Altered Neural Network Dynamics During the Visual Processing in Early Deaf Individuals

Introduction As a consequence of a sensory deprivation, the neural circuits serving spared sensory systems (e.g. visual cortical areas in case of deafness) can undergo functional changes (Bavelier and Neville 2002; Pavani and Röder 2012). This phenomenon is named intramodal plasticity. Deaf individuals and cochlear implant (CI) users have been shown to display a reduced responsiveness of the visual cortex following a visual onset as compared to hearing controls (e.g. Bottari et al., 2014; Sandmann et al., 2012), as well as an enhanced adaptation to repeated visual stimuli (Chen et al.,2017). The present work used a visual Mismatch Negativity (vMMN) paradigm to test whether early deafness impacts on visual stimuli-specific adaptation and on the response to a novel, unattended visual stimulus. We focused on the neural oscillatory dynamics of visual adaptation and vMMN in deaf individuals and hearing controls. Methods Brain responses in 11 early deaf adults (female N = 7; mean age = 36.64 years, SD = 9.99) and 11 matched hearing controls (female N = 6; mean age = 33.09 years; SD = 6.85) were measured during processing of a continuous stream of moving visual stimuli. A deviant stimulus (change in motion direction) was always preceded by at least three repetitions of standard stimuli (S1, S2 and S3). Neural oscillatory activity was measured by Electroencephalography (EEG). The analysis focused on the neural network dynamics between 4 and 40 Hz. The Signal was analysed between -100 to 400 ms after stimulus onset. A cluster-based permutation approach across the entire frequency range and electrodes was adopted for all comparisons. Results The group comparisons revealed: (1) during the adaptation phase (S1 vs. S3; S2 vs. S3), between about 200 and 300ms after stimulus onset, the deaf group displayed a significant reduction in the non-phase-locked (induced) theta power as compared to hearing controls; (2) no between group differences emerged in phase-locked (evoked) visual response; (3) in response to deviant stimuli (vMMN), a significant increase of the induced alpha power was found in deaf individuals as compared to hearing controls, between about 50 and 150 ms after stimulus onset. Conclusions These results provide novel evidence for two intramodal changes as a result of auditory deprivation: (1) greater adaptation to repeated visual stimuli, (2) stronger early-automatic attentional response in case of a novel visual event. Both mechanisms are compatible with a more efficient response in the visual cortical areas in early deaf individuals as compared to hearing controls.