The meridian effect on the cortical magnification factor for visual word form identification
33.529, Sunday, May 12, 8:30 am - 12:30 pm, Vista Ballroom
Li-Ting Tsai1, Chien-Chung Chen2, Yuh Jang1, Kuo-Meng Liao3; 1School of Occupational Therapy, College of Medicine, National Taiwan University, Taiwan., 2Department of Psychology, National Taiwan University, Taiwan., 3Department of Endocrinology and Metabolism, Zhongxiao Branch, Taipei City Hospital, Taiwan.
Previous studies of visual word form identification focused on the performance in the foveal vision. However, several eye diseases, such as age-related macular degeneration and diabetic retinopathy, would cause central vision problems. Improving peripheral form vision, such as eccentric viewing training and peripheral perceptual learning, is an important visual rehabilitation method for people with central visual deficit. The alternative location of foveal vision depends on the deficit position and personal preferred retinal loci. To develop a tool to assess the ability of word identification in all meridians, we thus measured the size threshold for identifying visual word forms in the fovea and eccentric vision on vertical and horizontal meridians. The stimuli were16 traditional Chinese characters selected for similarity in legibility and coverage of different spatial configurations. The characters were presented at the fovea, and at 1 degree, 2 degrees, 4 degrees, 6 degrees, and 8 degrees eccentricity in the upper, lower, right, and left meridians. The task of the observer was to press a key to indicate the corresponding character. Five observers with normal visual acuity were tested either monocularly, or binocularly. At every location, the size threshold was measured with a staircase procedure at four to seven contrast levels (10-80%). The fitted cortical magnification factors ("E2") on the vertical meridian was significantly smaller than those on the horizontal meridian. E2 on the upper median was smaller than that on the lower meridian. Such asymmetry was not found between the left and the right meridians. E2 in the binocular condition depended on both meridian and contrast. In sum, there is different meridian effect on the cortical magnification factors between the vertical and horizontal meridians and between monocular and binocular viewing. This latter suggests that a nonlinear binocular contrast summation process was involved in the meridian effect.