Auditory activation of cortical visual areas in cats after early visual deprivation

Gray matter volume of the feline cerebral cortex and structural plasticity following perinatal deafness

In response to sensory deprivation, the brain adapts according to contemporary demands to efficiently navigate a modified perceptual environment. This reorganization may result in improved processing of the remaining senses-a phenomenon referred to as compensatory crossmodal plasticity. One approach to explore this neuroplasticity is to consider the macrostructural changes in neural tissue that mirror this functional optimization. The current study is the first of its kind to measure MRI-derived gray matter (GM) volumes of control felines (n=30), while additionally identifying volumetric differences in response to perinatal deafness (30 ototoxically-deafened cats). To accomplish this purpose, regional and morphometric methods were performed in parallel. The regional analysis evaluated volumetric alterations of global GM, as well as the volumes of 146 regions of interest (ROIs) and 12 functional subgroupings of these ROIs. Results revealed whole-brain GM preservation; however, somatosensory and visual cortices exhibited an overall increase in volume. On a smaller scale, this analysis uncovered two auditory ROIs (second auditory cortex, A2, and ventral auditory field, VAF) that decreased in volume alongside two visual regions (anteromedial lateral suprasylvian area, AMLS and splenial visual area, SVA) that increased-all localized within the right hemisphere. Comparatively, the findings of tensor-based morphometry (TBM) generally aligned with those of the ROI-based method, as this voxel-wise approach demonstrated clusters of expansion coincident with visual- and somatosensory-related loci; although, it failed to detect any GM reductions following deafness. As distinct differences were identified in each analysis, the current study highlights the importance of employing multiple methods when exploring MRI volumetry. Overall, this study proposes that volumetric alterations within sensory loci allude to a redistribution of cortical space arising from modified perceptual demands following auditory deprivation.

Connectome alterations following perinatal deafness in the cat

Following sensory deprivation, areas and networks in the brain may adapt and reorganize to compensate for the loss of input. These adaptations are manifestations of compensatory crossmodal plasticity, which has been documented in both human and animal models of deafness-including the domestic cat. Although there are abundant examples of structural plasticity in deaf felines from retrograde tracer-based studies, there is a lack of diffusion-based knowledge involving this model compared to the current breadth of human research. The purpose of this study was to explore white matter structural adaptations in the perinatally-deafened cat via tractography, increasing the methodological overlap between species. Plasticity was examined by identifying unique group connections and assessing altered connectional strength throughout the entirety of the brain. Results revealed a largely preserved connectome containing a limited number of group-specific or altered connections focused within and between sensory networks, which is generally corroborated by deaf feline anatomical tracer literature. Furthermore, five hubs of cortical plasticity and altered communication following perinatal deafness were observed. The limited differences found in the present study suggest that deafness-induced crossmodal plasticity is largely built upon intrinsic structural connections, with limited remodeling of underlying white matter.

Brain Morphological Modifications in Congenital and Acquired Auditory Deprivation: A Systematic Review and Coordinate-Based Meta-Analysis

Neuroplasticity following deafness has been widely demonstrated in both humans and animals, but the anatomical substrate of these changes is not yet clear in human brain. However, it is of high importance since hearing loss is a growing problem due to aging population. Moreover, knowing these brain changes could help to understand some disappointing results with cochlear implant, and therefore could improve hearing rehabilitation. A systematic review and a coordinate-based meta-analysis were realized about the morphological brain changes highlighted by MRI in severe to profound hearing loss, congenital and acquired before or after language onset. 25 papers were included in our review, concerning more than 400 deaf subjects, most of them presenting prelingual deafness. The most consistent finding is a volumetric decrease in gray matter around bilateral auditory cortex. This change was confirmed by the coordinate-based meta-analysis which shows three converging clusters in this region. The visual areas of deaf children is also significantly impacted, with a decrease of the volume of both gray and white matters. Finally, deafness is responsible of a gray matter increase within the cerebellum, especially at the right side. These results are largely discussed and compared with those from deaf animal models and blind humans, which demonstrate for example a much more consistent gray matter decrease along their respective primary sensory pathway. In human deafness, a lot of other factors than deafness could interact on the brain plasticity. One of the most important is the use of sign language and its age of acquisition, which induce among others changes within the hand motor region and the visual cortex. But other confounding factors exist which have been too little considered in the current literature, such as the etiology of the hearing impairment, the speech-reading ability, the hearing aid use, the frequent associated vestibular dysfunction or neurocognitive impairment. Another important weakness highlighted by this review concern the lack of papers about postlingual deafness, whereas it represents most of the deaf population. Further studies are needed to better understand these issues, and finally try to improve deafness rehabilitation.

Aberrant development of excitatory circuits to inhibitory neurons in the primary visual cortex after neonatal binocular enucleation

The development of GABAergic interneurons is important for the functional maturation of cortical circuits. After migrating into the cortex, GABAergic interneurons start to receive glutamatergic connections from cortical excitatory neurons and thus gradually become integrated into cortical circuits. These glutamatergic connections are mediated by glutamate receptors including AMPA and NMDA receptors and the ratio of AMPA to NMDA receptors decreases during development. Since previous studies have shown that retinal input can regulate the early development of connections along the visual pathway, we investigated if the maturation of glutamatergic inputs to GABAergic interneurons in the visual cortex requires retinal input. We mapped the spatial pattern of glutamatergic connections to layer 4 (L4) GABAergic interneurons in mouse visual cortex at around postnatal day (P) 16 by laser-scanning photostimulation and investigated the effect of binocular enucleations at P1/P2 on these patterns. Gad2-positive interneurons in enucleated animals showed an increased fraction of AMPAR-mediated input from L2/3 and a decreased fraction of input from L5/6. Parvalbumin-expressing (PV) interneurons showed similar changes in relative connectivity. NMDAR-only input was largely unchanged by enucleation. Our results show that retinal input sculpts the integration of interneurons into V1 circuits and suggest that the development of AMPAR- and NMDAR-only connections might be regulated differently.

Task-specific reorganization of the auditory cortex in deaf humans

The principles that guide large-scale cortical reorganization remain unclear. In the blind, several visual regions preserve their task specificity; ventral visual areas, for example, become engaged in auditory and tactile object-recognition tasks. It remains open whether task-specific reorganization is unique to the visual cortex or, alternatively, whether this kind of plasticity is a general principle applying to other cortical areas. Auditory areas can become recruited for visual and tactile input in the deaf. Although nonhuman data suggest that this reorganization might be task specific, human evidence has been lacking. Here we enrolled 15 deaf and 15 hearing adults into an functional MRI experiment during which they discriminated between temporally complex sequences of stimuli (rhythms). Both deaf and hearing subjects performed the task visually, in the central visual field. In addition, hearing subjects performed the same task in the auditory modality. We found that the visual task robustly activated the auditory cortex in deaf subjects, peaking in the posterior-lateral part of high-level auditory areas. This activation pattern was strikingly similar to the pattern found in hearing subjects performing the auditory version of the task. Although performing the visual task in deaf subjects induced an increase in functional connectivity between the auditory cortex and the dorsal visual cortex, no such effect was found in hearing subjects. We conclude that in deaf humans the high-level auditory cortex switches its input modality from sound to vision but preserves its task-specific activation pattern independent of input modality. Task-specific reorganization thus might be a general principle that guides cortical plasticity in the brain.

Congenital Anophthalmia and Binocular Neonatal Enucleation Differently Affect the Proteome of Primary and Secondary Visual Cortices in Mice

In blind individuals, visually deprived occipital areas are activated by non-visual stimuli. The extent of this cross-modal activation depends on the age at onset of blindness. Cross-modal inputs have access to several anatomical pathways to reactivate deprived visual areas. Ectopic cross-modal subcortical connections have been shown in anophthalmic animals but not in animals deprived of sight at a later age. Direct and indirect cross-modal cortical connections toward visual areas could also be involved, yet the number of neurons implicated is similar between blind mice and sighted controls. Changes at the axon terminal, dendritic spine or synaptic level are therefore expected upon loss of visual inputs. Here, the proteome of V1, V2M and V2L from P0-enucleated, anophthalmic and sighted mice, sharing a common genetic background (C57BL/6J x ZRDCT/An), was investigated by 2-D DIGE and Western analyses to identify molecular adaptations to enucleation and/or anophthalmia. Few proteins were differentially expressed in enucleated or anophthalmic mice in comparison to sighted mice. The loss of sight affected three pathways: metabolism, synaptic transmission and morphogenesis. Most changes were detected in V1, followed by V2M. Overall, cross-modal adaptations could be promoted in both models of early blindness but not through the exact same molecular strategy. A lower metabolic activity observed in visual areas of blind mice suggests that even if cross-modal inputs reactivate visual areas, they could remain suboptimally processed.

Cross-modal plasticity in sensory deprived animal models: From the thalamocortical development point of view

Over recent decades, our understanding of the plasticity of the central nervous system has expanded enormously. Accordingly, it is now widely accepted that the brain can adapt to changes by reorganizing its circuitry, both in response to external stimuli and experience, as well as through intrinsic mechanisms. A clear example of this is the activation of a deprived sensory area and the expansion of spared sensory cortical regions in individuals who suffered peripheral sensory loss. Despite the efforts to understand these neuroplastic changes, the mechanisms underlying such adaptive remodeling remains poorly understood. Progress in understanding these events may be hindered by the highly varied data obtained from the distinct experimental paradigms analyzed, which include different animal models and neuronal systems, as well as studies into the onset of sensory loss. Here, we will establish the current state-of-the-art describing the principal observations made according to the time of sensory deprivation with respect to the development of the thalamocortical connectivity. We will review the experimental data obtained from animal models where sensory deprivation has been induced either before or after thalamocortical axons reach and invade their target cortical areas. The anatomical and functional effects of sensory loss on the primary sensory areas of the cortex will be presented. Indeed, we consider that the comparative approach of this review is a necessary step in order to help deciphering the processes that underlie sensory neuroplasticity, for which studies in animal models have been indispensable. Understanding these mechanisms will then help to develop restorative strategies and prostheses that will overcome the functional loss.

Rapid Changes in Cortical and Subcortical Brain Regions after Early Bilateral Enucleation in the Mouse

Functional sensory and motor areas in the developing mammalian neocortex are formed through a complex interaction of cortically intrinsic mechanisms, such as gene expression, and cortically extrinsic mechanisms such as those mediated by thalamic input from the senses. Both intrinsic and extrinsic mechanisms are believed to be involved in cortical patterning and the establishment of areal boundaries in early development; however, the nature of the interaction between intrinsic and extrinsic processes is not well understood. In a previous study, we used a perinatal bilateral enucleation mouse model to test some aspects of this interaction by reweighting sensory input to the developing cortex. Visual deprivation at birth resulted in a shift of intraneocortical connections (INCs) that aligned with ectopic ephrin A5 expression in the same location ten days later at postnatal day (P) 10. A prevailing question remained: Does visual deprivation first induce a change in gene expression, followed by a shift in INCs, or vice versa? In the present study, we address this question by investigating the neuroanatomy and patterns of gene expression in post-natal day (P) 1 and 4 mice following bilateral enucleation at birth. Our results demonstrate a rapid reduction in dorsal lateral geniculate nucleus (dLGN) size and ephrin A5 gene expression 24-hours post-enucleation, with more profound effects apparent at P4. The reduced nuclear size and diminished gene expression mirrors subtle changes in ephrin A5 expression evident in P1 and P4 enucleated neocortex, 11 and 8 days prior to natural eye opening, respectively. Somatosensory and visual INCs were indistinguishable between P1 and P4 mice bilaterally enucleated at birth, indicating that perinatal bilateral enucleation initiates a rapid change in gene expression (within one day) followed by an alteration of sensory INCs later on (second postnatal week). With these results, we gain a deeper understanding of how gene expression and sensory input together regulate cortical arealization and plasticity during early development.

Strain differences of the effect of enucleation and anophthalmia on the size and growth of sensory cortices in mice

Anophthalmia is a condition in which the eye does not develop from the early embryonic period. Early blindness induces cross-modal plastic modifications in the brain such as auditory and haptic activations of the visual cortex and also leads to a greater solicitation of the somatosensory and auditory cortices. The visual cortex is activated by auditory stimuli in anophthalmic mice and activity is known to alter the growth pattern of the cerebral cortex. The size of the primary visual, auditory and somatosensory cortices and of the corresponding specific sensory thalamic nuclei were measured in intact and enucleated C57Bl/6J mice and in ZRDCT anophthalmic mice (ZRDCT/An) to evaluate the contribution of cross-modal activity on the growth of the cerebral cortex. In addition, the size of these structures were compared in intact, enucleated and anophthalmic fourth generation backcrossed hybrid C57Bl/6J×ZRDCT/An mice to parse out the effects of mouse strains and of the different visual deprivations. The visual cortex was smaller in the anophthalmic ZRDCT/An than in the intact and enucleated C57Bl/6J mice. Also the auditory cortex was larger and the somatosensory cortex smaller in the ZRDCT/An than in the intact and enucleated C57Bl/6J mice. The size differences of sensory cortices between the enucleated and anophthalmic mice were no longer present in the hybrid mice, showing specific genetic differences between C57Bl/6J and ZRDCT mice. The post natal size increase of the visual cortex was less in the enucleated than in the anophthalmic and intact hybrid mice. This suggests differences in the activity of the visual cortex between enucleated and anophthalmic mice and that early in-utero spontaneous neural activity in the visual system contributes to the shaping of functional properties of cortical networks.

Linking neocortical, cognitive, and genetic variability in autism with alterations of brain plasticity: the Trigger-Threshold-Target model

The phenotype of autism involves heterogeneous adaptive traits (strengths vs. disabilities), different domains of alterations (social vs. non-social), and various associated genetic conditions (syndromic vs. nonsyndromic autism). Three observations suggest that alterations in experience-dependent plasticity are an etiological factor in autism: (1) the main cognitive domains enhanced in autism are controlled by the most plastic cortical brain regions, the multimodal association cortices; (2) autism and sensory deprivation share several features of cortical and functional reorganization; and (3) genetic mutations and/or environmental insults involved in autism all appear to affect developmental synaptic plasticity, and mostly lead to its upregulation. We present the Trigger-Threshold-Target (TTT) model of autism to organize these findings. In this model, genetic mutations trigger brain reorganization in individuals with a low plasticity threshold, mostly within regions sensitive to cortical reallocations. These changes account for the cognitive enhancements and reduced social expertise associated with autism. Enhanced but normal plasticity may underlie non-syndromic autism, whereas syndromic autism may occur when a triggering mutation or event produces an altered plastic reaction, also resulting in intellectual disability and dysmorphism in addition to autism. Differences in the target of brain reorganization (perceptual vs. language regions) account for the main autistic subgroups. In light of this model, future research should investigate how individual and sex-related differences in synaptic/regional brain plasticity influence the occurrence of autism.

Revisiting the adaptive and maladaptive effects of crossmodal plasticity

One of the most striking demonstrations of experience-dependent plasticity comes from studies of sensory-deprived individuals (e.g., blind or deaf), showing that brain regions deprived of their natural inputs change their sensory tuning to support the processing of inputs coming from the spared senses. These mechanisms of crossmodal plasticity have been traditionally conceptualized as having a double-edged sword effect on behavior. On one side, crossmodal plasticity is conceived as adaptive for the development of enhanced behavioral skills in the remaining senses of early-deaf or blind individuals. On the other side, crossmodal plasticity raises crucial challenges for sensory restoration and is typically conceived as maladaptive since its presence may prevent optimal recovery in sensory-re-afferented individuals. In the present review we stress that this dichotomic vision is oversimplified and we emphasize that the notions of the unavoidable adaptive/maladaptive effects of crossmodal reorganization for sensory compensation/restoration may actually be misleading. For this purpose we critically review the findings from the blind and deaf literatures, highlighting the complementary nature of these two fields of research. The integrated framework we propose here has the potential to impact on the way rehabilitation programs for sensory recovery are carried out, with the promising prospect of eventually improving their final outcomes.

Functional and structural changes throughout the auditory system following congenital and early-onset deafness: implications for hearing restoration

The absence of auditory input, particularly during development, causes widespread changes in the structure and function of the auditory system, extending from peripheral structures into auditory cortex. In humans, the consequences of these changes are far-reaching and often include detriments to language acquisition, and associated psychosocial issues. Much of what is currently known about the nature of deafness-related changes to auditory structures comes from studies of congenitally deaf or early-deafened animal models. Fortunately, the mammalian auditory system shows a high degree of preservation among species, allowing for generalization from these models to the human auditory system. This review begins with a comparison of common methods used to obtain deaf animal models, highlighting the specific advantages and anatomical consequences of each. Some consideration is also given to the effectiveness of methods used to measure hearing loss during and following deafening procedures. The structural and functional consequences of congenital and early-onset deafness have been examined across a variety of mammals. This review attempts to summarize these changes, which often involve alteration of hair cells and supporting cells in the cochleae, and anatomical and physiological changes that extend through subcortical structures and into cortex. The nature of these changes is discussed, and the impacts to neural processing are addressed. Finally, long-term changes in cortical structures are discussed, with a focus on the presence or absence of cross-modal plasticity. In addition to being of interest to our understanding of multisensory processing, these changes also have important implications for the use of assistive devices such as cochlear implants.

Sensitive and critical periods in visual sensory deprivation

While the demonstration of crossmodal plasticity is well established in congenital and early blind individuals, great debate still surrounds whether those who acquire blindness later in life can also benefit from such compensatory changes. No proper consensus has been reached despite the fact that a proper understanding of the developmental time course of these changes, and whether their occurrence is limited to—or within—specific time windows, is crucial to our understanding of the crossmodal phenomena. An extensive review of the literature reveals that while the majority of investigations to date have examined the crossmodal plasticity available to late blind individuals in quantitative terms, recent findings rather suggest that this reorganization also likely changes qualitatively compared to what is observed in early blindness. This obviously could have significant repercussions not only for the training and rehabilitation of blind individuals, but for the development of appropriate neuroprostheses designed to aid and potentially restore vision. Important parallels will also be drawn with the current state of research on deafness, which is particularly relevant given in the development of successful neuroprostheses (e.g., cochlear implants) for providing auditory input to the central nervous system otherwise aurally deafferented. Lastly, this paper will address important inconsistencies across the literature concerning the definition of distinct blind groups based on the age of blindness onset, and propose several alternatives to using such a categorization.

Compensatory plasticity and cross-modal reorganization following early visual deprivation

For human and non-human primates, vision is one of the most privileged sensory channels used to interact with the environment. The importance of vision is strongly embedded in the organization of the primate brain as about one third of its cortical surface is involved in visual functions. It is therefore not surprising that the absence of vision from birth, or the loss of vision later in life, has huge consequences, both anatomically and functionally. Studies in animals and humans, conducted over the past few decades, have demonstrated that the absence of vision causes massive structural changes that take place not only in the visually deprived cortex but also in other brain areas. These studies have further shown that the visually deprived cortex becomes responsive to a wide variety of non-visual sensory inputs. Recent studies even showed a role of the visually deprived cortex in cognitive processes. At the behavioral level, increases in acuity for auditory and tactile processes have been reported. The study of the congenitally blind brain also offers a unique model to gain better insights into the functioning of the normal sighted brain and to understand to what extent visual experience is necessary for the brain to develop its functional architecture. Finally, the study of the blind brain allows us to investigate how consciousness develops in the absence of vision. How does the brain of someone who has never had any visual perception form an image of the external world? In this paper, we discuss recent findings from animal studies as well as from behavioural and functional brain imaging studies in sighted and blind individuals that address these questions.

Impact of blindness onset on the functional organization and the connectivity of the occipital cortex

Contrasting the impact of congenital versus late-onset acquired blindness provides a unique model to probe how experience at different developmental periods shapes the functional organization of the occipital cortex. We used functional magnetic resonance imaging to characterize brain activations of congenitally blind, late-onset blind and two groups of sighted control individuals while they processed either the pitch or the spatial attributes of sounds. Whereas both blind groups recruited occipital regions for sound processing, activity in bilateral cuneus was only apparent in the congenitally blind, highlighting the existence of region-specific critical periods for crossmodal plasticity. Most importantly, the preferential activation of the right dorsal stream (middle occipital gyrus and cuneus) for the spatial processing of sounds was only observed in the congenitally blind. This demonstrates that vision has to be lost during an early sensitive period in order to transfer its functional specialization for space processing toward a non-visual modality. We then used a combination of dynamic causal modelling with Bayesian model selection to demonstrate that auditory-driven activity in primary visual cortex is better explained by direct connections with primary auditory cortex in the congenitally blind whereas it relies more on feedback inputs from parietal regions in the late-onset blind group. Taken together, these results demonstrate the crucial role of the developmental period of visual deprivation in (re)shaping the functional architecture and the connectivity of the occipital cortex. Such findings are clinically important now that a growing number of medical interventions may restore vision after a period of visual deprivation.

Cortical GABAergic interneurons in cross-modal plasticity following early blindness

Early loss of a given sensory input in mammals causes anatomical and functional modifications in the brain via a process called cross-modal plasticity. In the past four decades, several animal models have illuminated our understanding of the biological substrates involved in cross-modal plasticity. Progressively, studies are now starting to emphasise on cell-specific mechanisms that may be responsible for this intermodal sensory plasticity. Inhibitory interneurons expressing γ-aminobutyric acid (GABA) play an important role in maintaining the appropriate dynamic range of cortical excitation, in critical periods of developmental plasticity, in receptive field refinement, and in treatment of sensory information reaching the cerebral cortex. The diverse interneuron population is very sensitive to sensory experience during development. GABAergic neurons are therefore well suited to act as a gate for mediating cross-modal plasticity. This paper attempts to highlight the links between early sensory deprivation, cortical GABAergic interneuron alterations, and cross-modal plasticity, discuss its implications, and further provide insights for future research in the field.

Indirect pathway between the primary auditory and visual cortices through layer V pyramidal neurons in V2L in mouse and the effects of bilateral enucleation

Visual cortical areas are activated by auditory stimuli in blind mice. Direct heteromodal cortical connections have been shown between the primary auditory cortex (A1) and primary visual cortex (V1), and between A1 and secondary visual cortex (V2). Auditory afferents to V2 terminate in close proximity to neurons that project to V1, and potentially constitute an effective indirect pathway between A1 and V1. In this study, we injected a retrograde adenoviral vector that expresses enhanced green fluorescent protein under a synapsin promotor in V1 and biotinylated dextran amine as an anterograde tracer in A1 to determine: (i) whether A1 axon terminals establish synaptic contacts onto the lateral part of V2 (V2L) neurons that project to V1; and (ii) if this indirect cortical pathway is altered by a neonatal enucleation in mice. Complete dendritic arbors of layer V pyramidal neurons were reconstructed in 3D, and putative contacts between pre-synaptic auditory inputs and postsynaptic visual neurons were analysed using a laser-scanning confocal microscope. Putative synaptic contacts were classified as high-confidence and low-confidence contacts, and charted onto dendritic trees. As all reconstructed layer V pyramidal neurons received auditory inputs by these criteria, we conclude that V2L acts as an important relay between A1 and V1. Auditory inputs are preferentially located onto lower branch order dendrites in enucleated mice. Also, V2L neurons are subject to morphological reorganizations in both apical and basal dendrites after the loss of vision. The A1-V2L-V1 pathway could be involved in multisensory processing and contribute to the auditory activation of the occipital cortex in the blind rodent.

Brain development: critical periods for cross-sensory plasticity

Recent work has shown that visual deprivation of humans during a critical period leads to motion area MT+ responding to auditory motion. This cross-sensory plasticity, an important form of brain reorganization, may be mediated by top-down brain circuits from pre-frontal cortex.

What are the Effects of Severe Visual Impairment on the Cortical Organization and Connectivity of Primary Visual Cortex?

The organization and connections of the primary visual area (V1) were examined in mice that lacked functional rods (Gnat−/−), but had normal cone function. Because mice are nocturnal and rely almost exclusively on rod vision for normal behaviors, the Gnat−/− mice used in the present study are considered functionally blind. Our goal was to determine if visual cortex is reorganized in these mice, and to examine the neuroanatomical connections that may subserve reorganization. We found that most neurons in V1 responded to auditory, or some combination of auditory, somatosensory, and/or visual stimulation. We also determined that cortical connections of V1 in Gnat−/− mice were similar to those in normal animals, but even in normal animals, there is sparse input from auditory cortex (AC) to V1. An important observation was that most of the subcortical inputs to V1 were from thalamic nuclei that normally project to V1 such as the lateral geniculate (LG), lateral posterior (LP), and lateral dorsal (LD) nuclei. However, V1 also received some abnormal subcortical inputs from the anterior thalamic nuclei, the ventral posterior, the ventral lateral and the posterior nuclei. While the vision generated from the small number of cones appears to be sufficient to maintain most of the patterns of normal connectivity, the sparse abnormal thalamic inputs to VI, existing inputs from AC, and possibly abnormal inputs to LG and LP may be responsible for generating the alterations in the functional organization of V1.

Analysis of c-fos and zif268 expression reveals time-dependent changes in activity inside and outside the lesion projection zone in adult cat area 17 after retinal lesions

Retinal lesions induce a topographic reorganization in the corresponding lesion projection zone (LPZ) in the visual cortex of adult cats. To gain a better insight into the reactivation dynamics, we investigated the alterations in cortical activity throughout area 17. We implemented in situ hybridization and real-time polymerase chain reaction to analyze the spatiotemporal expression patterns of the activity marker genes zif268 and c-fos. The immediate early gene (IEG) data confirmed a strong and permanent activity decrease in the center of the LPZ as previously described by electrophysiology. A recovery of IEG expression was clearly measured in the border of the LPZ. We were able to register reorganization over 2.5-6 mm. We also present evidence that the central retinal lesions concomitantly influence the activity in far peripheral parts of area 17. Its IEG expression levels appeared dependent of time and distance from the LPZ. We therefore propose that coupled changes in activity occur inside and outside the LPZ. In conclusion, alterations in activity reporter gene expression throughout area 17 contribute to the lesion-induced functional reorganization.

Adult deafness induces somatosensory conversion of ferret auditory cortex

In response to early or developmental lesions, responsiveness of sensory cortex can be converted from the deprived modality to that of the remaining sensory systems. However, little is known about capacity of the adult cortex for cross-modal reorganization. The present study examined the auditory cortices of animals deafened as adults, and observed an extensive somatosensory conversion within as little as 16 days after deafening. These results demonstrate that cortical cross-modal reorganization can occur after the period of sensory system maturation.

Responses of inferior collicular cells to species-specific vocalizations in normal and enucleated rats

The inferior colliculus (IC) is an obligatory relay for the ascending and descending auditory pathways. Cells in this brainstem structure not only analyze auditory stimuli but they also play a major role in multi-modal integration of auditory and visual information. The aim of the present study was to determine whether cells in the central nucleus of the inferior colliculus (CNIC) of normal rats respond selectively to complex auditory signals, such as species-specific vocalizations, and compare their responses to those obtained in neonatal bilateral enucleated (P2-P3) adult rats. Extra-cellular recordings were carried out in anesthetized normal and enucleated rats using auditory stimuli (pure tones, broadband noise and vocalizations) presented in free field in a semi-anechoic chamber. The results indicate that most cells in the CNIC of both groups respond selectively to species-specific vocalizations better than to the same but inverted sounds. No significant differences were found between the normal and enucleated rat groups in their responses to broadband noise and pure tones.

Visual deprivation alters the development of cortical multisensory integration

It has recently been demonstrated that the maturation of normal multisensory circuits in the cortex of the cat takes place over an extended period of postnatal life. Such a finding suggests that the sensory experiences received during this time may play an important role in this developmental process. To test the necessity of sensory experience for normal cortical multisensory development, cats were raised in the absence of visual experience from birth until adulthood, effectively precluding all visual and visual-nonvisual multisensory experiences. As adults, semichronic single-unit recording experiments targeting the anterior ectosylvian sulcus (AES), a well-defined multisensory cortical area in the cat, were initiated and continued at weekly intervals in anesthetized animals. Despite having very little impact on the overall sensory representations in AES, dark-rearing had a substantial impact on the integrative capabilities of multisensory AES neurons. A significant increase was seen in the proportion of multisensory neurons that were modulated by, rather than driven by, a second sensory modality. More important, perhaps, there was a dramatic shift in the percentage of these modulated neurons in which the pairing of weakly effective and spatially and temporally coincident stimuli resulted in response depressions. In normally reared animals such combinations typically give rise to robust response enhancements. These results illustrate the important role sensory experience plays in shaping the development of mature multisensory cortical circuits and suggest that dark-rearing shifts the relative balance of excitation and inhibition in these circuits.

Auditory responses in the visual cortex of neonatally enucleated rats

A number of studies on humans and animals have demonstrated better auditory abilities in blind with respect to sighted subjects and have tried to define the mechanisms through which this compensation occurs. The aim of the present study, therefore, was to examine the participation of primary visual cortex (V1) to auditory processing in early enucleated rats. Here we show, using gaussian noise bursts, that about a third of the cells in V1 responded to auditory stimulation in blind rats and most of these (78%) had ON-type responses and low spontaneous activity. Moreover, they were distributed throughout visual cortex without any apparent tonotopic organization. Optimal frequencies determined using pure tones were rather high but comparable to those found in auditory cortex of blind and sighted rats. On the other hand, sensory thresholds determined at these frequencies were higher and bandwidths were wider in V1 of the blind animals. Blind and sighted rats were also stimulated for 60 min with gaussian noise, their brains removed and processed for c-Fos immunohistochemistry. Results revealed that c-Fos positive cells were not only present in auditory cortex of both groups of rats but there was a 10-fold increase in labeled cells in V1 and a fivefold increase in secondary visual cortex (V2) of early enucleated rats in comparisons to sighted ones. Also, the pattern of distribution of these labeled cells across layers suggests that the recruitment of V1 could originate at least in part through inputs arising from the thalamus. The ensemble of results appears to indicate that cross-modal compensation leading to improved performance in the blind depends on cell recruitment in V1 but probably also plastic changes in lower- and higher-order visual structures and possibly in the auditory system.

Early blindness results in abnormal corticocortical and thalamocortical connections

Studies in congenitally blind and bilaterally enucleated individuals show that an early loss of sensory driven activity can lead to massive functional reorganization. However, the anatomical substrate for this functional reorganization is unknown. In the present study, we examined patterns of corticocortical and thalamocortical connections in adult opossums that had been bilaterally enucleated neonatally, prior to the formation of retinogeniculate and geniculocortical connections. We show that in addition to normal thalamocortical projection patterns from visual nuclei, enucleated animals also receive input from nuclei associated with the somatosensory (ventral posterior nucleus, VP), auditory (medial geniculate nucleus, MGN), motor (ventrolateral nucleus, VL), and limbic/hippocampal systems (anterior dorsal nucleus, AD; and anterior ventral nucleus, AV). Likewise, in addition to normal corticocortical projections to area 17, bilaterally enucleated opossums also receive input from auditory, somatosensory, and multimodal cortex. These aberrant patterns of thalamocortical and corticocortical connections can account for alterations in functional organization observed in the visual cortex of bilateral enucleated animals, and indicate that factors extrinsic to the cortex play a large role in cortical field development and evolution. On the other hand, the maintenance of normal patterns of connections in the absence of visual input suggests that there are formidable constraints imposed on the developing cortex that highly restrict the types of evolutionary change possible.

Crossmodal audio-visual interactions in the primary visual cortex of the visually deprived cat: a physiological and anatomical study

Blind individuals often demonstrate enhanced non-visual perceptual abilities. Neuroimaging and transcranial magnetic stimulation experiments have suggested that computations carried out in the occipital cortex may underlie these enhanced somatosensory or auditory performances. Thus, cortical areas that are dedicated to the analysis of the visual scene may, in the blind, acquire the capacity to participate in other sensory processing. However, the neural substrate that underlies this transfer of function is not fully characterized. Here we studied the synaptic and anatomical basis of this phenomenon in cats that were visually deprived by dark rearing, either early visually deprived after birth (EVD), or late visually deprived after the end of the critical period (LVD); data were compared with those obtained in normally reared cats (controls). The presence of synaptic and spike responses to auditory stimulation was examined by means of intracellular recordings in area 17 and the border between areas 17 and 18. While none of the cells recorded in control and LVD cats showed responses to sound, 14% of the cells recorded in EVD cats showed both subthreshold synaptic responses and suprathreshold spike responses to auditory stimuli. Synaptic responses were of small amplitude, but well time-locked to the stimuli and had an average latency of 30+/-12ms. In an attempt to identify the origin of the inputs carrying auditory information to the visual cortex, wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was injected in the visual cortex and retrograde labeling examined in the cortex and thalamus. No significant retrograde labeling was found in auditory cortical areas. However, the proportion of neurons projecting from supragranular layers of the posteromedial and posterolateral parts of the lateral suprasylvian region to V1 was higher than that in control cats. Retrograde labeling in the lateral geniculate nucleus showed no difference in the total number of neurons between control and visually deprived cats, but there was a higher proportion of labeling in C-laminae in deprived cats. Labeled cells were not found in the medial geniculate nucleus, a thalamic relay for auditory information, in either control or visually deprived cats. Finally, immunohistochemistry of the visual cortex of deprived cats revealed a striking decrease in pavalbumin- and calretinin-positive neurons, the functional implications of which we discuss.

Enhanced sensitivity to echo cues in blind subjects

Many studies have reported that blind people compensate for their visual deficit by sharpening auditory processes. Here we compare the sensitivity to echo cues between blind and sighted subjects. In the first experiment, the blind subjects were more accurate than the sighted subjects in localizing an object on the basis of echo cues. To ensure that enhanced echolocalization abilities were not only due to the fact that blind individuals are more used to consciously paying attention to echo cues and are more familiar with this kind of tasks than sighted subjects, we tested both groups of subjects in a simple azimuthal localization task of auditory stimuli. In this second experiment, we evaluated the influence of irrelevant echo signals on auditory localization by placing the subjects and the sound sources at different positions in a sound reverberant room. Results revealed that blind subjects exhibit a higher sensitivity to echo signals than sighted subjects.

Thermal comfort: multisensory interactions?

Experimentally, the effects of environmental conditions upon human capabilities have been studied most often through the imposition of a single stressor in isolation. Although it seems to be a common belief that thermal comfort can be influenced by concomitant stimulation of non-tactile sensorial modalities, few studies have succeeded in delineating non-tactile stimulations, which interact with thermal sensation and thermal comfort. Here we briefly overview neurophysiological and behavioural findings in multisensory influences on thermal sensation and thermal comfort.

Involvement of the superior temporal cortex and the occipital cortex in spatial hearing: evidence from repetitive transcranial magnetic stimulation

The processing of auditory spatial information in cortical areas of the human brain outside of the primary auditory cortex remains poorly understood. Here we investigated the role of the superior temporal gyrus (STG) and the occipital cortex (OC) in spatial hearing using repetitive transcranial magnetic stimulation (rTMS). The right STG is known to be of crucial importance for visual spatial awareness, and has been suggested to be involved in auditory spatial perception. We found that rTMS of the right STG induced a systematic error in the perception of interaural time differences (a primary cue for sound localization in the azimuthal plane). This is in accordance with the recent view, based on both neurophysiological data obtained in monkeys and human neuroimaging studies, that information on sound location is processed within a dorsolateral "where" stream including the caudal STG. A similar, but opposite, auditory shift was obtained after rTMS of secondary visual areas of the right OC. Processing of auditory information in the OC has previously been shown to exist only in blind persons. Thus, the latter finding provides the first evidence of an involvement of the visual cortex in spatial hearing in sighted human subjects, and suggests a close interconnection of the neural representation of auditory and visual space. Because rTMS induced systematic shifts in auditory lateralization, but not a general deterioration, we propose that rTMS of STG or OC specifically affected neuronal circuits transforming auditory spatial coordinates in order to maintain alignment with vision.

Vertical sound localization in blind humans

It is widely held that early-blind people compensate their visual loss by a general sharpening of spatial hearing. The present study reports a possible exception to this view: when the vertical position (elevation) of a sound source had to be localized, four out of six early-blind subjects exhibited systematic deviations in pointing, while two early-blind subjects were as accurate as sighted controls. On the other hand, blind and sighted individuals were able to judge relative positions of different sound locations with similar precision. These results suggest that visual experience may be used to accurately calibrate the relation between the vertical coordinates of auditory space and body, but is not needed to develop sufficiently high resolution of spatial hearing.

Speech processing activates visual cortex in congenitally blind humans

Neurophysiological recordings and neuroimaging data in blind and deaf animals and humans suggest that perceptual functions may be organized differently after sensory deprivation. It has been argued that neural plasticity contributes to compensatory performance in blind humans, such as faster speech processing. The present study employed functional magnetic resonance imaging (fMRI) to map language-related brain activity in congenitally blind adults. Participants listened to sentences, with either an easy or a more difficult syntactic structure, which were either semantically meaningful or meaningless. Results show that blind adults not only activate classical left-hemispheric perisylvian language areas during speech comprehension, as did a group of sighted adults, but that they additionally display an activation in the homologueous right-hemispheric structures and in extrastriate and striate cortex. Both the perisylvian and occipital activity varied as a function of syntactic difficulty and semantic content. The results demonstrate that the cerebral organization of complex cognitive systems such as the language system is significantly shaped by the input available.

Auditory activation of "visual" cortical areas in the blind mole rat (Spalax ehrenbergi)

The mole rat (Spalax ehrenbergi) is a subterranean rodent whose adaptations to its fossorial life include an extremely reduced peripheral visual system and an auditory system suited for the perception of vibratory stimuli. We have previously shown that in this blind rodent the dorsal lateral geniculate nucleus, the primary visual thalamic nucleus of sighted mammals, is activated by auditory stimuli. In this report we focus on the manifestation of this cross-modal compensation at the cortical level. Cyto- and myeloarchitectural analyses of the occipital area showed that despite the almost total blindness of the mole rat this area has retained the organization of a typical mammalian primary visual cortex. Application of the metabolic marker 2-deoxyglucose and electrophysiological recording of evoked field potentials and single-unit activity disclosed that a considerable part of this area is activated by auditory stimuli. Previous neuronal tracing studies had revealed the origin of the bulk of this auditory input to be the dorsal lateral geniculate nucleus which itself receives auditory input from the inferior colliculus.

Plastic changes in the central auditory system after hearing loss, restoration of function, and during learning

Traditionally the auditory system was considered a hard-wired sensory system; this view has been challenged in recent years in light of the plasticity of other sensory systems, particularly the visual and somatosensory systems. Practical experience in clinical audiology together with the use of prosthetic devices, such as cochlear implants, contributed significantly to the present view on the plasticity of the central auditory system, which was originally based on data obtained in animal experiments. The loss of auditory receptors, the hair cells, results in profound changes in the structure and function of the central auditory system, typically demonstrated by a reorganization of the projection maps in the auditory cortex. These plastic changes occur not only as a consequence of mechanical lesions of the cochlea or biochemical lesions of the hair cells by ototoxic drugs, but also as a consequence of the loss of hair cells in connection with aging or noise exposure. In light of the aging world population and the increasing amount of noise in the modern world, understanding the plasticity of the central auditory system has its practical consequences and urgency. In most of these situations, a common denominator of central plastic changes is a deterioration of inhibition in the subcortical auditory nuclei and the auditory cortex. In addition to the processes that are elicited by decreased or lost receptor function, the function of nerve cells in the adult central auditory system may dynamically change in the process of learning. A better understanding of the plastic changes in the central auditory system after sensory deafferentation, sensory stimulation, and learning may contribute significantly to improvement in the rehabilitation of damaged or lost auditory function and consequently to improved speech processing and production.

Cross-modal neuroplasticity in neonatally enucleated hamsters: structure, electrophysiology and behaviour

Potential auditory compensation in neonatally bilaterally enucleated Syrian hamsters was explored anatomically, electrophysiologically and behaviourally. Gross morphology of the visual cortex appeared normal and no obvious cytoarchitectural malformation was discerned. However, enucleation induced a significant increase in the spontaneous firing rate of visual cortex cells. Further, auditory stimuli elicited field potentials and single unit responses in the visual cortex of enucleated, but not normal, animals. About 63% of the cells isolated in the visual cortex of 16 enucleated hamsters responded to at least one type of auditory stimulus. Most of the responses were less vigorous and less time-locked than those of auditory cortex cells, and thresholds were typically higher. Projection tracing with WGA-HRP disclosed reciprocal connections between the visual cortex and the dorsal lateral geniculate nucleus in both intact and enucleated animals. However, in the enucleated animals retrogradely labelled cells were also found in the inferior colliculus, the major midbrain auditory nucleus. Behaviourally determined auditory sensitivity across the hearing range did not differ between enucleated and intact hamsters. Minimum audible angle, as determined by a conditioned suppression task, ranged from around 17 to 22 degrees, with no significant difference between normal and enucleated animals. The two groups also did not differ with regard to the direction of their unconditioned head orientating response to intermittent noise. However, the enucleated animals showed a more vigorous response and were slower to habituate to the noise. These results show that bilateral enucleation of newborn hamsters results in auditory activation of visual targets, in addition to the typical activation of the intact auditory pathway. Behaviourally it appears that enucleated hamsters, compared with their normal littermates, are slower to habituate in their response to an unexpected source of sound.

Visual, auditory and bimodal activity in the banks of the lateral suprasylvian sulcus in the cat

In addition to visually driven cells we found within the lateral suprasylvian visual cortex of cats a considerable number of auditory and/or bimodal cells. Most of the visually driven cells were direction and orientation selective with responses that were neither highly stimulus time locked nor very stable. Most of the auditory responses were also not very stable, had relatively high thresholds, and were readily habituated. Previous studies have suggested that populations of cells within the lateral suprasylvian area are specialized for the analysis of optic flow fields (Rauscheker et al., 1987; Sherk et al., 1995). Given that a remarkable proportion of cells within this area can be also driven by auditory stimuli, we hypothesize that the 'optic flow' model may be extended to the bimodal domain rather than restricted to visual clues only. This, however, remains to be corroborated experimentally.

Transsexual limb transplants in fiddler crabs and expression of novel sensory capabilities

We used transsexual limb transplants in fiddler crabs to examine how peripheral sensory structures interact with the central nervous system (CNS) to produce a sexually dimorphic behavior. Female and male chemosensory feeding claws were transplanted onto male hosts in place of nonfeeding, nonchemosensory claws. Successfully transplanted claws retain donor morphologies and contain chemosensory neurons. Neurons in successfully transplanted female feeding claws express the enhanced sensitivity to chemical cues seen in female, but not male, neurons in claws of normal animals. When chemically stimulated, the transplanted claws evoke feeding behavior not observed in normal males, even though the sensory neurons in the transplanted limb project to the host's sexually dimorphic neuropil not known to receive chemosensory input. Behavioral sensitivity is directly related to the sensitivity of peripheral neurons in the transplanted feeding claw. Thus, the interactions between peripheral neurons and their targets may restructure the CNS so that novel sensory capabilities are expressed, and this can produce sexually dimorphic behaviors.

Audiovisual integration of letters in the human brain

Letters of the alphabet have auditory (phonemic) and visual (graphemic) qualities. To investigate the neural representations of such audiovisual objects, we recorded neuromagnetic cortical responses to auditorily, visually, and audiovisually presented single letters. The auditory and visual brain activations first converged around 225 ms after stimulus onset and then interacted predominantly in the right temporo-occipito-parietal junction (280345 ms) and the left (380-540 ms) and right (450-535 ms) superior temporal sulci. These multisensory brain areas, playing a role in audiovisual integration of phonemes and graphemes, participate in the neural network supporting the supramodal concept of a "letter." The dynamics of these functions bring new insight into the interplay between sensory and association cortices during object recognition.

Congenital auditory deprivation reduces synaptic activity within the auditory cortex in a layer-specific manner

The present study investigates the functional deficits of naive auditory cortices in adult congenitally deaf cats. For this purpose, their auditory system was stimulated electrically using cochlear implants. Synaptic currents in cortical layers were revealed using current source density analyses. They were compared with synaptic currents found in electrically stimulated hearing cats. The naive auditory cortex showed significant deficits in synaptic activity in infragranular cortical layers. Furthermore, there was also a deficit of synaptic activities at longer latencies (>30 ms). The 'cortical column' was not activated in the well-defined sequence found in normal hearing cats. These results demonstrate functional deficits as a consequence of congenital auditory deprivation. Similar deficits are likely in congenitally deaf children.

Cross-modal plasticity of the corticothalamic circuits in rats enucleated on the first postnatal day

Reorganization of the reciprocal corticothalamic connections was studied as a possible anatomical substrate of the cross-modal compensation of the missing visual input of the visual cortex by somatosensory-evoked activities in neonatally enucleated rats. The use of quantitative retrograde tract-tracing techniques revealed that the contribution of the lateral posterior thalamic nucleus (LP) is significantly increased following enucleation, while that of the dorsolateral geniculate and the lateral dorsal nuclei is decreased in the thalamocortical afferentation of a region in visual cortical area 17. In contrast with the control rats, a dense terminal arborization of afferents was labelled in the LP after the injection of anterograde tracer into the barrel cortex of the enucleated rats. The injection of anterograde tracer into the visual cortex also demonstrated a massive afferentation into the LP of the enucleated rats. Visual and somatosensory corticothalamic afferents exhibited similar ultrastructural features in the LP after enucleation, but their synaptic organizations differed as regards the diameter of the postsynaptic dendrites. Taken together with the previous observations, these results suggest a central role for the LP in the transmission of the somatosensory-evoked activities to the visual cortex after early blindness.

Improved auditory spatial acuity in visually deprived ferrets

We have examined the effects on auditory spatial acuity in the horizontal plane of depriving ferrets of patterned visual cues by binocular eyelid suture in infancy or for a comparable period in adulthood. Minimum audible angles (MAAs) were measured for 500-, 100- and 40-ms broadband noise bursts at the midline and at 45 degrees to one side. A logistic regression analysis revealed no consistent difference between the midline MAAs of normal and infant lid-sutured ferrets. However, the lateral field MAAs of the infant-deprived group were significantly smaller and showed less inter-subject variability than those of normal-sighted ferrets. The animals deprived in adulthood were tested in the lateral field only, firstly 6 months after binocular eyelid suture and again after a further 10 months. For the first test, the MAAs achieved by these animals with 500- and 100-ms noise bursts were significantly smaller than the normal values and no different from those of the infant-deprived group. A significant improvement in performance at the two shortest stimulus durations (100 and 40 ms) was observed when the adult-deprived animals were re-tested. Their second-test MAAs did not differ from those of the infant-deprived group at any of the three stimulus durations used, and both groups achieved significantly better scores than the normal-sighted control animals. These results show that prolonged visual deprivation in both juvenile and adult ferrets can lead to a significant improvement in auditory spatial acuity in the lateral sound field. This is consistent with reports that congenitally blind humans can localize peripheral sounds more accurately than normal controls.