Endophenotype trait domains for advancing gene discovery in autism spectrum disorder

Autism spectrum disorder (ASD) is associated with a diverse range of etiological processes, including both genetic and non-genetic causes. For a plurality of individuals with ASD, it is likely that the primary causes involve multiple common inherited variants that individually account for only small levels of variation in phenotypic outcomes. This genetic landscape creates a major challenge for detecting small but important pathogenic effects associated with ASD. To address similar challenges, separate fields of medicine have identified endophenotypes, or discrete, quantitative traits that reflect genetic likelihood for a particular clinical condition and leveraged the study of these traits to map polygenic mechanisms and advance more personalized therapeutic strategies for complex diseases. Endophenotypes represent a distinct class of biomarkers useful for understanding genetic contributions to psychiatric and developmental disorders because they are embedded within the causal chain between genotype and clinical phenotype, and they are more proximal to the action of the gene(s) than behavioral traits. Despite their demonstrated power for guiding new understanding of complex genetic structures of clinical conditions, few endophenotypes associated with ASD have been identified and integrated into family genetic studies. In this review, we argue that advancing knowledge of the complex pathogenic processes that contribute to ASD can be accelerated by refocusing attention toward identifying endophenotypic traits reflective of inherited mechanisms. This pivot requires renewed emphasis on study designs with measurement of familial co-variation including infant sibling studies, family trio and quad designs, and analysis of monozygotic and dizygotic twin concordance for select trait dimensions. We also emphasize that clarification of endophenotypic traits necessarily will involve integration of transdiagnostic approaches as candidate traits likely reflect liability for multiple clinical conditions and often are agnostic to diagnostic boundaries. Multiple candidate endophenotypes associated with ASD likelihood are described, and we propose a new focus on the analysis of "endophenotype trait domains" (ETDs), or traits measured across multiple levels (e.g., molecular, cellular, neural system, neuropsychological) along the causal pathway from genes to behavior. To inform our central argument for research efforts toward ETD discovery, we first provide a brief review of the concept of endophenotypes and their application to psychiatry. Next, we highlight key criteria for determining the value of candidate endophenotypes, including unique considerations for the study of ASD. Descriptions of different study designs for assessing endophenotypes in ASD research then are offered, including analysis of how select patterns of results may help prioritize candidate traits in future research. We also present multiple candidate ETDs that collectively cover a breadth of clinical phenomena associated with ASD, including social, language/communication, cognitive control, and sensorimotor processes. These ETDs are described because they represent promising targets for gene discovery related to clinical autistic traits, and they serve as models for analysis of separate candidate domains that may inform understanding of inherited etiological processes associated with ASD as well as overlapping neurodevelopmental disorders.

Somesthetic, Visual, and Auditory Feedback and Their Interactions Applied to Upper Limb Neurorehabilitation Technology: A Narrative Review to Facilitate Contextualization of Knowledge

Reduced hand dexterity is a common component of sensorimotor impairments for individuals after stroke. To improve hand function, innovative rehabilitation interventions are constantly developed and tested. In this context, technology-based interventions for hand rehabilitation have been emerging rapidly. This paper offers an overview of basic knowledge on post lesion plasticity and sensorimotor integration processes in the context of augmented feedback and new rehabilitation technologies, in particular virtual reality and soft robotic gloves. We also discuss some factors to consider related to the incorporation of augmented feedback in the development of technology-based interventions in rehabilitation. This includes factors related to feedback delivery parameter design, task complexity and heterogeneity of sensory deficits in individuals affected by a stroke. In spite of the current limitations in our understanding of the mechanisms involved when using new rehabilitation technologies, the multimodal augmented feedback approach appears promising and may provide meaningful ways to optimize recovery after stroke. Moving forward, we argue that comparative studies allowing stratification of the augmented feedback delivery parameters based upon different biomarkers, lesion characteristics or impairments should be advocated (e.g., injured hemisphere, lesion location, lesion volume, sensorimotor impairments). Ultimately, we envision that treatment design should combine augmented feedback of multiple modalities, carefully adapted to the specific condition of the individuals affected by a stroke and that evolves along with recovery. This would better align with the new trend in stroke rehabilitation which challenges the popular idea of the existence of an ultimate good-for-all intervention.

A Computational Analysis of Neural Mechanisms Underlying the Maturation of Multisensory Speech Integration in Neurotypical Children and Those on the Autism Spectrum

Failure to appropriately develop multisensory integration (MSI) of audiovisual speech may affect a child's ability to attain optimal communication. Studies have shown protracted development of MSI into late-childhood and identified deficits in MSI in children with an autism spectrum disorder (ASD). Currently, the neural basis of acquisition of this ability is not well understood. Here, we developed a computational model informed by neurophysiology to analyze possible mechanisms underlying MSI maturation, and its delayed development in ASD. The model posits that strengthening of feedforward and cross-sensory connections, responsible for the alignment of auditory and visual speech sound representations in posterior superior temporal gyrus/sulcus, can explain behavioral data on the acquisition of MSI. This was simulated by a training phase during which the network was exposed to unisensory and multisensory stimuli, and projections were crafted by Hebbian rules of potentiation and depression. In its mature architecture, the network also reproduced the well-known multisensory McGurk speech effect. Deficits in audiovisual speech perception in ASD were well accounted for by fewer multisensory exposures, compatible with a lack of attention, but not by reduced synaptic connectivity or synaptic plasticity.

The potential for developing a tactile communication system based on Blissymbolics

OBJECTIVE

To be useful for tactile communication, tactile stimuli need to be discriminable from each other. The objective of this study was to determine whether raised-line renderings of Blissymbols have the capacity for being developed into a tactile communication system as measured by their tactile discriminability.

METHODS

Tactile discrimination of Blissymbols was measured by performance on a task in which participants were asked to feel a target raised-line Blissymbol and then to find the target within an array containing the target and raised-line Blissymbol foils.

RESULTS

The vast majority of tactile Blissymbols had tactile discrimination scores of 90% accuracy or better.

CONCLUSION

Most raised-line Blissymbols can be tactilely discriminated from each other, indicating that they have the potential for being developed into a tactile communication system.

Laminar profile of visual response properties in ferret superior colliculus

In the superior colliculus (SC), visual afferent inputs from various sources converge in a highly organized way such that all layers form topographically aligned representations of contralateral external space. Despite this anatomical organization, it remains unclear how the layer-specific termination of different visual input pathways is reflected in the nature of visual response properties and their distribution across layers. To uncover the physiological correlates underlying the laminar organization of the SC, we recorded multiunit and local field potential activity simultaneously from all layers with dual-shank multichannel linear probes. We found that the location of spatial receptive fields was strongly conserved across all visual responsive layers. There was a tendency for receptive field size to increase with depth in the SC, with superficial receptive fields significantly smaller than deep receptive fields. Additionally, superficial layers responded significantly faster than deeper layers to flash stimulation. In some recordings, flash-evoked responses were characterized by the presence of gamma oscillatory activity (40-60 Hz) in multiunit and field potential signals, which was strongest in retinorecipient layers. While SC neurons tended to respond only weakly to full-field drifting gratings, we observed very similar oscillatory responses to the offset of grating stimuli, suggesting gamma oscillations are produced following light offset. Oscillatory spiking activity was highly correlated between horizontally distributed neurons within these layers, with oscillations temporally locked to the stimulus. Together, visual response properties provide physiological evidence reflecting the laminar-specific termination of visual afferent pathways in the SC, most notably characterized by the oscillatory entrainment of superficial neurons.

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.

Superior sensation: superior colliculus participation in rat vibrissa system

BACKGROUND

The superior colliculus, usually considered a visuomotor structure, is anatomically positioned to perform sensorimotor transformations in other modalities. While there is evidence for its potential participation in sensorimotor loops of the rodent vibrissa system, little is known about its functional role in vibrissa sensation or movement. In anesthetized rats, we characterized extracellularly recorded responses of collicular neurons to different types of vibrissa stimuli.

RESULTS

Collicular neurons had large receptive fields (median = 14.5 vibrissae). Single units displayed responses with short latencies (5.6 +/- 0.2 msec, median = 5.5) and relatively large magnitudes (1.2 +/- 0.1 spikes/stimulus, median = 1.2). Individual neurons could entrain to repetitive vibrissa stimuli delivered at < or = 20 Hz, with little reduction in phase locking, even when response magnitude was decreased. Neurons responded preferentially to vibrissa deflections at particular angles, with 43% of the cells having high (> or = 5) angular selectivity indices.

CONCLUSION

Results are consistent with a proposed role of the colliculus in somatosensory-mediated orienting. These properties, together with the connections of the superior colliculus in sensorimotor loops, are consistent with its involvement in orienting, alerting and attentive functions related to the vibrissa system.

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.

Some certainty for the “zone of uncertainty”? Exploring the function of the zona incerta

The zona incerta (ZI), first described over a century ago by Auguste Forel as a "region of which nothing certain can be said," forms a collection of cells that derives from the diencephalon. To this day, we are still not certain of the precise function of this "zone of uncertainty" although many have been proposed, from controlling visceral activity to shifting attention and from influencing arousal to maintaining posture and locomotion. In this review, I shall outline the recent advances in the understanding of the structure, connectivity and functions of the ZI. I will then focus on a possible and often neglected global role for the ZI, one that links its diverse functions together. In particular, I aim to highlight the idea that the ZI forms a primal center of the diencephalon for generating direct responses (visceral, arousal, attention and/or posture-locomotion) to a given sensory (somatic and/or visceral) stimulus. With this global role in mind, I will then address recent results indicating that abnormal ZI activity manifests in clinical symptoms of Parkinson disease.

Neural correlates of behavioral preference for culturally familiar drinks

Coca-Cola (Coke) and Pepsi are nearly identical in chemical composition, yet humans routinely display strong subjective preferences for one or the other. This simple observation raises the important question of how cultural messages combine with content to shape our perceptions; even to the point of modifying behavioral preferences for a primary reward like a sugared drink. We delivered Coke and Pepsi to human subjects in behavioral taste tests and also in passive experiments carried out during functional magnetic resonance imaging (fMRI). Two conditions were examined: (1) anonymous delivery of Coke and Pepsi and (2) brand-cued delivery of Coke and Pepsi. For the anonymous task, we report a consistent neural response in the ventromedial prefrontal cortex that correlated with subjects' behavioral preferences for these beverages. In the brand-cued experiment, brand knowledge for one of the drinks had a dramatic influence on expressed behavioral preferences and on the measured brain responses.

Visual feedback alters the variations in corticospinal excitability that arise from rhythmic movements of the opposite limb

Augmented visual feedback can have a profound bearing on the stability of bimanual coordination. Indeed, this has been used to render tractable the study of patterns of coordination that cannot otherwise be produced in a stable fashion. In previous investigations (Carson et al. 1999), we have shown that rhythmic movements, brought about by the contraction of muscles on one side of the body, lead to phase-locked changes in the excitability of homologous motor pathways of the opposite limb. The present study was conducted to assess whether these changes are influenced by the presence of visual feedback of the moving limb. Eight participants performed rhythmic flexion-extension movements of the left wrist to the beat of a metronome (1.5 Hz). In 50% of trials, visual feedback of wrist displacement was provided in relation to a target amplitude, defined by the mean movement amplitude generated during the immediately preceding no feedback trial. Motor potentials (MEPs) were evoked in the quiescent muscles of the right limb by magnetic stimulation of the left motor cortex. Consistent with our previous observations, MEP amplitudes were modulated during the movement cycle of the opposite limb. The extent of this modulation was, however, smaller in the presence of visual feedback of the moving limb (FCR omega2=0.41; ECR omega2=0.29) than in trials in which there was no visual feedback (FCR omega2=0.51; ECR omega2=0.48). In addition, the relationship between the level of FCR activation and the excitability of the homologous corticospinal pathway of the opposite limb was sensitive to the vision condition; the degree of correlation between the two variables was larger when there was no visual feedback of the moving limb. The results of the present study support the view that increases in the stability of bimanual coordination brought about by augmented feedback may be mediated by changes in the crossed modulation of excitability in homologous motor pathways.

Multimodality in the superior colliculus: an information theoretic analysis

The deep superior colliculus (DSC) integrates multisensory input and triggers an orienting movement toward the source of stimulation (target). It would seem reasonable to suppose that input of an additional modality should always increase the amount of information received by a DSC neuron concerning a target. However, of all DSC neurons studied, only about one half in the cat and one-quarter in the monkey were multimodal. The rest received only unimodal input. Multimodal DSC neurons show the properties of multisensory enhancement, in which the neural response to an input of one modality is augmented by input of another modality, and of inverse effectiveness, in which weaker unimodal responses produce a higher percentage enhancement. Previously, we demonstrated that these properties are consistent with the hypothesis that DSC neurons use Bayes' rule to compute the posterior probability that a target is present given their stochastic sensory inputs. Here we use an information theoretic analysis of our Bayesian model to show that input of an additional modality may indeed increase target information, but only if input received from the initial modality does not completely reduce uncertainty concerning the presence of a target. Unimodal DSC neurons may be those whose unimodal input fully reduces target uncertainty and therefore have no need for input of another modality.

Nonvisual influences on visual-information processing in the superior colliculus

Although visually responsive neurons predominate in the deep layers of the superior colliculus (SC), the majority of them also receive sensory inputs from nonvisual sources (i.e. auditory and/or somatosensory). Most of these 'multisensory' neurons are able to synthesize their cross-modal inputs and, as a consequence, their responses to visual stimuli can be profoundly enhanced or depressed in the presence of a nonvisual cue. Whether response enhancement or response depression is produced by this multisensory interaction is predictable based on several factors. These include: the organization of a neuron's visual and nonvisual receptive fields; the relative spatial relationships of the different stimuli (to their respective receptive fields and to one another); and whether or not the neuron is innervated by a select population of cortical neurons. The response enhancement or depression of SC neurons via multisensory integration has significant survival value via its profound impact on overt attentive/orientation behaviors. Nevertheless, these multisensory processes are not present at birth, and require an extensive period of postnatal maturation. It seems likely that the sensory experiences obtained during this period play an important role in crafting the processes underlying these multisensory interactions.