Source Localisation of Visual Evoked Potentials in Congenitally Deaf Individuals

Previous studies have suggested that individuals deprived of auditory input can compensate with specific superior abilities in the remaining sensory modalities. To better understand the neural basis of deafness-induced changes, the present study used electroencephalography to examine visual functions and cross-modal reorganization of the auditory cortex in deaf individuals. Congenitally deaf participants and hearing controls were presented with reversing chequerboard stimuli that were systematically modulated in luminance ratio. The two groups of participants showed similar modulation of visual evoked potential (VEP) amplitudes (N85, P110) and latencies (P110) as a function of luminance ratio. Analysis of VEPs revealed faster neural processing in deaf participants compared with hearing controls at early stages of cortical visual processing (N85). Deaf participants also showed higher amplitudes (P110) than hearing participants. In contrast to our expectations, the results from VEP source analysis revealed no clear evidence for cross-modal reorganization in the auditory cortex of deaf participants. However, deaf participants tended to show higher activation in posterior parietal cortex (PPC). Moreover, modulation of PPC responses as a function of luminance was also stronger in deaf than in hearing participants. Taken together, these findings are an indication of more efficient neural processing of visual information in the deaf, which may relate to functional changes, in particular in multisensory parietal cortex, as a consequence of early auditory deprivation.     

Auditory-somatosensory multisensory processing in auditory association cortex: an fMRI study

Using high-field (3 Tesla) functional magnetic resonance imaging (fMRI), we demonstrate that auditory and somatosensory inputs converge in a subregion of human auditory cortex along the superior temporal gyrus. Further, simultaneous stimulation in both sensory modalities resulted in activity exceeding that predicted by summing the responses to the unisensory inputs, thereby showing multisensory integration in this convergence region. Recently, intracranial recordings in macaque monkeys have shown similar auditory-somatosensory convergence in a subregion of auditory cortex directly caudomedial to primary auditory cortex (area CM). The multisensory region identified in the present investigation may be the human homologue of CM. Our finding of auditory-somatosensory convergence in early auditory cortices contributes to mounting evidence for multisensory integration early in the cortical processing hierarchy, in brain regions that were previously assumed to be unisensory.     

Cross-modal regulation of synaptic AMPA receptors in primary sensory cortices by visual experience

Lack of a sensory input not only alters the cortical circuitry subserving the deprived sense, but also produces compensatory changes in the functionality of other sensory modalities. Here we report that visual deprivation produces opposite changes in synaptic function in primary visual and somatosensory cortices in rats, which are rapidly reversed by visual experience. This type of bidirectional cross-modal plasticity is associated with changes in synaptic AMPA receptor subunit composition.     

Cross-modal plasticity in specific auditory cortices underlies visual compensations in the deaf

When the brain is deprived of input from one sensory modality, it often compensates with supranormal performance in one or more of the intact sensory systems. In the absence of acoustic input, it has been proposed that cross-modal reorganization of deaf auditory cortex may provide the neural substrate mediating compensatory visual function. We tested this hypothesis using a battery of visual psychophysical tasks and found that congenitally deaf cats, compared with hearing cats, have superior localization in the peripheral field and lower visual movement detection thresholds. In the deaf cats, reversible deactivation of posterior auditory cortex selectively eliminated superior visual localization abilities, whereas deactivation of the dorsal auditory cortex eliminated superior visual motion detection. Our results indicate that enhanced visual performance in the deaf is caused by cross-modal reorganization of deaf auditory cortex and it is possible to localize individual visual functions in discrete portions of reorganized auditory cortex.     

Visual cortex activation in late-onset, Braille naive blind individuals: an fMRI study during semantic and phonological tasks with heard words

Visual cortex activity in the blind has been shown in Braille literate people, which raise the question of whether Braille literacy influences cross-modal reorganization. We used fMRI to examine visual cortex activation during semantic and phonological tasks with auditory presentation of words in two late-onset blind individuals who lacked Braille literacy. Multiple visual cortical regions were activated in the Braille naive individuals. Positive BOLD responses were noted in lower tier visuotopic (e.g., V1, V2, VP, and V3) and several higher tier visual areas (e.g., V4v, V8, and BA 37). Activity was more extensive and cross-correlation magnitudes were greater during the semantic compared to the phonological task. These results with Braille naive individuals plausibly suggest that visual deprivation alone induces visual cortex reorganization. Cross-modal reorganization of lower tier visual areas may be recruited by developing skills in attending to selected non-visual inputs (e.g., Braille literacy, enhanced auditory skills). Such learning might strengthen remote connections with multisensory cortical areas. Of necessity, the Braille naive participants must attend to auditory stimulation for language. We hypothesize that learning to attend to non-visual inputs probably strengthens the remaining active synapses following visual deprivation, and thereby, increases cross-modal activation of lower tier visual areas when performing highly demanding non-visual tasks of which reading Braille is just one example.     

Multisensory processing in "unimodal" neurons: cross-modal subthreshold auditory effects in cat extrastriate visual cortex

Historically, the study of multisensory processing has examined the function of the definitive neuron type, the bimodal neuron. These neurons are excited by inputs from more than one sensory modality, and when multisensory stimuli are present, they can integrate their responses in a predictable manner. However, recent studies have revealed that multisensory processing in the cortex is not restricted to bimodal neurons. The present investigation sought to examine the potential for multisensory processing in nonbimodal (unimodal) neurons in the retinotopically organized posterolateral lateral suprasylvian (PLLS) area of the cat. Standard extracellular recordings were used to measure responses of all neurons encountered to both separate- and combined-modality stimulation. Whereas bimodal neurons behaved as predicted, the surprising result was that 16% of unimodal visual neurons encountered were significantly facilitated by auditory stimuli. Because these unimodal visual neurons did not respond to an auditory stimulus presented alone but had their visual responses modulated by concurrent auditory stimulation, they represent a new form of multisensory neuron: the subthreshold multisensory neuron. These data also demonstrate that bimodal neurons can no longer be regarded as the exclusive basis for multisensory processing.     

Functional topography of converging visual and auditory inputs to neurons in the rat superior colliculus

We have used a slice preparation of the infant rat midbrain to examine converging inputs onto neurons in the deeper multisensory layers of the superior colliculus (dSC). Electrical stimulation of the superficial visual layers (sSC) and of the auditory nucleus of the brachium of the inferior colliculus (nBIC) evoked robust monosynaptic responses in dSC cells. Furthermore, the inputs from the sSC were found to be topographically organized as early as the second postnatal week and thus before opening of the eyes and ear canals. This precocious topography was found to be sculpted by GABAA-mediated inhibition of a more widespread set of connections. Tracer injections in the nBIC, both in coronal slices as well as in hemisected brains, confirmed a robust projection originating in the nBIC with distinct terminals in the proximity of the cell bodies of dSC neurons. Combined stimulation of the sSC and nBIC sites revealed that the presumptive visual and auditory inputs are summed linearly. Finally, whereas either input on its own could manifest a significant degree of paired-pulse facilitation, temporally offset stimulation of the two sites revealed no synaptic interactions, indicating again that the two inputs function independently. Taken together, these data provide the first detailed intracellular analysis of convergent sensory inputs onto dSC neurons and form the basis for further exploration of multisensory integration and developmental plasticity.     

Cortex controls multisensory depression in superior colliculus

Multisensory depression is a fundamental index of multisensory integration in superior colliculus (SC) neurons. It is initiated when one sensory stimulus (auditory) located outside its modality-specific receptive field degrades or eliminates the neuron's responses to another sensory stimulus (visual) presented within its modality-specific receptive field. The present experiments demonstrate that the capacity of SC neurons to engage in multisensory depression is strongly dependent on influences from two cortical areas (the anterior ectosylvian and rostral lateral suprasylvian sulci). When these cortices are deactivated, the ability of SC neurons to synthesize visual-auditory inputs in this way is compromised; multisensory responses are disinhibited, becoming more vigorous and in some cases indistinguishable from responses to the visual stimulus alone. Although obtaining a more robust multisensory SC response when cortex is nonfunctional than when it is functional may seem paradoxical, these data may help explain previous observations that the loss of these cortical influences permits visual orientation behavior in the presence of a normally disruptive auditory stimulus.     

Corticofugal modulation of the midbrain frequency map in the bat auditory system

The auditory system, like the visual and somatosensory systems, contains topographic maps in its central neural pathways. These maps can be modified by sensory deprivation, injury and experience in both young and adult animals. Such plasticity has been explained by changes in the divergent and convergent projections of the ascending sensory system. Another possibility, however, is that plasticity may be mediated by descending corticofugal connections. We have investigated the role of descending connections from the cortex to the inferior colliculus of the big brown bat. Electrical stimulation of the auditory cortex causes a downward shift in the preferred frequencies of collicular neurons toward that of the stimulated cortical neurons. This results in a change in the frequency map within the colliculus. Moreover, similar changes can be induced by repeated bursts of sound at moderate intensities. Thus, one role of the mammalian corticofugal system may be to modify subcortical sensory maps in response to sensory experience.     

Multisensory processing via early cortical stages: Connections of the primary auditory cortical field with other sensory systems

It is still a popular view that primary sensory cortices are unimodal, but recent physiological studies have shown that under certain behavioral conditions primary sensory cortices can also be activated by multiple other modalities. Here, we investigate the anatomical substrate, which may underlie multisensory processes at the level of the primary auditory cortex (field AI), and which may, in turn, enable AI to influence other sensory systems. We approached this issue by means of the axonal transport of the sensitive bidirectional neuronal tracer fluorescein-labeled dextran which was injected into AI of Mongolian gerbils (Meriones unguiculatus). Of the total number of retrogradely labeled cell bodies (i.e. cells of origin of direct projections to AI) found in non-auditory sensory and multisensory brain areas, approximately 40% were in cortical areas and 60% in subcortical structures. Of the cell bodies in the cortical areas about 82% were located in multisensory cortex, viz., the dorsoposterior and ventroposterior, posterior parietal cortex, the claustrum, and the endopiriform nucleus, 10% were located in the primary somatosensory cortex (hindlimb and trunk region), and 8% in secondary visual cortex. The cortical regions with retrogradely labeled cells also contained anterogradely labeled axons and their terminations, i.e. they are also target areas of direct projections from AI. In addition, the primary olfactory cortex was identified as a target area of projections from AI. The laminar pattern of corticocortical connections suggests that AI receives primarily cortical feedback-type inputs and projects in a feedforward manner to its target areas. Of the labeled cell bodies in the subcortical structures, approximately 90% were located in multisensory thalamic, 4% in visual thalamic, and 6% in multisensory lower brainstem structures. At subcortical levels, we observed a similar correspondence of retrogradely labeled cells and anterogradely labeled axons and terminals in visual (posterior limitans thalamic nucleus) and multisensory thalamic nuclei (dorsal and medial division of the medial geniculate body, suprageniculate nucleus, posterior thalamic cell group, zona incerta), and in the multisensory nucleus of the brachium of the inferior colliculus. Retrograde, but not anterograde, labeling was found in the multisensory pontine reticular formation, particularly in the reticulotegmental nucleus of the pons. Conversely, anterograde, but no retrograde, labeling was found in the visual laterodorsal and lateroposterior thalamic nuclei, in the multisensory peripeduncular, posterior intralaminar, and reticular thalamic nuclei, as well as in the multisensory superior and pericentral inferior colliculi (including cuneiform and sagulum nucleus), pontine nuclei, and periaqueductal gray. Our study supports the notion that AI is not merely involved in the analysis of auditory stimulus properties but also in processing of other sensory and multisensory information. Since AI is directly connected to other primary sensory cortices (viz. the somatosensory and olfactory ones) multisensory information is probably also processed in these cortices. This suggests more generally, that primary sensory cortices may not be unimodal.     

Neural mechanisms for synthesizing sensory information and producing adaptive behaviors

The ability to integrate information from different sensory systems is a fundamental characteristic of the brain. Because different bits of information are derived from different sensory channels, their synthesis markedly enhances the detection and identification of external stimuli. The neural substrate for such “multisensory” integration is provided by neurons that receive convergent input from two or more sensory modalities. Many such multisensory neurons are found in the superior colliculus (SC), a midbrain structure that plays a significant role in overt attentive and orientation behaviors. The various principles governing the integration of visual, auditory, and somatosensory inputs in SC neurons have been explored in several species. Thus far, the evidence suggests a remarkable conservation of integrative features during vertebrate evolution. One of the most robust of these principles is based on spatial relationships: a striking enhancement in activity is induced in a multisensory neuron when two different sensory stimuli (e.g., visual and auditory) are in spatial concordance, whereas a profound response depression can be induced when these cues are spatially discordant. The most extensive physiological observations have been made in cat, and in this species the same principles that have been shown to govern multisensory integration at the level of the individual SC neuron have also been shown to govern overt attentive and orientation responses to multisensory stimuli. Most surprising, however, is the critical role played by association (i.e. anterior ectosylvian) cortex in facilitating these midbrain processes. In the absence of the modulating corticotectal influences, multisensory SC neurons in cat are unable to integrate the different sensory cues converging upon them in an adult-like fashion, and are unable to mediate overt multisensory behaviors. This situation appears quite similar to that observed during early postnatal life. When multisensory SC neurons first appear, they are able to respond to multiple sensory inputs but are unable to synthesize these inputs to significantly enhance or degrade their responses. During ontogeny, individual multisensory neurons develop this capacity abruptly, but at very different ages, until the mature population condition is reached after several postnatal months. It appears likely that the abrupt onset of this capacity in any individual SC neuron reflects the maturation of inputs from anterior ectosylvian cortex. Presumably, the functional coupling of cortex with an individual SC neuron is essential to initiate and maintain that neuron’s capability for multisensory integration throughout its life.     

Receptor organ damage causes loss of cortical surround inhibition without topographic map plasticity

Following restricted peripheral damage, reorganization of adult sensory or motor cortex is believed to depend on loss of surround inhibition, which unmasks latent inputs to the deprived cortex. Here I demonstrate that limited damage to auditory receptors causes loss of functional surround inhibition in the cortex, unmasking of latent inputs and significantly altered neural coding. However, these changes do not lead to plasticity of the cortical map, defined by the most sensitive input from the receptor surface to each cortical location. Thus, in sensory cortex, loss of surround inhibition as a consequence of receptor organ damage does not necessarily result in cortical map plasticity.     

The role of anterior ectosylvian cortex in cross-modality orientation and approach behavior

Physiological and behavioral studies in cat have shown that corticotectal influences play important roles in the information-processing capabilities of superior colliculus (SC) neurons. While corticotectal inputs from the anterior ectosylvian sulcus (AES) play a comparatively small role in the unimodal responses of SC neurons, they are particularly important in rendering these neurons capable of integrating information from different sensory modalities (e.g., visual and auditory). The present experiments examined the behavioral consequences of depriving SC neurons of AES inputs, and thereby compromising their ability to integrate visual and auditory information. Selective deactivation of a variety of other cortical areas (posterolateral lateral suprasylvian cortex, PLLS; primary auditory cortex, AI; or primary visual cortex, 17/18) served as controls. Cats were trained in a perimetry device to ignore a brief, low-intensity auditory stimulus but to orient toward and approach a nearthreshold visual stimulus (a light-emitting diode, LED) to obtain food. The LED was presented at different eccentricities either alone (unimodal) or combined with the auditory stimulus (multisensory). Subsequent deactivation of the AES, with focal injections of a local anesthetic, had no effect on responses to unimodal cues regardless of their location. However, it profoundly, though reversibly, altered orientation and approach to multisensory stimuli in contralateral space. The characteristic enhancement of these responses observed when an auditory cue was presented in spatial correspondence with the visual stimulus was significantly degraded. Similarly, the inhibitory effect of a spatially disparate auditory cue was significantly ameliorated. The observed effects were specific to AES deactivation, as similar effects were not obtained with deactivation of PLLS, AI or 17/18, or saline injections into the AES. These observations are consistent with postulates that specific cortical-midbrain interactions are essential for the synthesis of multisensory information in the SC, and for the orientation and localization behaviors that depend on this synthesis.     

Neuroanatomical identification of crossmodal auditory inputs to interneurons in somatosensory cortex

Multisensory convergence is the first, requisite step in the process that generates neural responses to events involving more than one sensory modality. Although anatomical studies have documented the merging of afferents from different sensory modalities within a given area, they do not provide insight into the architecture of connectivity at the neuronal level that underlies multisensory processing. In fact, few anatomical studies of multisensory convergence at the neuronal level have been conducted. The present study used a combination of tract-tracing, immunocytochemistry, and confocal microscopic techniques to examine the connections related to crossmodal auditory cortical inputs to somatosensory area SIV. Axons labeled from auditory cortex were found in contact with immunolabeled interneurons in SIV, some of which also colocalized vesicular glutamate transporter 1, indicating the presence of an active, glutamatergic synapse. No specific subtype of inhibitory interneuron appeared to be targeted by the crossmodal contacts. These results provide insight into the structural basis for multisensory processing at the neuronal level and offer anatomical evidence for the direct involvement of inhibitory interneurons in multisensory processing.     

The influence of visual and auditory receptive field organization on multisensory integration in the superior colliculus

The spatial register of the different receptive fields of multisensory neurons in the superior colliculus (SC) plays a significant role in determining the responses of these neurons to cross-modal stimulus combinations. Spatially coincident visual-auditory stimuli fall within these overlapping receptive fields and generally produce response enhancements that exceed the individual modality-specific responses and can exceed their sum. Yet, in this context, it has not been clear how "spatial coincidence" is operationally defined. Given the large size of SC receptive fields, visual and auditory stimuli could be within their respective receptive fields even when there are substantial spatial disparities between them. Indeed, previous observations have raised the possibility that there may be a second level of determinism in how SC neurons deal with the relative spatial locations of within-field cross-modal stimuli; specifically, that multisensory response enhancements become progressively weaker as the within-field visual and auditory stimuli become increasingly disparate. While the present experiments demonstrated that SC multisensory neurons have heterogeneous receptive fields, and that the greatest number of impulses evoked were by stimuli that fell within the area of cross-modal receptive field overlap, they also indicate that there is no systematic relationship between cross-modal stimulus disparity and the magnitude of multisensory response enhancement. Thus, two within-field cross-modal stimuli produced the same proportionate change (i.e., multisensory response enhancement) when they were widely disparate as they did when they overlapped one another in space. These observations indicate that cross-modal spatial coincidence can be defined operationally by the borders of an SC neuron's receptive fields regardless of the size of those receptive fields and/or the absolute spatial disparity between within-field cross-modal stimuli. Electronic Publication     

Spatial and cross-modal attention alter responses to unattended sensory information in early visual and auditory human cortex

Attending to a visual or auditory stimulus often requires irrelevant information to be filtered out, both within the modality attended and in other modalities. For example, attentively listening to a phone conversation can diminish our ability to detect visual events. We used functional magnetic resonance imaging (fMRI) to examine brain responses to visual and auditory stimuli while subjects attended visual or auditory information. Although early cortical areas are traditionally considered unimodal, we found that brain responses to the same ignored information depended on the modality attended. In early visual area V1, responses to ignored visual stimuli were weaker when attending to another visual stimulus, compared with attending to an auditory stimulus. The opposite was true in more central visual area MT+, where responses to ignored visual stimuli were weaker when attending to an auditory stimulus. Furthermore, fMRI responses to the same ignored visual information depended on the location of the auditory stimulus, with stronger responses when the attended auditory stimulus shared the same side of space as the ignored visual stimulus. In early auditory cortex, responses to ignored auditory stimuli were weaker when attending a visual stimulus. A simple parameterization of our data can describe the effects of redirecting attention across space within the same modality (spatial attention) or across modalities (cross-modal attention), and the influence of spatial attention across modalities (cross-modal spatial attention). Our results suggest that the representation of unattended information depends on whether attention is directed to another stimulus in the same modality or the same region of space.     

Sensory input directs spatial and temporal plasticity in primary auditory cortex.

The cortical representation of the sensory environment is continuously modified by experience. Changes in spatial (receptive field) and temporal response properties of cortical neurons underlie many forms of natural learning. The scale and direction of these changes appear to be determined by specific features of the behavioral tasks that evoke cortical plasticity. The neural mechanisms responsible for this differential plasticity remain unclear partly because important sensory and cognitive parameters differ among these tasks. In this report, we demonstrate that differential sensory experience directs differential plasticity using a single paradigm that eliminates the task-specific variables that have confounded direct comparison of previous studies. Electrical activation of the basal forebrain (BF) was used to gate cortical plasticity mechanisms. The auditory stimulus paired with BF stimulation was systematically varied to determine how several basic features of the sensory input direct plasticity in primary auditory cortex (A1) of adult rats. The distributed cortical response was reconstructed from a dense sampling of A1 neurons after 4 wk of BF-sound pairing. We have previously used this method to show that when a tone is paired with BF activation, the region of the cortical map responding to that tone frequency is specifically expanded. In this report, we demonstrate that receptive-field size is determined by features of the stimulus paired with BF activation. Specifically, receptive fields were narrowed or broadened as a systematic function of both carrier-frequency variability and the temporal modulation rate of paired acoustic stimuli. For example, the mean bandwidth of A1 neurons was increased (+60%) after pairing BF stimulation with a rapid train of tones and decreased (-25%) after pairing unmodulated tones of different frequencies. These effects are consistent with previous reports of receptive-field plasticity evoked by natural learning. The maximum cortical following rate and minimum response latency were also modified as a function of stimulus modulation rate and carrier-frequency variability. The cortical response to a rapid train of tones was nearly doubled if BF stimulation was paired with rapid trains of random carrier frequency, while no following rate plasticity was observed if a single carrier frequency was used. Finally, we observed significant increases in response strength and total area of functionally defined A1 following BF activation paired with certain classes of stimuli and not others. These results indicate that the degree and direction of cortical plasticity of temporal and receptive-field selectivity are specified by the structure and schedule of inputs that co-occur with basal forebrain activation and suggest that the rules of cortical plasticity do not operate on each elemental stimulus feature independently of others.     

Two cortical areas mediate multisensory integration in superior colliculus neurons

The majority of multisensory neurons in the cat superior colliculus (SC) are able to synthesize cross-modal cues (e.g., visual and auditory) and thereby produce responses greater than those elicited by the most effective single modality stimulus and, sometimes, greater than those predicted by the arithmetic sum of their modality-specific responses. The present study examined the role of corticotectal inputs from two cortical areas, the anterior ectosylvian sulcus (AES) and the rostral aspect of the lateral suprasylvian sulcus (rLS), in producing these response enhancements. This was accomplished by evaluating the multisensory properties of individual SC neurons during reversible deactivation of these cortices individually and in combination using cryogenic deactivation techniques. Cortical deactivation eliminated the characteristic multisensory response enhancement of nearly all SC neurons but generally had little or no effect on a neuron's modality-specific responses. Thus, the responses of SC neurons to combinations of cross-modal stimuli were now no different from those evoked by one or the other of these stimuli individually. Of the two cortical areas, AES had a much greater impact on SC multisensory integrative processes, with nearly half the SC neurons sampled dependent on it alone. In contrast, only a small number of SC neurons depended solely on rLS. However, most SC neurons exhibited dual dependencies, and their multisensory enhancement was mediated by either synergistic or redundant influences from AES and rLS. Corticotectal synergy was evident when deactivating either cortical area compromised the multisensory enhancement of an SC neuron, whereas corticotectal redundancy was evident when deactivation of both cortical areas was required to produce this effect. The results suggest that, although multisensory SC neurons can be created as a consequence of a variety of converging tectopetal afferents that are derived from a host of subcortical and cortical structures, the ability to synthesize cross-modal inputs, and thereby produce an enhanced multisensory response, requires functional inputs from the AES, the rLS, or both.     

Multisensory orientation behavior is disrupted by neonatal cortical ablation

The integration of visual and auditory information can significantly amplify the sensory responses of superior colliculus (SC) neurons and the behaviors that depend on them. This response amplification depends on the development of SC inputs that are derived from two regions of cortex: the anterior ectosylvian sulcus (AES) and the rostral lateral suprasylvian sulcus (rLS). Neonatal ablation of these cortico-collicular areas has been shown to disrupt the development of the multisensory enhancement capabilities of SC neurons and the present results demonstrate that it also precludes the development of the normal multisensory enhancements in orientation behavior. Animals with neonatal ablation of AES and rLS were tested at maturity and found unable to benefit from the combination of visual and auditory cues in their efforts to localize targets in contralesional space. In contrast, their ipsilesional multisensory orientation capabilities were indistinguishable from those of normal animals. However, when only one of these cortical areas was removed during early life, later behavioral consequences were negligible. Whether similar compensatory processes would occur in adult animals remains to be determined. These observations, coupled with those from previous studies, also suggest that a surprisingly high proportion of SC neurons capable of multisensory integration must be present for orientation behavior benefits to be realized. Compensatory mechanisms can achieve this if early lesions spare either AES or rLS, but even the impressive plasticity of the neonatal brain cannot compensate for the early loss of both of them.     

Sensory modality-specific homeostatic plasticity in the developing optic tectum

We found a previously unknown form of homeostatic synaptic plasticity in multisensory neurons in the optic tectum of Xenopus laevis tadpoles. Individual tectal neurons are known to receive converging inputs from multiple sensory modalities. We observed that long-term alterations in either visual or mechanosensory activity in vivo resulted in homeostatic changes specific to each sensory modality. In contrast with typical forms of homeostatic synaptic plasticity, such as synaptic scaling, we found that this type of plasticity occurred in a pathway-specific manner that is more reminiscent of hebbian-type plasticity.