PSYCHOPHYSICS OF READING .14. THE PAGE NAVIGATION PROBLEM IN USING MAGNIFIERS

Most people with low vision require magnification to read, A magnifier 's field of view often contains only a few letters at a time, Page nav igation is the process by which the reader moves the magnifier from wo rd to word, and from the end of one line to the beginning of the next line, Page navigation takes time and reduces reading speed, The major questions addressed in this paper are: (1) What role does page navigat ion play in limiting reading speed? and (2) Are the window width requi rements for reading (number of characters in the field for a criterion performance level) increased by the need for page navigation? We meas ured the reading speeds of three normal-vision and seven low-vision su bjects in two ways: with drifting-text requiring no page navigation, a nd with a closed-circuit TV (CCTV) magnifier which required page navig ation, We built special hardware to record the location of the CCTV's magnified field in the text, These recordings were used to separate fo rward-reading time (left-to-right movement through the text) from retr ace time (navigational movement), For normal-vision subjects, forward- reading and retrace times were about equal, For low-vision subjects, r etrace times were shorter than forward-reading times, indicating that the forward-reading performance was limited by visual, not navigationa l, demands, The retrace time did have an impact, however, ranging from 17 to 50% of the overall time, The window requirements for reading wi th page navigation (CCTV) were larger than those for reading without p age navigation (drifting-text). The difference was more than a factor of three for normal-vision subjects and close to a factor of two for l ow-vision subjects (10 characters for CCTV vs 5.2 characters for drift ing-text for 85% of maximum reading speed), Copyright (C) 1996 Elsevie r Science Ltd.