Purchase this article with an account.
Vladimir A. Silveira, Givago S. Souza, Bruno D. Gomes, Anderson R. Rodrigues, Luiz Carlos L. Silveira; Achromatic Discrimination Of Gabor Functions: Space And Spatial Frequency Joint Entropy For The Human Visual System Estimated From Psychometric Functions. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1882.
Download citation file:
© 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 black-and-white Gabor functions.
Stimuli were driven by Visage system (CRS). Stimuli were displayed in a CRT monitor (Mitsubishi Diamond Pro 2070SB CRT). Reference stimuli were horizontal, achromatic, stationary sine wave gratings of 0.4 (n = 3), 2 (n = 6), and 10 cpd (n = 3) enveloped by Gaussian functions of 1º standard deviation. The reference stimulus was presented for 1 s, followed by a blank screen with the same mean luminance and chromaticity for 1 s, a test stimulus with different spatial frequency or standard deviation for another 1 s, and again a blank screen with a tone to indicate that the subject had to signal if both stimuli were equal or different by pressing a CB6 Response Box (CRS). We tested 21 different test stimuli around 1º of standard deviation and 19 different test stimuli around the reference spatial frequency (0.4, 2, or 10 cpd) at 100%, 10%, 5%, and 2% Michelson contrasts. Space or spatial frequency entropy were taken as the standard deviations of the Gaussian functions fitted to the data of space or spatial frequency discrimination, and the joint entropy was then estimated by multiplying the spatial frequency entropy by the square root of the space entropy. The results were then compared with the theoretical limit for one dimensional Gabor functions (1/4π) (Daugman et al., 1985).
For all spatial frequencies, mean joint entropy decreased as a function of Michelson contrast. Space 1º, spatial frequency 0.4 cpd: mean joint entropy reached values below the theoretical limit at 100% contrast. Space 1º, spatial frequency 2 cpd: mean joint entropy reached values below the theoretical limit between 5% and 100% contrast. Space 1º, spatial frequency 10 cpd: mean joint entropy remained higher than theoretical limit for all contrast conditions.
Joint entropy may reach values below the theoretical limit 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). This effect may explain the results for high contrasts at intermediate spatial frequencies (2 cpd). For very low or very high spatial frequencies, a single visual pathway may be in charge of measuring luminance contrast.
This PDF is available to Subscribers Only