Navegando por Assunto "Entropia conjunta"

Agora exibindo 1 - 2 de 2

Acesso aberto (Open Access)
Division of labor between M and P visual pathways: different visual pathways minimize joint entropy differently
(2008-06) SILVEIRA, Luiz Carlos de Lima; SAITO, Cézar Akiyoshi; MELLO JÚNIOR, Harold Dias de; SILVEIRA, Vladímir de Aquino; SOUZA, Givago da Silva; RODRIGUES, Anderson Raiol; SILVA FILHO, Manoel da
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.
Acesso aberto (Open Access)
Entropia conjunta de espaço e reqüência espacial estimada através da discriminação de estímulos espaciais com luminância e cromaticidade moduladas por funções de Gábor: implicações para o processamento paralelo de informação no sistema visual humano
(Universidade Federal do Pará, 2013-12-06) SILVEIRA, Vladímir de Aquino; SOUZA, Givago da Silva; http://lattes.cnpq.br/5705421011644718
The objective of this study was to estimate the joint entropy of the human visual system in the domains of space and spatial frequency by using psychometric functions. The psychometric functions were obtained from stimulus discrimination that had luminance or chromaticity modulated by Gábor functions. The method consisted in evaluating the entropy in the space domain by testing subject capacity to discriminate stimuli that differed only in their spatial extent and in evaluating the entropy in the spatial frequency domain by testing subject capacity to discriminate stimuli that differed only in their spatial frequency. The joint entropy was then estimated from these two individual entropy values. Three visual conditions were studied: achromatic, chromatic without fine tuning correction of equiluminance, and chromatic with full equiluminance correction by using heterochromatic flickker photometry. Four subjects were tested in all conditions, two additional subjects were tested in the chromatic condition without fine equiluminance adjustment and a seventh subject also performed the acrhomatic test. All subjects were examined by an ophthalmologist, their eyes and visual system were considered normals, and presented no report, symptoms or signs of visual dysfunctions or diseases that could have affected their visual system. The subjects had their normal or corrected visual acuity of 20/30 minimum. The work was approved by the Comissão de Ética em Pesquisa (Núcleo de Medicina Tropical, UFPA) and followed the recomendations of the Helsinki Declaration. The Gábor functions used for luminance or chromaticity modulation comprised unidimensional horizontal sinusoidal gratings, modulated in the vertical direction, with bidimensional Gaussian envelopes whose spatial extent was measured by their standard deviation. Stimuli were generated by using a software written in Pascal in a Delphi 7 Enterprise environment. A Dell Precision 390 Workstation was used together with a CRS VSG ViSaGe stimulus generator to display the stimuli in a CRT monitor, 20”, 800 x 600 pixels, 120 Hz, RGB, Mitsubishi Diamond Pro 2070SB. In the achromatic experiments, the stimuli were generated by white luminance modulation (CIE1931: x = 0.270, y = 0.280; CIE1976: u’ = 0.186, v’ = 0.433), 44,5 cd/m2 mean luminance. In the chromatic experiments, mean luminance was kept in 15 cd/m2, and two series of red-green stimuli were used. In the first series, two chromaticities defined on the M-L axes of the DKL color space were used (CIE1976: green, u’=0.131, v’=0.380; red, u’=0.216, v’=0.371). In the second series, two chromaticities were defined along a red-green horizontal axis across the CIE1976 color space (CIE1976: green, u’=0.150, v’=0.480; red, u’=0.255, v’=0.480). Throughout the experiment, the reference stimuli comprised gratings with three different spatial frequencies (0.4, 2, and 10 cycles per degree) and a Gaussian envelope with 1 degree standard deviation. The test stimuli comprised 19 different spatial frequencies in the region of the reference spatial frequency and 21 different Gaussian envelopes in the region of the reference standard deviation. In the achromatic condition, four levels of Michelson contrast were studied: 2%, 5%, 10% e 100%. In the two chromatic conditions, the highest level of pooled cone contrast allowed by the CRT gamut was used, 17%. The procedure consisted of a two interval forced choice with the following steps: i) 1 s display of the reference stimulus; ii) 1 s replacement of the reference stimulus by a background with the same luminance and chromaticity; iii) 1 s display of the test stimulus which differed from the reference stimulus either in spatial frequency or spatial extent, together with a beep to tell the subject that it was now neccessary to provide a response if the two stimuli were equal or different; iv) replacement of the test stimulus by the background. The spatial extent or spatial frequency of the test stimulus was randomly changed from trial to trial by usind the method of constant stimuli. In a series comprising 300 trials, the spatial frequency was changed while in another series also comprising 300 trials, the spatial extent was changed, each test stimulus in each series being displayed at least 10 times. The subject response in every trial was stored as correct or incorrect for further use to estimate the psychometric function. The experimental data of the psychometric functions for spatial extent and spatial frequency at each contrast level, which corresponded to percent of correct responses, were fitted with Gaussian functions using the Least Square Method. For each contrast level, the spatial extent entropy and spatial frequency entropy were estimated from the standard deviations of these Gaussian functions. The joint entropy was then calculated by multiplying the square root of the spatial extent entropy by the spatial frequency entropy. The joint entropy values were compared with the theoretical minimum predicted for linear systems, 1/4π or 0.0796. For low and intermediate spatial frequencies at high contrasts, the joint entropy reached very low levels, below this minimum, suggesting that there were nonlinear interactions between two or more visual mechanisms. This phenomenon occurred in all conditions (achromatic, chromatic, and chromatic with fine equiluminance adjustment) and was more pronounced for spatial frequency 0.4 cycles / degree. A possible explanation for this phenomenon is the occurrence of nonlinear interactions between the retino-geniculo-striate visual pathways, such as the K, M, and P pathways, in the primary visual area or in higher levels of neural processing of visual information.