Navegando por Assunto "Flicker"

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Efeitos da adaptação ao flicker de luminância sobre o potencial cortical provocado visual
(Universidade Federal do Pará, 2015-08-20) LOUREIRO, Terezinha Medeiros Gonçalves de; SOUZA, Givago da Silva; http://lattes.cnpq.br/5705421011644718
Visual Evoked Potential (VEP) has been a useful method to evaluate spatial vision in humans. Sustained observation of a visual stimulus produces several changes in neural responses at different processing levels in visual system. Previous studies has elucidated how primary visual cortex processing spatial information. Many others studies has also suggested about the contribution of parallel pathways M and P activation on the visual cortical responses evoked by a stimuli that excite only one of these pathways. Cortical excitation through a kind of stimulus that promotes one or both preferential adaptation could be a valuable approach to study activity from M and P pathways interactions on the visual responses. The purpose of this study is to evaluate the effects of luminance flicker adaptation on cortical responses elicited under favorable conditions of joint or differential M and P pathways activation, leading to an increase or decrease cortical responses. Eight subjects (20.25 ± 1.5) with normal vision acuity or corrected to 20/20 were tested. VEPs were recorded under three conditions of visual stimulation with no adaptation: sinusoidal gratings at 0.4, 2 and 10cpd presented at 1 Hz pattern-reversal stimulus (test stimuli). Other conditions was elicited by two-dimensional Gaussian mask adaptation stimulus with luminance variation in time domain (flicker) presented at 5 Hz, 10 Hz and 30 Hz temporal modulation. The experiment consisted on VEPs records above occipital scalp elicited by 8 seconds of adaptation stimulus followed by 2 seconds test stimuli. Cortical responses were evaluated in the time and temporal frequencies domain. In the time domain were measured latency and the P1 component amplitude (peak-line), while in the temporal frequency domain were evaluated amplitudes of alpha, beta and gamma frequency bands present in the in the records. VEPs elicited by the test stimuli were compared between flicker adaptation and no adaptation conditions. Main findings consisted on flicker adaptation that occurred differently at spatial frequencies domain. Results showed P1 component in all stimulation conditions and flicker adaptation at lower spatial frequency (0.4 cpd) in all time conditions. It has also showed a reduction at alpha band energy and an increase in the gamma band at same condition. This study concluded that flicker adaptation led to VEP amplitude decreased due to loss of alpha oscillations energy and gamma band energy increased at 0.4 cpd, and it represented a modification on the balance between M and P visual pathways.