KEYWORDS: Visualization, Brain, Emotion, Electrodes, Electroencephalography, Signal processing, Information visualization, Frequency response, Data processing, Time-frequency analysis
The brain has the ability to evaluate unattended social information, such as facial expressions, and reassign attentional resources to specific relevant features. Two neuronal mechanisms could account for such facial emotional processing: one slow and accurate system that can be measured around 170 ms and one fast and imprecise system that is triggered around 90 ms which could support early negative emotional processing for automatic/unattended and peripheral stimulation. Evidence that these mechanisms exist for positive affective processing is scarce. The present study investigated the neural correlates of unattended negative and positive emotional processing using the rapid presentation of unilateral and bilateral peripheral facial expressions. Hence, we measured the electrophysiological correlates of unattended fear, happy and neutral faces presented in the left and right hemifields of neurotypical individuals using a frequency tagging paradigm and electroencephalography. Frequency stimulations of 5.8 Hz and 11 Hz were chosen to induce Steady-State Visual Evoked Potential (SSVEP) occurring at 170 ms and 90 ms, respectively. The SSVEP amplitudes showed that unattended positive and negative information in the periphery was processed at early stages and increased the brain's response to attended salient emotional stimuli in posterior visual regions. These results suggest that emotional stimuli presented outside the attentional focus elicit increased brain activity, particularly in posterior regions which could be altered in disorders of social recognition.
Attentional processing in the absence of conscious vision has yet to be understood in terms of neurophysiological mechanisms. Therefore, we used the visual mismatch negativity (vMMN) to determine if automatic detection of changes can be followed by an attentional switch without visual awareness. Random moving dots changing in direction were presented in the periphery, while participants carried out an effortful Stroop test in the central visual field to fully engage their attention on this primary task. The results revealed a posterior vMMN at 200 ms that was maximal in the parietal regions, revealing an automatic detection of change in the absence of visual awareness related to a dorsal/magnocellular pathway. Moreover, a frontal and central positivity, with a more pronounced activity in the left frontal areas was found at 300 ms possibly reflecting (1) unconscious attentional switch, (2) inhibition of explicit attentional switch by the left frontal areas acting on the right frontal areas via interhemispheric connections (3) inhibition of explicit attentional switch by the frontal areas acting on the central area via top-down connections. In conclusion, our results showed that vMMN could be a useful tool to study detection of changes and attentional mechanisms in the absence of visual consciousness.
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