What retinal area scans text when central vision is lost?
 
Sharma MK1, Timberlake GT1,2, Gobert D1,2 , Schuchard RA3, Maino JH1,2
 
1Kansas City VA Medical Center, 2University of Kansas Medical Center, 3Atlanta VA Medical Center
 
Objective: When normally sighted individuals read, high-speed saccadic eye movements shift text characters onto the retinal area of highest acuity, the fovea, where they are "inspected" and processed. When foveal vision is lost due to macular disease, many people are still able to read (slowly) using a peripheral retinal area. The retinal location of this peripheral "text scanning area" is not known and cannot be deduced from eye movement recordings. Our objective was to determine the retinal area used to scan text by digital image analysis of Scanning Laser Ophthalmoscope (SLO) video images that show text on the retina during reading.
 
Methods: Participants with normal vision or with bilateral macular scotomas read simple three-line sentences presented in the SLO. Videotaped images showing text on the retina during reading were digitized. Retinal "text maps" were created by measuring the coordinates (in pixels) of a retinal vessel landmark in each video frame. The coordinates were then used to shift a white-on-black image of the text. After each shift, grayscale values were added. In the resulting image, bright regions correspond to retinal areas on which text was placed most frequently; darker regions correspond to areas on which text was infrequently placed (see inset). Final retinal maps show areas used for text scanning and for fixating a small target.
 
Results: As expected, the brightest area of the text map corresponded to the central fovea for normally sighted subjects. Their retinal text maps also showed a highly ordered five-line structure, a result of scanning three text lines. Subjects with central visual loss all used a unique peripheral retinal area to scan text characters. In some instances, the brightest area of the text map corresponded to the retinal area the subject also used to fixate small visual stimuli. In other instances, the fixation area and text scanning area did not correspond. Retinal text maps of those with central visual loss lacked the orderly five-line structure, were larger, and less distinct than those of normal readers.
 
(Sharma)
 
Conclusions: We have shown that the retinal area used to scan text during reading can be accurately delineated using SLO retinal text maps. These maps are equivalent to a photographic time exposure in which the retina is the "film." Subjects without foveal vision use a consistent, unique retinal area to scan text. This area is not always the same as the area they use to inspect small, stationary objects. Rehabilitation specialists who wish to improve the reading abilities of patients with macular disease may find it useful to understand the retinal location a patient uses for scanning text in relation to the scotomatous area of central visual loss.
 
Funding Acknowledgment: Supported by VARR&D Grants C838-RA & C1740R.