Psychophysics of reading--II

AbstractVery 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 inlow 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-fieldloss and cloudy vs clear ocular media. Peakreading 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% ofthe 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 provideestimates ofthe,best reading performance to be expected from low-vision observers withcharacteristic forms ofvision 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.