Showing papers by "Eli Peli published in 1996"

Contrast perception across changes in luminance and spatial frequency.

Abstract: The interaction of the effects of luminance and spatial frequency on perception of suprathreshold contrast was studied with use of a contrast-matching paradigm. Four subjects matched the appearance of Gabor patches at different luminances and spatial frequencies. The contrast of a 1-octave Gabor test patch at one of five frequencies [1–16 cycles/degree (c/deg) in 1-octave steps] and at one of seven mean luminance levels (0.5–50 cd/m2 in 1/3-log-unit steps) was matched, by the method of adjustment to a standard patch of 3 c/deg at 50 cd/m2 at a nominal contrast of 0.3. For each block of trials the spatial frequency of the test patch was randomly changed (three repetitions at each frequency per block) while the luminance was fixed. The subject regularly shifted fixation between the two targets in response to a metronome tone every 1.5 s. Contrast constancy was demonstrated across the entire luminance range tested for all but the two highest frequencies. For 8 c/deg the perceived test contrast was reduced only when the luminance was less than 2 cd/m2. For 16 c/deg, perceived contrast decreased linearly (with a slope of −1/2 on a log scale) with decreases in luminance across the entire luminance range. As at threshold, reduction in luminance across the levels commonly available on a CRT display has only minimal effects on low-frequency suprathreshold contrast perception. However, the apparent contrast of high-frequency features, in binocular free-viewing conditions, is rapidly reduced with a local reduction in screen luminance. This effect has important implications for visual models used in image-quality analysis.

Evaluating Visual Information Provided by Audio Description.

Abstract: 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...

The role of context in reading with central field loss.

Abstract: It has been suggested that readers with central field loss (CFL) may not be able to use context to facilitate reading in the same way that normally sighted readers do because their processing capacity is fully utilized decoding the degraded visual stimulus. If true, this could account for their slow reading, even when text is appropriately magnified. Readers with CFL and normally sighted, age-matched controls read sentences and lists of random words from two dynamic text displays. We used sentence-gain (the ratio of reading rates for sentences to random words) to assess the use of context. Sentence-gain was equivalent across groups. Therefore, reduced reliance on context cannot explain the slower reading rates of people with CFL.

Test of a model of foveal vision by using simulations

Abstract: The appearance of four different images from three different distances was simulated by using the individual contrast sensitivity functions (CSF's) of normally sighted observers. The simulations were generated by using the observers' CSF's as a threshold in a pyramidal vision model of band-limited local contrast [J. Opt. Soc. Am A 7, 2030 (1990)]. Simulations based on CSF's obtained in an orientation discrimination task underestimated the observer's sensitivity in discriminating the images. Simulations based on CSF's obtained in a detection task provided a good estimate of observer's performance. The testing method was shown to be sensitive enough to be affected by the high-frequency residual, which is frequently ignored in visual models and simulations. An image-dependence effect found when the high-frequency residual was present was eliminated when the residual artifact was removed. The simulations based on the pyramidal vision model accurately predicted the distance at which they were discriminated from the original image, and thus this model may also serve as the basis for image-quality metrics. The testing method developed can also be used to determine the type of CSF that best represents observer performance in a task.

The necessary field of view to read with an optimal stand magnifier.

Abstract: BACKGROUND For most people with low vision, some form of magnification is necessary to read. Using a magnifier reduces the number of letters that can be seen simultaneously (field of view), which has been shown to decrease reading rates. This study sought to determine how many letters are necessary to attain maximal reading rates with a stand magnifier METHODS Younger and older normally-sighted and visually-impaired observers read short passages using a fiber optic stand magnifier (taper). The optical properties of this magnifier allowed the field of view to be precisely varied. Each subject read using at least four field sizes (3, 5, 9, and 13 characters visible) while reading speed was measured. RESULTS Reading rates continued to increase with as many as 13 characters visible, regardless of age or vision status. In addition, reading with the taper was slower for all subject groups than reading without the magnifier. CONCLUSIONS This study confirms reports that reading rates increase as the field of view increases when reading from a page of text The need for this large field of view relative to other low vision reading aids (i.e., computer-generated scroll displays) is likely the result of the readers' need to actively navigate across the page of text. low vision, magnifier, field of view, reading rates, taper

Visually impaired observers require a larger window than normally sighted observers to read from a scroll display.

Abstract: BACKGROUND As text is magnified, the number of letters that can be displayed with a magnifier or other reading aid at any one time (window size) reduced. There is a great deal of evidence that reading rates increase as the number of letters displayed increases. The minimum number of proportionally spaced letters necessary to read at maximal rates from a scroll display, in which the text is continuously panned across a television screen, is not known. METHODS Visually impaired (VI)and normally sighted (NA) observers were asked to read either sentences or random words scrolled across a computer screen. The window size was varied so that from one to 12 characters were visible, on average, at any one time. Each subject read with six different window sizes within the range of one to 12 characters visible. RESULTS For the NA group, a window of four to five characters was necessary to read at maximal rates. VI subjects, as a group, required a significantly larger window of six to seven characters to reach maximal reading rates. CONCLUSIONS The window size requirement for the NA group is close to the size other investigators have reported for a fixed-width font However, the VI group required a larger window than did the NA group, which has not been previously reported. This requirement could result from the additional stimulation available with a larger window to entrain the eyes' motion when reading from a passively scrolled display. low vision, reading rates, scroll display, computer screen, window size

Contrast of slightly complex patterns: computing the perceived contrast of Gabor patches

Abstract: The local contrast in an image may be approximated by the contrast of a Gabor patch of varying phase and bandwidth. In a search for a metric for such local contrast, perceived (apparent) contrast, as indicated by matching of such patterns, were compared here to the physical contrast calculated by a number of methods. The 2 cycles/deg 1-octave Gabor patch stimuli of different phases were presented side by side separated by 4 degrees. During each session the subjects (n equals 5) were adapted to the average luminance, and four different contrast levels (0.1, 0.3, 0.6, and 0.8) were randomly interleaved. The task was repeated at four mean luminance levels between 0.75 and 37.5 cd/m2. The subject's task was to indicate which of the two patterns was lower in contrast. Equal apparent contrast was determined by fitting a psychometric function to the data from 40 to 70 presentations. There was no effect of mean luminance on the subjects settings. The matching results rejected the hypothesis that either the Michelson formula or the King-Smith & Kulikowski contrast (CKK equals (Lmax-Laverage)/Laverage) was used by the subjects to set the match. The use of the Nominal contrast (the Michelson contrast of the underlying sinusoid) as an estimate of apparent contrast could not be rejected. In a second experiment the apparent contrast of a 1-octave Gabor patch was matched to the apparent contrast of a 2-octave Gabor patch (of Nominal contrast of 0.1, 0.3, 0.6, 0.8) using the method of adjustment. The result of this experiment rejected the prediction of the Nominal contrast definition. The local band limited contrast measure (Peli, 1990), when used with the modifications suggested by Lubin (1995), as an estimate of apparent contrast could not be rejected by the results of either experiment. These results suggest that a computational contrast measure based on multi scale bandpass filtering is a better estimate of apparent perceived contrast than any of the other measures tested.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.