O papel da sincronização da actividade neuronal no tálamo e córtex visual de mamíferos.

Os neurofisiologistas da visão tentam identificar os mecanismos celulares e de sistema que estão subjacentes à percepção visual. Esta é reconhecida como um processo construtivo, em que o mundo físico externo é organizado em representações de objectos. O nosso sistema visual tende a integrar os sinais provenientes da retina em superfícies e finalmente em objectos perceptuais. Isto significa que o mesmo estímulo físico pode, em situações ambíguas, originar representações alternativas e distintas do mundo visual, de acordo com diferentes critérios de organização perceptual implementados ao nível neuronal. O nosso trabalho veio mostrar que oscilações sincrónicas ocorrem simultaneamente em diversos níveis do sistema visual de uma forma que reflecte propriedades globais do estímulo visual. Identificámos mecanismos distintos de sincronização a nível cortical e subcortical, correlacionados com diferentes atributos do estímulo visual. Demonstrámos subsequentemente que a actividade de células isoladas ou grupos de células no córtex visual do gato pode modificar-se em função das possíveis interpretações perceptuais e de acordo com leis psicofísicas que regem a visão natural. Representações neuronais distintas parecem assim evoluir dinamicamente através de uma estratégia de sincronização e dessincronização. Um objecto seria assim representado por uma população neuronal sincronizada e a sua cisão em dois ocasionaria a emergência de duas populações dessincronizadas entre si. Efectuámos por fim estudos de imagiologia funcional em humanos (Ressonância Magnética Funcional) que mostraram que diferentes padrões de actividade cortical emergem consoante o tipo de interpretação perceptual, mesmo quando o estímulo visual permanece físicamente constante. A variação da dinâmica de activação cerebral para estímulos ambíguos, que induzem alternâncias perceptuais na presença de um padrão de estimulação constante na retina, mostra que o córtex cria representações dinâmicas da cena visual, muito para além de uma mera recriação fiel mas passiva do mundo exterior

Neural Responses of the Anterior Ventral Occipitotemporal Cortex in Developmental Dyslexia: Beyond the Visual Word Form Area

For the past 2 decades, neuroimaging studies in dyslexia have pointed toward a hypoactivation of the ventral occipitotemporal cortex (VOTC), a region that has been closely associated to reading through the extraction of a representation of words which is invariant to position, size, font or case. However, most of the studies are confined to the visual word form area (VWFA), while recent studies have demonstrated a posterior-to-anterior gradient of print specificity along the VOTC. In our study, the whole VOTC, partitioned into three main patches of cortex, is assessed in dyslexic and control adults. A total of 30 participants were included in this study (14 developmental dyslexics and 16 age- and education-matched controls). The design consisted of alternately viewed blocks of stimuli from a given class (words, consonant strings, phase-scrambled words, phasescrambled consonant strings, small checkerboards, large checkerboards). The analyzed contrast was print stimuli (words and consonants) versus scrambled stimuli and checkerboards. RESULTS. Corroborating previous findings, our results showed underactivation to print stimuli in the VWFA of dyslexics. Additionally, differences between dyslexics and controls were also found, particularly in an area of the anterior partition of the VOTC, suggesting a relevant role of this area in word processing. CONCLUSIONS. In sum, our study goes beyond the underactivation hypothesis in the VWFA of dyslexics and indicates that a particular area on the anterior fusiform region might be particularly involved in the reading deficits in dyslexia, demonstrating the involvement of multiple areas within VOTC in reading processes

Effects of anodal multichannel transcranial direct current stimulation (tDCS) on social-cognitive performance in healthy subjects: A randomized sham-controlled crossover pilot study

Recent studies suggest that temporoparietal junction (TPJ) modulation can influence attention and social cognition performance. Nevertheless, no studies have used multichannel transcranial direct current stimulation (tDCS) over bilateral TPJ to estimate the effects on these neuropsychological functions. The project STIPED is using optimized multichannel stimulation as an innovative treatment approach for chronic pediatric neurodevelopmental disorders, namely in children/adolescents with Autism Spectrum Disorder (ASD). In this pilot study, we aim to explore whether anodal multichannel tDCS coupled with a Joint Attention Task (JAT) influences social-cognitive task performance relative to sham stimulation, both in an Emotion Recognition Task (ERT) and in a Mooney Faces Detection Task (MFDT), as well as to evaluate this technique's safety and tolerability. Twenty healthy adults were enrolled in a randomized, single-blinded, sham-controlled, crossover study. During two sessions, participants completed the ERT and the MFDT before and after 20min of sham or anodal tDCS over bilateral TPJ. No significant differences on performance accuracy and reaction time were found between stimulation conditions for all tasks, including the JAT. A significant main time effect for overall accuracy and reaction time was found for the MFDT. Itching was the most common side effect and stimulation conditions detection was at chance level. Results suggest that multichannel tDCS over bilateral TPJ does not affect performance of low-level emotional recognition tasks in healthy adults. Although preliminary safety and tolerability are demonstrated, further studies over longer periods will be pursued to investigate the clinical efficacy in children/adolescents with ASD, where social cognition impairments are preponderant

A Fundamental Distinction in Early Neural Processing of Implicit Social Interpretation in Schizophrenia and Bipolar Disorder

Background: Social cognition impairment is a key phenomenon in serious mental disorders such as schizophrenia (SCZ) and bipolar disorder (BPD). Although genetic and neurobiological studies have suggested common neural correlates, here we hypothesized that a fundamental dissociation of social processing occurs at an early level in these conditions. Methods: Based on the hypothesis that key structures in the social brain, namely the temporoparietal junction, should present distinctive features in SCZ and BPD during low-level social judgment, we conducted a case-control study in SCZ (n = 20) and BPD (n = 20) patients and controls (n = 20), using task-based fMRI during a Theory of Mind (ToM) visual paradigm leading to interpretation of social meaning based on simple geometric figures. Results: We found opposite neural responses in two core ToM regions: SCZ patients showed social content-related deactivation (relative to controls and BPD) of the right supramarginal gyrus, while the opposite pattern was found in BPD; reverse patterns, relative to controls and SCZ, were found in the left posterior superior temporal gyrus, a region involved in inferring other’s intentions. Receiver-operating-characteristic curve analysis showed 88% accuracy in discriminating the two clinical groups based on these neural responses. Conclusions: These contrasting activation patterns of the temporoparietal junction in SCZ and BPD represent mechanistic differences of social cognitive dysfunction that may be explored as biomarkers or therapeutic targets

Optical Properties Influence Visual Cortical Functional Resolution After Cataract Surgery and Both Dissociate From Subjectively Perceived Quality of Vision

Purpose: To investigate the relation between optical properties, population receptive fields (pRFs), visual function, and subjectively perceived quality of vision after cataract surgery. Methods: The study includes 30 patients who had recently undergone bilateral sequential cataract surgery. We used functional magnetic resonance imaging and pRF modelling methods to assess pRF sizes across visual cortical regions (V1-V3). Subjects also performed a complete ophthalmologic and psychophysical examination and answered a quality of vision questionnaire. Results: Subjects with worse optical properties had, as predicted, larger pRF sizes. In addition, analysis in the primary visual cortex revealed significantly larger mean pRF sizes for operated subjects with worse contrast sensitivity (P = 0.038). In contrast, patients who scored high in the subjective "bothersome" dimension induced by dysphotic symptoms had surprisingly lower pRF size fitting interception (P = 0.012) and pRF size fitting slopes (P = 0.020), suggesting a dissociation between objective quality of vision and subjective appraisal. Conclusions: Optical properties of the eye influence pRF size. In particular, visual aberrations have a negative impact on visual cortical processing. A novel dissociation between subjective reports of quality of vision and pRF sizes was further identified. This suggests that patients with better cortical resolution may have a negative subjective response possibly because of improved perception of dysphotic phenomena. pRF properties represent a valuable quantitative measure to objectively evaluate quality of vision but do not necessarily predict subjective complaints