Does color improve object recognition? If so, is the improvement greater for images with low spatial resolution in which there is less shape information? Do people with low visual acuity benefit more from color? Three experiments measured reaction time (RT) and accuracy for naming food objects displayed in 4 types of images: gray scale or color, and high or low spatial resolution (produced by blur). Normally sighted Ss had faster RTs with color, but the improvement was not significantly greater for images with low spatial resolution. Low vision subjects were also faster with color, but the difference did not depend significantly on acuity. In 2 additional experiments, it was found that the faster RTs for color stimuli were related to objects' prototypicality but not to their color diagnosticity. It was concluded that color does improve object recognition, and the mechanism is probably sensory rather than cognitive in origin.
The visual span in reading is the number of characters that can be recognized at a glance. The shrinking visual span hypothesis attributes reading deficits in low vision, and slow reading in normal vision at low contrast, to a reduction in the visual span. This hypothesis predicts that reading time (msec/word) becomes increasingly dependent on word length as text contrast decreases. We tested and confirmed this prediction using the rapid serial visual presentation (RSVP) method. Estimates of the visual span ranged from about 10 characters for high-contrast text to less than two characters for low-contrast text. Eye-movement recordings showed that longer reading times at low contrast are partitioned about equally between prolonged fixation times and an increased number of saccades (presumably related to a reduced visual span). RSVP measurements for six out of seven low-vision subjects revealed a strong dependence of reading time on word length, as expected from reduced visual spans.
There is a growing consensus that clinical evaluation of the real-world consequences of eye disease requires new performance-based tests. This is because Snellen acuity and other common clinical tests are often poor predictors of everyday function. Ahn and Legge [(1995) Vision Research, 35, 1931-1938] validated a computerized test of reading speed by showing that it provides an accurate prediction of low-vision reading performance with magnifiers. Here, we describe development of a printed-card version of the test suitable for clinical use. This printed-card test retains key design features of the validated computerized test, including the same set of sentences and display format. Data from 23 low-vision subjects showed that a very simple testing procedure using printed cards and a stop watch could be used effectively to estimate reading speed. Reading speed based on a single card was quite accurate (SD equal to about 18% of the mean) and showed no practice effects from one card to the next. Reading speeds obtained with printed cards correlated highly (r = 0.887) with those from computerized testing. We conclude that a simple test, using printed cards, can be used to obtain useful estimates of low-vision reading speed.
Text can be depicted by luminance contrast (i.e., differences in luminance between characters and background) or by color contrast (i.e., differences in chromaticity). We used a psychophysical method to measure the reading speeds of eight normal and ten low-vision subjects for text displayed on a color monitor. Reading speed was measured as a function of luminance contrast, color contrast (derived from mixtures of red and green), and combinations of the two. When color contrast is high, normal subjects can read as rapidly as with high luminance contrast (greater than 300 words/min). Curves of reading speed versus contrast have the same shape for the two forms of contrast and are superimposed when contrast is measured in multiples of a threshold value. When both color and luminance contrast are present, there is no sign of additive interaction, and performance is determined by the form of contrast yielding the highest reading rate. Our findings suggest that color contrast and luminance contrast are coded in similar ways in the visual system but that the neural signals used in letter recognition are carried by different pathways for color and luminance. We found no advantages of color contrast for low-vision reading. For text composed of 6 degrees characters, all low-vision subjects read better with luminance contrast than with color contrast.
The term graphical perception refers to the part played by visual perception in analyzing graphs. Computer graphics have stimulated interest in the perceptual pros and cons of different formats for displaying data. One way of evaluating the effectiveness of a display is to measure the efficiency (as defined by signal-detection theory) with which an observer extracts information from the graph. We measured observers' efficiencies in detecting differences in the means or variances of pairs of data sets sampled from Gaussian distributions. Sample size ranged from 1 to 20 for viewing times of 0.3 or 1 sec. The samples were displayed in three formats: numerical tables, scatterplots, and luminance-coded displays. Efficiency was highest for the scatterplots (approximately equal to 60% for both means and variances) and was only weakly dependent on sample size and exposure time. The pattern of results suggests parallel perceptual computation in which a constant proportion of the available information is used. Efficiency was lowest for the numerical tables and depended more strongly on sample size and viewing time. The results suggest serial processing in which a fixed amount of the available information is processed in a given time.
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