatory neurons in the most superficial lamina of the SC are contextually modulated, monotonically increasing their response from suppression by the same-direction surround to maximal potentiation by an oppositely-moving surround. The degree of this potentiation declines with depth in the SC. Inhibitory neurons are suppressed by any surround at all depths. These response modulations in both neuronal populations are much more prominent to direction contrast than to phase, temporal frequency, or static orientation contrast, suggesting feature-specific saliency encoding in the mouse SC. Together, our findings provide evidence supporting locally generated feature representations in the SC, and lay the foundations towards a mechanistic and evolutionary understanding of their emergence.

Whole-brain high in-plane resolution fMRI using accelerated EPIK for enhanced characterisation of functional areas at 3T

The relatively high imaging speed of EPI has led to its widespread use in dynamic MRI studies such as functional MRI. An approach to improve the performance of EPI, EPI with Keyhole (EPIK), has been previously presented and its use in fMRI was verified at 1.5T as well as 3T. The method has been proven to achieve a higher temporal resolution and smaller image distortions when compared to single-shot EPI. Furthermore, the performance of EPIK in the detection of functional signals was shown to be comparable to that of EPI. For these reasons, we were motivated to employ EPIK here for high-resolution imaging. The method was optimised to offer the highest possible in-plane resolution and slice coverage under the given imaging constraints: fixed TR/TE, FOV and acceleration factors for parallel imaging and partial Fourier techniques. The performance of EPIK was evaluated in direct comparison to the optimised protocol obtained from EPI. The two imaging methods were applied to visual fMRI experiments involving sixteen subjects. The results showed that enhanced spatial resolution with a whole-brain coverage was achieved by EPIK (1.00 mm × 1.00 mm; 32 slices) when compared to EPI (1.25 mm × 1.25 mm; 28 slices). As a consequence, enhanced characterisation of functional areas has been demonstrated in EPIK particularly for relatively small brain regions such as the lateral geniculate nucleus (LGN) and superior colliculus (SC); overall, a significantly increased t-value and activation area were observed from EPIK data. Lastly, the use of EPIK for fMRI was validated with the simulation of different types of data reconstruction methods.

Simultaneous GCaMP6-based fiber photometry and fMRI in rats

BACKGROUND

Understanding the relationship between neural and vascular signals is essential for interpretation of functional MRI (fMRI) results with respect to underlying neuronal activity. Simultaneously measuring neural activity using electrophysiology with fMRI has been highly valuable in elucidating the neural basis of the blood oxygenation-level dependent (BOLD) signal. However, this approach is also technically challenging due to the electromagnetic interference that is observed in electrophysiological recordings during MRI scanning.

NEW METHOD

Recording optical correlates of neural activity, such as calcium signals, avoids this issue, and has opened a new avenue to simultaneously acquire neural and BOLD signals.

RESULTS

The present study is the first to demonstrate the feasibility of simultaneously and repeatedly acquiring calcium and BOLD signals in animals using a genetically encoded calcium indicator, GCaMP6. This approach was validated with a visual stimulation experiment, during which robust increases of both calcium and BOLD signals in the superior colliculus were observed. In addition, repeated measurement in the same animal demonstrated reproducible calcium and BOLD responses to the same stimuli.

COMPARISON WITH EXISTING METHOD(S)

Taken together, simultaneous GCaMP6-based fiber photometry and fMRI recording presents a novel, artifact-free approach to simultaneously measuring neural and fMRI signals. Furthermore, given the cell-type specificity of GCaMP6, this approach has the potential to mechanistically dissect the contributions of individual neuron populations to BOLD signal, and ultimately reveal its underlying neural mechanisms.

CONCLUSIONS

The current study established the method for simultaneous GCaMP6-based fiber photometry and fMRI in rats.

An fMRI Investigation of Preparatory Set in the Human Cerebral Cortex and Superior Colliculus for Pro- and Anti-Saccades

Previous studies have identified several cortical regions that show larger BOLD responses during preparation and execution of anti-saccades than pro-saccades. We confirmed this finding with a greater BOLD response for anti-saccades than pro-saccades during the preparation phase in the FEF, IPS and DLPFC and in the FEF and IPS in the execution phase. We then applied multi-voxel pattern analysis (MVPA) to establish whether different neural populations are involved in the two types of saccade. Pro-saccades and anti-saccades were reliably decoded during saccade execution in all three cortical regions (FEF, DLPFC and IPS) and in IPS during saccade preparation. This indicates neural specialization, for programming the desired response depending on the task rule, in these regions. In a further study tailored for imaging the superior colliculus in the midbrain a similar magnitude BOLD response was observed for pro-saccades and anti-saccades and the two saccade types could not be decoded with MVPA. This was the case both for activity related to the preparation phase and also for that elicited during the execution phase. We conclude that separate cortical neural populations are involved in the task-specific programming of a saccade while in contrast, the SC has a role in response preparation but may be less involved in high-level, task-specific aspects of the control of saccades.

Using a white matter reference to remove the dependency of global signal on experimental conditions in SPECT analyses

Proportional scaling models are often used in functional imaging studies to remove confounding of local signals by global effects. It is generally assumed that global effects are uncorrelated with experimental conditions. However, when the global effect is estimated by the global signal, defined as the intracerebral average, incorrect inference may result from the dependency of the global signal on preexisting conditions or experimental manipulations. In this paper, we propose a simple alternative method of estimating the global effect to be used in a proportional scaling model. Specifically, by defining the global signal with reference strictly to a white matter region within the centrum semiovale, the dependency is removed in experiments where white matter is unaffected by the disease effect or experimental treatments. The increase in the ability to detect changes in regional blood flow is demonstrated in a SPECT study of healthy and ill Gulf War veterans in whom it is suspected that brain abnormalities influence the traditional calculation of the global signal. Controlling for the global effect, ill veterans have significantly lower intracerebral averages than healthy controls (P = 0.0038), evidence that choice of global signal has an impact on inference. Scaling by the modified global signal proposed here results in an increase in sensitivity leading to the identification of several regions in the insula and frontal cortex where ill veterans have significantly lower SPECT emissions. Scaling by the traditional global signal results in the loss of sensitivity to detect these regional differences. Advantages of this alternative method are its computational simplicity and its ability to be easily integrated into existing analysis frameworks such as SPM.

Neural correlates of auditory-visual stimulus onset asynchrony detection

Intersensory temporal synchrony is an ubiquitous sensory attribute that has proven to be critical for binding multisensory inputs, sometimes erroneously leading to dramatic perceptual illusions. However, little is known about how the brain detects temporal synchrony between multimodal sensory inputs. We used positron emission tomography to demonstrate that detecting auditory-visual stimulus onset asynchrony activates a large-scale neural network of insular, posterior parietal, prefrontal, and cerebellar areas with the highest and task-specific activity localized to the right insula. Interregional covariance analysis further showed significant task-related functional interactions between the insula, the posterior thalamus, and superior colliculus. Based on these results and the available electrophysiological and anatomical connectivity data in animals, we propose that the insula, via its known short-latency connections with the tectal system, mediates temporally defined auditory-visual interaction at an early stage of cortical processing permitting phenomena such as the ventriloquist and the McGurk illusions.

Functional adaptation of reactive saccades in humans: a PET study

It is known that the saccadic system shows adaptive changes when the command sent to the extraocular muscles is inappropriate. Despite an abundance of supportive psychophysical investigations, the neurophysiological substrate of this process is still debated. The present study addresses this issue using H2(15)O positron emission tomography (PET). We contrasted three conditions in which healthy human subjects were required to perform saccadic eye movements toward peripheral visual targets. Two conditions involved a modification of the target location during the course of the initial saccade, when there is suppression of visual perception. In the RAND condition, intra-saccadic target displacement was random from trial-to-trial, precluding any systematic modification of the primary saccade amplitude. In the ADAPT condition, intra-saccadic target displacement was uniform, causing adaptive modification of the primary saccade amplitude. In the third condition (stationary, STAT), the target remained at the same location during the entire trial. Difference images reflecting regional cerebral-blood-flow changes attributable to the process of saccadic adaptation (ADAPT minus RAND; ADAPT minus STAT) showed a selective activation in the oculomotor cerebellar vermis (OCV; lobules VI and VII). This finding is consistent with neurophysiological studies in monkeys. Additional analyses indicated that the cerebellar activation was not related to kinematic factors, and that the absence of significant activation within the frontal eye fields (FEF) or the superior colliculus (SC) did not represent a false negative inference. Besides the contribution of the OCV to saccadic adaptation, we also observed, in the RAND condition, that the saccade amplitude was significantly larger when the previous trial involved a forward jump than when the previous trial involved a backward jump. This observation indicates that saccade accuracy is constantly monitored on a trial-to-trial basis. Behavioral measurements and PET observations (RAND minus STAT) suggest that this single-trial control of saccade amplitude may be functionally distinct from the process of saccadic adaptation.

Brain stem blood flow, pupillary response, and outcome in patients with severe head injuries

OBJECTIVE

Acute pupillary dilation in a head-injured patient is a neurological emergency. Pupil dilation is thought to be the result of uncal herniation causing mechanical compression of the IIIrd cranial nerve and subsequent brain stem compromise. However, not all patients with herniation have fixed and dilated pupils, and not all patients with nonreactive, enlarged pupils have uncal herniation. Therefore, we have tested an alternative hypothesis that a decrease in brain stem blood flow (BBF) is a more frequent cause of mydriasis and brain stem symptomatology after severe head injury. We determined the relation of BBF to outcome and pupillary response in patients with severe head injuries.

METHODS

One hundred sixty-two patients with a Glasgow Coma Scale score of 8 or less underwent stable xenon computed tomographic blood flow determination at the level of the superior colliculus, and this blood flow was correlated with pupillary features, intracranial pressure, computed tomographic scan pathology, and outcome.

RESULTS

A BBF of less than 40 ml/100 g/min was significantly associated with poor outcome (P < 0.009). In patients with bilaterally nonreactive pupils, the BBF was 30.5+/-16.8 ml/100 g/min, and in those with normally reactive pupils, the BBF was 43.8+/-18.7 ml/100 g/min (P < 0.001). Intracranial pressure and the presence of a brain stem lesion observed on the computed tomographic scan did not correlate with BBF, pupillary size, or reactivity. Unfavorable outcome at 12 months was directly related to age (P = 0.062) and inversely related to pupillary responsiveness (P = 0.0006), pupil size (P = 0.005), and BBF of less than 40 ml/100 g/min (P = 0.009).

CONCLUSION

These findings suggest that pupillary dilation is associated with decreased BBF and that ischemia, rather than mechanical compression of the IIIrd cranial nerve, is an important causal factor. More important, pupil dilation may be an indicator of ischemia of the brain stem. If cerebral blood flow and cerebral perfusion pressure can be rapidly restored in the patient with severe head injury who has dilated pupils, the prognosis may be good.

A subcortical pathway to the right amygdala mediating "unseen" fear

Neuroimaging studies have shown differential amygdala responses to masked ("unseen") emotional stimuli. How visual signals related to such unseen stimuli access the amygdala is unknown. A possible pathway, involving the superior colliculus and pulvinar, is suggested by observations of patients with striate cortex lesions who show preserved abilities to localize and discriminate visual stimuli that are not consciously perceived ("blindsight"). We used measures of right amygdala neural activity acquired from volunteer subjects viewing masked fear-conditioned faces to determine whether a colliculo-pulvinar pathway was engaged during processing of these unseen target stimuli. Increased connectivity between right amygdala, pulvinar, and superior colliculus was evident when fear-conditioned faces were unseen rather than seen. Right amygdala connectivity with fusiform and orbitofrontal cortices decreased in the same condition. By contrast, the left amygdala, whose activity did not discriminate seen and unseen fear-conditioned targets, showed no masking-dependent changes in connectivity with superior colliculus or pulvinar. These results suggest that a subcortical pathway to the right amygdala, via midbrain and thalamus, provides a route for processing behaviorally relevant unseen visual events in parallel to a cortical route necessary for conscious identification.

Cavernous angioma with olivary hypertrophy and contralateral cerebellar diaschisis

We describe a 66-year-old man with a 20-year history of ataxic gait who suddenly developed diplopia on rightward gaze. Neurologic examination revealed right hemi-ataxia and hemi-hypesthesia, and left internuclear ophthalmoplegia. MRI showed a cavernous angioma in the left tectum, mild right cerebellar atrophy, and left interior olivary hypertrophy. Single photon emission computed tomography (SPECT) imaging demonstrated contralateral cerebellar diaschisis. We discuss the findings and review the literature concerning contralateral cerebellar diaschisis.

MR vs CT in progressive supranuclear palsy

Nine progressive supranuclear palsy (PSP) patients were studied with computerized tomography (CT) and magnetic resonance (MR) in order to determine the efficacy of each in detecting atrophy of the brainstem. Three additional PSP patients were evaluated with MRI for quantitative (electronic) measurements of the colliculi, pons and midbrain tegmentum. Both CT and MRI were equally effective in demonstrating midbrain atrophy. The MR was able to utilize the sagittal view to visualize thinning of the collicular (quadrigeminal) plate, a useful sign in PSP. Atrophy of the thinned collicular plate is more pronounced in the superior colliculus, one of the most common sites of pathology in PSP. The MR is able to make quantitative measurements of the degree of atrophy of the colliculi, pons and midbrain tegmentum.

Transient increase in the in vivo binding of the benzodiazepine antagonist [3H]flumazenil in deafferented visual areas of the adult mouse brain

Flumazenil is an imidazobenzodiazepine, an antagonist of central benzodiazepine (BDZ) receptors. BDZ binding sites are a modulatory component located on the gamma-aminobutyric acid (GABA) receptor macromolecule. We studied the effect of monocular enucleation on [3H]flumazenil binding in deprived and intact visual areas and nonvisual areas of the adult mouse brain under in vivo conditions. [3H]flumazenil binding was examined at seven time points up to 56 days postenucleation. In some monocularly deprived mice, changes in local blood flow accompanied with the BDZ receptor response were evaluated by coinjection of [3H]flumazenil and 99mTc-HMPAO. Monocular enucleation produced a transient increase in [3H]flumazenil binding in the deprived visual cortex and superior colliculus. At 17 days postenucleation, [3H]flumazenil binding in the anterior and posterior portions of the visual cortex and the superior colliculus increased by 28%, 15% and 23%, respectively, and declined to control levels at 45 days postenucleation. The increase in [3H]flumazenil was accompanied with a decrease in blood flow. Alterations in BDZ receptors and blood flow were selective to deprived visual structures. The regional correlation between the metabolic deficit and the BDZ response provides further support that the increase in BDZ receptor binding is confined to regions of reduced neuronal activity. [11C]flumazenil is an excellent radiotracer for in vivo imaging of benzodiazepine receptors in human brain using positron emission tomography (PET). This study suggests the suitability of [11C]flumazenil for in vivo PET study of BDZ receptor response to deafferentation of visual structures in human brain.