Multimodal cortical sensory pathways revealed by sequential transcranial electrical stimulation in mice

Neural Correlates of Multisensory Detection Behavior: Comparison of Primary and Higher-Order Visual Cortex

We act upon stimuli in our surrounding environment by gathering the multisensory information they convey and by integrating this information to decide on a behavioral action. We hypothesized that the anterolateral secondary visual cortex (area AL) of the mouse brain may serve as a hub for sensorimotor transformation of audiovisual information. We imaged neuronal activity in primary visual cortex (V1) and AL of the mouse during a detection task using visual, auditory, and audiovisual stimuli. We found that AL neurons were more sensitive to weak uni- and multisensory stimuli compared to V1. Depending on contrast, different subsets of AL and V1 neurons showed cross-modal modulation of visual responses. During audiovisual stimulation, AL neurons showed stronger differentiation of behaviorally reported versus unreported stimuli compared to V1, whereas V1 showed this distinction during unisensory visual stimulation. Thus, neural population activity in area AL correlates more closely with multisensory detection behavior than V1.

Reconsidering the Border between the Visual and Posterior Parietal Cortex of Mice

The posterior parietal cortex (PPC) contributes to multisensory and sensory-motor integration, as well as spatial navigation. Based on primate studies, the PPC is composed of several subdivisions with differing connection patterns, including areas that exhibit retinotopy. In mice the composition of the PPC is still under debate. We propose a revised anatomical delineation in which we classify the higher order visual areas rostrolateral area (RL), anteromedial area (AM), and Medio-Medial-Anterior cortex (MMA) as subregions of the mouse PPC. Retrograde and anterograde tracing revealed connectivity, characteristic for primate PPC, with sensory, retrosplenial, orbitofrontal, cingulate and motor cortex, as well as with several thalamic nuclei and the superior colliculus in the mouse. Regarding cortical input, RL receives major input from the somatosensory barrel field, while AM receives more input from the trunk, whereas MMA receives strong inputs from retrosplenial, cingulate, and orbitofrontal cortices. These input differences suggest that each posterior PPC subregion may have a distinct function. Summarized, we put forward a refined cortical map, including a mouse PPC that contains at least 6 subregions, RL, AM, MMA and PtP, MPta, LPta/A. These anatomical results set the stage for a more detailed understanding about the role that the PPC and its subdivisions play in multisensory integration-based behavior in mice.

Multisensory integration in the mouse cortical connectome using a network diffusion model

Having a structural network representation of connectivity in the brain is instrumental in analyzing communication dynamics and neural information processing. In this work, we make steps towards understanding multisensory information flow and integration using a network diffusion approach. In particular, we model the flow of evoked activity, initiated by stimuli at primary sensory regions, using the asynchronous linear threshold (ALT) diffusion model. The ALT model captures how evoked activity that originates at a given region of the cortex “ripples through” other brain regions (referred to as an activation cascade). We find that a small number of brain regions–the claustrum and the parietal temporal cortex being at the top of the list–are involved in almost all cortical sensory streams. This suggests that the cortex relies on an hourglass architecture to first integrate and compress multisensory information from multiple sensory regions, before utilizing that lower dimensionality representation in higher level association regions and more complex cognitive tasks.

Having a structural network representation of connectivity in the brain is instrumental in analyzing communication dynamics and neural information processing. In this work, we make steps towards understanding multisensory information flow and integration using a network diffusion approach. In particular, we model the flow of evoked activity, initiated by stimuli at primary sensory regions, using the asynchronous linear threshold (ALT) diffusion model. The ALT model captures how evoked activity that originates at a given region of the cortex “ripples through” other brain regions (referred to as an activation cascade). We apply the ALT model to the mouse connectome provided by the Allen Institute for Brain Science. A first result, using functional datasets based on voltage-sensitive dye (VSD) imaging, is that the ALT model, despite its simplicity, predicts the temporal ordering of each sensory activation cascade quite accurately. We further apply this model to study multisensory integration and find that a small number of brain regionsthe claustrum and the parietal temporal cortex being at the top of the listare involved in almost all cortical sensory streams. This suggests that the cortex relies on an hourglass architecture to first integrate and compress multisensory information from multiple sensory regions, before utilizing that lower dimensionality representation in higher level association regions and more complex cognitive tasks.

Approaches to Understanding Multisensory Dysfunction in Autism Spectrum Disorder

Abnormal sensory responses are a DSM-5 symptom of autism spectrum disorder (ASD), and research findings demonstrate altered sensory processing in ASD. Beyond difficulties with processing information within single sensory domains, including both hypersensitivity and hyposensitivity, difficulties in multisensory processing are becoming a core issue of focus in ASD. These difficulties may be targeted by treatment approaches such as "sensory integration," which is frequently applied in autism treatment but not yet based on clear evidence. Recently, psychophysical data have emerged to demonstrate multisensory deficits in some children with ASD. Unlike deficits in social communication, which are best understood in humans, sensory and multisensory changes offer a tractable marker of circuit dysfunction that is more easily translated into animal model systems to probe the underlying neurobiological mechanisms. Paralleling experimental paradigms that were previously applied in humans and larger mammals, we and others have demonstrated that multisensory function can also be examined behaviorally in rodents. Here, we review the sensory and multisensory difficulties commonly found in ASD, examining laboratory findings that relate these findings across species. Next, we discuss the known neurobiology of multisensory integration, drawing largely on experimental work in larger mammals, and extensions of these paradigms into rodents. Finally, we describe emerging investigations into multisensory processing in genetic mouse models related to autism risk. By detailing findings from humans to mice, we highlight the advantage of multisensory paradigms that can be easily translated across species, as well as the potential for rodent experimental systems to reveal opportunities for novel treatments. LAY SUMMARY: Sensory and multisensory deficits are commonly found in ASD and may result in cascading effects that impact social communication. By using similar experiments to those in humans, we discuss how studies in animal models may allow an understanding of the brain mechanisms that underlie difficulties in multisensory integration, with the ultimate goal of developing new treatments. Autism Res 2020, 13: 1430-1449. © 2020 International Society for Autism Research, Wiley Periodicals, Inc.

Excitatory Neuronal Hubs Configure Multisensory Integration of Slow Waves in Association Cortex

Multisensory integration (MSI) is a fundamental emergent property of the mammalian brain. During MSI, perceptual information encoded in patterned activity is processed in multimodal association cortex. The systems-level neuronal dynamics that coordinate MSI, however, are unknown. Here, we demonstrate intrinsic hub-like network activity in the association cortex that regulates MSI. We engineered calcium reporter mouse lines based on the fluorescence resonance energy transfer sensor yellow cameleon (YC2.60) expressed in excitatory or inhibitory neurons. In medial and parietal association cortex, we observed spontaneous slow waves that self-organized into hubs defined by long-range excitatory and local inhibitory circuits. Unlike directional source/sink-like flows in sensory areas, medial/parietal excitatory and inhibitory hubs had net-zero balanced inputs. Remarkably, multisensory stimulation triggered rapid phase-locking mainly of excitatory hub activity persisting for seconds after the stimulus offset. Therefore, association cortex tends to form balanced excitatory networks that configure slow-wave phase-locking for MSI. VIDEO ABSTRACT.

Feedback inhibition derived from the posterior parietal cortex regulates the neural properties of the mouse visual cortex

Feedback regulation from the higher association areas is thought to control the primary sensory cortex, contribute to the cortical processing of sensory information, and work for higher cognitive functions such as multimodal integration and attentional control. However, little is known about the underlying neural mechanisms. Here, we show that the posterior parietal cortex (PPC) persistently inhibits the activity of the primary visual cortex (V1) in mice. Activation of the PPC causes the suppression of visual responses in V1 and induces the short-term depression, which is specific to visual stimuli. In contrast, pharmacological inactivation of the PPC or disconnection of cortical pathways from the PPC to V1 results in an effect of transient enhancement of visual responses in V1. Two-photon calcium imaging demonstrated that the cortical disconnection caused V1 excitatory neurons an enhancement of visual responses and a reduction of orientation selectivity index (OSI). These results show that the PPC regulates the response properties of V1 excitatory neurons. Our findings reveal one of the functions of the PPC, which may contribute to higher brain functions in mice.

Asymmetric Direct Reciprocal Connections Between Primary Visual and Somatosensory Cortices of the Mouse

Several studies show direct connections between primary sensory cortices involved in multisensory integration. The purpose of this study is to understand the microcircuitry of the reciprocal connections between visual and somatosensory cortices. The laminar distribution of retrogradely labeled cell bodies in V1 and in the somatosensory cortex both in (S1BF) and outside (S1) the barrel field was studied to provide layer indices in order to determine whether the connections are of feedforward, feedback or lateral type. Single axons were reconstructed and the size of their swellings was stereologically sampled. The negative layer indices in S1 and S1BF and the layer index near zero in V1 indicate that the connection from S1BF to V1 is of feedback type while the opposite is of lateral type. The greater incidence of larger axonal swellings in the projection from V1 to S1BF strongly suggests that S1BF receives a stronger driver input from V1 and that S1BF inputs to V1 have a predominant modulatory influence.

Tomographic optical imaging of cortical responses after crossing nerve transfer in mice

To understand the neural mechanisms underlying the therapeutic effects of crossing nerve transfer for brachial plexus injuries in human patients, we investigated the cortical responses after crossing nerve transfer in mice using conventional and tomographic optical imaging. The distal cut ends of the left median and ulnar nerves were connected to the central cut ends of the right median and ulnar nerves with a sciatic nerve graft at 8 weeks of age. Eight weeks after the operation, the responses in the primary somatosensory cortex (S1) elicited by vibratory stimulation applied to the left forepaw were visualized based on activity-dependent flavoprotein fluorescence changes. In untreated mice, the cortical responses to left forepaw stimulation were mainly observed in the right S1. In mice with nerve crossing transfer, cortical responses to left forepaw stimulation were observed in the left S1 together with clear cortical responses in the right S1. We expected that the right S1 responses in the untreated mice were produced by thalamic inputs to layer IV, whereas those in the operated mice were mediated by callosal inputs from the left S1 to layer II/III of the right S1. To confirm this hypothesis, we performed tomographic imaging of flavoprotein fluorescence responses by macroconfocal microscopy. Flavoprotein fluorescence responses in layer IV were dominant compared to those in layer II/III in untreated mice. In contrast, responses in layer II/III were dominant compared to those in layer IV in operated mice. The peak latency of the cortical responses in the operated mice was longer than that in the untreated mice. These results confirmed our expectation that drastic reorganization in the cortical circuits was induced after crossing nerve transfer in mice.

Effect of temperature on FAD and NADH-derived signals and neurometabolic coupling in the mouse auditory and motor cortex

Tight coupling of neuronal metabolism to synaptic activity is critical to ensure that the supply of metabolic substrates meets the demands of neuronal signaling. Given the impact of temperature on metabolism, and the wide fluctuations of brain temperature observed during clinical hypothermia, we examined the effect of temperature on neurometabolic coupling. Intrinsic fluorescence signals of the oxidized form of flavin adenine dinucleotide (FAD) and the reduced form of nicotinamide adenine dinucleotide (NADH), and their ratios, were measured to assess neural metabolic state and local field potentials were recorded to measure synaptic activity in the mouse brain. Brain slice preparations were used to remove the potential impacts of blood flow. Tight coupling between metabolic signals and local field potential amplitudes was observed at a range of temperatures below 29 °C. However, above 29 °C, the metabolic and synaptic signatures diverged such that FAD signals were diminished, but local field potentials retained their amplitude. It was also observed that the declines in the FAD signals seen at high temperatures (and hence the decoupling between synaptic and metabolic events) are driven by low FAD availability at high temperatures. These data suggest that neurometabolic coupling, thought to be critical for ensuring the metabolic health of the brain, may show temperature dependence, and is related to temperature-dependent changes in FAD supplies.

Reconsidering Tonotopic Maps in the Auditory Cortex and Lemniscal Auditory Thalamus in Mice

The auditory thalamus and auditory cortex (AC) are pivotal structures in the central auditory system. However, the thalamocortical mechanisms of processing sounds are largely unknown. Investigation of this process benefits greatly from the use of mice because the mouse is a powerful animal model in which various experimental techniques, especially genetic tools, can be applied. However, the use of mice has been limited in auditory research, and thus even basic anatomical knowledge of the mouse central auditory system has not been sufficiently collected. Recently, optical imaging combined with morphological analyses has enabled the elucidation of detailed anatomical properties of the mouse auditory system. These techniques have uncovered fine AC maps with multiple frequency-organized regions, each of which receives point-to-point thalamocortical projections from different origins inside the lemniscal auditory thalamus, the ventral division of the medial geniculate body (MGv). This precise anatomy now provides a platform for physiological research. In this mini review article, we summarize these recent achievements that will facilitate physiological investigations in the mouse auditory system.

Large-scale imaging of cortical dynamics during sensory perception and behavior

Sensory-driven behaviors engage a cascade of cortical regions to process sensory input and generate motor output. To investigate the temporal dynamics of neural activity at this global scale, we have improved and integrated tools to perform functional imaging across large areas of cortex using a transgenic mouse expressing the genetically encoded calcium sensor GCaMP6s, together with a head-fixed visual discrimination behavior. This technique allows imaging of activity across the dorsal surface of cortex, with spatial resolution adequate to detect differential activity in local regions at least as small as 100 μm. Imaging during an orientation discrimination task reveals a progression of activity in different cortical regions associated with different phases of the task. After cortex-wide patterns of activity are determined, we demonstrate the ability to select a region that displayed conspicuous responses for two-photon microscopy and find that activity in populations of individual neurons in that region correlates with locomotion in trained mice. We expect that this paradigm will be a useful probe of information flow and network processing in brain-wide circuits involved in many sensory and cognitive processes.

Population Response Propagation to Extrastriate Areas Evoked by Intracortical Electrical Stimulation in V1

The mouse visual system has multiple extrastriate areas surrounding V1 each with a distinct representation of the visual field and unique functional and connectivity profiles, which are believed to form two parallel processing streams, similar to the ventral and dorsal streams in primates. At the same time, mouse visual areas have a high degree of interconnectivity, in particular V1 sends input to all higher visual areas. The study of these direct connections can further our understanding of the cortical processing of visual signals in the early mammalian cortex. Several studies have been published about the anatomy of these connections, but an in vivo electrophysiological characterization and comparison of the transmission to multiple extrastriate areas has not yet been reported. We used intracortical electrical stimulation combined with RH1691 VSD imaging in adult C57BL/6 mice in urethane anesthesia to analyze interareal transmission from V1 to extrastriate areas in superficial cortical layers. We found seven extrastriate response sites (five lateral, two medial) in a spatial pattern similar to area maps of the mouse visual cortex and, by shifting the location of V1 stimulation, demonstrated that the evoked responses in LM and AL were in accordance with the visuotopic mappings of these areas known from anatomy and in vivo studies. These two sites, considered to be gateways to their processing streams, had shorter latencies and faster transmission speeds than other extrastriate response sites. Short latency differences between response sites, and that TTX injection into LM reduced but did not eliminate other extrastriate responses indicated that the evoked cortical activity was, at least partially, transmitted directly from V1 to extrastriate areas. This study reports on analysis of interareal transmission from V1 to multiple extrastriate areas in mouse using intracortical electrical stimulation in vivo.

Wide-field Ca(2+) imaging reveals visually evoked activity in the retrosplenial area

Due to recent advances of genetic manipulation, mouse brain has become a useful model for studying brain function, which demands whole brain functional mapping techniques in the mouse brain. In the present study, to finely map visual responsive areas in the mouse brain, we combined high-resolution wide-field optical imaging with transgenic mice containing the genetically encoded Ca²⁺ indicator, GCaMP3. With the high signal amplitude of GCaMP3 expressing in excitatory neurons, this system allowed neural activity to be observed with relatively fine spatial resolution and cell-type specificity. To evaluate this system, we examined whether non-visual areas exhibited a visual response over the entire surface of the mouse hemisphere. We found that two association areas, the retrosplenial area (RS) and secondary motor/anterior cingulate area (M2/AC), were significantly responsive to drifting gratings. Examination using gratings with distinct spatiotemporal frequency parameters revealed that the RS strongly responded to high-spatial and low-temporal frequency gratings. The M2/AC exhibited a response property similar to that of the RS, though it was not statistically significant. Finally, we performed cellular imaging using two-photon microscopy to examine orientation and direction selectivity of individual neurons, and found that a minority of neurons in the RS clearly showed visual responses sharply selective for orientation and direction. These results suggest that neurons in RS encode visual information of fine spatial details in images. Thus, the present study shows the usefulness of the functional mapping method using a combination of wide-field and two-photon Ca²⁺ imaging, which allows for whole brain mapping with high spatiotemporal resolution and cell-type specificity.

Visual system plasticity in mammals: the story of monocular enucleation-induced vision loss

The groundbreaking work of Hubel and Wiesel in the 1960’s on ocular dominance plasticity instigated many studies of the visual system of mammals, enriching our understanding of how the development of its structure and function depends on high quality visual input through both eyes. These studies have mainly employed lid suturing, dark rearing and eye patching applied to different species to reduce or impair visual input, and have created extensive knowledge on binocular vision. However, not all aspects and types of plasticity in the visual cortex have been covered in full detail. In that regard, a more drastic deprivation method like enucleation, leading to complete vision loss appears useful as it has more widespread effects on the afferent visual pathway and even on non-visual brain regions. One-eyed vision due to monocular enucleation (ME) profoundly affects the contralateral retinorecipient subcortical and cortical structures thereby creating a powerful means to investigate cortical plasticity phenomena in which binocular competition has no vote.In this review, we will present current knowledge about the specific application of ME as an experimental tool to study visual and cross-modal brain plasticity and compare early postnatal stages up into adulthood. The structural and physiological consequences of this type of extensive sensory loss as documented and studied in several animal species and human patients will be discussed. We will summarize how ME studies have been instrumental to our current understanding of the differentiation of sensory systems and how the structure and function of cortical circuits in mammals are shaped in response to such an extensive alteration in experience. In conclusion, we will highlight future perspectives and the clinical relevance of adding ME to the list of more longstanding deprivation models in visual system research.

A novel behavioral paradigm to assess multisensory processing in mice

Human psychophysical and animal behavioral studies have illustrated the benefits that can be conferred from having information available from multiple senses. Given the central role of multisensory integration for perceptual and cognitive function, it is important to design behavioral paradigms for animal models to provide mechanistic insights into the neural bases of these multisensory processes. Prior studies have focused on large mammals, yet the mouse offers a host of advantages, most importantly the wealth of available genetic manipulations relevant to human disease. To begin to employ this model species for multisensory research it is necessary to first establish and validate a robust behavioral assay for the mouse. Two common mouse strains (C57BL/6J and 129S6/SvEv) were first trained to respond to unisensory (visual and auditory) stimuli separately. Once trained, performance with paired audiovisual stimuli was then examined with a focus on response accuracy and behavioral gain. Stimulus durations varied from 50 ms to 1 s in order to modulate the effectiveness of the stimuli and to determine if the well-established "principle of inverse effectiveness" held in this model. Response accuracy in the multisensory condition was greater than for either unisensory condition for all stimulus durations, with significant gains observed at the 300 ms and 100 ms durations. Main effects of stimulus duration, stimulus modality and a significant interaction between these factors were observed. The greatest behavioral gain was seen for the 100 ms duration condition, with a trend observed that as the stimuli became less effective, larger behavioral gains were observed upon their pairing (i.e., inverse effectiveness). These results are the first to validate the mouse as a species that shows demonstrable behavioral facilitations under multisensory conditions and provides a platform for future mechanistically directed studies to examine the neural bases of multisensory integration.

Visual cortical areas of the mouse: comparison of parcellation and network structure with primates

Brains have evolved to optimize sensory processing. In primates, complex cognitive tasks must be executed and evolution led to the development of large brains with many cortical areas. Rodents do not accomplish cognitive tasks of the same level of complexity as primates and remain with small brains both in relative and absolute terms. But is a small brain necessarily a simple brain? In this review, several aspects of the visual cortical networks have been compared between rodents and primates. The visual system has been used as a model to evaluate the level of complexity of the cortical circuits at the anatomical and functional levels. The evolutionary constraints are first presented in order to appreciate the rules for the development of the brain and its underlying circuits. The organization of sensory pathways, with their parallel and cross-modal circuits, is also examined. Other features of brain networks, often considered as imposing constraints on the development of underlying circuitry, are also discussed and their effect on the complexity of the mouse and primate brain are inspected. In this review, we discuss the common features of cortical circuits in mice and primates and see how these can be useful in understanding visual processing in these animals.