The physical contrast of simple images such as sinusoidal gratings or a single patch of light on a uniform background is well defined and agrees with the perceived contrast, but this is not so for complex images. Most definitions assign a single contrast value to the whole image, but perceived contrast may vary greatly across the image. Human contrast sensitivity is a function of spatial frequency; therefore the spatial frequency content of an image should be considered in the definition of contrast. In this paper a definition of local band-limited contrast in images is proposed that assigns a contrast value to every point in the image as a function of the spatial frequency band. For each frequency band, the contrast is defined as the ratio of the bandpass-filtered image at the frequency to the low-pass image filtered to an octave below the same frequency (local luminance mean). This definition raises important implications regarding the perception of contrast in complex images and is helpful in understanding the effects of image-processing algorithms on the perceived contrast. A pyramidal image-contrast structure based on this definition is useful in simulating nonlinear, threshold characteristics of spatial vision in both normal observers and the visually impaired.

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Clinicians will find these novel diagrams, diagramming techniques, and analyses valuable when prescribing prismatic aids for hemianopia and when designing new prism devices for patients with various types of field loss.

Application of image processing for the visually impaired is discussed. Image degradation in the low vision patient's visual system can be specified as a transfer function obtained by measurements of contrast sensitivity. The effectiveness of adaptive image enhancement for printed pictures is demonstrated using an optically simulated cataractous lens.

Retinal nerve fiber layer (RNFL) photographs from 26 patients with Alzheimer's disease and 23 normal, age-matched, control subjects were reviewed for quality and abnormalities by two observers. A higher proportion of Alzheimer's patients showed RNFL abnormalities when compared to control subjects. There was some disagreement between the two observers regarding quality and frequency of abnormalities, reflecting suboptimal quality of the photographs obtained in patients with advanced Alzheimer's disease. Although these findings add to the clinical and histopathological evidence that ganglion cell degeneration occurs in Alzheimer's disease, the difficulty in obtaining and evaluating retinal nerve fiber layer photographs, especially in advanced cases, may limit the clinical usefulness of retinal nerve fiber layer analysis in such patients.

Short segments of two TV programs without audio description (AD) were presented to 25 subjects with low vision and 24 subjects with normal vision, and 29 additional subjects heard only the standard audio portions. The subjects then answered questions based on the visual information contained in the AD of the programs. The subjects with normal vision performed the best, followed by those with low vision and those who heard only the audio portion; all performed at better than chance levels. The results indicate that although AD may provide information on visual details to visually impaired audiences, some of the information in the AD may be obtained from the standard audio portion.

PurposePeripheral field loss (PFL) due to retinitis pigmentosa, choroideremia, or glaucoma often results in a highly constricted residual central field, which makes it difficult for patients to avoid collision with approaching pedestrians. We developed a virtual environment to evaluate the ability of patients to detect pedestrians and judge potential collisions. We validated the system with both PFL patients and normally sighted subjects with simulated PFL. We also tested whether properly placed high-power prisms may improve pedestrian detection.MethodsA virtual park-like open space was rendered using a driving simulator (configured for walking speeds), and pedestrians in testing scenarios appeared within and outside the residual central field. Nine normally sighted subjects and eight PFL patients performed the pedestrian detection and collision judgment tasks. The performance of the subjects with simulated PFL was further evaluated with field of view expanding prisms.ResultsThe virtual system for testing pedestrian detection and collision judgment was validated. The performance of PFL patients and normally sighted subjects with simulated PFL were similar. The prisms for simulated PFL improved detection rates, reduced detection response times, and supported reasonable collision judgments in the prism-expanded field; detections and collision judgments in the residual central field were not influenced negatively by the prisms.ConclusionsThe scenarios in a virtual environment are suitable for evaluating PFL and the impact of field of view expanding devices.Translational RelevanceThis study validated an objective means to evaluate field expansion devices in reproducible near-real-life settings.

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