Browsing by Author "Castelhano, Joao"

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    Oscillations or Synchrony? Disruption of Neural Synchrony despite Enhanced Gamma Oscillations in a Model of Disrupted Perceptual Coherence
    (2015) Castelhano, Joao; Bernardino, Inés; Rebola, José; Rodríguez B., Eugenio; Castelo Branco, Miguel
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    The Dual Facet of Gamma Oscillations: Separate Visual and Decision Making Circuits as Revealed by Simultaneous EEG/fMRI
    (2014) Castelhano, Joao; Duarte, Isabel Catarina; Wibral, Michael; Rodriguez, Eugenio; Castelo-Branco, Miguel
    It remains an outstanding question whether gamma-band oscillations reflect unitary cognitive processes within the same task. EEG/MEG studies do lack the resolution or coverage to address the highly debated question whether single gamma activity patterns are linked with multiple cognitive modules or alternatively each pattern associates with a specific cognitive module, within the same coherent perceptual task. One way to disentangle these issues would be to provide direct identification of their sources, by combining different techniques. Here, we directly examined these questions by performing simultaneous EEG/fMRI using an ambiguous perception paradigm requiring holistic integration. We found that distinct gamma frequency sub-bands reflect different neural substrates and cognitive mechanisms when comparing object perception states vs. no categorical perception. A low gamma sub-band (near 40 Hz) activity was tightly related to the decision making network, and in particular the anterior insula. A high gamma sub-band (approximate to 60 Hz) could be linked to early visual processing regions. The demonstration of a clear functional topography for distinct gamma sub-bands within the same task shows that distinct gamma-band modulations underlie sensory processing and perceptual decision mechanisms. Hum Brain Mapp 35:5219-5235, 2014. 2014 Wiley Periodicals, Inc.
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    To Perceive or Not Perceive: The Role of Gamma-band Activity in Signaling Object Percepts
    (2013) Castelhano, Joao; Rebola, Jose; Leitao, Bruno; Rodriguez, Eugenio; Castelo-Branco, Miguel
    The relation of gamma-band synchrony to holistic perception in which concerns the effects of sensory processing, high level perceptual gestalt formation, motor planning and response is still controversial. To provide a more direct link to emergent perceptual states we have used holistic EEG/ERP paradigms where the moment of perceptual "discovery'' of a global pattern was variable. Using a rapid visual presentation of short-lived Mooney objects we found an increase of gamma-band activity locked to perceptual events. Additional experiments using dynamic Mooney stimuli showed that gamma activity increases well before the report of an emergent holistic percept. To confirm these findings in a data driven manner we have further used a support vector machine classification approach to distinguish between perceptual vs. non perceptual states, based on time-frequency features. Sensitivity, specificity and accuracy were all above 95%. Modulations in the 30-75 Hz range were larger for perception states. Interestingly, phase synchrony was larger for perception states for high frequency bands. By focusing on global gestalt mechanisms instead of local processing we conclude that gamma-band activity and synchrony provide a signature of holistic perceptual states of variable onset, which are separable from sensory and motor processing.