Ausência de efeito do ruído de luminância sobre as respostas corticais provocadas por estímulos pseudoisocromáticos

Abstract

Luminance noise in a pseudoisochromatic stimulus is featured by the spatial random distribution of the different luminance values. This characteristic makes possible to eliminate brightness clues between target and background of the stimuli, enabling that the perception of the differences between them occurs due their chromatic content. It has been observed that changes in the parameters of the luminance noise modify the perceptual discrimination between the color of the target and background of the stimulus. It is unknown where and how the luminance and color information interact to drive the perceptual modifications. The aim of this dissertation was to evaluate the influence of the chances in the luminance noise on the cortical responses evoked by pseudoisochromatic stimulus. This research was approved by the Ethics Committee for Humans Research of the Tropical Medicine Institute, Federal University of Pará, report #991.803. The sample comprised by 16 healthy trichromat participants, ranged between 24 e 36 years (26.5 ± 3.4 years), nine women, seven men. The stimuli were generated in the ViSaGe system and were composed by 6° red-green pseudoisochromatic gratings 6°, spatial frequency of 2 cpd in a patch of 7.25°. A mosaic composed by 2448 circles varying in size and luminance. Two experiments tested two different ways of manipulation of the luminance noise, keeping the same mean luminance (15 cd/m2) of the stimulus at all stimulus conditions: (i) In the first experiment, 16 participants of the sample were tested. There was variation of the luminance noise amplitude among 4 conditions: 20 cd/m2; 12 cd/m2; 4 cd/m2 and 1 cd/m2, keeping the same number of discrete values of luminance at all conditions (5 values). (ii) In the second experiment, 7 participants of the sample were tested. There was variation of the luminance level among three conditions: 2, 5 e 11 levels, keeping the luminance noise amplitude of 20 cd/m2 at all conditions. The cortical activity was recorded by goldsurface electrodes place at Oz (active electrode), Fpz (reference electrode) and Fz (ground electrode). The electrical signal was amplified 50000 times, digitized at 1000 Hz and filtered between 0.1 and 100 Hz. The response magnitude was measured between the baseline and the negative component peak (N1) at 120 ms and of the positive components P1 and P2, at 70 and 230 ms, respectively. The time to response was quantified as the duration between the stimulus onset to the component peaks. It was used one-way ANOVA to compare the component amplitude and latency evoked by the different stimulus conditions. The first experimente result showed that there was no significant effect of the changes of the luminance noise amplitude on the amplitude from P1 (F[3,60] = 0.6452, p = 0.5929), N1 (F[3,60] = 0.0744, p = 0.97) e P2 (F[3,60] = 0.5183, p = 0.67655). The same result happens to the latency comparison, there was no significant effect of the luminance noise amplitude changes on the component latencies from P1 (F[3,60] = 1.0987, p = 0.3578), N1 (F[3,60] = 0.0509, p = 0.98) e P2 (F[3,60] = 1.0384, p = 0.3831). As observed in the first experiment, the results of the second experiment showed that there was no significant effect of the luminance noise level manipulation of the stimulus on the components amplitude P1 (F[2,18] = 0.1847, p = 0.8339), N1 (F[2,18] = 0.121, p = 0.89) e P2 (F[2,18] = 0.1057, p = 0.8998), as well as no significant effect of the luminance level modifications on the component latencies P1 (F[2,18] = 0.8327, p = 0.5457), N1 (F[2,18] = 0.347, p = 0.71) e P2 (F[2,18] = 1.1412, p = 0.3423). The main finding of this dissertation was that, although luminance noise changes reveals changes in psychophysical responses, they do not change on the cortical responses. It is suggested that the alteration of the chromatic discrimination observed psychophysically due the manipulation of the luminance noise should not happen in the primary visual cortex, but probably in morpho-functional substrates in higher cortical areas.