Effects of early binocular enucleation on auditory and somatosensory coding in the superior colliculus of the rat

Interactions between Guidance Cues and Neuronal Activity: Therapeutic Insights from Mouse Models

Topographic mapping of neural circuits is fundamental in shaping the structural and functional organization of brain regions. This developmentally important process is crucial not only for the representation of different sensory inputs but also for their integration. Disruption of topographic organization has been associated with several neurodevelopmental disorders. The aim of this review is to highlight the mechanisms involved in creating and refining such well-defined maps in the brain with a focus on the Eph and ephrin families of axon guidance cues. We first describe the transgenic models where ephrin-A expression has been manipulated to understand the role of these guidance cues in defining topography in various sensory systems. We further describe the behavioral consequences of lacking ephrin-A guidance cues in these animal models. These studies have given us unexpected insight into how neuronal activity is equally important in refining neural circuits in different brain regions. We conclude the review by discussing studies that have used treatments such as repetitive transcranial magnetic stimulation (rTMS) to manipulate activity in the brain to compensate for the lack of guidance cues in ephrin-knockout animal models. We describe how rTMS could have therapeutic relevance in neurodevelopmental disorders with disrupted brain organization.

Development of multisensory integration from the perspective of the individual neuron

The ability to use cues from multiple senses in concert is a fundamental aspect of brain function. It maximizes the brain’s use of the information available to it at any given moment and enhances the physiological salience of external events. Because each sense conveys a unique perspective of the external world, synthesizing information across senses affords computational benefits that cannot otherwise be achieved. Multisensory integration not only has substantial survival value but can also create unique experiences that emerge when signals from different sensory channels are bound together. However, neurons in a newborn’s brain are not capable of multisensory integration, and studies in the midbrain have shown that the development of this process is not predetermined. Rather, its emergence and maturation critically depend on cross-modal experiences that alter the underlying neural circuit in such a way that optimizes multisensory integrative capabilities for the environment in which the animal will function.

Alignment of multimodal sensory input in the superior colliculus through a gradient-matching mechanism

The superior colliculus (SC) is a midbrain structure that integrates visual, somatosensory, and auditory inputs to direct head and eye movements. Each of these modalities is topographically mapped and aligned with the others to ensure precise behavioral responses to multimodal stimuli. While it is clear that neural activity is instructive for topographic alignment of inputs from the visual cortex (V1) and auditory system with retinal axons in the SC, there is also evidence that activity-independent mechanisms are used to establish topographic alignment between modalities. Here, we show that the topography of the projection from primary somatosensory cortex (S1) to the SC is established during the first postnatal week. Unlike V1-SC projections, the S1-SC projection does not bifurcate when confronted with a duplicated retinocollicular map, showing that retinal input in the SC does not influence the topography of the S1-SC projection. However, S1-SC topography is disrupted in mice lacking ephrin-As, which we find are expressed in graded patterns along with their binding partners, the EphA4 and EphA7, in both S1 and the somatosensory recipient layer of the SC. Together, these data support a model in which somatosensory inputs into the SC map topographically and establish alignment with visual inputs in the SC using a gradient-matching mechanism.

Effects of vibrissectomy during early postnatal ontogenesis in rat pups on behavioral development

The effects of daily trimming of the whiskers in rat pups from day 2 or day 9 to day 20 on eye opening time and the formation of early behavioral reactions, i.e., walking, rearing, grooming, and manipulatory activity, were studied in rat pups subjected to deprivation and siblings reared with them. Rat pups subjected to vibrissectomy from day 2 of life and their siblings showed acceleration of eye opening, maturation of grooming, and rearing without support; the formation of manipulatory activity was delayed, as were changes in the structures of correlational relationships between the times at which behavioral reactions formed and eye opening occurred. Vibrissectomy from day 9 of life eliminated most correlational relationships in deprived animals without impairing the dynamics of the development of early behavioral reactions. Siblings showed delayed formation of manipulatory activity, though changes in the structure of correlational relationships were less significant than in deprived animals.

Somatosensory cross-modal plasticity in the superior colliculus of visually deafferented rats

The effects of neonatal visual deafferentation on the final adult pattern of cortico-collicular connections from the rat primary somatosensory cortex barrel field were studied by injecting an anterograde tracer (BDA) into different locations of the barrel cortex. Collicular afferents originating in the barrel cortex normally end in the intermediate collicular strata (SGI and SAI). However, neonatal visual deafferentation caused an invasion of abundant somatosensory cortical afferents into the lateral portions of the superficial collicular strata (SGS and SO). Moreover, anterograde-labelled fibers in the intermediate strata were more densely packed in visually deafferented animals. In order to study the activity of the altered somatosensory cortico-collicular connection, the effects of two different types of whisker stimuli on c-fos expression in the SC were analyzed (apomorphine treatment and enriched environment exploration). In stimulated control animals, c-fos expression was clearly evident in neurons of the intermediate layers 2 h after whisker stimulation. Similar stimulation in adult animals that underwent neonatal visual deafferentation triggered higher levels of c-fos expression in the superficial collicular layers that were invaded by cortico-collicular axonal branches. In exploration experiments, increased levels of c-fos expression were also detected in lateral parts of the intermediate layers of visually deafferented animals. These results suggest that the ascending fibers of somatosensory cortical origin can recruit deafferented superficial collicular neurons that enabling them to participate in extravisual behavioural responses mediated by collicular circuits.

Early visual deprivation alters multisensory processing in peripersonal space

Multisensory peripersonal space develops in a maturational process that is thought to be influenced by early sensory experience. We investigated the role of vision in the effective development of audiotactile interactions in peripersonal space. Early blind (EB), late blind (LB) and sighted control (SC) participants were asked to lateralize auditory, tactile and audiotactile stimuli. The experiment was conducted with the hands uncrossed or crossed over the body midline in order to alter the relationship between personal and peripersonal spatial representations. First, we observed that the crossed posture results in a greater detrimental effect for tactile performance in sighted subjects but a greater deficit in auditory performance in early blind ones. This result is interpreted as evidence for a visually driven developmental process that automatically remaps tactile and proprioceptive spatial representation into an external framework. Second, we demonstrate that improved reaction times observed in the bimodal conditions in SC and LB exceeds that predicted by probability summation in both conditions of postures, indicating neural integration of different sensory information. In EB, nonlinear summation was obtained in the uncrossed but not in the crossed posture. We argue that the default use of an anatomically anchored reference system in EB prevents effective audiotactile interactions in the crossed posture due to the poorly aligned spatial coordinates of these two modalities in such conditions. Altogether, these results provide compelling evidence for the critical role of early vision in the development of multisensory perception and action control in peripersonal space.

Visual deprivation modifies auditory directional tuning in the inferior colliculus

The aim of this study was to assess whether early visual deprivation could modulate the auditory directional tunings of single neurons in the central nucleus of the inferior colliculus of the rat. Extracellular recordings were carried out in normal and early bilaterally enucleated rats. Direction-specific auditory neurons were found in both groups, and no evidence was found for a topographical order of best azimuthal direction. Although the distribution of best azimuthal direction was unaltered in enucleated rats, our data suggest that early visual deprivation modifies the width of auditory directional receptive fields in the central nucleus of the inferior colliculus. This suggests that visual input plays a substantial role in refining auditory receptive fields in the inferior colliculus.