Brain plasticity and auditory spatial adaptation in patients with unilateral hearing loss.

The ability to localize sounds in patients with Unilateral Hearing Loss (UHL) is usually disrupted due to alteration in the integration of binaural cues. Nonetheless, some patients are able to compensate deficit using adaptive strategies. In this study, we explored the neural correlates underlying this adaptation. Twenty-one patients with UHL were separated into 3 groups using cluster analysis based on their binaural performance. The resulting clusters were referred to as better, moderate, and poorer performers cluster (BPC, MPC, and PPC). We measured the mismatch negativity (MMN) elicited by deviant sounds located at 10°, 20°, and 100° from a standard positioned at 50° ipsilateral to the deaf ear. The BPC exhibited significant MMN for all 3 deviants, similar to normal hearing (NH) subjects. In contrast, there was no significant MMN for 10° and 20° deviants for the PPC and for NH when one ear was plugged and muffed. Scalp distribution was maximal over central regions in BPC, while PPC showed more frontal MMN distribution. Thus, the BPC exhibited a contralateral activation pattern, similar to NH, while the PPC exhibited more symmetrical hemispheric activation. MMN can be used as a neural marker to reflect spatial adaptation in patients with UHL.

Auditory cortical plasticity after cochlear implantation in asymmetric hearing loss is related to spatial hearing: a PET H215O study.

In asymmetric hearing loss (AHL), the normal pattern of contralateral hemispheric dominance for monaural stimulation is modified, with a shift towards the hemisphere ipsilateral to the better ear. The extent of this shift has been shown to relate to sound localization deficits. In this study, we examined whether cochlear implantation to treat postlingual AHL can restore the normal functional pattern of auditory cortical activity and whether this relates to improved sound localization. The auditory cortical activity was found to be lower in the AHL cochlear implanted (AHL-CI) participants. A cortical asymmetry index was calculated and showed that a normal contralateral dominance was restored in the AHL-CI patients for the nonimplanted ear, but not for the ear with the cochlear implant. It was found that the contralateral dominance for the nonimplanted ear strongly correlated with sound localization performance (rho = 0.8, P < 0.05). We conclude that the reorganization of binaural mechanisms in AHL-CI subjects reverses the abnormal lateralization pattern induced by the deafness, and that this leads to improved spatial hearing. Our results suggest that cochlear implantation enables the reconstruction of the cortical mechanisms of spatial selectivity needed for sound localization.

Intensity patterns at the peaks of brain activity in fMRI and PET are highly correlated with neural models of spatial integration.

Spatial integration during the brain's cognitive activity prompts changes in energy used by different neuroglial populations. Nevertheless, the organisation of such integration in 3D -brain activity remains undescribed from a quantitative standpoint. In response, we applied a cross-correlation between brain activity and integrative models, which yielded a deeper understanding of information integration in functional brain mapping. We analysed four datasets obtained via fundamentally different neuroimaging techniques (functional magnetic resonance imaging [fMRI] and positron emission tomography [PET]) and found that models of spatial integration with an increasing input to each step of integration were significantly more correlated with brain activity than models with a constant input to each step of integration. In addition, marking the voxels with the maximal correlation, we found exceptionally high intersubject consistency with the initial brain activity at the peaks. Our method demonstrated for the first time that the network of peaks of brain activity is organised strictly according to the models of spatial integration independent of neuroimaging techniques. The highest correlation with models integrating an increasing at each step input suggests that brain activity reflects a network of integrative processes where the results of integration in some neuroglial populations serve as an input to other neuroglial populations.

Neuroinflammation and depression: A review.

Some recent clinical and preclinical evidence suggests that neuroinflammation is a key factor that interacts with the three neurobiological correlates of major depressive disorder: depletion of brain serotonin, dysregulation of the hypothalamus-pituitary-adrenal (HPA) axis and alteration of the continuous production of adult-generated neurons in the dentate gyrus of the hippocampus. This review discusses the main players in brain immunity as well as how inflammation interacts with the above three mechanisms. It is reported that kynurenine (KYN) pathway alteration in favour of its excitotoxic component and HPA axis dysregulation have the common effect of increasing extracellular glutamate levels and glutamate neurotransmission, which can impact hippocampal neurogenesis. This pathophysiological cascade appears to be triggered or sustained and reinforced by any chronic inflammatory condition involving increased circulating markers of inflammation that are able to cross the blood-brain barrier and activate microglia; it can also be the consequence of primary brain neuroinflammation, such as in neurodegenerative disorders with early manifestations that are frequently depressive symptoms. Further recent data indicate that primary microglial activation may also result from a direct impact of chronic stress on vascular function. The intricated dynamic crosstalk between neuroinflammation and other relevant neurobiological correlates of depression add to evidence that neuroinflammation may be a key therapeutic target for future therapeutic strategies in major depressive disorder.

Brain immune cells characterization in UCMS exposed P2X7 knock-out mouse.

BACKGROUND: Several lines of evidence suggest that neuroinflammation might be a key neurobiological mechanism of depression. In particular, the P2X7 receptor (P2X7R), an ATP-gated ion channel involved in activation of the pro-inflammatory interleukin IL-1ß, has been shown to be a potential new pharmacological target in depression. The aim of this study was to explore the impact of unpredictable chronic mild stress (UCMS) on behavioural changes, hippocampal neurogenesis, and cellular characterisation of brain immune cells, in P2X7R Knock-Out (KO) mice. METHODS: P2X7R KO and wild-type (WT) mice were subjected to a 6-week UCMS protocol and received a conventional oral antidepressant (15 mg.kg-1 fluoxetine) or water per os. The mice then underwent behavioural tests consisting of the tail suspension test (TST), the elevated plus maze (EPM) test, the open field test, the splash test and the nest building test (week 7). Doublecortin immunostaining (DCX) of brain slices was used to assess neurogenesis in the dentate gyrus. Iba1 and TMEM119 immunostaining was used to characterise brain immune cells, Iba1 as a macrophage marker (including microglial cells) and TMEM119 as a potential specific resident microglial cells marker. RESULTS: After a 6-week UCMS exposure, P2X7R KO mice exhibited less deterioration of their coat state, spent a significantly smaller amount of time immobile in the TST and spent a larger amount of time in the open arms of the EPM. As expected, adult ventral hippocampal neurogenesis was significantly decreased by UCMS in WT mice, while P2X7R KO mice maintained ventral hippocampal neurogenesis at similar levels in both control and UCMS conditions. In stress-related brain regions, P2X7R KO mice also exhibited less recruitment of Iba1+/TMEM119+ and Iba1+/TMEM119- cells in the brain. The ratio between these two staining patterns revealed that brain immune cells were mostly composed of Iba1+/TMEM119+ cells (87 to 99%), and this ratio was affected neither by P2X7R genetic depletion nor by antidepressant treatment. DISCUSSION: Behavioural patterns, neurogenesis levels and density of brain immune cells in P2X7R KO mice after exposure to UCMS significantly differed from control conditions. Brain immune cells were mostly increased in brain regions known to be sensitive to UCMS exposure in WT but not in P2X7R KO mice. Considering Iba1+/TMEM119- staining might characterize peripheral immune cells, the ratio between Iba1+/TMEM119+ cells and IBA1+/TMEM119- cells, suggests that the rate of peripheral immune cells recruitment may not be modified neither by P2X7R gene expression nor by antidepressant treatment.

Unique Features of Subcortical Circuits in a Macaque Model of Congenital Blindness.

There is an extensive modification of the functional organization of the brain in the congenital blind human, although there is little understanding of the structural underpinnings of these changes. The visual system of macaque has been extensively characterized both anatomically and functionally. We have taken advantage of this to examine the influence of congenital blindness in a macaque model of developmental anophthalmia. Developmental anophthalmia in macaque effectively removes the normal influence of the thalamus on cortical development leading to an induced "hybrid cortex (HC)" combining features of primary visual and extrastriate cortex. Here we show that retrograde tracers injected in early visual areas, including HC, reveal a drastic reduction of cortical projections of the reduced lateral geniculate nucleus. In addition, there is an important expansion of projections from the pulvinar complex to the HC, compared to the controls. These findings show that the functional consequences of congenital blindness need to be considered in terms of both modifications of the interareal cortical network and the ascending visual pathways.

Stimulus-specific information is represented as local activity patterns across the brain.

Modern neuroimaging represents three-dimensional brain activity, which varies across brain regions. It remains unknown whether activity of different brain regions has similar spatial organization to reflect similar cognitive processes. We developed a rotational cross-correlation method allowing a straightforward analysis of spatial activity patterns distributed across the brain in stimulation specific contrast images. Results of this method were verified using several statistical approaches on real and simulated random datasets. We found, for example, that the seed patterns in the fusiform face area were robustly correlated to brain regions involved in face-specific representations. These regions differed from the non-specific visual network meaning that activity structure in the brain is locally preserved in stimulus-specific regions. Our findings indicate spatially correlated perceptual representations in cerebral activity and suggest that the 3D coding of the processed information is organized using locally preserved activity patterns across the brain. More generally, our results demonstrate that information is represented and shared in the local spatial configurations of brain activity.

The 'Yin' and the 'Yang' of the kynurenine pathway: excitotoxicity and neuroprotection imbalance in stress-induced disorders.

The amino-acid tryptophan (TRY) is converted into kynurenine (KYN) and subsequent metabolites by the tryptophan/catabolites (TRY/CAT) pathway (kynurenine pathway). 'Excito-toxic' and 'neuro-protective' metabolites are produced, which modulate the glutamatergic neurotransmission. The TRY/CAT pathway is activated by hypothalamic-pituitary-adrenal endocrine induction during stress by corticoids hormones, and the excitotoxic branch of the TRY/CAT pathway is activated by proinflammatory cytokines. During stress and major depressive disorders, it is generally accepted that inflammation induces an imbalance toward the excitotoxic branch of the TRY/CAT pathway, causing changes in brain connectivity in corticolimbic structures and therefore psychocognitive abnormalities. In neurodegenerative diseases, the activation of the oxidative branch of the TRY/CAT pathway has been frequently reported. We propose a comprehensive survey of the TRY/CAT pathway (kynurenine pathway) abnormalities in stress and inflammation-induced MDD and neurodegenerative diseases. As TRY/CAT pathway is a common feature of stress, inflammation, affective disorders, and neurodegenerative diseases, we discuss the status of the TRY/CAT pathway as a possible link among chronic stress, inflammation, depressive disorders and neurodegenerative diseases. This review does not claim to be exhaustive, but in a pharmacological perspective, it will be proposed that modulation of the excitotoxicity/neuroprotection balance is a valuable strategy for new and more effective treatments of mood disorders.

How Areal Specification Shapes the Local and Interareal Circuits in a Macaque Model of Congenital Blindness.

There is little understanding of the structural underpinnings of the functional reorganization of the cortex in the congenitally blind human. Taking advantage of the extensive characterization of the macaque visual system, we examine in macaque the influence of congenital blindness resulting from the removal of the retina during in utero development. This effectively removes the normal influence of the thalamus on cortical development leading to an induced hybrid cortex (HC) combining features of primary visual and extrastriate cortex. Retrograde tracers injected in HC reveal a local, intrinsic connectivity characteristic of higher order areas and show that the HC receives a uniquely strong, purely feedforward projection from striate cortex but no ectopic inputs, except from subiculum, and entorhinal cortex. Statistical modeling of quantitative connectivity data shows that HC is relatively high in the cortical hierarchy and receives a reinforced input from ventral stream areas while the overall organization of the functional streams are conserved. The directed and weighted anophthalmic cortical graph from the present study can be used to construct dynamic and structural models. These findings show how the sensory periphery governs cortical phenotype and reveal the importance of developmental arealization for understanding the functional reorganization in congenital blindness.

Rediscovering TNAP in the Brain: A Major Role in Regulating the Function and Development of the Cerebral Cortex.

The presence of alkaline phosphatase (AP) activity in the neural tissue has been described decades ago. However, only recent studies clarified the isotype, regional distribution and subcellular localization of the AP expressed in the cerebral cortex of diverse mammalian species including the human. In the primate brain the discovery that the bone AP isotype (TNAP) is expressed provided the opportunity of a deeper understanding of the role of this enzyme in neuronal functions based on the knowledge acquired by studying the role of the enzyme in hypophosphatasia, mostly in bone mineralization. TNAP exhibits widespread substrate specificity and, in the brain, it is potentially involved in the regulation of molecules which play fundamental roles in signal transmission and development. In light of these observations, the localization of TNAP in the human cerebral cortex is of high significance when considering that epilepsy is often diagnosed in hypophosphatasia. Here we overview our results on the identification of TNAP in the primate cerebral cortex: TNAP exhibits a noticeably high activity in the synapses and nodes of Ranvier, is specifically present in layer 4 of the sensory cortices and additionally in layer 5 of prefrontal, temporal and other associational areas in human. Our studies also indicate that bone AP activity depends on the level of sensory input and that its developmental time-course exhibits characteristic regional differences. The relevance of our findings regarding human cortical physiology and brain disorders are discussed.

The role of long-range connections on the specificity of the macaque interareal cortical network.

We investigated the influence of interareal distance on connectivity patterns in a database obtained from the injection of retrograde tracers in 29 areas distributed over six regions (occipital, temporal, parietal, frontal, prefrontal, and limbic). One-third of the 1,615 pathways projecting to the 29 target areas were reported only recently and deemed new-found projections (NFPs). NFPs are predominantly long-range, low-weight connections. A minimum dominating set analysis (a graph theoretic measure) shows that NFPs play a major role in globalizing input to small groups of areas. Randomization tests show that (i) NFPs make important contributions to the specificity of the connectivity profile of individual cortical areas, and (ii) NFPs share key properties with known connections at the same distance. We developed a similarity index, which shows that intraregion similarity is high, whereas the interregion similarity declines with distance. For area pairs, there is a steep decline with distance in the similarity and probability of being connected. Nevertheless, the present findings reveal an unexpected binary specificity despite the high density (66%) of the cortical graph. This specificity is made possible because connections are largely concentrated over short distances. These findings emphasize the importance of long-distance connections in the connectivity profile of an area. We demonstrate that long-distance connections are particularly prevalent for prefrontal areas, where they may play a prominent role in large-scale communication and information integration.

Brain prediction of auditory emphasis by facial expressions during audiovisual continuous speech.

The visual cues involved in auditory speech processing are not restricted to information from lip movements but also include head or chin gestures and facial expressions such as eyebrow movements. The fact that visual gestures precede the auditory signal implicates that visual information may influence the auditory activity. As visual stimuli are very close in time to the auditory information for audiovisual syllables, the cortical response to them usually overlaps with that for the auditory stimulation; the neural dynamics underlying the visual facilitation for continuous speech therefore remain unclear. In this study, we used a three-word phrase to study continuous speech processing. We presented video clips with even (without emphasis) phrases as the frequent stimuli and with one word visually emphasized by the speaker as the non-frequent stimuli. Negativity in the resulting ERPs was detected after the start of the emphasizing articulatory movements but before the auditory stimulus, a finding that was confirmed by the statistical comparisons of the audiovisual and visual stimulation. No such negativity was present in the control visual-only condition. The propagation of this negativity was observed between the visual and fronto-temporal electrodes. Thus, in continuous speech, the visual modality evokes predictive coding for the auditory speech, which is analysed by the cerebral cortex in the context of the phrase even before the arrival of the corresponding auditory signal.

Quality of life and auditory performance in adults with asymmetric hearing loss.

We evaluated the relationship between binaural hearing deficits and quality of life. The study included 49 adults with asymmetric hearing loss (AHL), and 11 adult normal-hearing listeners (NHL) served as controls. Speech reception thresholds (SRT) were assessed with the French Matrix Test. Quality of life was evaluated with the Speech, Spatial and Qualities of Hearing Scale (SSQ) and the Glasgow Health Status Inventory. Speech recognition in noise was significantly poorer for AHL subjects [-0.12 dB signal-to-noise ratio (SNR) in dichotic (with speech presented to the poorer ear and noise to the better ear), -1.72 dB in diotic and -6.84 dB in reverse-dichotic conditions] compared to NHL (-4.98 dB in diotic and -9.58 dB in dichotic conditions). Scores for quality-of-life questionnaires were significantly below norms. Significant correlations were found between the SRT for the dichotic condition and the SSQ total score (r = -0.38, p = 0.01), and pure-tone average thresholds for both groups.

Speech prosody perception in cochlear implant users with and without residual hearing.

OBJECTIVES: The detection of fundamental frequency (F0) variations plays a prominent role in the perception of intonation. Cochlear implant (CI) users with residual hearing might have access to these F0 cues. The objective was to study if and how residual hearing facilitates speech prosody perception in CI users. DESIGN: The authors compared F0 difference limen (F0DL) and question/statement discrimination performance for 15 normal-hearing subjects (NHS) and two distinct groups of CI subjects, according to the presence or absence of acoustic residual hearing: one "combined group" (n = 11) with residual hearing and one CI-only group (n = 10) without any residual hearing. To assess the relative contribution of the different acoustic cues for intonation perception, the sensitivity index d' was calculated for three distinct auditory conditions: one condition with original recordings, one condition with a constant F0, and one with equalized duration and amplitude. RESULTS: In the original condition, combined subjects showed better question/statement discrimination than CI-only subjects, d' 2.44 (SE 0.3) and 0.91 (SE 0.25), respectively. Mean d' score of NHS was 3.3 (SE 0.06). When F0 variations were removed, the scores decreased significantly for combined subjects (d' = 0.66, SE 0.51) and NHS (d' = 0.4, SE 0.09). Duration and amplitude equalization affected the scores of CI-only subjects (mean d' = 0.34, SE 0.28) but did not influence the scores of combined subjects (d' = 2.7, SE 0.02) or NHS (d' = 3.3, SE 0.33). Mean F0DL was poorer in CI-only subjects (34%, SE 15) compared with combined subjects (8.8%, SE 1.4) and NHS (2.4%, SE 0.05). In CI subjects with residual hearing, intonation d' score was correlated with mean residual hearing level (r = -0.86, n = 11, p < 0.001) and mean F0DL (r = 0.84, n = 11, p < 0.001). CONCLUSION: Where CI subjects with residual hearing had thresholds better than 60 dB HL in the low frequencies, they displayed near-normal question/statement discrimination abilities. Normal listeners mainly relied on F0 variations which were the most effective prosodic cue. In comparison, CI subjects without any residual hearing had poorer F0 discrimination and showed a strong deficit in speech prosody perception. However, this CI-only group appeared to be able to make some use of amplitude and duration cues for statement/question discrimination.

Visual activity predicts auditory recovery from deafness after adult cochlear implantation.

Modern cochlear implantation technologies allow deaf patients to understand auditory speech; however, the implants deliver only a coarse auditory input and patients must use long-term adaptive processes to achieve coherent percepts. In adults with post-lingual deafness, the high progress of speech recovery is observed during the first year after cochlear implantation, but there is a large range of variability in the level of cochlear implant outcomes and the temporal evolution of recovery. It has been proposed that when profoundly deaf subjects receive a cochlear implant, the visual cross-modal reorganization of the brain is deleterious for auditory speech recovery. We tested this hypothesis in post-lingually deaf adults by analysing whether brain activity shortly after implantation correlated with the level of auditory recovery 6 months later. Based on brain activity induced by a speech-processing task, we found strong positive correlations in areas outside the auditory cortex. The highest positive correlations were found in the occipital cortex involved in visual processing, as well as in the posterior-temporal cortex known for audio-visual integration. The other area, which positively correlated with auditory speech recovery, was localized in the left inferior frontal area known for speech processing. Our results demonstrate that the visual modality's functional level is related to the proficiency level of auditory recovery. Based on the positive correlation of visual activity with auditory speech recovery, we suggest that visual modality may facilitate the perception of the word's auditory counterpart in communicative situations. The link demonstrated between visual activity and auditory speech perception indicates that visuoauditory synergy is crucial for cross-modal plasticity and fostering speech-comprehension recovery in adult cochlear-implanted deaf patients.

Amygdala hyperactivation relates to eating behaviour: a potential indicator of food addiction in Prader-Willi syndrome.

Prader-Willi syndrome is a rare neurodevelopmental genetic disorder characterized by various endocrine, cognitive and behavioural problems. The symptoms include an obsession for food and reduced satiety, which leads to hyperphagia and morbid obesity. Neuropsychological studies have reported that Prader-Willi patients display altered social interactions with a specific weakness in interpreting social information and responding to them, a symptom close to that observed in autism spectrum disorders. In the present case-control study, we hypothesized that brain regions associated with compulsive eating behaviour would be abnormally activated by food-related odours in Prader-Willi syndrome, as these can stimulate the appetite and induce hunger-related behaviour. We conducted a brain imaging study using the olfactory modality because odours have a high-hedonic valence and can cause stronger emotional reactions than other modalities. Further, the olfactory system is also intimately associated with the endocrine regulation of energy balance and is the most appropriate modality for studies of Prader-Willi syndrome. A total of 16 Prader-Willi participants were recruited for this study, which is a significant achievement given the low incidence rate of this rare disease. The second group of 11 control age-matched subjects also participated in the brain imaging study. In the MRI scanner, using an MRI-compatible olfactometer during 56 block sessions, we randomly presented two odours (tulip and caramel), which have different hedonic valence and a different capacity to arouse hunger-related behaviour. Our results demonstrate that Prader-Willi participants have abnormal activity in the brain reward system that regulates eating behaviour. Indeed, we found that these patients had right amygdala activity up to five times higher in response to a food odour (caramel) compared with the tulip odour. In contrast, age-matched control participants had similar activity levels in response to both odours. The amygdala activity levels were found to be associated with the severity of the hyperphagia in Prader-Willi patients. Our results provide evidence for functional alteration of the right amygdala in Prader-Willi syndrome, which is part of the brain network involved in food addiction modulated by the ghrelin and oxytocin systems, which may drive the hyperphagia. Our study provides important new insights into the functioning of emotion-related brain circuits and pathology, and it is one of the few to explore the dysfunction of the neural circuits involved in emotion and addiction in Prader-Willi syndrome. It suggests new directions for the exploration and remediation of addictive behaviours.

Audiovisual Training in Virtual Reality Improves Auditory Spatial Adaptation in Unilateral Hearing Loss Patients.

Unilateral hearing loss (UHL) leads to an alteration of binaural cues resulting in a significant increment of spatial errors in the horizontal plane. In this study, nineteen patients with UHL were recruited and randomized in a cross-over design into two groups; a first group (n = 9) that received spatial audiovisual training in the first session and a non-spatial audiovisual training in the second session (2 to 4 weeks after the first session). A second group (n = 10) received the same training in the opposite order (non-spatial and then spatial). A sound localization test using head-pointing (LOCATEST) was completed prior to and following each training session. The results showed a significant decrease in head-pointing localization errors after spatial training for group 1 (24.85° ± 15.8° vs. 16.17° ± 11.28°; p < 0.001). The number of head movements during the spatial training for the 19 participants did not change (p = 0.79); nonetheless, the hand-pointing errors and reaction times significantly decreased at the end of the spatial training (p < 0.001). This study suggests that audiovisual spatial training can improve and induce spatial adaptation to a monaural deficit through the optimization of effective head movements. Virtual reality systems are relevant tools that can be used in clinics to develop training programs for patients with hearing impairments.

Capturing Visual Attention With Perturbed Auditory Spatial Cues.

Lateralized sounds can orient visual attention, with benefits for audio-visual processing. Here, we asked to what extent perturbed auditory spatial cues-resulting from cochlear implants (CI) or unilateral hearing loss (uHL)-allow this automatic mechanism of information selection from the audio-visual environment. We used a classic paradigm from experimental psychology (capture of visual attention with sounds) to probe the integrity of audio-visual attentional orienting in 60 adults with hearing loss: bilateral CI users (N = 20), unilateral CI users (N = 20), and individuals with uHL (N = 20). For comparison, we also included a group of normal-hearing (NH, N = 20) participants, tested in binaural and monaural listening conditions (i.e., with one ear plugged). All participants also completed a sound localization task to assess spatial hearing skills. Comparable audio-visual orienting was observed in bilateral CI, uHL, and binaural NH participants. By contrast, audio-visual orienting was, on average, absent in unilateral CI users and reduced in NH listening with one ear plugged. Spatial hearing skills were better in bilateral CI, uHL, and binaural NH participants than in unilateral CI users and monaurally plugged NH listeners. In unilateral CI users, spatial hearing skills correlated with audio-visual-orienting abilities. These novel results show that audio-visual-attention orienting can be preserved in bilateral CI users and in uHL patients to a greater extent than unilateral CI users. This highlights the importance of assessing the impact of hearing loss beyond auditory difficulties alone: to capture to what extent it may enable or impede typical interactions with the multisensory environment.

Effects of contrast and contrast adaptation on static receptive field features in macaque area V1.

The spatiotemporal features of the "static" receptive field (RF), as revealed with flashing bars or spots, determine other RF properties. We examined how some of these static RF features vary with contrast and contrast adaptation in area V1 of the anesthetized macaque monkey. RFs were mapped with light and dark flashing bars presented at three different contrasts, with the low and medium contrasts eliciting approximately 1/3 and 2/3 of the high-contrast response amplitude. The main results are as follows: 1) RF widths decreased when contrast decreased; however, the amount of decrease was less than that expected from an iceberg model and closer to the expectation of a contrast invariance of the RF width. 2) Area tuning experiments with drifting gratings showed an opposite effect of contrast: an increase in preferred stimulus diameter when contrast decreased. This implies that the effect of contrast on preferred stimulus size is not predictable from the static RF. 3) Contrast adaptation attenuated the effect of contrast on RF amplitude but did not significantly modify RF width. 4) RF subregion overlap was only marginally affected by changes in contrast and contrast adaptation; the classification of cells as simple and complex, when established from subregion overlap, appears to be robust with respect to changes in contrast and adaptation state. Previous studies have shown that the spatiotemporal features of the RF depend largely on the stimuli used to map the RF. This study shows that contrast is one elemental feature that contributes to the dynamics of the RF.

Evolution of crossmodal reorganization of the voice area in cochlear-implanted deaf patients.

Psychophysical and neuroimaging studies in both animal and human subjects have clearly demonstrated that cortical plasticity following sensory deprivation leads to a brain functional reorganization that favors the spared modalities. In postlingually deaf patients, the use of a cochlear implant (CI) allows a recovery of the auditory function, which will probably counteract the cortical crossmodal reorganization induced by hearing loss. To study the dynamics of such reversed crossmodal plasticity, we designed a longitudinal neuroimaging study involving the follow-up of 10 postlingually deaf adult CI users engaged in a visual speechreading task. While speechreading activates Broca's area in normally hearing subjects (NHS), the activity level elicited in this region in CI patients is abnormally low and increases progressively with post-implantation time. Furthermore, speechreading in CI patients induces abnormal crossmodal activations in right anterior regions of the superior temporal cortex normally devoted to processing human voice stimuli (temporal voice-sensitive areas-TVA). These abnormal activity levels diminish with post-implantation time and tend towards the levels observed in NHS. First, our study revealed that the neuroplasticity after cochlear implantation involves not only auditory but also visual and audiovisual speech processing networks. Second, our results suggest that during deafness, the functional links between cortical regions specialized in face and voice processing are reallocated to support speech-related visual processing through cross-modal reorganization. Such reorganization allows a more efficient audiovisual integration of speech after cochlear implantation. These compensatory sensory strategies are later completed by the progressive restoration of the visuo-audio-motor speech processing loop, including Broca's area.