Psychophysics of reading. III. A fiberscope low-vision reading aid.

Existing reading aids providing magnification in excess of 6X (24 diopters) are awkward to use, expensive, or not portable. The authors have developed a new type of reading aid, a low-resolution fiberscope, that provides magnification up to 40X (160 diopters), and is easy to use, inexpensive, and portable. The fiberscope consists of an objective-lens assembly, a flexible bundle of optical fibers, and an eyepiece that together transmit an image from the page to the reader's eye. Complete construction details are included. The design is based on the authors' previous findings that reading requires a field of only four characters and a resolution of only 2 cycles/character. After only half an hour of training, a diverse group of low-vision readers read with the fiberscope at rates between 12 and 95 words/minute. Low-vision readers with central field loss had a median reading rate of only 18 words/min, whereas those with intact central fields had a median reading rate of 67 words/min, not far short of the median rate of 73 words/min for normal readers using the fiberscope. This is consistent with the authors' previous findings that low-vision readers with central field loss read much more slowly than those with intact central fields, even when optimal magnification is provided. The authors conclude that the fiberscope may be very useful as an inexpensive portable reading aid for people requiring high magnification.

Psychophysics of reading--I. Normal vision.

This paper is about the visual requirements for reading with normal vision. It is the first in a series devoted to the psychophysics of reading with normal and low vision. We have measured reading rates for text scanned across the face of a TV monitor while varying parameters that are important in current theories of pattern vision. Our results provide estimates of the stimulus parameters required for optimal reading of scanned text. We have found that maximum reading rates are achieved for characters subtending 0.3 degree to 2 degrees. Contrast polarity (black-on-white vs white-on-black text) has no effect. Reading rate increases with field size, but only up to 4 characters, independent of character size. When text is low-pass spatial-frequency filtered, reading rate increases with bandwidth, but only up to two cycles/character, independent of character size. When text is matrix sampled, reading rate increases with sample density, but only up to a critical sample density which depends on character size. The critical sample density increases from about 4 X 4 samples/character for 0.1 degree characters to more than 20 X 20 samples/character for 24 degrees characters. We suggest that one spatial-frequency channel suffices for reading.

Psychophysics of reading--II. Low vision.

Very little is known about the effects of visual impairment on reading. We used psychophysical methods to study reading by 16 low-vision observers. Reading rates were measured for text scanned across the face of a TV monitor while varying parameters that are likely to be important in low vision: angular character size, number of characters in the field, number of dots composing each character, contrast polarity (white-on-black vs black-on-white text), and character spacing. Despite diverse pathologies and degrees of vision loss in our sample, several major generalizations emerged. There is a wide variation in peak reading rates among low-vision observers, but 64% of the variance can be accounted for by two major distinctions: intact central fields vs central-field loss and cloudy vs clear ocular media. Peak reading rates for observers with central-field loss were very low (median 25 words/minute), while peak reading rates for observers with intact central fields were at least 90 words/minute (median 130 words/minute). Most low-vision readers require magnification to obtain characters of optimal size. Sloan M acuity was a better predictor of optimal character size than Snellen acuity, accounting for 72% of the variance. Low-vision reading is similar to normal reading in several respects. For example, both show the same dependence on the number of characters in the field. Our results provide estimates of the best reading performance to be expected from low-vision observers with characteristic forms of vision loss, and the stimulus parameters necessary for optimal performance. These results will be useful in the development of clinical tests of low vision, and in the design of low-vision reading aids.