O processamento visual de contrastes s radiações eletromagnéticas dentro da faixa de comprimentos de ondas entre aproximadamente 380 nm e 760 nm vindas do ambiente entram em contato com fotorreceptores na retina e podem ser convertidas em sinais eletroquímicos (Luo, 1999). Esses sinais são transmitidos e codificados através de redes neuronais que levam, entre outros efeitos, à geração da percepção visual consciente. Desde a recepção da energia eletromagnética até a geração da percepção consciente, ocorrem diversos processos de codificação da informação cuja compreensão é fundamental para entender-se como a visão funciona. Os processos mais distais são mais bem entendidos, enquanto aqueles que ocorrem progressivamente mais centralmente são da mesma forma progressivamente menos entendidos.Nos fotorreceptores, a codificação da informação luminosa ocorre através da transdução da quantidade de fótons absorvidos pelos fotopigmentos em amplitudes de alterações graduadas do potencial de membrana da célula fotorreceptora Pugh Jr., 1992;2006). As respostas elétricas dos fotorreceptores aos estímulos luminosos são hiperpolarizantes (Naka; Rushton, 1966). Em síntese, os fotorreceptores funcionam como contadores dos fótons absorvidos, em outras palavras, do número de eventos de absorção de fótons pelos fotopigmentos que levam a fotoisomerizações desses fotopigmentos e ativação da cascata de fototransdução Pugh Jr., 1992; Yau, 1994).A partir da próxima etapa do processamento visual, a sinapse entre fotorreceptores e células bipolares, inicia-se uma transformação da informação luminosa. As células bipolares comparam as informações oriundas de diferentes fotorreceptores, fazendo surgir pela primeira vez dentro do processamento visual a informação de contraste (Werblin; Dowling, 1969; Zhang; Wu, 2009). As células bipolares fazem a codificação de contraste graças à organização de seus campos receptivos com diferentes propriedades de ativação / inibição entre as áreas centrais e periféricas desses campos. A estrutura do campo receptivo com antagonismo funcional entre seu centro e periferia é o circuito básico para o processamento da informação espacial no sistema visual. A geração da oponência entre centro e periferia dos campos receptivos das células bipolares tem origem em acoplamentos entre células bipolares e na combinação de várias vias sinápticas mediadas por células horizontais e amácrinas (para maiores detalhes
We used psychometric functions to estimate the joint entropy for space discrimination and spatial frequency discrimination. Space discrimination was taken as discrimination of spatial extent. Seven subjects were tested. Gábor functions comprising unidimensionalsinusoidal gratings (0.4, 2, and 10 cpd) and bidimensionalGaussian envelopes (1°) were used as reference stimuli. The experiment comprised the comparison between reference and test stimulithat differed in grating's spatial frequency or envelope's standard deviation. We tested 21 different envelope's standard deviations around the reference standard deviation to study spatial extent discrimination and 19 different grating's spatial frequencies around the reference spatial frequency to study spatial frequency discrimination. Two series of psychometric functions were obtained for 2%, 5%, 10%, and 100% stimulus contrast. The psychometric function data points for spatial extent discrimination or spatial frequency discrimination were fitted with Gaussian functions using the least square method, and the spatial extent and spatial frequency entropies were estimated from the standard deviation of these Gaussian functions. Then, joint entropy was obtained by multiplying the square root of space extent entropy times the spatial frequency entropy. We compared our results to the theoretical minimum for unidimensional Gábor functions, 1/4π or 0.0796. At low and intermediate spatial frequencies and high contrasts, joint entropy reached levels below the theoretical minimum, suggesting non-linear interactions between two or more visual mechanisms. We concluded that non-linear interactions of visual pathways, such as the M and P pathways, could explain joint entropy values below the theoretical minimum at low and intermediate spatial frequencies and high contrasts. These non-linear interactions might be at work at intermediate and high contrasts at all spatial frequencies once there was a substantial decrease in joint entropy for these stimulus conditions when contrast was raised.
Visual perception and action are strongly linked with parallel processing channels connecting the retina, the lateral geniculate nucleus, and the input layers of the primary visual cortex. Achromatic vision is provided by at least two of such channels formed by the M and P neurons. These cell pathways are similarly organized in primates having different lifestyles, including species that are diurnal, nocturnal, and which exhibit a variety of color vision phenotypes. We describe the M and P cell properties by 3D Gábor functions and their 3D Fourier transform. The M and P cells occupy different loci in the Gábor information diagram or Fourier Space. This separation allows the M and P pathways to transmit visual signals with distinct 6D joint entropy for space, spatial frequency, time, and temporal frequency. By combining the M and P impacts on the cortical neurons beyond V1 input layers, the cortical pathways are able to process aspects of visual stimuli with a better precision than it would be possible using the M or P pathway alone. This performance fulfils the requirements of different behavioral tasks.
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