Contrast (vision)

About: Contrast (vision) is a research topic. Over the lifetime, 10379 publications have been published within this topic receiving 221480 citations.

Spatial processing of visual information in the movement-detecting pathway of the fly

Abstract: 1. Spatial processing of visual signals in the fly's movement-detecting pathway was studied by recording the responses of directionally-selective movement-detecting (DSMD) neurons in the lobula plate. The summarized results pertain to a type of neuron which preferentially responds to horizontal movement directed toward the animal's midline. Three kinds of visual stimuli were used: moving gratings, reversing-contrast gratings and reversing-contrast bars. 2. Contrast-sensitivity functions were measured for reversing-contrast gratings. With horizontally-oriented gratings, sensitivity is maximum at the lowspatial-frequency end and falls off toward high frequencies. With vertically-oriented gratings, sensitivity is maximum at an intermediate spatial frequency (Fig. 7). These results are consistent with a neural organization in which the DSMD neuron receives its input through an array of small-field (“ sampling”) units, each unit having a receptive field comprising an excitatory centre and horizontally-extending inhibitory flanks (Fig. 17). 3. Threshold contrast functions were measured for reversing-contrast bars (Figs. 11 and 12). The results for horizontally-oriented bars differ from those for vertically-oriented bars in a way that is consistent with the hypothesized neural organization. 4. Response to horizontally-moving, verticallyoriented gratings of various spatial frequencies were measured (Figs. 13 and 14) and the results used to infer the azimuthal angle\(\widetilde{\Delta \phi }\) between the visual axes of sampling units participating in directionally-selective movement detection (Fig. 18). At a mean luminance of 10 cd/m2, the inferred value of\(\widetilde{\Delta \phi }\) is approximately equal to the angle between the visual axes of adjacent ommatidia of a horizontal row, in the frontal eye region (Figs. 14, 18). 5. When the level of ambient light is decreased, the response characteristics of the DSMD neuron change in a way which suggests that, within the eye, the neural representation of the visual scene becomes coarser than the ommatidial mosaic. When mean luminance is lowered by 3 log units (from 10 cd/m2 to 0.01 cd/m2) the altered response characteristics suggest neuronal modifications such that the excitatory centres of the sampling units' receptive fields become 50% wider (Figs. 7 and 17), the inhibitory flanks become weaker and more diffuse, and\(\widetilde{\Delta \phi }\) increases by 30% (Figs. 14 and 18). Neuronal mechanisms that might mediate such changes are proposed and discussed. 6. The experimentally-measured characteristics of the DSMD neuron are compared with theoretically-predicted characteristics of an ideal movement detector, designed for optimum performance. This comparison suggests that the fly's movement-detecting pathway prefilters visual signals in such a way as to extract the most reliable movement cues, and that it analyzes the filtered information in a way that achieves maximum directional selectivity. The characteristics of the movement-detecting pathway vary with luminance in a way that ensures the best attainable performance at each level of ambient light (Fig. 21).

A framework for inverse tone mapping

Abstract: In recent years many tone mapping operators (TMOs) have been presented in order to display high dynamic range images (HDRI) on typical display devices. TMOs compress the luminance range while trying to maintain contrast. The inverse of tone mapping, inverse tone mapping, expands a low dynamic range image (LDRI) into an HDRI. HDRIs contain a broader range of physical values that can be perceived by the human visual system. We propose a new framework that approximates a solution to this problem. Our framework uses importance sampling of light sources to find the areas considered to be of high luminance and subsequently applies density estimation to generate an expand map in order to extend the range in the high luminance areas using an inverse tone mapping operator. The majority of today’s media is stored in the low dynamic range. Inverse tone mapping operators (iTMOs) could thus potentially revive all of this content for use in high dynamic range display and image based lighting (IBL). Moreover, we show another application that benefits quick capture of HDRIs for use in IBL.

Contrast sensitivity in evaluation of visual impairment due to diabetes

Abstract: Spatial contrast sensitivity of 19 diabetics with different degrees of visual impairment was studied. It was found that contrast sensitivity at intermediate and low spatial frequencies may decrease without corresponding loss of visual acuity. In advanced cases of diabetes the opposite may be true: contrast sensitivity was better than expected on the basis of visual acuity. Thus both visual acuity and contrast sensitivity measurements are useful in the evaluation of the nature of visual impairment due to diabetic eye disease.