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Vladimir A. Silveira, Givago S. Souza, Bruno D. Gomes, Anderson R. Rodrigues, Luiz Carlos L. Silveira; Chromatic Discrimination Of Gabor Functions: Space And Spatial Frequency Joint Entropy For The Human Visual System Estimated From Psychometric Functions. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6392.
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© ARVO (1962-2015); The Authors (2016-present)
To estimate space and spatial frequency joint entropy of the human visual system by using psychometric functions obtained from discrimination of chromatic Gabor functions. To compare the results with previous results obtained with similar achromatic stimuli.
Five subjects were tested in all conditions. Stimuli were written in PASCAL, using Delphi 7 Enterprise environment, in a Dell Precision 390 Workstation. VSG Visage stimulus generator (CRS) was used. Stimuli were displayed in a Mitsubishi Diamond Pro 2070SB CRT, 20", 800 x 600 pixels, 120 Hz frame rate. Two series of red-green chromatic stimuli were used: two chromaticities defined along the DKL space M-L axis (CIE1976: red, u’=0.216, v’=0.371; green, u’=0.131, v’=0.38); and two chromaticities defined along a horizontal axis centered in CIE1976 coordinates (red, u’=0.255, v’=0.48; u’=0.15, v’: 0.48). The highest color contrast allowed by the monitor gamut was used: 14% (DKL) and 30.2% (CIE1976). Mean luminance, 36 cd/m2. Procedures were the same as previously used to study achromatic stimulus discrimination (Silveira et al., 2011, ARVO Abstracts).
Joint entropy values ranged as follows. Space 1º, spatial frequency 0.4 cpd: mean joint entropy was 0.0254 (DKL), 0.0259 (CIE1976), and 0.0221(achromatic). Space 1º, spatial frequency 2 cpd: mean joint entropy was 0.0881 (DKL), 0.0809 (CIE1976), and 0.0599 (achromatic). Space 1º, spatial frequency 10 cpd: mean joint entropy was 0.7216 (DKL), 0.8882 (CIE1976), and 0.6344(achromatic). Joint entropy was similar for high contrast color stimuli specified either in DKL or CIE1976 color spaces. It remained above the theoretical limit for one dimensional Gabor functions (1/4π) (Daugman et al., 1985) between 2 cpd and 10 cpd, but it fell below the theoretical minimum for very low spatial frequencies (0.4 cpd). The main difference between chromatic and achromatic stimuli occurred at 2 cpd: while for chromatic stimuli joint entropy remained above the theoretical minimum, for achromatic stimuli it fell below the minimum.
Joint entropy may reach values below the theoretical minimum only if non-linear interactions between two or more visual pathways, such as the M and P pathways, occur at higher visual system levels (Silveira, 1998; Silveira et al., 2008). For chromatic stimuli at very low spatial frequencies, M and P pathways may cooperate for the stimulus detection.
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